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Physics at the LHC and sLHC
Nuclear Instruments and Methods in Physics Research A 636 (2011) S1–S7
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Physics at the LHC and sLHC Karl Jakobs Physikalisches Institut, Hermann-Herder Str. 3, 79104 Freiburg, Germany
On behalf of the ATLAS and CMS Collaborations a r t i c l e in fo
abstract
Available online 4 June 2010
Over the coming years the experiments at the Large Hadron Collider (LHC) will explore the TeV energy scale and will most likely answer fundamental questions of particle physics. In the focus of the experiments are the search for physics beyond the Standard Model and the investigation of the nature of electroweak symmetry breaking. In order to fully exploit the physics potential of the LHC facility a high luminosity upgrade with luminosities of 1035 cm 2 s 1 is envisaged. In the present article the physics potential of the LHC is summarized. For various physics scenarios it is also discussed how an upgraded high luminosity collider, the so called super-LHC (sLHC), could extend the physics reach of the LHC. & 2010 Elsevier B.V. All rights reserved.
Keywords: LHC sLHC Higgs physics Search for supersymmetry
1. Introduction After a construction time of more than 10 years, the Large Hadron Collider (LHC) will deliver first proton–proton collisions this year. After running in an initial phase at a centre-of-mass energy of 7 TeV, it is anticipated that the design energy of 14 TeV and the design luminosity of 1034 cm 2 s 1 will be reached. This machine will open up the possibility to explore the TeV energy range, which plays a key role in the investigation of the electroweak symmetry breaking and in the search for physics beyond the Standard Model. The experiments ATLAS and CMS have been designed and optimized to cover a large spectrum of possible physics signatures. In the main focus will be the search for the Higgs boson as well as for particles predicted by supersymmetry or technicolor theories, new gauge bosons and searches for composite quarks and leptons. Besides the discovery potential for new physics the experiments also have a large potential to perform precision measurements of Standard Model parameters, like measurements of the W and top quark masses or triple gauge boson couplings. Many of the Standard Model measurements will, in particular at the beginning of the data-taking period, be used for a detailed understanding of the detector performance and for an optimization of reconstruction algorithms. In order to exploit the full physics capabilities of the LHC a high-luminosity upgrade to reach luminosities of 1035 cm 2 s 1, hereafter referred to as super-LHC (sLHC), is planned. The physics reach of this collider will certainly depend on the scenarios realized in nature and can be more precisely determined once new physics at the LHC will have been established. However, such a high-luminosity machine will in general extend the discovery
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reach in the high-mass region (e.g. quark compositeness, new heavy gauge bosons, multi-TeV squarks and gluinos, extra dimensions) and improve the accuracy in the determination of the parameters of new physics possibly discovered at the LHC. In addition, the sensitivity to rare processes (e.g. rare Higgs boson decays, Higgs boson pair production, multi gauge boson production and FCNC top-quark decays) can be extended and the accuracy in the determination of Standard Model parameters (e.g. triple and quartic gauge couplings, couplings of the Higgs boson to fermions) can be significantly improved. In the following the physics potential of the LHC experiments is briefly summarized. For several scenarios it is discussed what additional insight could be gained at the sLHC. It should be stressed that the sLHC predictions are preliminary and indicative since the running conditions of the sLHC machine and the performance of the upgraded detectors are not yet precisely known. For details on the LHC physics potential the reader is referred to Refs. [1–3], the physics estimates of the sLHC are based on the report published in Ref. [4].
2. Status of the accelerator and the experiments The two general purpose experiments ATLAS and CMS have been assembled and installed prior to the first operation of the LHC in September 2008. As an example the insertion of the ATLAS endcap calorimeter into the toroid system is shown in Fig. 1. The CMS experiment has first been assembled and many detector components have been commissioned in the surface hall, before the various slices of the experiment were lowered down into the experimental hall. Meanwhile the commissioning is well advanced and both experiments have used the time since the LHC machine
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Table 1 Cross-sections at a centre-of-mass energy of 14 TeV and approximate numbers of expected events per second and per year for some important physics processes at low luminosity (1033 cm 2 s 1). Process
s (pb)
Events/s
Events/year
W-ev Z-e þ e tt
1.5 104 1.5 103 800 5 108
15 1.5 0.8 5 105
108 107 107 1012
1
0.001
104
10 105
0.01 100
105 109
bb g~ g~ ðmg~ ¼ 1 TeV=c2 Þ H (mH ¼ 200 GeV/c2) Inclusive jets pT 4200 GeV=c
Fig. 1. Picture of the installation of the ATLAS detector before the endcap calorimeter system is moved into the toroid system (May 2006). Four of the eight superconducting toroid coils are visible.
accident in September 2008 to collect large samples of cosmic events. They allowed to align and calibrate the various subdetector at a level higher than anticipated. Both experiments look forward to record first collisions before the end of the year 2009. On the machine side an extensive repair and consolidation programme was carried out. In total 39 dipole and 14 quadrupole magnets were replaced, nearly 900 new helium pressure release ports were installed, electrical interconnections repaired and the quench detection system improved. It is expected that first collision data for physics will be provided at a centre-of-mass energy of 7 TeV in 2009/10 and that an integrated luminosity of about 200 pb 1 can be collected.
3. LHC running scenarios and cross-sections It is expected that after the first run in 2009/10 an initial luminosity of 1033 cm 2 s 1 (hereafter called low luminosity) can be achieved. During the further years of operation, this value should rise to the design luminosity of 1034 cm 2 s 1 (hereafter called high luminosity). Integrated luminosities of 10 and 100 fb 1 per year should therefore be collected at low and high luminosity respectively. The expected cross-sections at a centre-of-mass energy of 14 TeV and the corresponding event rates at low luminosity are given in Table 1. For the reduced centre-of-mass energy of 7 TeV the reduction in cross-sections depends on the energy scale of the final state and is largest for heavy objects with masses in the TeV range. For the pair production of top quarks, for example, a reduction factor of about four is found. In the initial phase at 14 TeV at low luminosity, almost 50 W and five Z bosons decaying to lepton pairs will be produced every second, as well as one tt pair and 500,000 bb pairs. The copious tt production will constitute a significant background for many searches of new physics signals since it may lead to characteristic final states with leptons, jets and missing transverse energy Emiss . On the other T hand it can be used to determine important detector performance parameters in the early running. Unless stated otherwise, the physics reach discussed in the following is based on a centre-of-mass energy of 14 TeV.
extraction of trigger and reconstruction efficiencies as well as for the understanding of the measurement of the missing transverse energy, Emiss , which is vital for the search for physics beyond the T Standard Model. The initial measurements of the W and Z boson cross-sections are expected to be limited by uncertainties on the luminosity which are estimated to be at the level of 5–10% during the first 1–2 years. It is also expected that a top production signal can already be established after a few months of running at a luminosity of 1032 cm 2 s 1. Such a signal can be identified after applying simple cuts, i.e. by requiring one lepton with pT 420 GeV=c and four jets with pT 4 40 GeV=c within jZj o 2:5 without b-tagging and kinematical fits. Such a sample is ideally suited to establish the jet energy scale via the W-qq and t-Wb decay chains appearing in tt events as well as to determine the b-tagging performance from early data [3]. In addition, the experiments must be open and unbiased for early surprises and unexpected discoveries with early data. Therefore unbiased inclusive measurements of the lepton, dilepton, jet- and missing transverse energy spectra are essential. To illustrate the performance, a 5sdiscovery of a new heavy vector boson Z 0 with Standard-Model-like couplings and with a mass of 1 (1.5) TeV/c2 will be possible after collecting integrated luminosities of 70 pb 1 (300 pb 1) at a centre-of-mass energy of 14 TeV [2,3]. First data on QCD jet production will be used to test perturbative QCD and to look for deviations from the Standard Model. Given the new energy regime, higher transverse jet energies where, e.g. first signs of compositeness could show up, can be rapidly probed. Even if relatively large jet energy scale uncertainties of the order of 10% for early data are assumed, compositeness scales of L ¼ 3 TeV can already be probed with data corresponding to an integrated luminosity of only 10 pb 1 at 14 TeV [2,5]. This is close to the present Tevatron reach, where compositeness scales of 2.9 TeV are excluded at the 95% C.L. [6]. The sensitivity at the LHC is largely increased with higher integrated luminosities and compositeness scales of L ¼ 40 and 60 TeV can be excluded at the 95% C.L. for 300 and 3000 fb 1 respectively [4]. It should be noted that the accumulation of 3000 fb 1 requires running the sLHC for several years.
5. Precision measurements of standard model parameters 5.1. Measurement of the W mass
4. Standard model measurements with early data pffiffi Even during the early running phase at s ¼ 7 TeV important Standard Model reference signals can be established. The observation of the W and Z bosons will be important for the
At the time of the LHC start-up, the W mass will be known with a precision of 25 MeV=c2 from measurements at the Tevatron and LEP2. The motivation to improve this result is mainly that precise measurements of the W mass, of the top mass
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and of the Higgs mass will provide stringent tests of the consistency of the underlying theory. At the LHC, 60 million well-reconstructed W-‘v decays (where ‘ ¼ e or m) should be collected by each experiment in one year of data taking at low luminosity. The statistical error on the W mass measurement is therefore expected to be small ð o 2 MeV=c2 Þ. The systematic error will arise mainly from the Monte Carlo reliability in reproducing the data, i.e. the physics and the detector performance. Uncertainties related to the physics result from the limited knowledge of the WpT spectrum, structure functions, the W width and from W radiative decays. Uncertainties related to the detector result from the limited knowledge of the absolute lepton energy scale and the detector energy/ momentum resolution and response. Many of these uncertainties (lepton scale, detector resolution and response and the WpT spectrum) will be constrained in the experiment by using the high-statistics sample of leptonic Z decays. First studies [1] indicated that the expected uncertainties on the W mass measurement are at a level of 15–20 MeV/c2, mainly dominated by the knowledge of the the lepton energy scale. In a recent study the principle limits of the precision of the W mass measurement at the LHC have been investigated [7]. Here, the method to use the very large sample of Z-‘‘ decays to fix the lepton energy scale is refined and carried out in a more differential way to uniform the lepton energy scale across the whole detector acceptance. In addition, kinematical distributions for the W production are constrained by the corresponding distributions from Z decays. If it can be assumed that the radiative corrections are knows with LEP-precision, the total error on the W mass is estimated to reach values around 6–7 MeV/c2. This should probably be considered as an experimental limit which sets at the same time the scale for the required precision of radiative corrections. Since this measurement is limited by systematic uncertainties, no further improvement is expected at the sLHC, where the relevant uncertainties will be difficult to control.
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sensitivity to FCNC top-quark decays [4]. In particular in the case of t-qZ the improvement is almost linear with the luminosity due to the very low background level and branching ratios of the order of 10 6 are achievable. In order to reach this sensitivity, the b-tagging performance of the detectors must be kept at its present value, which requires superb semiconductor tracking detectors at the sLHC.
6. Search for supersymmetry 6.1. The standard LHC scenario If Supersymmetry (SUSY) [8] exists at the electroweak scale, its discovery at the LHC should be straightforward. The SUSY crosssection is dominated by gluinos and squarks, which are strongly produced with cross-sections comparable to the Standard Model backgrounds at the same Q2. Gluinos and squarks then decay via a series of steps into the lightest supersymmetric particle (LSP), which may itself decay, if R-parity is violated. These decay chains lead to a variety of signatures involving multiple jets, leptons, photons, heavy flavours, W and Z bosons, and missing transverse energy. The combination of a large production cross-section and distinctive signatures makes it easy to separate SUSY from the Standard Model background. In a first step of SUSY searches at the LHC multijet events with large missing transverse momentum will be studied. An excess at large Emiss would provide sensitivity to squarks and gluinos up to T the TeV energy range. The discovery range for squarks and gluinos is shown in Fig. 2 as a function of the universal scalar mass m0 and of the universal gaugino mass m1/2 in the framework of the minimal supergravity (mSUGRA) model [9]. Already for an integrated luminosity of only 1 fb 1, the reach in the jets + Emiss T channel extends to squark and gluino masses of the order of
3000 miss
E T + jets 2500
q (4
ly excluded
g(4000)
Theoretical
1500
5.3. Rare top-quark decays
1000 Most rare decays of the top quark expected in the Standard Model are beyond reach at the LHC. However, there is a large class of theories beyond the Standard Model where branching ratios for decays of top quarks induced by flavour-changing neutral currents (FCNC) be as large as 10 5–10 6. Studies of the ATLAS and CMS collaborations [1,2] of various FCNC decay modes (t-qg, t-qZ and t-qg where q¼u or c) indicate that the data which can be collected with a luminosity of 1034 cm 2 s 1 are not sufficient to explore these models. With a detector performance comparable to that expected at 1034 cm 2 s 1, the sLHC should enhance by a large factor the
√s = 28 TeV : 100 fb-1
:
2000 m1/2 (GeV)
At the LHC, top quark measurements will benefit from the large tt event samples, so that not only the mass and the production cross-section, but also branching ratios, couplings and rare decays can be studied in detail. The best channel for the top mass measurement will most likely be tt production with one W decaying leptonically and the other one hadronically. The top mass will be determined from the hadronic part of the decay, as the invariant mass of the three jets originating from the same top (mt ¼ mjjb). The associated leptonic top decay will be used to trigger the events and to suppress backgrounds. All together, a total uncertainty in the range of 1 GeV=c2 or better should be achieved [2,3]. Also in this case more data at the sLHC do not offer obvious improvements.
m( τ ) < m( ~ 0 χ1) 1
5.2. Measurement of the top quark mass
000
)
√s = 14 TeV : 1000 fb-1 , 2000 fb-1 g(3000) q (3 00
0)
g(2500)
q(
√s = 14 TeV : 100 fb -1 , 200 fb-1
25 0
0)
500 0
500
1000
1500
2000
m0 (GeV) Fig. 2. The expected 5s LHC discovery contours for squarks and gluinos in the (m0/m1/2) parameter plane of mSUGRA for various assumptions on the integrated luminosity and centre-of-mass energy. The mSUGRA parameters are chosen to be A0 ¼0, tanb ¼ 10, and m o 0.
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1.5 TeV/c2. These mass limits can be extended up to about 2.5 TeV/c2 for an integrated luminosity of 100 fb 1. SUSY cascade decays give also rise to lepton, b-jet and tau signatures. Selecting various multilepton final states similar regions in the (m0, m1/2) plane can be probed [2,3]. The main challenge at the LHC is not to discover SUSY but to separate the many SUSY processes that occur and to measure the masses and other properties of the SUSY particles. The approach followed by the collaborations has been to investigate in detail the signatures for particular points in the parameter spaces of the minimal supergravity (mSUGRA) model, Gauge Mediated SUSY breaking (GMSB) models [10], and R-parity violating models [8]. Methods such as looking for kinematic endpoints for mass distributions and using these to determine combinations of masses have proven generally useful. As an example, the production of q~ L q~ R , followed by the decays q~ L -qw02 , w02 -‘~ R ‘, ‘~ R -‘w01 , can be selected in an inclusive way by requiring two leptons in the final state with the same flavour and opposite sign, large Emiss and jet multiplicity [2,3]. The dilepton T mass distribution shows a very sharp end-point, which is due to the kinematic properties of the decay and depends on the masses of the particles involved, the two lightest neutralinos and the slepton, through a simple kinematic relation. The position of the end-point can be measured with a precision of 500 MeV/c2 (0.5%) with an integrated luminosity of 30 fb 1, thus providing a combined constraint on the three masses mentioned above. Another example is the search for the h-bb decay in SUSY events. If the two-body decay w02 -w01 h is kinematically allowed, it generally has a substantial branching ratio because the light neutralinos are dominantly gauginos. In many cases it is possible to reconstruct the h-bb as a resonance peak in the SUSY event sample, after applying simple kinematic cuts. An Emiss requirement T usually suppresses the Standard Model background. This signal may be easier to detect than h-gg and so it may provide the discovery mode for the light Higgs boson, although the gg signal is still important to measure the mass precisely. The Higgs signal can also provide a good starting point for further analysis of SUSY particles. Given the success in extracting precise measurements for the points studied and the large number of SUSY events expected at the LHC, the experiments are likely not just to discover SUSY, if it exists, but to make many precise measurements. The starting point in this study will be to look for characteristic deviations from the Standard Model. In SUGRA and some other models, there would be events with multiple jets and leptons plus large Emiss . In T GMSB models, there would be events with prompt photons or quasi-stable sleptons. In R-parity violating models, there would be events with very high jet multiplicity and/or leptons. Any such signal would point to possible classes of models and would indicate the rough mass scale. The next step would be to use partial reconstruction methods, like the ones discussed above, to try to constrain as many combinations of masses as possible.
6.2. Extensions at the sLHC It can be seen from Fig. 2 that a luminosity upgrade would extend the mass reach for squarks and gluinos from about 2.5 TeV (standard LHC) to about 3 TeV (sLHC). This performance does not require major detector upgrades because these inclusive searches are based mainly on calorimetric measurements of high-pT jets and large missing transverse energy. On the other hand, reconstruction of more exclusive decay chains which may be rate-limited at the LHC, such as some cascade decays of heavy gauginos, could become possible at the sLHC provided the full detector functionality is preserved. Two points of mSUGRA space where the squark and gluino masses
exceed 2 TeV/c2 have been studied in some detail [4,11]. The expected event rates are small at the standard LHC and detailed SUSY studies will not be possible. The tenfold increase in rate at the sLHC allows to extract also for such cases valuable information about the underlying theory.
7. The search for the Higgs boson 7.1. Standard Model Higgs boson The Standard Model Higgs boson is searched for at the LHC in various decay channels, the choice of which is given by the signal rates and the signal-to-background ratios in the various mass regions. The search strategies and background rejection methods have been established through many studies over the past years [1–3]. The search is challenging in the intermediate mass region, mH o2mz . Even though the natural width of the Standard Model Higgs boson in this mass range is narrow, the backgrounds from tt and continuum ZZ or WW production are relatively large and thus, an excellent detector performance in terms of energy resolution and background rejection is required [1–3]. Originally, inclusive final states have been considered, among them the well established H-gg and H-ZZ ðÞ -‘‘ ‘‘ decay channels. In addition, more exclusive channels have been considered in the low mass region by searching for Higgs boson decays in bb or gg in association with a lepton from a decay of an accompanying W or Z boson or a top quark. The search can be extended by using the vector boson fusion mode, which was proposed in the literature several years ago [12]. In vector boson fusion events the Higgs boson is accompanied by two jets in the forward regions of the detector, originating from the initial quarks that emit the vector bosons. On the other hand, central jet activity is suppressed due to the lack of colour exchange between the initial state quarks. This is in contrast to most background processes, where there is color flow in the t-channel. Jet tagging in the forward region of the detector together with a veto of jet activity in the central region are therefore useful tools to enhance the signal-to-background ratio. Studies have shown that the vector boson fusion production provides the only way to get access to the important H-tt mode for the Standard Model Higgs boson. A detection of this decay mode is particularly important to extract information on the Higgs boson couplings to fermions, as discussed below. However, a discovery in this final state is not easy and requires integrated luminosities of the order of 30 fb 1. The sensitivity in terms of luminosity required to make a 5s discovery in the ATLAS experiment is shown in Fig. 3 for the combination of a few important discovery channels. A comparable significance can be achieved in the CMS experiment [2]. A Standard Model Higgs boson can be discovered at the LHC over the full mass range from the LEP2 lower limit [13] up to the TeV range with a high significance. Over a large fraction of the mass range the discovery of a Standard Model Higgs boson will be possible in two or more independent channels. It should be noted that the associated ttH production with the subsequent decay of the Higgs boson into a bb pair has disappeared from the discovery plots of both the ATLAS and CMS collaborations. In recent studies [2,3], based on a more realistic simulation of the b-tagging performance as well as on explicit matrix element calculations [14], the large backgrounds from ttjj and ttbb production have been estimated more reliably. In addition, there is a combinatorial background (ambiguities in the association of the tagged b jets to the top quarks and Higgs boson) from the signal itself. If the large systematic uncertainties on these backgrounds are taken into account, the signal
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significance 10
13 ATLAS
9
H H H H
8
Luminosity [fb−1]
7
→ → → →
γγ ZZ∗ → 4l ττ WW → eνμν
12 11 10 9 8
6
7
5
6 4
5 4
3
3
2
2 1
1
0 120 140 160 180 200 220 240 260 280 300 mH [GeV] Fig. 3. Significance contours for different Standard Model Higgs boson masses and integrated luminosities. The thick curve represents the 5s discovery contour. The median significance is shown with a colour according to the legend. The hatched area below 2 fb 1 indicates the region where the approximations used in the statistical combination are not accurate, although they are expected to be conservative [3].
significance is too weak and this channel does not contribute to the discovery potential. However, there is still hope that the dominant bb decay mode can be detected in the low mass region. In a recent study [15] it was proposed to search for this decay mode in the associated WH and ZH production, where the two bosons are back-to-back at high transverse momenta. The selected phase-space region corresponds to only a small fraction of the total WH and ZH cross-section (about 5% fox pT 4200 GeV=c), but kinematic acceptance is larger, while backgrounds are reduced. The use of jet finding and b-tagging was adopted to identify the characteristic structure of a fast-moving Higgs boson that decays into a bb pair with a small separation angle. The ATLAS collaboration has presented a first study of the sensitivity based on a more realistic simulation of the detector and trigger [16]. The analysis considers the three decay modes WH-‘n bb, ZH-‘‘ bb and ZH-nn bb and employs modern jet reconstruction and decomposition techniques. In a cut-based analysis the combined sensitivity of these channels for an integrated luminosity of 30 fb 1 was estimated to reach 3:0s after systematic uncertainties were taken into account. This indicates that this channel can be reinstated as one of the promising search and measurement channels for the low mass Standard Model Higgs boson at the LHC. 7.2. Higgs boson parameters After a possible observation of a Higgs boson at the LHC it will be important to establish its nature. Besides a precise measurement of its mass, which enters electroweak precision tests, a determination of the spin and CP-quantum numbers is important. To establish that the Higgs mechanism is at work, measurements of the couplings of the Higgs boson to fermions and bosons as well as a demonstration of the Higgs boson self coupling are vital. A precise measurement of the Higgs boson mass can be achieved over a wide mass range using the electromagnetic gg or 4‘ final states. Information on the spin and CP quantum number can be inferred from the angular distribution of the leptons in the H-ZZ-4‘ mode or from the azimuthal separation Dfjj of the
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tagging jets in vector boson fusion events (for a more detailed discussion see Ref. [17] and references therein). By performing a global fit where for a given Higgs boson mass the full information of all accessible production and decay channels is used, ratios of Higgs boson couplings to the HWW coupling (normalization channel) can be extracted [18,19]. Assuming an integrated luminosity of 300 fb 1 it has been estimated that the HZZ/HWW and Htt/HWW coupling ratios can be measured with a precision of 10–20% in the mass range from 1202180 GeV=c2 . The sensitivity to the top Yukawa coupling enters via the strong dependence of the gluon fusion production cross-section on this coupling. These measurements can be improved at the sLHC. Of particular importance in that context is the possibility to detect further rare Standard Model decay modes, like the H-Z g and H-mm decay modes [4]. In particular the latter is important since it provides access to an additional fermionic decay. To fully establish the Higgs mechanism, it must be demonstrated that the shape of the Higgs potential has the form required for electroweak symmetry breaking. The coupling strength gHHH enters in the production rate of Higgs boson pairs and it has been suggested to look in the mass region mH 4140 GeV=c2 for HH-WW WW decays [20]. Due to the limited production rates no measurement of the Higgs boson serf coupling will be possible at the LHC. It is still very difficult to extract information on the couplings using the tenfold increase in statistics at the sLHC. There might be sensitivity for mass values around 160 GeV/c2 [4], however, more detailed studies including realistic simulations of the detector performance and of pile-up at the sLHC are needed to extract more solid conclusions. 7.3. Supersymmetric Higgs bosons The LHC experiments have also a large potential in the investigation of the MSSM Higgs sector. If the SUSY mass scale is large and supersymmetric particles do not appear in Higgs boson decays, the full parameter space in the conventional ðmA ,tanbÞ plane can be explored. The regions covered with a significance of more than 5s are shown in Fig. 4 for the combination of both experiments and for an integrated luminosity of 300 fb 1. The maximal mixing scenario [1] has been assumed. It has been shown that this coverage can also been achieved for other mixing scenarios [17]. In addition to the channels discussed for the Standard Model case, the MSSM Higgs search relies heavily on the bbH=A-bb tt channel and on the direct and associated H-gg channels. Over a large fraction of the parameter space more than one Higgs boson and/or more than one decay mode would be accessible, except in the region of large mA and intermediate tanb. In this region only the lightest Higgs boson h can be observed, unless the heavier Higgs bosons (H, A, H 7 ) have detectable decay modes into SUSY particles. This means that the LHC cannot promise a complete and model-independent observation of the heavy part of the MSSM Higgs spectrum, although the observation of sparticles (e.g. squarks and gluinos) will clearly indicate that additional Higgs bosons should exist. The sLHC should be able to extend somewhat the region over which at least one heavy Higgs boson can be discovered in addition to the light Higgs boson h. The extended discovery reach is illustrated in Fig. 4 by the rightmost contour in the plot. 7.4. Strongly-coupled vector boson system In the absence of a scalar Higgs boson, the principal probe for the mechanism of electroweak symmetry breaking will be gauge
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sLHC is shown. The resonance is at the limit of the observation at the LHC, with 6.6 events expected for an integrated luminosity of 300 fb 1 over a background of about 2.2 events around pffiffiffithe region of the peak [4]. At the sLHC a signal significance of S= B 10 can be reached. For the discussion of more resonant processes the reader is referred to Ref. [4]. A study of nonresonant processes, such as the W + W + production, will require a few years of sLHC running and a good understanding of the underlying backgrounds.
8. Other physics beyond the standard model The ATLAS and CMS experiments will also be sensitive to a variety of other possible extensions of the Standard Model. Discovery limits for other phenomena are summarized below, assuming an integrated luminosity of 100 fb 1. For more details the reader is referred to Refs. [1–3].
Technicolor resonances can be searched for in their decays to a
Fig. 4. The combined sensitivity of the ATLAS and CMS experiments for the discovery of MSSM Higgs bosons (in the case of maximal mixing). The excluded regions with a significance of 5s in the (mA, tanb) plane are shown. In the different regions the number of supersymmetric Higgs bosons that can be detected is indicated. In the region to the left of the rightmost contour at least two Higgs bosons can be discovered at the sLHC (for 3000 fb 1 per experiment and both experiments combined).
pair of gauge bosons, or to a techi-pion and a gauge boson. The sensitivity for these resonances extends up to the TeV range. Although the technicolor parameter space is very large, there is a number of potential channels which allow for combinations of signatures to help in understanding the nature of the resonances. Excited quarks should be detected up to masses in the order of 5–6 TeV/c2. The discovery potential for first generation leptoquarks extends up to 1:5 TeV=c2 (assuming BRðLQ -eq ¼ 0:5ÞÞ. New vector bosons (W 0 and Z 0 ) should be detectable up to masses in the order of 5–6 TeV/c2.
The mass reach for these new physics signatures can typically be extended by 30% at the sLHC. In addition, the sLHC will also allow to improve significantly on the measurement of triple gauge boson couplings (which have not been discussed in this article) and will allow to reach an accuracy comparable with the size of electroweak and possibly SUSY virtual corrections [4].
9. Conclusions
Fig. 5. Expected signal and background for a 1.5 TeV/c2 WZ resonance in the leptonic decay channel for 3000 fb 1 (sLHC).
boson scattering at high energies. In order to explore such processes final states containing pairs of gauge bosons with large invariant mass need to be measured. It has been shown that the experiments at the LHC will be sensitive to the presence of resonances, such as rlike vector resonances in the WZ system, up to masses around 1.5 TeV/c2 [1,4]. In Fig. 5 the expected signal for a 1.5 TeV/c2 resonance at the
The ATLAS and CMS experiments are well matched to search for physics beyond the Standard Model and to investigate the nature of electroweak symmetry breaking at the LHC. The large event samples, which will be produced for many physics processes, will allow in addition to perform precision measurements of important parameters of the Standard Model. The sLHC appears as a natural extension to fully exploit the physics potential of the LHC facility. It gives access to higher masses for supersymmetry or other scenarios beyond the Standard Model. It might have a vital role to play in the investigation of the nature of electroweak symmetry breaking. Regardless whether or not a Higgs-like resonance will be detected at the LHC, the scattering of longitudinal gauge bosons at high energy must be studied. For many key physics processes at the sLHC the basic particle signatures including b-tagging and forward jet tagging have to be present. Therefore the new or upgraded tracking detectors, discussed at this conference, have to work. References [1] ATLAS Collaboration, Detector and Physics Performance Technical Design Report, CERN/LHCC/99-15 (1999).
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[2] CMS Collaboration, CMS Physics Technical Design Report, Vol II: Physics Performance, CERN/LHCC/2006-021 (2006), J. Phys. G Nucl. Part. Phys. 34 (2006) 995. [3] ATLAS Collaboration, Expected performance of the ATLAS experiment: detector, trigger, physics, CERN Report, CERN-OPEN-2008-020, arXiv:0901.0512. [4] F. Gianotti, et al., Eur. Phys. J. C 39 (2005) 293 hep-ph/0204087. [5] CMS Collaboration, CMS search plans and sensitivity to new physics using dijets, CMS note, CMS PAS SBM-07-001 (2007). [6] DØ Collaboration, V.M. Abazov, et al., Fermilab-PUB-09-326-E, Phys. Rev. Lett., arXiv:0906.4819, submitted for publication. [7] N. Besson, et al., Eur. Phys. J. C 57 (3) (2008) 627 SN-ATLAS-2008-070, arXiv:0805.2093. [8] For a review, see for example: S.P. Martin, hep-ph/9709356 (1997) and references therein. [9] See for example K. Inoue, et al., Prog. Theor. Phys. 68 (1982) 927; L. Alvarez-Gaume, J. Polchinski, M.B. Wise, Nucl. Phys. B 221 (1983) 495. [10] See for example M. Dine, W. Fischler, M. Srednicki, Nucl. Phys. B 189 (1981) 575; M. Dine, et al., Phys. Rev. D 53 (1996) 2658. [11] I. Hinchliffe, F.E. Paige, hep-ph/0201141.
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[12] D.L. Rainwater, D. Zeppenfeld, J. High Energy Phys. 12 (1997) 5; D.L. Rainwater, D. Zeppenfeld, K. Hagiwara, Phys. Rev. D 59 (1999) 014037; T. Plehn, D.L. Rainwater, D. Zeppenfeld, Phys. Rev. D 61 (2000) 093005; D.L. Rainwater, D. Zeppenfeld, Phys. Rev. D 60 (1999) 113004; D.L. Rainwater, D. Zeppenfeld, Phys. Rev. D 61 (2000) 099901 Erratum; N. Kauer, T. Plehn, D.L. Rainwater, D. Zeppenfeld, Phys. Lett. B 503 (2001) 113. [13] ALEPH, DELPHI, L3 and OPAL Collaborations, Phys. Lett. B 565 (2003) 61. [14] M. Mangano, et al., J. High Energy Phys. 07 (2003) 001. [15] J.M. Butterworth, et al., Phys. Rev. Lett. 100 (2008) 242001. [16] ATLAS Collaboration, ATLAS public note, ATL-PHYS-PUB-2009-088 (2009). [17] K. Jakobs, Eur. Phys. J. C 59 (2009) 463. ¨ [18] M. Duhrssen, Prospects for the measurement of Higgs boson coupling parameters in the mass range from 110–190 GeV/c2, ATLAS note, ATL-PHYS2003-030. ¨ [19] M. Duhrssen, et al., Phys. Rev. D 70 (2004) 113009. [20] U. Baur, T. Plehn, D. Rainwater, Phys. Rev. D 69 (2004) 053004 hep-ph/ 0310056.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Physics at the LHC and sLHC Karl Jakobs Physikalisches Institut, Hermann-Herder Str. 3, 79104 Freiburg, Germany
On behalf of the ATLAS and CMS Collaborations a r t i c l e in fo
abstract
Available online 4 June 2010
Over the coming years the experiments at the Large Hadron Collider (LHC) will explore the TeV energy scale and will most likely answer fundamental questions of particle physics. In the focus of the experiments are the search for physics beyond the Standard Model and the investigation of the nature of electroweak symmetry breaking. In order to fully exploit the physics potential of the LHC facility a high luminosity upgrade with luminosities of 1035 cm 2 s 1 is envisaged. In the present article the physics potential of the LHC is summarized. For various physics scenarios it is also discussed how an upgraded high luminosity collider, the so called super-LHC (sLHC), could extend the physics reach of the LHC. & 2010 Elsevier B.V. All rights reserved.
Keywords: LHC sLHC Higgs physics Search for supersymmetry
1. Introduction After a construction time of more than 10 years, the Large Hadron Collider (LHC) will deliver first proton–proton collisions this year. After running in an initial phase at a centre-of-mass energy of 7 TeV, it is anticipated that the design energy of 14 TeV and the design luminosity of 1034 cm 2 s 1 will be reached. This machine will open up the possibility to explore the TeV energy range, which plays a key role in the investigation of the electroweak symmetry breaking and in the search for physics beyond the Standard Model. The experiments ATLAS and CMS have been designed and optimized to cover a large spectrum of possible physics signatures. In the main focus will be the search for the Higgs boson as well as for particles predicted by supersymmetry or technicolor theories, new gauge bosons and searches for composite quarks and leptons. Besides the discovery potential for new physics the experiments also have a large potential to perform precision measurements of Standard Model parameters, like measurements of the W and top quark masses or triple gauge boson couplings. Many of the Standard Model measurements will, in particular at the beginning of the data-taking period, be used for a detailed understanding of the detector performance and for an optimization of reconstruction algorithms. In order to exploit the full physics capabilities of the LHC a high-luminosity upgrade to reach luminosities of 1035 cm 2 s 1, hereafter referred to as super-LHC (sLHC), is planned. The physics reach of this collider will certainly depend on the scenarios realized in nature and can be more precisely determined once new physics at the LHC will have been established. However, such a high-luminosity machine will in general extend the discovery
E-mail address: [email protected] 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.077
reach in the high-mass region (e.g. quark compositeness, new heavy gauge bosons, multi-TeV squarks and gluinos, extra dimensions) and improve the accuracy in the determination of the parameters of new physics possibly discovered at the LHC. In addition, the sensitivity to rare processes (e.g. rare Higgs boson decays, Higgs boson pair production, multi gauge boson production and FCNC top-quark decays) can be extended and the accuracy in the determination of Standard Model parameters (e.g. triple and quartic gauge couplings, couplings of the Higgs boson to fermions) can be significantly improved. In the following the physics potential of the LHC experiments is briefly summarized. For several scenarios it is discussed what additional insight could be gained at the sLHC. It should be stressed that the sLHC predictions are preliminary and indicative since the running conditions of the sLHC machine and the performance of the upgraded detectors are not yet precisely known. For details on the LHC physics potential the reader is referred to Refs. [1–3], the physics estimates of the sLHC are based on the report published in Ref. [4].
2. Status of the accelerator and the experiments The two general purpose experiments ATLAS and CMS have been assembled and installed prior to the first operation of the LHC in September 2008. As an example the insertion of the ATLAS endcap calorimeter into the toroid system is shown in Fig. 1. The CMS experiment has first been assembled and many detector components have been commissioned in the surface hall, before the various slices of the experiment were lowered down into the experimental hall. Meanwhile the commissioning is well advanced and both experiments have used the time since the LHC machine
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Table 1 Cross-sections at a centre-of-mass energy of 14 TeV and approximate numbers of expected events per second and per year for some important physics processes at low luminosity (1033 cm 2 s 1). Process
s (pb)
Events/s
Events/year
W-ev Z-e þ e tt
1.5 104 1.5 103 800 5 108
15 1.5 0.8 5 105
108 107 107 1012
1
0.001
104
10 105
0.01 100
105 109
bb g~ g~ ðmg~ ¼ 1 TeV=c2 Þ H (mH ¼ 200 GeV/c2) Inclusive jets pT 4200 GeV=c
Fig. 1. Picture of the installation of the ATLAS detector before the endcap calorimeter system is moved into the toroid system (May 2006). Four of the eight superconducting toroid coils are visible.
accident in September 2008 to collect large samples of cosmic events. They allowed to align and calibrate the various subdetector at a level higher than anticipated. Both experiments look forward to record first collisions before the end of the year 2009. On the machine side an extensive repair and consolidation programme was carried out. In total 39 dipole and 14 quadrupole magnets were replaced, nearly 900 new helium pressure release ports were installed, electrical interconnections repaired and the quench detection system improved. It is expected that first collision data for physics will be provided at a centre-of-mass energy of 7 TeV in 2009/10 and that an integrated luminosity of about 200 pb 1 can be collected.
3. LHC running scenarios and cross-sections It is expected that after the first run in 2009/10 an initial luminosity of 1033 cm 2 s 1 (hereafter called low luminosity) can be achieved. During the further years of operation, this value should rise to the design luminosity of 1034 cm 2 s 1 (hereafter called high luminosity). Integrated luminosities of 10 and 100 fb 1 per year should therefore be collected at low and high luminosity respectively. The expected cross-sections at a centre-of-mass energy of 14 TeV and the corresponding event rates at low luminosity are given in Table 1. For the reduced centre-of-mass energy of 7 TeV the reduction in cross-sections depends on the energy scale of the final state and is largest for heavy objects with masses in the TeV range. For the pair production of top quarks, for example, a reduction factor of about four is found. In the initial phase at 14 TeV at low luminosity, almost 50 W and five Z bosons decaying to lepton pairs will be produced every second, as well as one tt pair and 500,000 bb pairs. The copious tt production will constitute a significant background for many searches of new physics signals since it may lead to characteristic final states with leptons, jets and missing transverse energy Emiss . On the other T hand it can be used to determine important detector performance parameters in the early running. Unless stated otherwise, the physics reach discussed in the following is based on a centre-of-mass energy of 14 TeV.
extraction of trigger and reconstruction efficiencies as well as for the understanding of the measurement of the missing transverse energy, Emiss , which is vital for the search for physics beyond the T Standard Model. The initial measurements of the W and Z boson cross-sections are expected to be limited by uncertainties on the luminosity which are estimated to be at the level of 5–10% during the first 1–2 years. It is also expected that a top production signal can already be established after a few months of running at a luminosity of 1032 cm 2 s 1. Such a signal can be identified after applying simple cuts, i.e. by requiring one lepton with pT 420 GeV=c and four jets with pT 4 40 GeV=c within jZj o 2:5 without b-tagging and kinematical fits. Such a sample is ideally suited to establish the jet energy scale via the W-qq and t-Wb decay chains appearing in tt events as well as to determine the b-tagging performance from early data [3]. In addition, the experiments must be open and unbiased for early surprises and unexpected discoveries with early data. Therefore unbiased inclusive measurements of the lepton, dilepton, jet- and missing transverse energy spectra are essential. To illustrate the performance, a 5sdiscovery of a new heavy vector boson Z 0 with Standard-Model-like couplings and with a mass of 1 (1.5) TeV/c2 will be possible after collecting integrated luminosities of 70 pb 1 (300 pb 1) at a centre-of-mass energy of 14 TeV [2,3]. First data on QCD jet production will be used to test perturbative QCD and to look for deviations from the Standard Model. Given the new energy regime, higher transverse jet energies where, e.g. first signs of compositeness could show up, can be rapidly probed. Even if relatively large jet energy scale uncertainties of the order of 10% for early data are assumed, compositeness scales of L ¼ 3 TeV can already be probed with data corresponding to an integrated luminosity of only 10 pb 1 at 14 TeV [2,5]. This is close to the present Tevatron reach, where compositeness scales of 2.9 TeV are excluded at the 95% C.L. [6]. The sensitivity at the LHC is largely increased with higher integrated luminosities and compositeness scales of L ¼ 40 and 60 TeV can be excluded at the 95% C.L. for 300 and 3000 fb 1 respectively [4]. It should be noted that the accumulation of 3000 fb 1 requires running the sLHC for several years.
5. Precision measurements of standard model parameters 5.1. Measurement of the W mass
4. Standard model measurements with early data pffiffi Even during the early running phase at s ¼ 7 TeV important Standard Model reference signals can be established. The observation of the W and Z bosons will be important for the
At the time of the LHC start-up, the W mass will be known with a precision of 25 MeV=c2 from measurements at the Tevatron and LEP2. The motivation to improve this result is mainly that precise measurements of the W mass, of the top mass
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and of the Higgs mass will provide stringent tests of the consistency of the underlying theory. At the LHC, 60 million well-reconstructed W-‘v decays (where ‘ ¼ e or m) should be collected by each experiment in one year of data taking at low luminosity. The statistical error on the W mass measurement is therefore expected to be small ð o 2 MeV=c2 Þ. The systematic error will arise mainly from the Monte Carlo reliability in reproducing the data, i.e. the physics and the detector performance. Uncertainties related to the physics result from the limited knowledge of the WpT spectrum, structure functions, the W width and from W radiative decays. Uncertainties related to the detector result from the limited knowledge of the absolute lepton energy scale and the detector energy/ momentum resolution and response. Many of these uncertainties (lepton scale, detector resolution and response and the WpT spectrum) will be constrained in the experiment by using the high-statistics sample of leptonic Z decays. First studies [1] indicated that the expected uncertainties on the W mass measurement are at a level of 15–20 MeV/c2, mainly dominated by the knowledge of the the lepton energy scale. In a recent study the principle limits of the precision of the W mass measurement at the LHC have been investigated [7]. Here, the method to use the very large sample of Z-‘‘ decays to fix the lepton energy scale is refined and carried out in a more differential way to uniform the lepton energy scale across the whole detector acceptance. In addition, kinematical distributions for the W production are constrained by the corresponding distributions from Z decays. If it can be assumed that the radiative corrections are knows with LEP-precision, the total error on the W mass is estimated to reach values around 6–7 MeV/c2. This should probably be considered as an experimental limit which sets at the same time the scale for the required precision of radiative corrections. Since this measurement is limited by systematic uncertainties, no further improvement is expected at the sLHC, where the relevant uncertainties will be difficult to control.
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sensitivity to FCNC top-quark decays [4]. In particular in the case of t-qZ the improvement is almost linear with the luminosity due to the very low background level and branching ratios of the order of 10 6 are achievable. In order to reach this sensitivity, the b-tagging performance of the detectors must be kept at its present value, which requires superb semiconductor tracking detectors at the sLHC.
6. Search for supersymmetry 6.1. The standard LHC scenario If Supersymmetry (SUSY) [8] exists at the electroweak scale, its discovery at the LHC should be straightforward. The SUSY crosssection is dominated by gluinos and squarks, which are strongly produced with cross-sections comparable to the Standard Model backgrounds at the same Q2. Gluinos and squarks then decay via a series of steps into the lightest supersymmetric particle (LSP), which may itself decay, if R-parity is violated. These decay chains lead to a variety of signatures involving multiple jets, leptons, photons, heavy flavours, W and Z bosons, and missing transverse energy. The combination of a large production cross-section and distinctive signatures makes it easy to separate SUSY from the Standard Model background. In a first step of SUSY searches at the LHC multijet events with large missing transverse momentum will be studied. An excess at large Emiss would provide sensitivity to squarks and gluinos up to T the TeV energy range. The discovery range for squarks and gluinos is shown in Fig. 2 as a function of the universal scalar mass m0 and of the universal gaugino mass m1/2 in the framework of the minimal supergravity (mSUGRA) model [9]. Already for an integrated luminosity of only 1 fb 1, the reach in the jets + Emiss T channel extends to squark and gluino masses of the order of
3000 miss
E T + jets 2500
q (4
ly excluded
g(4000)
Theoretical
1500
5.3. Rare top-quark decays
1000 Most rare decays of the top quark expected in the Standard Model are beyond reach at the LHC. However, there is a large class of theories beyond the Standard Model where branching ratios for decays of top quarks induced by flavour-changing neutral currents (FCNC) be as large as 10 5–10 6. Studies of the ATLAS and CMS collaborations [1,2] of various FCNC decay modes (t-qg, t-qZ and t-qg where q¼u or c) indicate that the data which can be collected with a luminosity of 1034 cm 2 s 1 are not sufficient to explore these models. With a detector performance comparable to that expected at 1034 cm 2 s 1, the sLHC should enhance by a large factor the
√s = 28 TeV : 100 fb-1
:
2000 m1/2 (GeV)
At the LHC, top quark measurements will benefit from the large tt event samples, so that not only the mass and the production cross-section, but also branching ratios, couplings and rare decays can be studied in detail. The best channel for the top mass measurement will most likely be tt production with one W decaying leptonically and the other one hadronically. The top mass will be determined from the hadronic part of the decay, as the invariant mass of the three jets originating from the same top (mt ¼ mjjb). The associated leptonic top decay will be used to trigger the events and to suppress backgrounds. All together, a total uncertainty in the range of 1 GeV=c2 or better should be achieved [2,3]. Also in this case more data at the sLHC do not offer obvious improvements.
m( τ ) < m( ~ 0 χ1) 1
5.2. Measurement of the top quark mass
000
)
√s = 14 TeV : 1000 fb-1 , 2000 fb-1 g(3000) q (3 00
0)
g(2500)
q(
√s = 14 TeV : 100 fb -1 , 200 fb-1
25 0
0)
500 0
500
1000
1500
2000
m0 (GeV) Fig. 2. The expected 5s LHC discovery contours for squarks and gluinos in the (m0/m1/2) parameter plane of mSUGRA for various assumptions on the integrated luminosity and centre-of-mass energy. The mSUGRA parameters are chosen to be A0 ¼0, tanb ¼ 10, and m o 0.
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1.5 TeV/c2. These mass limits can be extended up to about 2.5 TeV/c2 for an integrated luminosity of 100 fb 1. SUSY cascade decays give also rise to lepton, b-jet and tau signatures. Selecting various multilepton final states similar regions in the (m0, m1/2) plane can be probed [2,3]. The main challenge at the LHC is not to discover SUSY but to separate the many SUSY processes that occur and to measure the masses and other properties of the SUSY particles. The approach followed by the collaborations has been to investigate in detail the signatures for particular points in the parameter spaces of the minimal supergravity (mSUGRA) model, Gauge Mediated SUSY breaking (GMSB) models [10], and R-parity violating models [8]. Methods such as looking for kinematic endpoints for mass distributions and using these to determine combinations of masses have proven generally useful. As an example, the production of q~ L q~ R , followed by the decays q~ L -qw02 , w02 -‘~ R ‘, ‘~ R -‘w01 , can be selected in an inclusive way by requiring two leptons in the final state with the same flavour and opposite sign, large Emiss and jet multiplicity [2,3]. The dilepton T mass distribution shows a very sharp end-point, which is due to the kinematic properties of the decay and depends on the masses of the particles involved, the two lightest neutralinos and the slepton, through a simple kinematic relation. The position of the end-point can be measured with a precision of 500 MeV/c2 (0.5%) with an integrated luminosity of 30 fb 1, thus providing a combined constraint on the three masses mentioned above. Another example is the search for the h-bb decay in SUSY events. If the two-body decay w02 -w01 h is kinematically allowed, it generally has a substantial branching ratio because the light neutralinos are dominantly gauginos. In many cases it is possible to reconstruct the h-bb as a resonance peak in the SUSY event sample, after applying simple kinematic cuts. An Emiss requirement T usually suppresses the Standard Model background. This signal may be easier to detect than h-gg and so it may provide the discovery mode for the light Higgs boson, although the gg signal is still important to measure the mass precisely. The Higgs signal can also provide a good starting point for further analysis of SUSY particles. Given the success in extracting precise measurements for the points studied and the large number of SUSY events expected at the LHC, the experiments are likely not just to discover SUSY, if it exists, but to make many precise measurements. The starting point in this study will be to look for characteristic deviations from the Standard Model. In SUGRA and some other models, there would be events with multiple jets and leptons plus large Emiss . In T GMSB models, there would be events with prompt photons or quasi-stable sleptons. In R-parity violating models, there would be events with very high jet multiplicity and/or leptons. Any such signal would point to possible classes of models and would indicate the rough mass scale. The next step would be to use partial reconstruction methods, like the ones discussed above, to try to constrain as many combinations of masses as possible.
6.2. Extensions at the sLHC It can be seen from Fig. 2 that a luminosity upgrade would extend the mass reach for squarks and gluinos from about 2.5 TeV (standard LHC) to about 3 TeV (sLHC). This performance does not require major detector upgrades because these inclusive searches are based mainly on calorimetric measurements of high-pT jets and large missing transverse energy. On the other hand, reconstruction of more exclusive decay chains which may be rate-limited at the LHC, such as some cascade decays of heavy gauginos, could become possible at the sLHC provided the full detector functionality is preserved. Two points of mSUGRA space where the squark and gluino masses
exceed 2 TeV/c2 have been studied in some detail [4,11]. The expected event rates are small at the standard LHC and detailed SUSY studies will not be possible. The tenfold increase in rate at the sLHC allows to extract also for such cases valuable information about the underlying theory.
7. The search for the Higgs boson 7.1. Standard Model Higgs boson The Standard Model Higgs boson is searched for at the LHC in various decay channels, the choice of which is given by the signal rates and the signal-to-background ratios in the various mass regions. The search strategies and background rejection methods have been established through many studies over the past years [1–3]. The search is challenging in the intermediate mass region, mH o2mz . Even though the natural width of the Standard Model Higgs boson in this mass range is narrow, the backgrounds from tt and continuum ZZ or WW production are relatively large and thus, an excellent detector performance in terms of energy resolution and background rejection is required [1–3]. Originally, inclusive final states have been considered, among them the well established H-gg and H-ZZ ðÞ -‘‘ ‘‘ decay channels. In addition, more exclusive channels have been considered in the low mass region by searching for Higgs boson decays in bb or gg in association with a lepton from a decay of an accompanying W or Z boson or a top quark. The search can be extended by using the vector boson fusion mode, which was proposed in the literature several years ago [12]. In vector boson fusion events the Higgs boson is accompanied by two jets in the forward regions of the detector, originating from the initial quarks that emit the vector bosons. On the other hand, central jet activity is suppressed due to the lack of colour exchange between the initial state quarks. This is in contrast to most background processes, where there is color flow in the t-channel. Jet tagging in the forward region of the detector together with a veto of jet activity in the central region are therefore useful tools to enhance the signal-to-background ratio. Studies have shown that the vector boson fusion production provides the only way to get access to the important H-tt mode for the Standard Model Higgs boson. A detection of this decay mode is particularly important to extract information on the Higgs boson couplings to fermions, as discussed below. However, a discovery in this final state is not easy and requires integrated luminosities of the order of 30 fb 1. The sensitivity in terms of luminosity required to make a 5s discovery in the ATLAS experiment is shown in Fig. 3 for the combination of a few important discovery channels. A comparable significance can be achieved in the CMS experiment [2]. A Standard Model Higgs boson can be discovered at the LHC over the full mass range from the LEP2 lower limit [13] up to the TeV range with a high significance. Over a large fraction of the mass range the discovery of a Standard Model Higgs boson will be possible in two or more independent channels. It should be noted that the associated ttH production with the subsequent decay of the Higgs boson into a bb pair has disappeared from the discovery plots of both the ATLAS and CMS collaborations. In recent studies [2,3], based on a more realistic simulation of the b-tagging performance as well as on explicit matrix element calculations [14], the large backgrounds from ttjj and ttbb production have been estimated more reliably. In addition, there is a combinatorial background (ambiguities in the association of the tagged b jets to the top quarks and Higgs boson) from the signal itself. If the large systematic uncertainties on these backgrounds are taken into account, the signal
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significance 10
13 ATLAS
9
H H H H
8
Luminosity [fb−1]
7
→ → → →
γγ ZZ∗ → 4l ττ WW → eνμν
12 11 10 9 8
6
7
5
6 4
5 4
3
3
2
2 1
1
0 120 140 160 180 200 220 240 260 280 300 mH [GeV] Fig. 3. Significance contours for different Standard Model Higgs boson masses and integrated luminosities. The thick curve represents the 5s discovery contour. The median significance is shown with a colour according to the legend. The hatched area below 2 fb 1 indicates the region where the approximations used in the statistical combination are not accurate, although they are expected to be conservative [3].
significance is too weak and this channel does not contribute to the discovery potential. However, there is still hope that the dominant bb decay mode can be detected in the low mass region. In a recent study [15] it was proposed to search for this decay mode in the associated WH and ZH production, where the two bosons are back-to-back at high transverse momenta. The selected phase-space region corresponds to only a small fraction of the total WH and ZH cross-section (about 5% fox pT 4200 GeV=c), but kinematic acceptance is larger, while backgrounds are reduced. The use of jet finding and b-tagging was adopted to identify the characteristic structure of a fast-moving Higgs boson that decays into a bb pair with a small separation angle. The ATLAS collaboration has presented a first study of the sensitivity based on a more realistic simulation of the detector and trigger [16]. The analysis considers the three decay modes WH-‘n bb, ZH-‘‘ bb and ZH-nn bb and employs modern jet reconstruction and decomposition techniques. In a cut-based analysis the combined sensitivity of these channels for an integrated luminosity of 30 fb 1 was estimated to reach 3:0s after systematic uncertainties were taken into account. This indicates that this channel can be reinstated as one of the promising search and measurement channels for the low mass Standard Model Higgs boson at the LHC. 7.2. Higgs boson parameters After a possible observation of a Higgs boson at the LHC it will be important to establish its nature. Besides a precise measurement of its mass, which enters electroweak precision tests, a determination of the spin and CP-quantum numbers is important. To establish that the Higgs mechanism is at work, measurements of the couplings of the Higgs boson to fermions and bosons as well as a demonstration of the Higgs boson self coupling are vital. A precise measurement of the Higgs boson mass can be achieved over a wide mass range using the electromagnetic gg or 4‘ final states. Information on the spin and CP quantum number can be inferred from the angular distribution of the leptons in the H-ZZ-4‘ mode or from the azimuthal separation Dfjj of the
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tagging jets in vector boson fusion events (for a more detailed discussion see Ref. [17] and references therein). By performing a global fit where for a given Higgs boson mass the full information of all accessible production and decay channels is used, ratios of Higgs boson couplings to the HWW coupling (normalization channel) can be extracted [18,19]. Assuming an integrated luminosity of 300 fb 1 it has been estimated that the HZZ/HWW and Htt/HWW coupling ratios can be measured with a precision of 10–20% in the mass range from 1202180 GeV=c2 . The sensitivity to the top Yukawa coupling enters via the strong dependence of the gluon fusion production cross-section on this coupling. These measurements can be improved at the sLHC. Of particular importance in that context is the possibility to detect further rare Standard Model decay modes, like the H-Z g and H-mm decay modes [4]. In particular the latter is important since it provides access to an additional fermionic decay. To fully establish the Higgs mechanism, it must be demonstrated that the shape of the Higgs potential has the form required for electroweak symmetry breaking. The coupling strength gHHH enters in the production rate of Higgs boson pairs and it has been suggested to look in the mass region mH 4140 GeV=c2 for HH-WW WW decays [20]. Due to the limited production rates no measurement of the Higgs boson serf coupling will be possible at the LHC. It is still very difficult to extract information on the couplings using the tenfold increase in statistics at the sLHC. There might be sensitivity for mass values around 160 GeV/c2 [4], however, more detailed studies including realistic simulations of the detector performance and of pile-up at the sLHC are needed to extract more solid conclusions. 7.3. Supersymmetric Higgs bosons The LHC experiments have also a large potential in the investigation of the MSSM Higgs sector. If the SUSY mass scale is large and supersymmetric particles do not appear in Higgs boson decays, the full parameter space in the conventional ðmA ,tanbÞ plane can be explored. The regions covered with a significance of more than 5s are shown in Fig. 4 for the combination of both experiments and for an integrated luminosity of 300 fb 1. The maximal mixing scenario [1] has been assumed. It has been shown that this coverage can also been achieved for other mixing scenarios [17]. In addition to the channels discussed for the Standard Model case, the MSSM Higgs search relies heavily on the bbH=A-bb tt channel and on the direct and associated H-gg channels. Over a large fraction of the parameter space more than one Higgs boson and/or more than one decay mode would be accessible, except in the region of large mA and intermediate tanb. In this region only the lightest Higgs boson h can be observed, unless the heavier Higgs bosons (H, A, H 7 ) have detectable decay modes into SUSY particles. This means that the LHC cannot promise a complete and model-independent observation of the heavy part of the MSSM Higgs spectrum, although the observation of sparticles (e.g. squarks and gluinos) will clearly indicate that additional Higgs bosons should exist. The sLHC should be able to extend somewhat the region over which at least one heavy Higgs boson can be discovered in addition to the light Higgs boson h. The extended discovery reach is illustrated in Fig. 4 by the rightmost contour in the plot. 7.4. Strongly-coupled vector boson system In the absence of a scalar Higgs boson, the principal probe for the mechanism of electroweak symmetry breaking will be gauge
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sLHC is shown. The resonance is at the limit of the observation at the LHC, with 6.6 events expected for an integrated luminosity of 300 fb 1 over a background of about 2.2 events around pffiffiffithe region of the peak [4]. At the sLHC a signal significance of S= B 10 can be reached. For the discussion of more resonant processes the reader is referred to Ref. [4]. A study of nonresonant processes, such as the W + W + production, will require a few years of sLHC running and a good understanding of the underlying backgrounds.
8. Other physics beyond the standard model The ATLAS and CMS experiments will also be sensitive to a variety of other possible extensions of the Standard Model. Discovery limits for other phenomena are summarized below, assuming an integrated luminosity of 100 fb 1. For more details the reader is referred to Refs. [1–3].
Technicolor resonances can be searched for in their decays to a
Fig. 4. The combined sensitivity of the ATLAS and CMS experiments for the discovery of MSSM Higgs bosons (in the case of maximal mixing). The excluded regions with a significance of 5s in the (mA, tanb) plane are shown. In the different regions the number of supersymmetric Higgs bosons that can be detected is indicated. In the region to the left of the rightmost contour at least two Higgs bosons can be discovered at the sLHC (for 3000 fb 1 per experiment and both experiments combined).
pair of gauge bosons, or to a techi-pion and a gauge boson. The sensitivity for these resonances extends up to the TeV range. Although the technicolor parameter space is very large, there is a number of potential channels which allow for combinations of signatures to help in understanding the nature of the resonances. Excited quarks should be detected up to masses in the order of 5–6 TeV/c2. The discovery potential for first generation leptoquarks extends up to 1:5 TeV=c2 (assuming BRðLQ -eq ¼ 0:5ÞÞ. New vector bosons (W 0 and Z 0 ) should be detectable up to masses in the order of 5–6 TeV/c2.
The mass reach for these new physics signatures can typically be extended by 30% at the sLHC. In addition, the sLHC will also allow to improve significantly on the measurement of triple gauge boson couplings (which have not been discussed in this article) and will allow to reach an accuracy comparable with the size of electroweak and possibly SUSY virtual corrections [4].
9. Conclusions
Fig. 5. Expected signal and background for a 1.5 TeV/c2 WZ resonance in the leptonic decay channel for 3000 fb 1 (sLHC).
boson scattering at high energies. In order to explore such processes final states containing pairs of gauge bosons with large invariant mass need to be measured. It has been shown that the experiments at the LHC will be sensitive to the presence of resonances, such as rlike vector resonances in the WZ system, up to masses around 1.5 TeV/c2 [1,4]. In Fig. 5 the expected signal for a 1.5 TeV/c2 resonance at the
The ATLAS and CMS experiments are well matched to search for physics beyond the Standard Model and to investigate the nature of electroweak symmetry breaking at the LHC. The large event samples, which will be produced for many physics processes, will allow in addition to perform precision measurements of important parameters of the Standard Model. The sLHC appears as a natural extension to fully exploit the physics potential of the LHC facility. It gives access to higher masses for supersymmetry or other scenarios beyond the Standard Model. It might have a vital role to play in the investigation of the nature of electroweak symmetry breaking. Regardless whether or not a Higgs-like resonance will be detected at the LHC, the scattering of longitudinal gauge bosons at high energy must be studied. For many key physics processes at the sLHC the basic particle signatures including b-tagging and forward jet tagging have to be present. Therefore the new or upgraded tracking detectors, discussed at this conference, have to work. References [1] ATLAS Collaboration, Detector and Physics Performance Technical Design Report, CERN/LHCC/99-15 (1999).
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