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

2. 2 outline The initial experiment conditions The ATLAS Physics Programme The ATLAS detector & trigger B-physics potential with the nominal detector @ L=10 33 cm -2 s -1 ; Preliminary estimate of the B-physics potential with the initial detector and luminosity; Conclusions

3. 3 The initial experiment conditions Many uncertainties: 1.The LHC luminosity: the target initial luminosity was doubled to L= 2 x 10 33 cm -2 s -1 ; 2.The detector configuration; in particular the initial HLT/DAQ system bandwidth and processing power (resources limitations); 3.The physics rates (uncertainties on ,K and heavy flavour production cross-sections);

4. 4 The initial experiment conditions Results presented here refer to the nominal detector configuration and L= 1x10 33 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. 5 The ATLAS Physics Programme 1.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… 2.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… 3.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. 6 Cross-sections and rates huge range of cross-section values and rates –listed for 10 34 cm -2 s -1 –total   100 mb (10 9 Hz) –b production   0.7 mb (7  10 6 Hz) –W/Z production   200/60 nb (2/0.6 kHz) –Top production   0.8 nb (80 Hz) –SM Higgs (m H = 150 GeV)   30 pb (3 Hz) With branching ratios included –W  e 150 Hz –Z  ee 15 Hz –H   0.003 Hz

7. 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  b =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  b;

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

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

10. 10 The Atlas B-Physics Programme The main physics processes that can be studied: –CP violation: Asymmetry in B 0 d  J/  K 0 s  measurement of sin2  ; Asymmetry in B 0 s  J/   test of the SM; Asymmetry in B 0 d,s  h  h   measurement of  ; –B 0 s - B 0 s oscillations; –Rare B-decays with dimuons: B 0 d,s   +  , B 0 d   *0  +  , B 0 s   0  +  , … Also: –B-production cross-section measurement; –  b polarisation measurement; –Related to B-physics: direct J/ ,  production

11. 11 The B-trigger -1 L=1 x 10 33 cm -2 s -1 ; Level-1: single muon trigger p T > 6 GeV/c, |  |<2.5; –Rate is expected to be about 23 kHz; –dominated by in-flight decays of ,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. 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: 1.Either an ID full-scan; 2.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 E T >2 GeV; hadronic cluster: E T >5 GeV; it requires less processing power resources (but less efficient)

13. 13 The B-trigger -3 J/Psi      : two opposite muons p T 1 >6 GeV and p T 2 >3 GeV (  in TileCal); mass cuts; J/Psi  e  e  : two opposite-charge electrons with both p T 1 >1GeV; mass cuts; rate @ lvl2: 40 Hz (lvl1 mu8, L=1 x 10 33 cm -2 s -1 ); B  hadrons: example: B  h  h  : two opposite tracks with p T >4 GeV; mass cuts;

14. 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. 15 B 0 d  J/  K 0 s J/   l  l  reconstruction; mass resolution: 40 MeV (muons) and 60 MeV (electrons); K 0 s      : 4.5 – 7.0 MeV mass resolution; B 0 d : 3D kinematic fit applying vertex and mass constraint; B 0 d mass resolution: 19 (26) MeV; Background mainly from B decays with a J/  in the finals state; small contribution from false J/  ; B 0 d reconstruction; CDF has shown a similar signal;  b /  tot and prod. rate improved at LHC

16. 16 B 0 d  J/  K 0 s Flavour tagging: –Opposite-side tagging: muon (trigger) or electron (pt>5 GeV); D tag = 0.5; –Same-side tagging: B-  algorithm (charged meson associated with the B-hadron); D tag = 0.16; Event yeld for 30 fb  @ L=1x10 33 cm  s  J/  (ee)K 0 s signal back. J/  (     )K 0 s signal back. e tags - 5800 500  tags 14400 90011900 1100 B-  tags - 376100 13700

17. 17 B 0 d  J/  K 0 s J/  (ee)K 0 s J/  (     )K 0 s lepton tags 0.0180.023 B-  tags -0.015 Estimate of the statistical error of sin2  using a time-dependent analysis with an integrated luminosity of 30 fb . Overall statistical error: 10 fb -1 : 0.018 30 fb -1 : 0.010 Competitive with LHCb and B-factories Statistical Error:

18. 18 B 0 d  J/  K 0 s Systematic Error: analysis of control samples: –B +  J/  (  )K + –B 0 d  J/  (  )K *0 –Provide measurements of D tag, and A p. Invariant mass distribution for B 0 d  J/  K + with superimposed the Estimated background.

19. 19 B 0 d  J/  K 0 s Systematic errors –  D tag /D tag : 0.003 –  D back /D back : 0.006 –  A P : 0.0005; Global systematic error: < 0.01

20. 20 B 0 d,s  h  h   Expected to provide measurements of the CP asymmetry related to the angle . 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: B 0 d       B 0 d       B 0 s       B 0 s        b  p     b  p      A CP : about 0.1. Can provide cross-check of results from dedicate B-experiments

21. 21 B 0 s - B 0 s oscillations Processes considered: –B 0 s  D s   + and B 0 s  D s  a 1 + ; (D s    - ;        Event reconstruction includes vertices and masses reconstruction; Proper time resolution: rms=0.06 ps. Event yield in 30 fb  : –7100 B 0 s  D s   + and 2600 B 0 s  D s  a 1 + ; –Background: mainly from B 0 d  D s  a 1 +, B 0 d  D s   + (2200 events) and from the combinatorial background (11300 events)

22. 22 B 0 s - B 0 s oscillations  m s reach evaluated with the amplitude-fit technique; it is measurable with more than 5  if: –  m s < 22.5 ps -1 ; L=10 fb  1 ; –  m s < 29.5 ps -1 ; L=30 fb  1 ; The measurement significance as a function of  m s for L=30 fb  1 ;

23. 23 Rare decays with dimuons The decays B 0 s   +   and B 0 d   +   have very small BR but they can be selected by the atlas trigger even at the nominal LHC luminosity. With 130 fb  of data the reaction B 0 s   +   can be seen with 4.7  assuming the S.M. BR of 4.9 10  9. Another interesting class of reactions are exclusive decays such as B 0 s   0  +  , B 0 d   0  +  , B 0 d   *0  +  , … Detailed measurements of the decays can test the SM  search for new physics (eg A FB in decay)

24. 24 Initial ATLAS configuration 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. 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. 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. 27 T/DAQ Deferrals & LHC lum. The target initial luminosity was doubled to L= 2 x 10 33 cm -2 s -1 ; –  increase the low-p T inclusive muon trigger threshold; –include the low pt inclusive muon trigger with a low-pT dimuon trigger; Consequences: –CP violation:  sin2  : 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 1x10 33 cm -2 s -1 ;

28. 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 sin2  comparable to that of LHCb; –Measurement of the B 0 s - B 0 s oscillations; –Unique opportunity to search for rare B 0 s decays: potential indirect evidence of new physics.