The Large Hadron Collider with its two general-purpose proton-proton experiments Atlas and CMS provides ample opportunities for precision measurements of Standard Model parameters. Ultimate precisions of 5\:\mathrm{ MeV}$ for the $W boson mass and better than :\rm{GeV}$ for the top mass are expected. Sensitivities to triple gauge couplings will improve by orders of magnitude. Novel areas like single top production and high-mass Drell-Yan lepton pairs will be open to exploration

Røhne, O M

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EPJdirect (2002), to be processed by the editorial EPJdirectelectronic onlyc© Springer-Verlag 2002Prospects for Precision measurements at theLarge Hadron ColliderO. M. Røhne1CERN, Geneva, Switzerlande-mail: ole.rohne@cern.chAbstract. The Large Hadron Collider with its two general-purpose proton-protonexperiments Atlas and CMS provides ample opportunities for precision measurementsof Standard Model parameters. Ultimate precisions of 15 MeV for the W boson massand better than 1 GeV for the top mass are expected. Sensitivities to triple gaugecouplings will improve by orders of magnitude. Novel areas like single top productionand high-mass Drell-Yan lepton pairs will be open to exploration.PACS: not given1 IntroductionThe Large Hadron Collider currently under construction at CERN is often per-ceived primarily as a discovery facility. Indeed, colliding protons at a center-of-mass energy of 14TeV with a design luminosity of 1034cm−2s−1 will undoubtedlyallow searches for a wide range of new phenomena. However, the facility will alsolend itself to precision measurements of Standard Model parameters. This is par-ticularly, yet not uniquely, true for the initial low luminosity phase.Even in the low luminosity phase, each LHC experiment will collect 10fb−1=yr,which is comparable to the total integrated luminosity at Tevatron Run 2. Fur-thermore, as the center-of-mass energy is up by almost an order of magnitude,LHC has the advantage of high production cross-sections for key processes, ex-pected production rates are shown in table 1. The leading-order implication ofProcess  (pb) N (s−1) N (yr−1)W ! e 1:5 104 15  108Z ! e+e− 1:5 103 1:5  107tt 800 0:8  107Table 1. Examples of LHC production rates at 1033 cm−2s−1the vast event samples is that virtually all precision measurements will be sys-tematics limited. Secondly, the large event samples available for calibration andcontrols will in turn reduce systematics. Finally, being able to apply hard cutsyields well-understood signal samples, again reducing systematics.http://link.springer.de/link/service/journals/10105/index.htmlSN-ATLAS-2002-01415 Jan 2002Prospects for Precision measurements at the Large Hadron Collider 2When commissioned, the Atlas and CMS detectors will constitute powerfulsystems for precision physics. Perhaps the hardest challenge in terms of detectorsystematics will be to understand the lepton energy/momentum scale with theaccuracy required for the W mass measurements. Atlas has concluded [1] that ascale precision of 0:02% is possible, eventually using Z ! ‘‘ for calibration. A 1%precision on the jet energy scale should be possible using transverse energy bal-ance in Z=γ+jet events. The jet energy scale in hadronic top decays can be fixedin-situ using the W boson mass constraint. Knowing the absolute luminosity isof course important for all measurements involving cross-section. Combining in-formation from LHC itself with forward detectors and measurements of the Wand Z rates, the goal is to achieve better than 5% precision.2 W mass measurementIn the Standard Model the masses of the W boson, the top quark and the Higgsboson are related through radiative corrections:M2W =p2GF sin2 w(1−r)(1)r  m2t ;r  logMH :This relation currently constrains the Higgs mass; assuming the Higgs boson isdiscovered at LHC, equation 1 will be used as a test of the Standard Model.Following LEP2 and Tevatron Run 1, MW = 80:451 0:033 is still in a certainsense a poorly known precision parameter of the Standard Model. In order forMW and mt to have equal weights in a 2-test of equation 1, their precisionsshould be related by MW ’ 0:7  10−2mt. Thus, a precision of MW =15 MeV is needed to match a top mass measurement to mt = 2 GeV.Atlas has studied [2] the classical method for measuring MW at hadron col-liders. Leptonically decayingW bosons are selected requiring one isolated lepton,significant missing transverse energy and minimal hadronic activity. TheW massis extracted from the transverse massmT =√2p‘TpT(1− cos): (2)Here, the neutron transverse momentum pT is reconstructed using the transversemomenta of the lepton and hadronic recoil u, see figure 1. A negligible statisticalerror of statMW < 2 MeV can be reached after only one year running at low lumi-nosity, collecting 10 fb−1, and the experimental precision will be determined bysystematics. Table 2 shows a comparison of systematic errors at CDF Run 1B [3]with projections for one LHC experiments. Note that several contributions tothe systematics scale down with increased statistics, others are expected to beconstrained by LHC measurements. The lepton scale uncertainty is dominatingthe systematics and, as already mentioned, will probably be the most challeng-ing to control. Combining both leptonic channels from the two experiments, acombined precision of 15 MeV should be possible albeit challenging.http://link.springer.de/link/service/journals/10105/index.htmlProspects for Precision measurements at the Large Hadron Collider 3‘−‘uAtlas 10 fb−1Fig. 1. W mass measurement. Left panel: Transverse view of leptonic W decay.Right panel: Transverse mass distribution, without and with detector smearing.Source ofsystematic errorMWCDF Run IB84 pb−1e-channelMWAtlas 10 fb−1‘-channelLepton scale 75 MeV 15 MeVLepton resolution 25 MeV 5 MeV Known to < 1:5%pWT 15 MeV 5 MeV Constrain with pZTRecoil 37 MeV 5 MeV Constrain with Z dataPDFs 15 MeV < 10 MeV Constrained at LHCΓW (10 MeV) 7 MeV ΓW = 30 MeV from Run IIRadiative decays 20 MeV < 10 MeV Theoretical calculationsTotal 92 MeV < 25 MeV Per lepton species, per experimentTable 2. Systematic errors of W mass measurements at CDF Tevatron Run 1Bcompared to the expectations for the Atlas experiment at LHC. Similar perfor-mance is expected from CMS.http://link.springer.de/link/service/journals/10105/index.htmlProspects for Precision measurements at the Large Hadron Collider 4Alternative methods for measuring MW do exist and will be used for crosschecks. One possibility is to compare the transverse mass distributions of Wand Z boson decays. Being effectively a measurement of MW =MZ , importantsystematics cancel but for the price of a factor 10 loss of statistics. Anotheroption is to abandon the transverse mass and study the transverse momentumdistribution of the lepton. As this method does not depend on hadronic recoilreconstruction, it avoids the associated systematics and could even be used athigh luminosity. However, the measurement is highly sensitive to assumptionson the W boson transverse momentum spectrum, and would require improvedtheoretical understanding.3 Triple gauge couplingsGeneral charged TGCs are customarily parameterized assuming Lorentz andelectro-magnetic gauge invariance, and C and P conservation, yielding five in-dependent parameters: V , V , gZ1 (V = Z; γ). Likewise, neutral TGCs areparameterized under the assumptions of Lorentz and electro-magnetic gauge in-variance, and Bose symmetry. This gives eight parameters hVi (i = 1 : : : 4) for theZγγ/ZZγ vertex, and four parameters fVi (i = 4; 5) for the ZZγ/ZZZ vertex.The only triple gauge couplings present in the Standard Model are Z and γ .As naively adding anomalous couplings to the Standard Model generally violatesunitarity, the parameterization has to include form factors with a scale param-eter FF, interpreted as the scale of new physics. Running at a center of massenergy almost an order of magnitude above the Tevatron, the LHC experimentswill push the exploration limits from about 2 TeV to 10 TeV.Atlas has studied the possibility of measuring anomalous charged TGCs usingthe WZ and Wγ final states [4]. Neutral TGCs have been studied by CMS inthe Zγ final state [2], and by ATLAS in the ZZ final state [5]. The expectedsensitivities are summarized in table 3. Comparing to existing LEP2 results [6]and to expectations for Tevatron Run 2 [7], the LHC improvements range fromone to several orders of magnitude.4 Drell-Yan production of lepton pairsDrell-Yan produced lepton pairs have a distinct experimental signature andare accessible at LHC even at high luminosity. The cross section and forward-backward asymmetry are both functions of the invariant mass and the rapiditywhich are experimentally well-defined observables. An integrated luminosity of100 fb−1 at LHC will provide about 100 million events at the Z pole, and tensof thousand events with invariant mass above 400GeV. The cross section is sen-sitive to any new physics that might show up in the s-channel: New resonances,contact terms, R-parity violating SUSY etc. Figure 2 compares the expectednumber of lepton pair events at Tevatron Run 2 and LHC.Unlike e+e− and pp facilities, a proton-proton collider does not have a natu-rally defined forward direction for AFB measurements. However, as the incominganti-quark must be a sea-quark whose PDF peaks at low x, the incoming quark ishttp://link.springer.de/link/service/journals/10105/index.htmlProspects for Precision measurements at the Large Hadron Collider 5LEP2 Run 2 LHC 100 fb−1γ [−0:089;+0:020] − [−0:0031;+0:0030]Z − − [−0:0060;+0:0061]γ [−0:13;+0:13] − [−0:060;+0:062]Z − − [−0:11; 0:13]gZ1 [−0:074; 0:028] − [−0:0073;+0:0096]hZ3 [−0:20;+0:07] − 0:0007hZ4 [−0:05;+0:12] − 2 10−6fγ4 [−0:17;+0:19] 0:163 0:00073fZ4 [−0:31;+0:28] 0:159 0:00062fγ5 [−0:36;+0:40] [−0:179;+0:170] 0:00074fZ5 [−0:36;+0:39] [−0:184;+0:162] 0:00062Table 3. LHC sensitivities to anomalous TGCs compared to LEP2 results andTevatron Run 2 expectationsFig. 2. Drell-Yan lepton pair cross sections at Tevatron and LHChttp://link.springer.de/link/service/journals/10105/index.htmlProspects for Precision measurements at the Large Hadron Collider 6most likely the one carrying the highest longitudinal momentum. Consequently,the forward direction can be tagged event by event according to the sign ofthe lepton pair rapidity. Due to the statistical nature of the direction tag, theasymmetry vanishes in central events and increases with increasing rapidity.The possibility of extracting the electoweak mixing angle sin2lepteff (M2Z) fromlepton pair forward-backward asymmetry at LHC has been studied by Atlas [8].Utilizing moderate electron identification capabilities in the very forward regionjj = 2:5 − 4:9, it is possible to measure sin2lepteff with a statistical error of1:410−4. Extracting the parameter with such a precision would require a factor10 improved understanding of parton distribution functions. Such knowledgemight become available from future measurements at HERA, the Tevatron Run2, and LHC itself. Alternatively, forward-backward asymmetry measurementscould be combined with the electroweak mixing angle measurements from LEPto further constrain PDFs.5 Top quark mass measurementsThe CDF and D0 experiments have measured the mass of the heaviest quarkto mt = 174:3  5:1 [9][10]. At Run 2 the precision is expected to improve to2− 3GeV. The prospects for further improved measurements at LHC have beenstudied using a variety of approaches, see [11][12] as well as [13][14]. The precisionagain turns out to be limited by systematic errors.For tagging purposes, top mass analyses at LHC require one top quark todecay semileptonically while the other is allowed to decay hadronically. Thebasic event topology is shown in figure 3. Reconstructing the invariant massATLAS 10 fb−1 full simulation0501001502000 100 200 300 400mjjb (GeV)σ = 13.4 GeVEvents/4 GeVFig. 3. Lepton plus jets tt decays. Left panel: Event topology. Right panel: Signaland combinatorial background distribution.of the hadronic top using the reconstructed bjj system and assuming a 1% jetcalibration, the resulting top mass precision of ’ 2 GeV turns out to be limitedby the uncertainty of final state radiation.http://link.springer.de/link/service/journals/10105/index.htmlProspects for Precision measurements at the Large Hadron Collider 7Several ways of reducing the impact of FSR have been explored. One promis-ing approach is to reconstruct also the semileptonically decaying top and do aconstrained kinematic fit using mjj = MW = m‘ and mjjb = m‘b. Arguingthat events with excessive final state radiation or energy lost to b-decay neutri-nos will give a bad fit, the reconstructed top mass is studied as a function ofthe 2 of the fit, see figure 4, left panel. Using Mfit(2 = 0) as the estimator, aprecision of 1GeV can be reached. The residual systematic errors are dominatedby the uncertainty on the b-jet calibration.Another option is to select tt events with a high transverse momentum andthen reconstruct the hadronically decaying top using a single large calorimetercone. This method requires an algorithm to subtract the underlying event contri-bution. As can be seen from the right panel of figure 4, a recalibration procedureis needed to get back the simulated top mass. The recalibration together withthe fact that Atlas has a non-compensating calorimeter limits the accuracy to2:3 GeV.ATLAS 10 fb−1 ATLAS 10 fb−1Fig. 4. Two approaches to reducing top mass systematics. Left panel: Recon-structed top mass as a function of 2 of constrained kinematic fit. Right panel:Top mass estimation using a single large calorimeter cone.CMS has pioneered a method to measure the top mass using semileptonictop decays with a J=Ψ() in the final state [15]. The b-jet from the oppositetop quark is required to contain yet another muon for tagging; the topology isillustrated in figure 5. Not depending on jet measurements, this method can alsobe used at high luminosity. Unfortunately, the branching suppression is severe,and an integrated luminosity of 500fb−1 is required to reach a statistical error of0:9GeV. The method has a significant dependence on the transverse momentumspectrum of the tt, and would thus require careful tuning of simulations to data.The main source of systematics is expected to be the uncertainty on the b-fragmentation, amounting to 0:4 GeV.Taking advantage of the complementary systematic errors of the above meth-ods, an ultimate precision of mt  1 GeV should be possible at LHC.http://link.springer.de/link/service/journals/10105/index.htmlProspects for Precision measurements at the Large Hadron Collider 8Fig. 5. Top mass measurement using the ‘J= final state.6 Single top productionWhile single top production is not yet observed at the Tevatron [16][17], theproduction rate will be significant at LHC. The relevant cross sections are shownin table 4, to be compared to cross sections for tt and non-top backgrounds. TheProcess  (pb)Wg-fusion 244Wt 60W  10tt 830Wbb 300W + jets 18000Table 4. Single top production, signal and background cross sectionsStandard Model has very specific predictions for electroweak single-top events:The top quarks will be produced fully left-polarized, and the branching fractionto longitudinally polarized W ’s is 69% with the remaining 31% left-polarized.Using 10 fb−1 of data, measurements of either polarization with better than 2%resolution should be possible [18].7 ConclusionsAssuming the challenges of systematic errors can be handled, the LHCexperiments will provide valuable precision measurements of importantStandard Model parameters.References1. The ATLAS Collaboration. Detector and Physics Performance TDR, chapter 12.The Atlas Collaboration, 1999.http://link.springer.de/link/service/journals/10105/index.htmlProspects for Precision measurements at the Large Hadron Collider 92. (ed. ) Altarelli, G. and (ed. ) Mangano, Michelangelo L. Standard Model physics(and more) at the LHC. Proceedings, Workshop, Geneva, Switzerland, May25-26, October 14-15, 1999. CERN Yellow Reports, 2000.3. T. Affolder et al. Measurement of the W boson mass with the Collider Detectorat Fermilab. Phys. Rev., D64:052001, 2001.4. M. Dobbs and M. Lefebvre. Probing the three vector-boson vertex in WVproduction. Atlas Physics Workshop 2001.5. S. Hassani. Probing neutral gauge boson self-interaction in ZZ production withATLAS. Atlas Physics Workshop 2001.6. The LEP Electroweak Working Group. A combination of preliminary electroweakmeasurements and constraints on the standard model. 2001.7. U. Baur and D. Rainwater. Probing neutral gauge boson self-interactions in Z Zproduction at hadron colliders. Phys. Rev., D62:113011, 2000.8. K Sliwa, S Riley, and U Baur. Effects of possible extensions to rapidity coverageof the ATLAS detector on the determination of sin2 θeff (m2z). ATLAS-PHYS,(018), 2000.9. Luc Demortier et al. Combining the top quark mass results for Run 1 from CDFand D0. FERMILAB-TM-2084.10. D.E. Groom et al. Review of Particle Physics. The European Physical Journal,C15:1+, 2000. http://pdg.lbl.gov.11. The ATLAS Collaboration. Detector and Physics Performance TDR, chapter 18.The Atlas Collaboration, 1999.12. Lars Sonnenschein. Top mass determination in the ttbar semileptonic decaychannel. CMS-NOTE-2001-001.13. P. Roy. Top mass measurement in the lepton+jets channel. Atlas PhysicsWorkshop 2001.14. F. Fassi and I. Efthymiopoulos. High pt top mass reconstrution in the singlelepton plus jets channel using a large calorimeter cluster. Atlas PhysicsWorkshop 2001.15. Avto Kharchilava. Top mass determination in leptonic final states with j/ψ.Physics Letters B, 471:73–38, 2000.16. D. Acosta et al. Search for single-top-quark production in p anti-p collisions ats**(1/2) = 1.8-tev. 2001.17. V. M. Abazov et al. Search for single top quark production at D0 using neuralnetworks. Phys. Lett., B517:282–294, 2001.18. B. Gonzalez-Pineiro, R. Brock, D. O’Neil, and M. Lefebvre. Top and Wpolarization in EW top production at LHC. ATLAS-PHYS, (018), 2000.http://link.springer.de/link/service/journals/10105/index.html

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