ATLAS and Astroparticle Physics

Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 25–32 www.elsevierphysics.com

ATLAS and Astroparticle Physics James L. Pinfolda∗ , a

Department of Physics, University of Alberta, Edmonton, Alberta T6G2N5, Canada The construction of the ATLAS detector at the LHC is nearing completion. This report will briefly summarize the progress, the technical challenges of the detector construction, and the status in the summer of 2006. The project is on track and the ATLAS Collaboration is poised to exploit the discovery potential of the LHC physics arena. Research into the fundamental nature of matter at the high energy frontier takes place in three main areas: accelerator based particle physics; high energy astrophysics; and, the cosmology of the early universe. The LHC project provides the laboratory to perform measurements of great importance for cosmic ray astrophysics and cosmology. Also, the study of astroparticle physics can have significant implications for collider physics at the LHC. This paper reviews some of the important synergistic links between astroparticle and LHC physics with the ATLAS detector.

1. Introduction The Large Hadron Collider (LHC) machine [1], currently under construction at the European Centre for Particle Physics CERN) near Geneva in Switzerland, will - in the summer of 2007 - start to collide proton beams at a centreof-mass-energy (Ecm ) of 900 GeV. A year later the LHC is scheduled to reach the unprecedented design Ecm of 14 TeV running at low luminosity, ∼ 1033 cm−2 s−1 . It is envisaged that by 2011 the LHC will b running at full design luminosity of ∼ 1034 cm−2 s−1 , taking 100 fb−1 of data per year. This luminosity will be achieved by filling each of the two LHC rings with 2835 bunches with 1011 particles in each bunch. The LHC machine beamline elements are currently being installed in the 27 km circumference Large Electron Positron (LEP) collider tunnel. The last magnet is due to be installed in March 2007. Achieving a beam energy of 7 TeV in the LEP tunnel is a major technological challenge requiring the bending power of roughly 1200 superconducting dipole magnets each providing a field of 8.3T. The LHC will also run in a heavy-ion collider mode at an Ecm of approximately 1000 TeV using lead ions. ∗ For

the ATLAS Collaboration.

0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.10.004

The LHC has four main intersection points housing the experiments: ALICE [2], ATLAS [3], CMS [4] and LHCb [5]. ATLAS and CMS are general purpose detectors with a very wide physics reach and the LHCb experiment is aimed specifically at the study of b-physics and CP-violation in the b-sector. Turning to heavy ion physics, ALICE will study ion-ion and proton-ion collisions. ATLAS and CMS will also take part in the heavy-ion program. There are three smaller, dedicated, detectors that are planning to take data at the LHC. The first is TOTEM [6], designed to measure the proton-proton total cross-section and play a role in the forward physics program of CMS. The second is MoEDAL [7],aimed at the search for the magnetic monopole and other highly ionizing particles. Last, but not least, there is LHCf [8] an experiment to study very forward particle emission in the LHC collider to provide data for the tuning of air showers simulations that are required to better understand ultra high energy cosmic rays interactions. The operation of the LHC is expected to follow the following schedule. There will be around 140-180 days of running per year; 100-120 days of proton collisions per year (4 × 106); plus, approximately round 40 days of heavy-ion and TOTEM

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running per year. The present LHC heavy ion program is divided into three phases:. LHC Phase I, is the ” baseline program” that envisages Pb ion collisions. LHC Phase II will see collisions with lighter ions ( He, O, Ar, Kr, In). The final phase would see so-called hybrid collisions e.g. p-A collisions with Pb, Ar and O ions. Exploiting the physics potential of the LHC means that we can answer, or at least illuminate, such fundamental open questions as: the generation of mass with or without the Higgs mechanism; unification of fundamental interactions; possible new physics such as supersymmetry, large extra dimensions and technicolour; and, reaching into cosmology, the nature of dark matter. The search for the Higgs boson is often used as an exemplar of TeV physics. With the expected LHC detectors performance described in the talk on the Standard Model (SM) 2. The ATLAS Detector The ATLAS Collaboration currently consists of 164 institutions from 35 countries, counting over 1830 scientific authors. Besides the construction and the integration of detector components for the general-purpose ATLAS detector, which have steadily progressed over the past years, the installation in the huge underground cavern (100m below ground, about 50m long with a diameter of about 35m) has now entered its final phase. The ATLAS detector has the typical structure of a general purpose collider detector and, on the whole, similar performance. All general purpose collider detectors have a cylindrical symmetry with two endcaps, to make the detector as hermetic as possible. The barrel and endcap regions are layered like an onion. The first layer is a low mass tracking region, immersed in a solenoidal magnetic field to provide a momentum measurement from the trajectory of the charged particle track. The vertex position of primary and secondary vertices of an event is also measured in the tracking layer. The presence of well separated secondary vertices is used to identify the decays of hadrons containing heavy flavour quarks. The next “high-Z” layer comprises the EM calorimetry the prime purpose of

which is to absorb and measure electronic and photonic energy. Of course this layer also measures the energy deposited by charged hadrons as they traverse to the hadron calorimeter. The hadron calorimeter layer consists of material with large hadronic interaction length such as iron or steel. It is designed to be deep enough to contain neutral and charged hadronic energy as well as shield the muon detectors from hadronic “punchthrough”. Energetic muons will easily traverse the tracking and calorimetry to be detected in the muon spectrometer. 2.1. ATLAS Magnet System The choice of the magnetic field configuration strongly influences the detector design and its physics reach. The ATLAS collaboration has chosen a magnet configuration based on superconducting air-core toroids for the muon momentum measurement, complemented by a superconducting solenoid (2T) for the tracking region. The inner tracking systems are immersed in a solenoidal B-field of 2(4)T in ATLAS(CMS). The tracking system is used to measure the trajectory, momentum and primary/secondary vertices (e.g. from b quark decays) of charged particles. It is also essential for electron (e) and τ identification and the calibration of the EM calorimeter using momentum/energy matching. The ATLAS inner tracker covers an η range of roughly ±2.5. The tracking must cope with high instantaneous rates and integrated rates. This gives rise to a large integrated dose that ranges from ∼1 Mrad to ∼30 Mrad, with a corresponding neutron fluence of 1014 to 1015 n/cm2 . Thus, cooling is required. 2.2. ATLAS Inner Detector In ATLAS precision tracking takes place in the pixel detector which has 3 layers in the barrel (radius =5,10,12 cm) and 3 pairs of forward/backwards disks (at a distance of 50, 56 and 65 cm from the centre). The pixel size is 50μm × 300/400μm. The pixel detector has a surface area of 2 m2 containing 8 × 107 channels. Next comes the semiconductor tracker which has four layers in the barrel region (radius = 28,36,43,50 cm) with pitch 80μm. Each endcap has twelve wheel pairs deployed as 3 modules, with the outer module ex-

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tending to ∼2.8 m from the centre. The radius of each wheel is ∼50 cm. There are a total of 6.3 × 106 channels of silicon covering an area of 61 m2 . The next detector is the Transition Radiation Tracker (TRT), The detecting elements are 4mm diameter straw tubes with a radiator to stimulate transition radiation from electrons. The barrel region of the TRT consists of long tubes (0 < |z| < 74 cm for 63 < r < 107 cm) and short tubes (40 < |z| < 74 for 56 < r < 63 cm ). In the endcap region we have 2 × 18 multi-plane wheels with active radius, 64 < r < 102 cm (for |z| < 2.8 m ) and 48 < r < 102 cm for (|z| > 2.8 m ). The overall track position accuracy in the middle of the TRT is 50μm and the pion rejection factor achieved is ∼100 at an electron efficiency of 90%. The design goals of the tracking system are roughly: • A high PT track reconstruction efficiency for isolated tracks >95% and within jets >90% • For isolated leptons in the ATLAS detector a momentum resolution ΔPT /PT ≤ 30% at 500 GeV and 1 − 2% at 20 GeV. • An impact parameter resolution at high PT (∼ 20 GeV) of ∼20 μm in the “bend” plane(r, φ)and around 100 μm in the beam (z) direction. 2.3. The Calorimetry ATLAS calorimetry consists of electromagnetic and hadronic calorimeters covering rapidity region up to |η| < 4.9 The EM calorimetry is only based on liquid argon technology, while hadronic calorimetry also uses scintillating tiles technology. 2.3.1. EM Calorimetry A high resolution EM calorimeter is employed to measure the energy and position of electrons and photons and aid in particle ID. The EM calorimetry is also used in the measurement of hadronic jets. ATLAS has employed a high granularity liquid-Argon sampling calorimeter which provides longitudinal and transverse segmentation and has a novel “accordion” structure. The depth of the EM calorimetry is greater than 24

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radiation lengths (Xo ). The design goal for the ATLAS EM calorimetry is to provide an √ energy resolution with a stochastic term of 10%/ E or better, with a constant term of 1% or better, for the reconstruction of the Higgs boson decays: H → γγ and H → 4e.

2.3.2. Hadron Calorimetry Hadronic calorimeters, along with the EM calorimetry, are required for the measurement of the energy and position of hadrons and jets and also the measurement of ETmiss . The missing transverse momentum vector can be used to infer the presence of weakly interacting neutral particles in the final state. The performance of the hadron calorimeter can be characterized by the jet-jet and ETmiss resolution, where the mass resolution depends also on the jet algorithm (including cone size), fragmentation and energy pileup. Studies, using the decays of Higgs boson to final states containing leptons and jets such as H → ZZ(W W ) → lljj, indicate that a calorimeter granularity of Δη ×Δφ is sufficient to measure the final state jet structure with high efficiency and adequate resolution. The ATLAS hadronic calorimeters cover the range |η| < 4.9 A Fe-scintillator sampling calorimeter was chosen for the “barrel” and “extended barrel ” region (|η| < 1.7 and Cu(W) liquid argon calorimeters for the “endcap/forward” (1.5 < |η| < 3.2/3.1 < |η| < 4.9) regions. An important parameter in the design of the hadronic calorimeter is its thickness since it has to provide good containment of hadronic showers and minimize “punch-through” to the muon system. The total thickness is 11 interaction lengths (λ) at η = 0, and is never less than 10λ across the acceptance of the hadron calorimetry. The design goals for jet reconstruction√envisage an hadronic energy resolution of ∼50%/ E (stochastic term) with a few percent constant term, for |η| < 3. For the forward region the √ design requires an energy resolution of ∼100/ E (scochastic term) with around a 10% constant term. The stopping power, energy resolution and large η-coverage of the hadron calorimetry make possible a good ETmiss measurement.

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2.4. Muon Spectrometry An outer muon spectrometer is used to identify and trigger on muons as well as measure muon momentum (combined with the inner detector). The ability to identify and trigger on muons down to low PT increases the acceptance for important physics processes such as: H → 4μ, a “gold-plated” search channel for the Higgs boson. In addition, muons can be identified in jets and can therefore can be used for b-tagging. The performance of the muon system is determined by: pattern recognition involving correlated (EM showers, punchthrough) and uncorrelated backgrounds ( neutrons and associated γ’s); momentum resolution; and the first level trigger, for example the rate is dominated by π/K decays up to ∼4 GeV and by heavy flavour decays from ∼4-25 GeV. The ATLAS muon detectors are contained in a toroidal magnetic field in air and cover a pseudorapidity range: |η| < 2.7. The muon spectrometer consists of monitored drift tubes and cathode strip chambers which incorporate high accuracy tracking (typically ∼50 μm per chamber), aligned to around 30μm. Thin gap chambers and resistive plate chambers are used for triggering in the endcap and barrel regions, respectively. The envisaged PT resolution, of the muon spectrometer combined with the inner tracking, ranges from a few percent at PTµ ∼100 GeV to around 10% at PTµ of 1 TeV. 2.4.1. Trigger and Data Acquisition System A major challenge to the ATLAS trigger system is the reduction of a 109 Hz interaction rate to a final output rate of around 100 Hz - with events size in the range 1 → 2 MB - to data storage, for further off-line analysis. This will result in multiPetabyte raw data sets being generated each year that will need to be processed and analyzed at centres across the world. The on-line data reduction takes place at three different trigger levels. The level-1 data rate is so high that the trigger must be implemented in dedicated electronic hardware. Local pattern recognition on prompt coarse grained data. Level-1 selects events at the rate of ∼105 Hz. At Level-2,

finer granularity and more precise measurements will be utilized in addition to event kinematics and topology. Sub-detector matching is also used to further refine the selection, resulting in a level2 rate of around 1000 Hz. At level-3, full event reconstruction and online analysis using the full calibration, will be performed for the first time. The final output rate is envisaged to be ∼100 Hz. 3. ATLAS Status The magnets - barrel toroids, end cap toroids and central solenoid plus cryogenics and related services - were constructed between 1998 and 2006. The magnet installation started in 2004 and will be completed in 2007. The muon spectrometer, including the precision measurement and trigger chambers in the barrel region plus the small and big forward wheels, were produced between 1998 and 2005. The installation of the muon spectrometer started in 2005 and will be completed in 2007. It took ten years to construct the calorimeter system - consisting of the liquid argon electromagnetic calorimetry in the barrel, endcap and forward region, plus a scintillating tile hadronic calorimeter in the barrel region - starting in 1996. Installation of the ATLAS calorimeter system stated in 2004 and was completed in 2006. The tracking detector system construction - including pixel detectors, silicon strip detectors and transition radiation tracker - started in 1999. The installation of the tracking system started in 2006 on a tight schedule that envisages completion in 2007. The trigger and data acquisition systems of ATLAS are being assembled at Point 1. Offline software and computing activities are underway using the Worldwide LHC Computing Grid (WLCG). Last, but not least, the surface infrastructure at point 1 has been completed. 4. The ATLAS Physics Program The physics reach of the LHC in the high PT physics arena, means that we can answer, or shed light on, such fundamental open questions as: the generation of mass; the unification of fundamental interactions; new physics such as SUSY, technicolor, the signals for EDs; and, the nature of

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dark matter. The search for the Higgs boson is often used as an exemplar of TeV physics. The LHC detector design performance will allow us to explore the SM Higgs mass range from the LEP200 limit all the way up to 1 TeV. Entering the world of SUSY, the same detector performance enables us to largely cover the various different signatures of SUSY particle production. ATLAS is developing a program of forward physics [10][11] that incorporates such areas as hard and soft diffractive processes, two-photon interactions and peripheral collisions, the measurement of the p-p total cross-section, low-x dynamics, forward physics phenomena, forward physics of proton-nucleus and nucleus-nucleus collisions, and a detailed investigation of the forward system of multiplicities, energy spectra and particle species. 5. ATLAS and Astroparticle Physics The physics arena in which LHC physics and astroparticle physics can have synergistic results can be divided into three areas. The first is what collider physicists would call low-PT physics studies. They include for example: diffractive processes; the measurement of the total pp crosssection, the detailed investigation of the highest energy particles produced in the forward regions: energy spectra, multiplicities, particle species, etc. These studies can provide important information to improve our understanding of the cosmic ray composition, interactions and phenomenology, the features of the air showers, and the nature of apparently new exotic states identified cosmic ray emulsion experiments, etc. The second area is, again using the terminology of collider physicists, high-PT physics studies. Such studies are the main motivation for building the LHC and are expected to add substantially to our knowledge of fundamental physics. A host of possible discoveries like the Higgs boson, supersymmetric particles, manifestations of extra dimensions, and in general new physics, should allow us to answer important questions, like the origin of the particle masses, the nature of dark matter composition, etc. This will obviously have a big impact also on astroparticle physics and cos-

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mology, and may even help disclosing the origin of HECR. On the other hand the recent and future precision measurements of the cosmic microwave background and future measurements made using cosmic-ray arrays and neutrino telescopes could provide physics results that confirm, complement and extend LHC results. The WMAP data opened up an era of precision cosmology that has reinforced the case for cold dark matter in the universe. If the predictions for ’standard’ SUSY scenarios are found to agree with observations at the LHC and are consistent with the interaction strengths and relic density as determined by astroparticle physics and cosmology, this concurrence will provide strong evidence that the DM is supersymmetric. However, if this agreement is not seen a whole host of possibilities arise. It is only through the combination of approaches in particle physics, astrophysics and cosmology that the identity of DM will be uncovered. Lastly, LHC detectors such as ATLAS and CMS will make available unprecedented area of fine grained underground detectors and magnetic field volume. These detectors can be used to make precise determination of the direction and momentum of large numbers of penetrating cosmic ray tracks within a very small area.The ATLAS detectors could be used to directly measure cosmic ray interactions as the LEP detectors were used in the COSMOLEP project [12]. In principle, trigger rates from the cosmic ray phenomena mentioned above are low enough that they can be run in conjunction with the standard trigger menus. Interesting cosmic ray phenomena that could be studied are that of, say, muon bundles and upward moving showers from high energy neutrino interactions in the Earth. In this way collider physics experiments can make a direct contribution to astroparticle physics experimentation. Because of the limited space available we shall only briefly expand on the synergies between collider Physics and astroparticle physics by considering the search for dark matter.

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5.1. The Search for Dark matter Stable neutral SUSY particles such as the neutralino (χ˜0 ) or gravitinos are promising candidates for cold dark matter (CDM). One of the simplest SUSY models, mSUGRA, is defined by six parameters: m0 , m1/2 , A0 , tan β, sign(μ) and the gravitino mass m3/2 . A slice through mSUGRA parameter space determines the various possibilities for the LSP. The LSP is either a stau (˜ τ ), excluded by the limits on the abundance of charged dark matter [13], or a χ˜0 where the χ˜0 ˜ ), Wino is a mixture of the gauginos, Bino (B ˜ ) and up- and down-type Higgsinos. The (W favoured candidate for CDM is the lightest χ˜0 in R-parity conserving SUSY models. Although the χ˜0 could have an important Higgsino admixture for m0 > 1TeV, it is mostly pure Bino in much of the parameter space. A comprehensive description of the nature of DM can only arise as a combination of understanding achieved from both astroparticle and collider results. An analysis of the mSUGRA parameter space enables us to reach the following general conclusions. In the high temperature very early universe all particles were in thermal equilibrium. As the universe expanded the χ˜0 interaction rate fell behind the expansion rate and the χ˜0 ’s were frozen out. As χ˜0 ’s are stable their thermal relic density survives to the present and the Boltzmann equation for s in a Friedmann-Robertson-Walker universe can be used to calculate the χ˜0 relic density Ω(χ˜01 )h2 we see today. Only relatively small regions of the mSUGRA model parameter space will give a low enough value of Ω(χ˜01 )h2 to be compatible with cosmological measurements and theory. The region at low m0 −m1/2 is called the ’bulk region’. Here the LSP has a mass of less than 200 GeV and thus this region is severely constrained by the searches for SUSY at LEP-2 and at the Tevatron. The lower limit on the Higgs boson mass obtained at LEP-2 [14]] as well as the LEP-2 limits on the chargino mass [15] significantly reduce the allowed part of the bulk region. The strip extending from the bulk region to large m1/2 , along the edge of the charged LSP

region, has an increased annihilation rate as the lightest slepton and LSP are almost mass degenerate. This is called the ’co-annihilation region’, where the process χ˜01 τ˜ → τ γ is enhanced. The cosmologically allowed co-annihilation region is now only a narrow strip which can be completely covered by LHC searches [16]. The portion of the phase space at large values of m0 and m1/2 is known as the “Rapid annihilation funnel”. In this region, for tanβ ≥ 30, the mass of the LSP is such that there is enhanced χ˜0 annihilation via a resonant intermediate (schannel) heavy Higgs boson (A) [17]. Processes with m0 and/or m1/2 greater than or roughly equal to ∼1 TeV can satisfy relic density constraints. The region compatible with cosmological constraints takes the form of a ‘funnel’ pointing towards large mA . Lastly, at large m0 and along the boundary beyond which EW symmetry breaking no longer occurs there is the ‘focus point’ (FP) region. In the FP region the LSP acquires a significant Higgsino content [18]. The annihilation cross-section is enhanced in this region since the Higgsino component can couple to the SM gauge bosons [19]. Also, the χ˜01 becomes nearly degenerate in mass with the χ˜02 and χ˜± 1 . Thus, additional annihilation and co-annihilation processes occur which, taken together, can reduce the neutralino relic densities to WMAP compatible values. The FP region can extend to large values of m0 giving rise to squarks, gluinos and sleptons that are too heavy to be observed at the LHC. In the stau co-annihilation region, the LHC can probe all the relevant parameter space for tanβ ≤ 45. Indirect searches for χ˜0 DM do not make a big contribution in this region of parameter space. The best situation arises when tanβ is large and there is some overlap with the A annihilation funnel. The large m1/2 and tanβ portion of parameter space is not, at present, accessible to any planned experiments, although this region seems to be consistent with the WMAP limits. A large part of the A annihilation funnel can be explored by the LHC. But, the large m1/2 section might not be accessible to any search experiments. The lower part of the annihilation funnel

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is accessible to a 1 TeV centre of mass energy linear e+ e− collider. Indirect searches for e+ ’s, p¯’s and γ’s produced by DM annihilation in the galactic core or halo can have a similar reach to that of the linear collider (LC). In the Focus Point (FP) region, the LHC can cover m1/2 values ranging up to 700 GeV, corresponding to ∼ 1.8 TeV. IceCube can explore the FP region for m1/2 ≤1400 GeV. Future planned direct DM search experiments should be able to access almost all the FP region. Nearly all of the mSUGRA regions allowed by the WMAP results can be covered by combining results from all the different search experiments described above. The exceptions are a few regions in the parameter space at large m1/2 piece of the stau coannihilation corridor or the A annihilation funnel. 6. Conclusion The LHC has a compelling and ambitious physics program that will mine the rich vein of TeV scale physics, with a direct discovery potential for new particles up to masses of 5 TeV. This means that we can answer, or at least illuminate, such fundamental open questions as: the generation of mass with or without the Higgs mechanism; unification of fundamental interactions; possible new physics such as supersymmetry, the nature of CP-violation, large extra dimensions and technicolour. The synergies between particle physics, astrophysics and cosmology in the next ten years should amplify our ability to make fast and deeper inroads into the terra incognita beyond the borders of our current knowledge. There is no doubt that a new frontier of fruitful collaboration lies before us. REFERENCES 1. The LHC Study Group.“The Large Hadron Collider conceptual design”, CERN/AC/9505,1995. 2. The ALICE Collaboration, “A Large Ion Collider Experiment, Technical Proposal”, CERN/LHCC/95-71, 1995

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