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B-physics with the initial ATLAS detector

B-physics with the initial ATLAS detector. Aleandro Nisati for the ATLAS Collaboration INFN Commissione Scientifica I February 3rd, 4th 2003. outline. The initial experiment conditions The ATLAS Physics Programme The ATLAS detector & trigger

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B-physics with the initial ATLAS detector

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  1. B-physics with the initial ATLAS detector Aleandro Nisati for the ATLAS Collaboration INFN Commissione Scientifica I February 3rd, 4th 2003

  2. outline • The initial experiment conditions • The ATLAS Physics Programme • The ATLAS detector & trigger • B-physics potential with the nominal detector @ L=1033 cm-2 s-1; • Preliminary estimate of the B-physics potential with the initial detector and luminosity; • Conclusions

  3. The initial experiment conditions Many uncertainties: • The LHC luminosity: the target initial luminosity was doubled to L= 2 x 1033 cm-2 s-1; • The detector configuration; in particular the initial HLT/DAQ system bandwidth and processing power (resources limitations); • The physics rates (uncertainties on p,K and heavy flavour production cross-sections);

  4. The initial experiment conditions • Results presented here refer to the nominal detector configuration and L= 1x1033 cm-2 s-1[Yellow Report CERN 2000-004] ; • The analysis with the initial detector layout (including the change of the B-layer radial position, re-evaluation of the material distribution in the ID) as a function of luminosity and the trigger conditions is on-going (within the Data Challenge project); however some preliminary indications on the degradation of the physics performances will be provided.

  5. The ATLAS Physics Programme • The most prominent issues for the LHC are the quest for the origin of the spontaneous symmetry-breaking mechanism (SM and MSSM) and the search for new physics: SuSy, Heavy Bosons, etc… • ATLAS (and CMS) is a general-purpose experiment optimized to maximize the potential discovery new physics: Higgs boson(s) , SuSy particles, W’ and Z’, etc… • However we have to consider that: • The LHC is a beauty factory  dedicated B-experiment (LHCb); • The ATLAS detector allows also a wide programme of B-physics studies, competitive with LHCb in some channels, “for free”…

  6. Cross-sections and rates • huge range of cross-section values and rates • listed for 1034 cm-2 s-1 • total • s 100 mb (109 Hz) • b production • s 0.7 mb (7*106 Hz) • W/Z production • s 200/60 nb (2/0.6 kHz) • Top production • s 0.8 nb (80 Hz) • SM Higgs (mH = 150 GeV) • s 30 pb (3 Hz) • With branching ratios included • W  en 150 Hz • Z  ee 15 Hz • H  gg 0.003 Hz

  7. B-simulation • Monte Carlo generator: PYTHIA 5.7/JETSET 7.4; • Flavour creation, flavour excitation and gluon splitting included; • CTEQ2L parton distribution; • Peterson function eb=0.007 • Full GEANT3 simulation of the detector response; in some case integrated with fast simulation; • Total inelastic cross-section: 80 mb; bb cross section: 500 mb;

  8. The ATLAS experiment Pixel module Muon Trigger Elx and Algor. LAr e.m. endcap module Tile Calorimeter Module(s) MDTchamber assembly RPC chambers

  9. The ATLAS Trigger/DAQ System Detectors 1 GHz interaction rate / 40 MHz bunch-crossing rate Front-end m 2 s latency Level 1 Pipelines <75 (100) kHz ~100 GB/s output data flow Readout Drivers O(10) ms latency Level 2 Readout Buffers O(1) kHz output rate O(1) GB/s output data flow Event Builder Buffers & ~ seconds latency Processing Event Filter Farms O(100) Hz output rate O(100) MB/s output data flow Data Storage LVL1 • Hardware based (FPGA and ASIC) • Coarse calorimeter granularity • Trigger muon detectors: RPCs and TGCs RoI Pointers • LVL2 • Region-of-Interest (RoI) • Specializedalgorithms • Fast selection with early rejection HLT EF • Full event available • Offline derived algorithms • Seeding by LVL2 • Best calibration / alignment • Latency less demanding

  10. The Atlas B-Physics Programme • The main physics processes that can be studied: • CP violation: • Asymmetry in B0d  J/Y K0s  measurement of sin2b; • Asymmetry in B0s  J/Yf; test of the SM; • Asymmetry in B0d,s  h+ h-  measurement of b+g; • B0s - B0s oscillations; • Rare B-decays with dimuons: B0d,sm+ m-, B0dK*0m+ m-, B0sf0m+ m -, … • Also: • B-production cross-section measurement; • Lb polarisation measurement; • Related to B-physics: direct J/Y, U production

  11. The B-trigger -1 • L=1 x 1033 cm-2 s-1; • Level-1: single muon trigger pT > 6 GeV/c, |h|<2.5; • Rate is expected to be about 23 kHz; • dominated by in-flight decays of p,K and heavy flavour muon production; • Dimuon trigger possibly with lower thresholds; • Raise thresholds for higher luminosities; • Level-2, step 1: confirm level-1 muon trigger in RoI; • Use precision muon system together with ID for momentum measurement  important rejection of in-flight decays; • Rate: about 5 kHz;

  12. The B-trigger -2 • Level-2, step 2: • Specific selections are applied for different channels; in all cases we perform a track reconstruction in the Inner Detector with: • Either an ID full-scan; • Or RoI-based ID track reconstruction. • ID full scan: unguided search for tracks in all Pixel system; track extrapolation to the SCT+TRT (electrons down to 1 GeV); • RoI approach: consider only regions with calorimeter activity tagged by level-1 system: example: em cluster ET>2 GeV; hadronic cluster: ET >5 GeV; it requires less processing power resources (but less efficient)

  13. The B-trigger -3 • J/Psim+m-: two opposite muons pT1>6 GeV and pT2 >3 GeV (m in TileCal); mass cuts; • J/Psie+e -: two opposite-charge electrons with both pT1 >1GeV; mass cuts; rate @ lvl2: 40 Hz (lvl1 mu8, L=1 x 1033 cm-2 s-1); • B hadrons: example: B h+ h-: two opposite tracks with pT>4 GeV; mass cuts;

  14. The B-trigger -4 • Event Filter: track refit, including a vertex fit; decay length and fit quality cuts are applied; • about a factor of 10 wrt LVL2 can be achieved by exclusive selections.

  15. B0d  J/Y K0s • J/Y l+l- reconstruction; mass resolution: 40 MeV (muons) and 60 MeV (electrons); • K0sp+p-: 4.5 – 7.0 MeV mass resolution; • B0d: 3D kinematic fit applying vertex and mass constraint; B0d mass resolution: 19 (26) MeV; • Background mainly from B decays with a J/Y in the finals state; small contribution from false J/Y ; B0d reconstruction; CDF has shown a similar signal; sb/stot and prod. rate improved at LHC

  16. Flavour tagging: Opposite-side tagging: muon (trigger) or electron (pt>5 GeV); Dtag = 0.5; Same-side tagging: B-p algorithm (charged meson associated with the B-hadron); Dtag = 0.16; Event yeld for 30 fb-1 @ L=1x1033cm-2 s-1 B0d  J/Y K0s

  17. B0d  J/Y K0s Statistical Error: Estimate of the statistical error of sin2b using a time-dependent analysis with an integrated luminosity of 30 fb-1. • Overall statistical error: • 10 fb-1: 0.018 • 30 fb-1: 0.010 • Competitive with LHCb and B-factories

  18. B0d  J/Y K0s Systematic Error: • analysis of control samples: • B+ J/Y(mm)K+ • B0dJ/Y(mm)K*0 • Provide measurements of Dtag, and Ap. Invariant mass distribution for B0d  J/Y K+ with superimposed the Estimated background.

  19. B0d  J/Y K0s • Systematic errors • dDtag/Dtag: 0.003 • dDback/Dback: 0.006 • dAP: 0.0005; Global systematic error: < 0.01

  20. B0d,s  h+ h- • Expected to provide measurements of the CP asymmetry related to the angle a. • ATLAS does not have an event-by-eveny particle identification, but can separate on statistical basis; the signals from all significant two-body decays of b-hadrons will overlap: B0dp+ p-, B0dK+ p-, B0sK- p+, B0sK+ K-, Lbpp-, LbpK-; dACP: about 0.1 . Can provide cross-check of results from dedicate B-experiments

  21. B0s - B0s oscillations • Processes considered: • B0s Ds-p+ and B0s Ds-a1+ ; (Ds -fp-; fK+K-) • Event reconstruction includes vertices and masses reconstruction; • Proper time resolution: rms=0.06 ps. • Event yield in 30 fb-1: • 7100 B0s Ds-p+ and 2600 B0s Ds-a1+ ; • Background: mainly from B0dDs-a1+, B0dDs-p+ (2200 events) and from the combinatorial background (11300 events)

  22. B0s - B0s oscillations • Dms reach evaluated with the amplitude-fit technique; it is measurable with more than 5s if: • Dms < 22.5 ps-1; L=10 fb-1; • Dms < 29.5 ps-1; L=30 fb-1; The measurement significance as a function of Dms for L=30 fb-1;

  23. Rare decays with dimuons • The decays B0sm+ m- and B0d  m+ m- have very small BR but they can be selected by the atlas trigger even at the nominal LHC luminosity. With 130 fb-1 of data the reaction B0sm+ m- can be seen with 4.7s assuming the S.M. BR of 4.9 10-9. • Another interesting class of reactions are exclusive decays such as B0sf0m+ m - , B0dr0m+ m- , B0dK*0m+ m- , … • Detailed measurements of the decays can test the SM  search for new physics (eg AFB in decay)

  24. Initial ATLASconfiguration • New radial position of the B-layer since the Yellow report (CERN 2000-004) • Limited resources and technical/schedule constraints • effect: detector staging and TDAQ staging. • Stage the following components (defer for 1-2 years) • The middle pixel layer (not the B-layer) • Outermost TRT wheels, half of the CSC layers • MDT chambers in transition region (EES, EEL) • Cryostat gap scintillators, part of high luminosity shielding • Reduction of Read-Out Drivers for LAr calorimeter

  25. Initial detector configuration • Main effect to the B-physics performance due to the detector layout wrt to the Yellow Book results comes from the change of the B-layer radial position (from 4.30 cm to 5.05 cm) and from the material in and before that layer (increased thickness of the beam pipe and pixel services); preliminary estimations with DC1 data analysis: • Impact parameter resolution and proper time resolution degraded by about 30%; • Mass resolutions degraded by about 15%; • Reconstruction efficiencies: no important degradation found; • Effects of the missing pixel layer under study.

  26. T/DAQ Deferrals • Temporary re-allocation of TDAQ sub-system resources will be used to fund overcosts in common projects • Would lead to drastic reduction in initial HLT/DAQ system if additional funds not obtained (only about 1/2 of the HLT/DAQ CORE budget remaining!) • Impact of deferrals on rate capability is difficult to estimate. • Evaluation of rate capability versus cost requires understanding behavior of HLT/DAQ (whose design is not yet complete) as a function of many parameters • At this time, we use a simplified cost model with significant uncertainties

  27. T/DAQ Deferrals & LHC lum. • The target initial luminosity was doubled to L= 2 x 1033 cm-2 s-1; •  increase the low-pT inclusive muon trigger threshold; • include the low pt inclusive muon trigger with a low-pT dimuon trigger; • Consequences: • CP violation: dsin2b: 0.010  0.015 (dimuon trigger only); • Mixing cannot be studied with dimuon trigger only; • Rare B decays: unaffected; • Restore the low lumi trigger menu as soon as L approaches values close to 1x1033 cm-2 s-1;

  28. Summary Although ATLAS is designed to probe the O(1TeV) energy scale, this experiment can make several useful measurements in the B-physics sector: • Sensitivity to sin2b comparable to that of LHCb; • Measurement of the B0s- B0s oscillations; • Unique opportunity to search for rare B0s decays: potential indirect evidence of new physics.

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