Neumeister, N

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CP Violation Studies in ATLAS and CMSNorbert NeumeisterInstitute for High Energy Physics ViennaNikolsdorfergasse 18, A-1050 Wien, AustriaABSTRACTThe large number of bb events at LHC will oer the possibility to study CP violation in theB system. The two general purpose detectors ATLAS and CMS, primarily designed to studyhigh pt-phenomena, are capable of doing B-physics and the possibility of measuring CP hasbeen investigated by both collaborations. We discuss the potential of the two detectors forthe measurement the angles alpha and beta of the unitarity triangle during the initial lowluminosity phase of the LHC.1 IntroductionThe coupling between up- and down-type quarks is de-scribed by the Cabibbo-Kobayashi-Maskawa (CKM) ma-trix [1]. The precise determination of elements of thismatrix is one of the primary goals of heavy-avour physics.In the Wolfenstein parametrization [2] the matrix iswritten approximately asV '0@1 122 A3(  i)  1 122A2A3(1    i)  A211A(1)Only Vuband Vtdhave a complex phase in this parametri-sation and are therefore the relevant terms which couldproduce CP violation. The unitarity of the CKM ma-trix implies a relation between the elementsVub+ Vtd= Vcb(2)which can be represented as a triangle in the   com-plex plane, with the three angles ; ; , see gure 1. Inprinciple all three angles of this unitarity triangle areaccessible to direct experimental measurements. Herewe will concentrate on the measurements of the angles and .There are a variety of ways in which CP can beviolated in the B system leading to  (B0! f) 6= (B0!f). The measurable decay rate asymmetryA = (B0! f)   (B0!f) (B0! f) +  (B0!f)(3)depends on the angle of the unitarity triangle. For ex-ample, A  sin 2 for B0d! J= K0sand A  sin 2 forB0d! + .Both time-integrated and time-dependent mea-surements of this asymmetry can be performed. Thetime-dependent measurement is done using the posi-tion of the reconstructed decay vertex of the B meson.1.00.80.60.40.200 0.2 0.4 0.6 0.8 1.0–0.2–0.4–0.6–0.8–1.0η95% C.L.αγ β ρFigure 1: Unitarity triangle of the CKM matrix withcurrent constraint.2 B Physics at the LHCThe initial luminosity of LHC is expected to be 1032to 1033cm 2s 1. B-physics studies will be easiest atthis initial luminosity, where pile-up eects are smalland vertex detectors very close to the beam pipe areexpected to survive for several years.For this study, we assume a center-of-mass en-ergy ofps = 14 TeV and an integrated luminosity of104pb 1, corresponding to 107seconds at a luminosityof 1033cm 2s 1. In this regime, we expect from 1.5 to2 underlying events per bunch-crossing.There is a large uncertainty in the estimationof the production cross-section of bb at LHC. For thisstudy we used bb= 500 b. The fraction of bb eventsat the LHC is thus bb=tot 0:5%. With such a cross-Page 1section, we anticipate about 51012bb events/year (107s).The experimental diculties are the high ratesand the large associated multiplicities. What is neededis a powerful trigger system to select the interestingmodes, detectors with high granularity, high-resolutionvertex detectors, ecient track reconstruction and goodmomentum resolution.3 DetectorsThe two general purpose detectors CMS and ATLASare extensively described in [3] and [4]. Both detectorsare designed to take full benet of the lower luminosity(1033cm 2s 1) to study B physics.The basis of the CMS detector is a long (13 m) su-perconducting solenoid with a 6 m bore and a uniform4 T eld, containing the inner tracker and all calorime-try. The most distinguishing feature of the ATLAS de-tectors is its large air-core toroid muon spectrometer.Inside the air-core muon spectrometer is the calorimetersystem. The inner tracking detector is located in 2 Tsolenoidal magnetic eld provided by a superconductingcoil that sites in front of the electromagnetic calorime-ter. ATLAS and CMS overall lengths are 44 m and 21 mapproximately, and their diameters are 22 m and 14 mrespectively. Both detectors have very good momentumresolution: pt=ptfor tracks with pt= 100 GeV is 2%for ATLAS and 1% for CMS.The two detectors have foreseen high-resolutionvertex detectors for B-physics. The main weakness ofthe two detectors for B physics compared to dedicatedB physics experiments is that they have no hadron iden-tication.ATLAS and CMS are using the centrally pro-duced b's while LHC-B [5], a dedicated B-experimentat the LHC is using forward b's.4 Measuring sin2To determine the angle  of the unitarity triangle themost appropriate decay channel is B0d! J= K0sfol-lowed by J= ! `+` (` = ; e) and K0s! + , as itis a CP eigenstate, and has the cleanest signature andthe most tractable background. Further advantages arethe relatively high branching ratio and the fact thattriggering on J= ! `+` is relatively easy. RecentCDF results are most encouraging in this respect [6].The experimentally measurable time-integrated asym-metry for this decay channel is related to sin 2:A = (B0d! J= K0s)   (B0d! J= K0s) (B0d! J= K0s) +  (B0d! J= K0s)= sin2 D xd1 + x2d(4)where D is the dilution factor and xd=(1 + x2d) is afactor due to time-integration (xdhas been measuredto be 0.71 [7]). The reconstructed B0dmeson is taggedas having been produced as a B0dor aB0dusing thecharge of lepton (e; ) from the semileptonic decay ofthe associated b.4.1 Trigger and Event SelectionIn principle there are 3 possibilities:(i) muon-tag with J= ! + (ii) muon-tag with J= ! e+e (iii) electron-tag with J= ! + The ATLAS collaboration has investigated all three pos-sibilities, while in CMS the electron modes are underactive study and only the mode J= ! + with -tag is included here.CMS:The trigger is provided by two low-ptmuons with rapidity-dependent trigger thresholds: pt> 4:5 GeV for 0:0 <jj  1:5, pt> 3:6 GeV for 1:5 < jj  2:0 andpt> 2:6 GeV for 2:0 < jj  2:5 The third muon isrequired to reach at least the rst muon station. Thefollowing further cuts are applied to all events: 's from K0swithin jj  2:4 and pt 0:7 GeV K0sdecay length in the transverse plane between2 and 40 cm, to avoid problems due to patternrecognition The reconstructed invariant masses of J= ,K0sandB0dmust lie within 2 of their known masses tag muon with pt> 4 GeVThe reconstructed masses have the following resolu-tions: 16 MeV for J= , 8.6 MeV forK0sand 12 (22) MeVfor B0dwith (without) mass constraints [8]. To obtainthe expected number of events in addition to the trig-ger eciency and the geometrical acceptance, a trackreconstruction eciency of 95% for triggered muons and90% for the third muon is considered. The K0srecon-struction eciency in the case of 2 's from K0swithpt 0:7 GeV folded with their momentum distributionis on average 35% [8].ATLAS:ATLAS is triggering at the rst-level on a single muonwith jj < 2:2 and pt> 6 GeV. In addition, the second-level trigger requires at least one of the following: (i)an e+e pair with pet> 1 GeV, (ii) an additional muonwith pt> 5 GeV and jj < 2:5 or (iii) an electronwith pt> 6 GeV and jj < 2:5. The total trigger ratefor this three triggers is about 1 kHz. The electrons areidentied in the ATLAS TRT. In the case J= ! + both electron tags and muon tags are used. The follow-ing cuts are applied to all events at analysis level: 's from K0swithin jj  2:5 and pt> 0:5 GeV K0sdecay length in the transverse plane between 1and 50 cmPage 2 tag lepton pt> 5 GeV for muons or pt> 6 GeVfor electrons the J= K0ssystem is required to have a mass withina 2 of the known B0dmass and a proper decaytime must exceed 0:5 psFig. 2 shows an B0d! J= K0s! e+e + eventwith an electron tag reconstructed in the ATLAS Innerdetector.e-pipi+e+-µ+Figure 2: B0d! J= K0s! e+e + event recon-structed in the ATLAS Inner detector.For -tagged J= ! e+e decays, a tag muonwith pt> 6 GeV and jj < 2:2, and two electrons withpt> 1 GeV were required. For -tagged J= ! + decays, a tag muon with pt> 5 GeV, and two othermuons with pt(1) > 5 GeV, pt(2) > 3 GeV were re-quired; in addition, one of the three muons was requiredto satisfy the trigger, having pt> 6 GeV, jj < 2:2. Forelectron-tagged J= ! + events, a tag electronwith pt> 5 GeV, and two muons with pt(1) > 6 GeV,j(1)j < 2:2, and pt(2) > 3 GeV were required.TheK0smass resolution is in the range of 3-8 MeVfor transverse decay lengths of 1-50 cm. The average re-construction eciency is 91% and the background un-der the K0speak is 6% [9]. The reconstructed J= masshas a resolution of 27 MeV and the resolution of the re-constructed B0dis 18 (35) MeV with (without) massconstraints [10]. To calculate the expected number ofevents a reconstruction eciency of 80% for leptons and95% for pions is taken into account.4.2 Tagging and DilutionThe avour of the produced B0dis tagged by the chargeof the lepton produced in the semileptonic decay of theother b-quark in the event. However this tagging is notfully ecient and the measured decay asymmetry is af-fected by dilution eects. The dominant contributionto dilution is from oscillations of B0dor B0sbefore decay-ing to leptons and from cascade decays. The dilutionfactor due to mixing can be written as:Dmix=Xipi11 + x2i(5)where piare the production rates for B, B0d, B0sandb. With p: pd: ps: p= 0:38 : 0:38 : 0:14 : 0:1,xd= 0:71 and xs= 10:0, the dilution factor due tomixing is Dmix= 0:73.The dilution factorDtagcan be dened as: Dtag=1  2w, where w is the fraction of wrong tags. Sourcesof mistags are: cascade decays (b! c! `) muons from additional b's and c's hadron decays (K, decays) (for muons) punch-through (for muons) conversions and Dalitz pairs (for electrons)Fig. 3 shows the fraction of wrong sign muons plotted asa function of the ptcut for various sources of mistags.The major contribution to w is cascade decays. Thecontribution from punch-through, hadron decays andfrom conversions is small. The fraction of mistagged353025201510500 2 4 6 8 10 12 14pT - cut% wrong tagsall, mixing includedall, no mixingfrom cascade decaysfrom K,pi-decaysfrom additional b, cFigure 3: Fraction of wrong tags as a function of thept-threshold of the tagging muon.Page 3events depends on the requirements placed on the asso-ciated lepton, the purity increases with increasing pt.The presence of background introduces a thirddilution factor Dbackground= NS=(NS+ NB), whereNSand NBare the number of signal and backgroundevents.The dilution factor in equation 4 can be writtenas: D = DmixDtagDbackground4.3 BackgroundThe background for this channel is dominated by acci-dental coincidences between a real J= and a K0sfromfragmentation. The backgrounds from B! J= Kand B0! J= K0peak below the signal mass peak.The signal to background ratio is about 1 : 10. Fig. 4shows the reconstructed mass distributions (signal andbackground) for CMS and ATLAS.4.4 Expected SensitivityThe statistical error on sin 2 for the time-integratedanalysis is calculated as:(sin 2) 1D  xd=(1 + x2d) pNtot(6)The results of the time-dependent and time-integratedJ= K0sanalyses are summarized in Table 1. It has tobe stressed out that the electron tag in CMS is underinvestigation and is not included yet. The theoreticaland systematical error is of the order of 1%.Parameter CMS ATLASBR[B0d! J= K0s] 3:3 10 43:75 10 4N [-tag, J= ! e+e ] { 10120N [-tag, J= ! + ] 8000 6590N [e-tag, J= ! + ] { 5680N(signal) 8000 22390N(background) 800 2590N(total) 8800 24980Dbackground0.91 0.90DtagDmix0.53 0.56(sin 2) time-dep. 0.045 0.018(sin 2) time-int. 0.048 0.020Table 1: Sensitivity to sin 2 for 104pb 1.5 Measuring sin2The most promising channel for the measurement of theangle  of the unitarity triangle is B0d! + . Thetrigger of such events is provided by the semileptonicdecay of the associated b-hadron in the event which isused to tag the avour of the produced B0d. The time-integrated asymmetry A for this decay channel is:A =N+ N N++N = D xd1 + x2d sin 2 (7)04.8 5.0 5.2 5.4 5.6Events / 10 MeV15002000 pµtag  > 5 GeV/cTppi  > 0.7 GeV/c T1000500pi piµ µ GeV– +M + –pp     bb     µtag + Bo + Xds J/ψ Ko pi–pi+µ+µ–| < 2.4ηpi,µ 104 pb–1CMSσ  : 16 MeV σ  : 22 MeV σ  : 12 MeV ψBBwith mass const.ATLASσ = 12 MeVσ = 18 MeVEvents / 10 MeVpi piµ µ GeV– +M + –Figure 4: B0d! J= K0sin CMS and ATLAS.Page 4where N+and N are the number of reconstructedevents with positively and negatively charged taggingmuons and D is the dilution factor discussed in sec-tion 4.2.The study of this decay channel is experimentallymore dicult than for B0d! J= K0sbecause one lacksthe clean signature of the leptonic J= decay.The rst level trigger for this analysis is an in-clusive muon trigger with pt> 9 GeV and jj < 2:4 inthe case of CMS and pt> 6 GeV and jj < 1:6 in thecase of ATLAS. In addition the ATLAS second leveltrigger requires an unlike-charge pair of particles withpt> 6 GeV and with the sum ptof the two particleslarger than 15 GeV.The branching ratio for the decay B0d! + was assumed to be 2 10 5.5.1 Background and Event SelectionWithout particle identication the largest backgroundis from two-body B-meson decays when the chargedhadrons are assigned pion mass. One has to rely onmass resolution to suppress this background. In par-ticular, the decay B0d! K peaks just below the B0dmass, while the decay B0s! K is just above. Thedecay B0s! KK is peaked on top of the signal; how-ever, the smaller B0sproduction probability suppressesthis background. An asymmetric contribution comesfrom the decay b! p. Other quasi-two body decayslike B0d! , B0s! K, B0s! K and b! Kcontribute less than 1%. Fig. 5 shows the contributionfrom the two-body decay backgrounds. In CMS the re-constructed mass has a resolution B= 25 MeV and intotal the two body decays contribute to a backgroundto signal ratio, B=(S + B), of  45% in a mass win-dow of 1. In ATLAS the + mass resolution is50 MeV.The combinatorial background adds an almostat background contribution, which is around 10% ofthe total.All other sources of background are highly sup-pressed using the following selection criteria: [8, 9]CMS: two opposite-sign hadrons with pht> 5 GeV withinjj < 2 distance between the two pions R < 1 isolation I < 0:1, where I is dened as thePptof the hadrons within a cone R < 1 around theB0ddirection, normalized to the ptof the B can-didate (only charged tracks with pt> 2 GeV areconsidered and the two pions are not included inthe sum) impact parameter signicance d=d> 2 for eachpion, where bis the impact parameter resolution  < 100 mrad where  is the angle in the transverseplane between the direction dened by the primaryand decay vertex and the reconstructed  sum-momentumATLAS: two hadrons with pt> 6 GeV and sum ptlargerthan 15 GeV scaled impact parameters of the two pions d=d> 3 angle between the reconstructed B-meson and theline joining the primary and secondary vertices inthe transverse plane < 6 transverse decay length > 300 m distance of closest approach of the two particles< 55 mFig. 6 shows the invariant mass distributions for the sig-nal and background after all cuts for CMS and ATLAS.B˚      pi+pi–d20004.8 5.0 5.2 5.4 5.6 5.8Mpi–pi+ GeVEvents / 10 MeV 4006001000800140016001200sB˚       KpisB˚       K+K–B˚       KpidSumΛb      ppiσ(Mpipi) = 27 MeVCMSFigure 5: B ! + mass resolution and 2-body decaybackgrounds.5.2 Expected SensitivityThe results of the time-dependent and time-integratedB0d! + analyses are summarized in Table 2. Itcan be seen, that performing a time-dependent mea-surement will improve the sensitivity. Theorectical un-certainties in this measurement are larger than in thecase of sin 2 because of contributions from possiblepenguin diagram decays.Page 5  pi+pi– 04.8 5.0 5.2 5.4 5.6 5.8Mpi+pi– GeVEvents / 10 MeV200pµ  > 9 G eV/c| ηpi,µ | <  2.4d Tppi  >  5 G eV/c T  pi+pi–  Bo d 104 pb–1pp     bb     µtag + Bo + X4006008001000120014001600CMSσ = 27 MeVATLASσ = 50 MeVMpi+pi– GeVEvents / 25 MeVFigure 6: B0d! + in CMS and ATLAS.Parameter CMS ATLASN [Bd! ] 4300 7120N [Bd! K] 1525 2890N [Bs! KK] 1250 1070N [Bs! K] 510 1495other backgrounds 515 1585N(background) 3800 7040N(total) 8100 14160Dbackground0.47 0.50DtagDmix0.57 0.56(sin 2) time-dep. 0.070 0.043(sin 2) time-int. 0.050 0.047Table 2: Sensitivity to sin 2 for 104pb 1.6 SummaryBoth collaborations, ATLAS and CMS, studied the pos-sibility of measuring CP violation. Even without par-ticle identication they perform well for B0d! + thanks to their good mass resolution and are are ableto compete with dedicated B physics experiments in thechannel B0d! J= K0s.AcknowledgementsI would like to thank Daniel Denegri for many usefuldiscussions and for the invitation to participate in thisconference.References[1] M. Kobayashi and T. Maskawa, Prog. Theor. Phys.49 (1973) 652; N. Cabibbo, Phys. Rev. Lett. 10(1963) 531.[2] L. Wolfenstein, Phys. Rev. Lett. 51 (1983) 1945.[3] CMS Collaboration, CERN-LHCC 94{38,LHCC/P 1, 1994.[4] ATLAS Collaboration, CERN-LHCC 94{43,LHCC/P 2, 1994.[5] LHC-B Collaboration, Letter of Intent,CERN/LHCC 95-5, LHCC/I8, August 1995.[6] J.C. Yun, B Physics Results at CDF, FERMILAB-CONF-95-346-E (Jul 1995).[7] H. Albrecht et al. (ARGUS Collaboration), Z.Phys. C55 (1992) 357.[8] D. Denegri et al., Int. J. Mod. Phys. A9 (1994)4211-4255.[9] ATLAS Collaboration, CERN-LHCC 93{51, 1994.[10] P. Eerola et al., ATLAS Internal Note, Phys-No-47, 1994.Page 6

CP violation studies in ATLAS and CMS

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