B-physics is an importany part of studies with the LHC beams at low luminosities. Effective B-meson reconstruction with exclusive semileptonic modes in the ATLAS can produce a clean sample of events for detailed physics analysis with first data. The analysis includes measurements of B-meson mass and proper lifetime to verify performance of the Inner Detector and to estimate background and trigger efficiencies for rare decays. The b-quark fragmentation with low pt, bbbar correlations and pt compensation will be studied in events with reconstructed B-mesons. The underlying event multiplisities in such events will be mesured and compared with data from Leading jet analysis

Boldyrev, A

Smirnova, L

CERN Document Server

ATL-PHYS-PROC-2009-13523November2009Physics with B-mesons in ATLASLidia Smirnova1, Alexey Boldyrev, on behalf of the ATLAS Collaboration**Moscow State University , E-mail: Lidia.Smirnova@cern.ch, Alexey.Boldyrev@cern.chAbstractB-physics is an important part of studies with the LHC beams at low luminosities. Ef-fective B-meson reconstruction in ATLAS can produce a clean sample of events for detailedphysics analysis with first data. The analysis includes measurements of B-meson massesand proper lifetimes to verify the performance of the Inner Detector, and to estimate back-grounds and trigger efficiencies for rare decays.XXXIX International Symposium on Multiparticle Dynamics4-9 September 2009Gomel RegionBelarus1Speaker1 IntroductionB-physics is one of the important fields that can be studied with the first ATLAS data. Until now the in-formation about B-meson production come from B-factories, high-energy electron-positron and electron-proton colliders and proton-antiproton colliders. The proton-proton collisions at the LHC will be the nextstep in studies of B-meson production in hadron interactions with large b-quark production cross sectionand high luminosity. The ATLAS experiment will be able to reconstruct exclusive B-meson decays.After the introduction, section two describes briefly the ATLAS detector and triggers, relevant for B-physics, with stress on di-muon channels. Reconstruction of inclusive J/ψ→ µµ decays and separationof J/ψ from direct and indirect (B-hadrons) production mechanisms are discussed. The efficiency ofB+-meson reconstruction with B+→ J/ψK+ decay mode is presented. The main results with B-physicsmeasurements with early data are summarized. All results described in this overview are for the LHCenergy 14 TeV.2 The ATLAS experimentATLAS is the largest particle detector at the LHC due to the superconducting air-core toroid, which isthe source of the outer magnetic field [1]. The length of detector is 46 m and its diameter is 26 m. Threelevels of the muon drift chambers are positioned inside this magnetic field among the toroidal coils. Theyprovide identification for muons in a wide region of transverse momenta (pT) starting from 3 GeVand aprecise determination of pT up to about 1 TeV. The Muon Spectrometer and the Inner Detector comprisethe ATLAS tracking system. The Inner Detector has a length of 6 m and is 2 m in diameter. It includesthree different detector technologies. Closest to the beam pipe there are three levels of pixel detectorswith 80 ·106 read out electronic channels. Further out there are four layers of silicon microstrip detectors.With the pixels they produce an average of seven precision hits for a charged particle track. The thirdcomponent of Inner Detector is the Transition Radiation Tracker (TRT) with thin proportional drift tubes4 mm diameters (straws). Each track crosses at most 36 straws. By selection of high-energy hits fromtransition radiation, the TRT permits to the identification of electrons among charged particles.The ATLAS Trigger consists of three levels. The Level 1 trigger operates with trigger muon chambersand calorimetry information, and provides a trigger rate at luminosities 1031−1033cm−2s−1 of about 75kHz. The Level 2 trigger algorithms decrease the event rate to approximately 2 kHz. The resources ofthe Event Filter as the third trigger step reduce the output event rate 200 Hz, with approximately 5-10%devoted to B-physics. Two different trigger algorithms are available to cover B-decay modes with twomuons in final state [2]. The topological di-muon trigger tunes two muons at Level 1 with separateregions of interest (RoI) around each muon direction. These are combined at Level 2 to calculate theinvariant mass. A vertex fit can also be applied. The second scheme is applied for early data and onemuon at Level 1, and wide the RoI to search for second muon. Invariant mass cuts applied after thesearch for the second muon candidate in the Inner Detector tracks, confirmed in muon detectors. Bothdescribed triggers will be selecting events with J/ψ decaying to muons. To measure trigger efficiencywith real data J/ψ → µµ decays will be used in the low pT region from a few GeV/c to 20 GeV/c bytag-and-probe method, illustrated in Figure 1, taken from [2]. With starting conditions two muon triggerwill be 6 GeV/c for first muon and 4 GeV/c for second muon (µ6 µ4).3 Inclusive J/ψ eventsThe exclusive B-decay channels will be selected by inclusive J/ψ triggers and subsequently fully re-constructed using offline tools. Understanding of inclusive Jpsi events is a first milestone in measuringexclusive decays of B-hadrons to J/ψ . The inclusive J/ψ events are composed of J/ψ directly produced1Figure 1: The overall efficiency of the Level 1 single muon trigger with respect to the offline selectionobtained by the tag-and-probe method (filled circles) and the efficiency estimated from a single muonMC sample (open circles). The curve is a fit to efficiency obtained by tag-and-probe method.in proton-proton collisions and from indirect once, mostly coming from B-decays. Positive displacementof J/ψ-vertex from primary interaction means non-zero life time and indicate the indirect J/ψ pro-duction. The simulated pseudo-proper time measurements for J/ψ → µµ decays is shown in Figure 2.The distribution for prompt J/ψ around zero has the dark shading and the sum of prompt and indirectJ/ψ candidates is shown by lighter shading. For the J/ψ events selected with pT thresholds 6 GeV and4 GeV for the harder and the softer muon respectively, the pseudo-proper time cut of > 0.15 ps givesbb→ J/ψX selection efficiency of 80% with 20% prompt J/ψ contamination.Figure 2: Pseudo-proper time distribution for reconstructed prompt J/ψ and the sum of prompt andindirect J/ψ candidates24 Early measurements of B+-mesonThe exclusive channel B+ → J/ψK+ can be measured at early stage of LHC running with integralluminosity ∼ 10 pb−1[2], due to the clear event topology and relatively large branching ratio (1.007±0.035) · 10−3. It can serve as a reference channel for searches of the very rare decay Bs → µµ , as acontrol channel for CP violation measurements due to the low direct CP violation of the channel [3].Measurements of such well-known masses and lifetimes [4] will be used for Inner Detector calibrationand alignment studies, and for searches of association particle production. The reconstructed B+ mesonmass is shown in Figure 3 with fit results. The expected mass resolution for B+-mesons is 42.2 ± 1.3MeV. One can sees that the background in the signal mass region is very low and the event sample withproduced B+-meson is clean enough for the further event analysis.With early data B+ production cross sections, both absolute and differential, depending on transversemomenta, can be measured. Given that a statistical precision of O (1%) will be reached with an inte-grated luminosity of 0.1 fb−1, the contribution of the systematics will dominate the uncertainties of thefirst measurements. This is the case even for the differential cross-section measurement. Although thestatistics in each pT bin is limited, the total uncertainty is dominated by the systematic uncertainties inthe branching ratio of the B+ → J/ψK+ and in the luminosity, which are of the same order. For theexclusive cross-section measurement in the B+→ J/ψK+ channel, the relative uncertainties of the dif-ferential and total cross-sections are given in Table 1. The first row of the table contains the quadratic sumof the statistical uncertainty corresponding to an integrated luminosity of 0.01 fb−1and the uncertaintyin the efficiency. The latter is based on the statistics of the Monte Carlo dataset used. The second row iscalculated by adding in quadrature the above uncertainty to the systematic uncertainty of the luminosityand the branching ratio for every pT bin.Figure 3: B+ mass fit with the both signal (red) and background (blue) contributions shown separatelyFor the high statistics pT bins as well as for the total cross-section, the total relative uncertaintyis dominated by systematic errors, originating mainly from the uncertainty in the luminosity, which isassumed to be 10 % for the initial phase, and the 10 % uncertainty in the branching ratio of B+→ J/ψK+.The effect of the assumed background shape on the measurements is estimated to be less than 1 %.Finally, the precision of the lifetime measurement, for the same integrated luminosity is 2.5 %, where nosystematic effects are taken into account.3pT range [GeV] pT ∈ [10,18] pT ∈ [18,26] pT ∈ [26,34] pT ∈ [34,42] pT ∈ [10, in f )stat.+A [%] 7.7 6.9 10.5 13.9 4.3total[%] 16.1 15.8 17.6 19.8 14.8Table 1: Statistical and total uncertainties for the B+ → J/ψK+ differentional and total cross-sectionmeasurements for an integrated luminosity of 0.01 fb−1. Total uncertainties include luminosity and BRsystematic uncertainties.5 Early measurements with B0d and B0s mesonsWith the early data, the decays B0d → J/ψK∗0 and Bs→ J/ψφ can be used to measure B hadron massesand lifetimes with sufficient precision to permit sensitive tests of the detector performance. In particular,the B0d lifetime can be determined with a relative statistical error of 10%, with an integrated luminosityof 10 pb−1, and the same precision will be achieved for the B0s lifetime with 150 pb−1[2]. The proposedmethod of a simultaneous fit of background and signal events allows a sensitive determination of themasses and decay times of B mesons. With early data, the optimal overall precision will be obtainedwith no cuts on the secondary vertex displacement. This is appropriate for the early data when theperformance of the detector and reconstruction algorithms may not be well understood. This strategyis consistent with that of the early B-physics triggers, where no displacement cuts on the J/ψ will beapplied at the trigger level. In the early data taking phase, the self-tagging decay B0d → J/ψK∗0 willbe used to calibrate the jet charge tag for jets containing a B0d . This will be of use for physics studiesinvolving B0d decays, but also this good understanding for the tagging performance for B0d→ J/ψK∗0 willallow the fragmentation modelling for Bs→ J/ψφ decays to be improved.6 ConclusionsWith early data the reconstruction of exclusive B-meson decays with two muons in the final state, espe-cially with J/ψ , will be done. The measurements of B hadron masses and lifetimes will help to improvethe detector performance. The analysis B+-decay and B0d → J/ψK∗0 decay will serve as a referencechannel to the Bs→ µµ analysis. The B0d-decays early physics studies will improve Bs→ J/ψφ decaysanalysis, where the CP violation parameters can be measured. The ATLAS B-physics first data is veryimportant for following new physics studies beyond SM.References[1] ATLAS Collaboration, The ATLAS Experiment at the CERN Large Hadron Collider, JINST 3(2008) S08003[2] ATLAS Collaboration, G. Aad et al., Expected Performance of the ATLAS Experiment, Detector,Trigger and Physics, CERN-OPEN-2008-020, 2008[3] V.M. Abazov et al., D0 Collaboration, Phys. Rev. Lett. 100 (2008) 211802[4] W.-M. Yao. et al., J. of Phys. G33 (2006) 14

Physics with B-mesons in ATLAS

B-physics is an important part of studies with the LHC beams at low luminosities. Effective B-meson reconstruction with exclusive semileptonic modes in the ATLAS can produce a clean sample of events for detail physics analysis with first data. The analysis includes measurements of B-meson mass and proper lifetime to verify performance of the Inner Detector and to estimate background and trigger efficiencies for rare decays. The b-quark fragmentation with low pt, bbbar correlations and pt compensation will be studied in events with reconstructed B-mesons. The underlying event multiplicities in such events will be measured and compared with data from Leading jet analysis

Physics with B-mesons in ATLAS

Abstract

Similar works

Full text

Available Versions

CERN Document Server

CERN Document Server