Parton distributions at 14TeV with ATLAS

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Nuclear Physics B (Proc. Suppl.) 133 (2004) 63 68

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Parton distributions at 14TeV with ATLAS A. M. Moraes a * and C. Buttar a aDepartment of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield $3 7RH, UK Accurate measurements at ATLAS will allow perturbative QCD to be tested in an energy regime never probed. Operating at the expected level of precision, ATLAS will measure SM cross sections and QCD related processes at the LHC which will further constrain the parton densities distributions. We present results from simulations of jet studies, direct photon production, Drell-Yan processes and heavy flavour production which indicate the potential to investigate the partonic structure of protons at ATLAS.

1. Introduction The study of high-energy scattering has led to a more complete picture of the constituents of matter and their interactions. The use of accelerators, both fixed target and circular, has proved to be not only a good source of information but an essential tool in the development of particle physics [1]. Most of our information about the structure and properties of hadrons and their constituent partons is based on the analysis of highenergy scattering data obtained through many experiments performed in the last fifty years. Although our knowledge on the partonic structure of hadrons accumulated so far allows successful descriptions of many aspects of particle interactions to be made, uncertainties associated to parton densities, especially those arising from DGLAP evolution to higher Q2 scales and from the parton distributions themselves at low-x values, still remain and limit the accuracy of theoretical predictions. Experiments designed to probe kinematic regions not yet explored can provide crucial information to reduce the current level of these uncertainties. The Large Hadron Collider (LHC), under construction at CERN in a 27 km circumference tunnel, will collide protons at a centre of mass energy of v G = 14 TeV [2]. The design luminosity (also referred to as"high luminosity") is 1034 cm-2s -1, *E-mail address: [email protected]. This work is supported by CAPES and PPARC. 0920-5632/$ see front matter © 2004 Published by Elsevier B.M doi: l 0.1016/j.nuclphysbps.2004.04.138

but during the first year the LHC will also run at a lower luminosity, namely 2 × 1033 cm-2s -1 (referred to as "low luminosity"). Besides the large potential to discover new physics, e.g. the Higgs boson and Supersymmetry (SUSY), the LHC will also test the predictive power of the Standard Model (SM) for particle interactions at an energy regime never probed before. Among the detectors being constructed at LHC's interaction points, the ATLAS detector has been designed to be a multi-purpose detector capable of making precise measurements during both low and high luminosity runs with a coverage of l~l -< 5 [3]. In order to explore the discovery potential for new physics at the LHC, this detector has been optimised to be especially precise for measurements of leptons (e and #), photons, jets and missing ET. In addition, ATLAS will also perform precise measurements for a large number of physics channels and in most cases, these measurements are expected to improve significantly on previous experiments [4]. Operating at the expected level of precision, ATLAS will measure SM cross-sections and QCD related processes at the LHC which will further constrain the parton densities distributions. We discuss some of these measurements in this paper. In section 2 we present the kinematic range of parton interactions at the LHC. Results from simulations of jet studies are discussed in section 3, and for direct photon production in section 4. DrellYan processes and heavy flavour production are

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discussed in sections 5 and 6 respectively. Finally, our conclusion are presented in section 7. 2. K i n e m a t i c reach o f L H C

At the LHC, essentially all physics processes, from precise measurements of electroweak processes and studies of heavy flavours to searches of physics beyond the SM, are connected to quarks and gluons interactions. A detailed understanding of QCD is therefore important for almost all physics processes to be studied at the LHC, as the production mechanisms and dynamics of particle interactions are mostly controlled by QCD. lo9

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Figure 1 illustrates the phase-space region in x and Q2 that can be probed at the LHC (upper triangle) [5]. LHC's large centre-of-mass energy, v ~ = 14 TeV, will allow parton interactions of considerably high-Q 2 to be probed. Varying M and y, we see that partons with different momentum fractions, x, can be reconstructed and in some cases, x can be more than an order of magnitude smaller than the smallest x values reached at HERA. Covering [r/I _< 5, ATLAS will be able to explore a large fraction of LHC's kinematic region and use this information to constrain parton distribution functions (pdf's). Some examples of how this can be done are presented in the sections below.

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At hadron colliders, jets are one of the most prominent signatures of a hard scattering process which has taken place in a given hadron-hadron collision. At the LHC, the production of jets will probe the partonic structure of protons at the smallest distance scales ever investigated.

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Figure 1. Values of x and Q2 probed in the production of a particle of mass M and rapidity y at the LHC. In a hard scattering where two partons have interacted and produced a particle of mass M and rapidity y, the fraction of momentum carried by each incoming parton (xa and x2 for partons 1 and 2 respectively) can be approximately reconstructed by using

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Figure 2. Inclusive jet cross-section as a function of EJTet for different ~ bins. Figure 2 shows the simulated inclusive jet crosssection as a function of the transverse energy of the leading jet, EJTet, for three r/bins: 0 < [r~l < 1, 1 < i~/[ < 2 a n d 2 < It/[ < 3 [3,6]. Statistical errors for a simulated sample corresponding to an integrated luminosity of 300 fb -1 are included in

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figure 2. Also shown in the figure is the prediction of NLO calculation [7]. Running at low-luminosity, about 105 events with a jet of ESTet > 1 TeV and .~ 103 with a jet of E~Tet > 2 TeV are expected per low-luminosity year at the LHC. This indicates that large statistical samples of high-ET jets will be produced even at the low luminosity stage. Experimental errors are expected to be dominated by systematic errors, especially those arising from uncertainties on the jet energy scale and luminosity. 10 8 :::

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with 7" = 0.5 x Irh - r}21. In figure 3 the expected range in x and Q2 for the di-jet differential cross-section measurement at ATLAS is shown [8]. Only bins containing more than 100 events for an integrated luminosity of 300 fb -1 are shown, xl,~ and Q2 are reconstructed using eqs. (2) and (3). Jets are pre-selected for Iol < 3.2 and ET > 180 GeV. The kinematic reach for Q2 = 105 GeV 2, which is above the HERA kinematic limit, covers values of x as low as 10 -3. For Q2 > 107 GeV 2 the region with x > 0.1 is covered. More studies are now being carried out to quantify uncertainties related to these measurements. 4. D i r e c t

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The detection of photons at the LHC will be a challenging task due to large background from jets containing, for example, a leading 7r° which can mimic the photon signature. At ATLAS, isolation cut techniques and highly granular LAr calorimeters for [r}[ < 2.5 should allow a large rejection factor against this background. The photon identification efficiency is expected to be higher than 80% at the low luminosity runs [3].

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Measurements of jet production cross-section are sensitive to both quark and gluon densities with the gluon contribution decreasing with increasing ET. Measurements of di-jet differential cross-section for different values of minimal ET for both jets and of the two jet pseudorapidities, rh,~, can be used to constrain the fraction of momenta xl,2 of the partons entering the hard scattering for a scale Q2. At leading order, the parton momenta Xl,2 and the hard scattering scale Q2 can be reconstructed

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Direct photon measurements can provide important constraints on parton distributions, especially on the gluon distribution in the proton. The production of direct photons at the LHC

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will be dominated by the QCD Comptom process, qg ~ q'7, in most of the kinematic region. Smaller contributions from annihilation, qq --+ g'y are also expected. Figure 4 shows the inclusive direct photon cross-section at leading order as a function of PT. It also includes a detector simulation using ATLFAST which requires the photon to be detected with E~h°t°n > 40 GeV and within Ir}[ < 2.5. At HERA, the acceptance for this channel corresponds to Q2 > 103 GeV 2 and x > 0.01. Photons detected at ATLAS with Ephoton ~T > 40 GeV within Ir/I < 2.5 will allow the region of Qg~ > 10 a GeV 2 and x > 5 × 10 -4 to be explored. The upper x value, xm,=, will depend on the number of high-Er photons detected, which is proportional to the integrated luminosity. For 30 fb -x , ,~ 104 events with E~h°t°n > 500 GeV are expected to be detected, corresponding to Xmax = 0.2. 5.

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processes

Drell-Yan pairs are produced by the annihilation of quark and anti-quarks via an intermediate vector boson. This probes the proton structure at a scale Q2 equal to the mass squared of the lepton pair. Thus, at the LHC quark and anti-quark densities can be constrained by measuring DrellYan lepton pair production and the production of W and Z bosons with leptonic decays to electron or muons.

Figure 5 shows the expected cross-section at leading order for Drell-Yan muon pairs as a function of the invariant mass of the muon pair, mu~. The result shown in figure 5 was obtained using the ATLFAST detector simulation requiring p~UOn > 6 GeV and ]r/ul < 2.5 . The resonance contribution due to production of the Z boson can be clearly seen. For 30 fb -1, 104 muon pair events with an invariant mass muu > 400 GeV are expected to be detected. With those events the range of Q2 > 1.6 x 105 GeV 2 and 2.3 x 10 -3 < x < 0.34 is covered for ]r/u[ < 2.5. W bosons will also be produced in large quantities at the LHC. For the same integrated luminosity (30 fb-1), 105 events containing a W with pW T > 400 GeV and decaying leptonically to electron or muon and neutrino, are expected to be detected within Ir/#[ < 2.5. The range in x and Q2 covered from W production is t h u s 3 x 10 -4 < x < 0.1 at Q ~ 6 x 10 s GeV [6]. At HERA, for Q~ > 104 GeV 2 the region kinematically accessible is restricted to x > 0.1. , - , 3000

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The rapidity distribution of W bosons, yW, will also provide information that can be used to constrain the quark and anti-quark densities in the proton. W + and W - will have different yW distributions reflecting the differences in the parton

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distributions leading to the production of a W + (ud--+ W +) or W - (dfi --+ W - ) . In figure 6 the expected shape yW for W + and W - production is shown. The production cross-section for W + is larger than for W-. The difference in the shape of the rapidity distribution should survive in the detectable decay lepton. Given the large statistics expected for W =L production at the LHC and their relatively clean signatures, accurate constraints to u02 ) and d(d) densities can be expected to be achieved through measurements at ALTAS. 6. H e a v y flavour production Due to its high centre-of-mass energy and luminosities, the LHC can be seen as a factory of heavy particles. Even during the low-luminosity stage, about 106 bb pairs will be produced every second. About 107 tt pairs will be produced in one year of low-luminosity run [4]. Such high rates for heavy quark production will provide valuable information for many aspects of the SM, including on patton densities. 10~ 1° ~ ::£

quarks at the LHC. Therefore measurements of heavy quark production cross-sections will provide constraints on the gluon density. The cross-section for heavy quark pair production at the LHC as a function of the transverse momentum pT Q is shown in figure 7 [3]. For pT Q> 50 GeV the cross-sections for charm and bottom production are verff similar and, in fact they only differ for small p ~ (pT Q < 20 GeV). The crosssection for t{ production has a significant difference to c~ and bb cross-sections which persists up to pT Q -~ 600 GeV. Measurements describing the momentum fraction carried by charm and bottom quarks, Xc and Xb, can also be performed by the LHC's multi-purpose detectors, ATLAS [3] and CMS [9]. Strategies to separate charm and bottom quarks are currently being investigated. One of theses strategies, originally developed by the CMS Collaboration~ consists in using quark flavour tagged subsample of photon-jet final states [10]. For photon-jet subsamples with charm or bottom flavoured jets the production mechanism is dominated by by quark-gluon scattering. The jet flavour can be identified using the inclusive muon PT spectrum in addition to b-jet identification using standard lifetime techniques. E

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Figure 8. Inclusive muon pT spectrum in selected photon-jet events originating from light and heavy quarks [10].

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The expected inclusive muon PT spectrum from the different initial quark flavours is shown in figure 8. Simulations for the LHC indicate that assuming p~ > 40 GeV and inclusive muons with PT of 5 - 10 GeV, 10~ c-photon events and 105 b-photon events should be accepted for an integrated luminosity of 10 fb -1. These events should allow Xc and xb to be determined for 0.001 < Xc(Xb) < 0.1 with an uncertainty of 4- 5-10% [10].

4.

5.

6. 7.

7. Conclusions

Running at very high centre-of-mass energies, the LHC experiments will test the predictive power of the Standard Model for particle interactions at an energy regime never probed before. LHC experiments will also beneft from large statistical samples of data for many interesting channels. This will considerably reduce the current levels of statistical uncertainties in many precise measurements with the measurements overall precision being limited by systematics effects. Performing precise measurements in a large number of physics channels such as jet measurements, direct photon production, DreU-Yan processes and heavy flavour production, in most cases, ATLAS is expected to improve significantly on previous experiments. As discussed in many examples in this note, these measurements should also lead to improvements in the knowledge of the proton structure. The results presented and discussed here come from contributions of different group members of the ATLAS Collaboration. REFERENCES

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