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B s -meson physics: perspectives at ATLAS & CMS

Explore Bs meson physics at Large Hadron Collider (LHC) through CP violation measurements, rare decays, and oscillations. Learn about the impact of the new LHC schedule on B-physics experiments.

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B s -meson physics: perspectives at ATLAS & CMS

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  1. Bs-meson physics: perspectives at ATLAS & CMS Roberto Covarelli (CERN) on behalf of the ATLAS and CMS collaborations VI B-physics meeting – Ferrara – 18-19 March 2009 Roberto Covarelli

  2. Outline • Heavy-Flavor Physics at ATLAS and CMS • The LHC foreseen schedule and its impact on experimental goals • The ATLAS and CMS detectors • Triggers dedicated to Heavy-Flavor physics • Level-1 and software-based triggers • Some selected topics: • CP violation measurements in Bs → J/y f (ATLAS and CMS) • Search for the rare decay Bs → m+ m- (ATLAS and CMS) • Search for the rare decay Bs → f m+ m-(CMS) • Measurement of Bs oscillations (ATLAS) • Conclusions Roberto Covarelli

  3. Heavy-flavor physics at LHC • A rich heavy-flavor physics program is foreseen at LHC, thanks to the huge heavy-quark pair production cross-section (sbb ~ 400 mb @ 10 TeV) • LHCb  experiment dedicated to B-physics • ATLAS and CMS experiments dedicated to high-pT physics • When studying exclusive B/Bs decay modes, trigger/reconstruction capabilities are limited to particular channels with peculiar signatures • At high luminosity probably limited to events including a muon pair • At low luminosity more trigger combinations could be effective (single muon, muon + low-ET jet, electron pair, track combinations…) • Anyway all channels suppressed even by minimal pT thresholds/cuts  challenging analyses Example: bb → X J/y → m+ m- @ 10 TeV Roberto Covarelli

  4. The new LHC schedule… • After inspection of damages caused by the Sep 2008 incident and accurate estimation of repair times, new accelerator schedule coming out of February meeting in Chamonix: • New start-up in autumn 2009 • Long physics run through autumn 2010 (skipping previously scheduled winter break) • Foreseeing ~10 “luminosity periods” according to changing beam conditions (intensity, b*, number of bunches …etc.) Roberto Covarelli

  5. LHC 2009/10 …and impact on B-physics • Energy decrease does not affect much theoretical cross-section: • sbb (10 TeV) ~ 400 mb  N(Bs) ~ 48 M/pb-1 • Low instantaneous luminosity has a large impact on event selection criteria, especially at trigger level • For all studies presented trigger strategies will be detailed, highlighting if / to which extent the analysis yields have been estimated using realistic low-luminosity thresholds Roberto Covarelli

  6. The ATLAS detector • Coverage: |h| < 2.5 and full f • 3-level trigger (1 hardware + 2 software) • Acceptance forces relatively high pT muons at trigger level • Good muon resolution in tracker + spectrometer Combined Muon Roberto Covarelli

  7. The CMS detector • Coverage: |h| < 2.4 and full f • 2-level trigger (Level 1 + High-Level Trigger) • Muon triggers expected to be efficient down to pT ~ 3 GeV/c • Excellent muon reconstruction performances due to: • 3.8-T magnetic field • all-silicon tracker pT resolution in silicon tracker (muons) Roberto Covarelli

  8. The ATLAS B-physics trigger • Level-1 (Thin Gap Chambers + RPC) • 2MU0 / 2MU6 streams: 2 Regions of Interest (RoI) in Dh x Df associated to any pT(m) / pT(m) > 6 GeV/c • Level-2 • confirm each m RoI with precision muon chamber and inner detector measurements • dimuon vertex reconstruction and cuts on invariant mass • EventFilter: • refit inner detector tracks • proper decay length cut • angular distribution cuts • Output rate (L = 1033 cm-2 s-1) ~ 10 Hz Single LVL1 efficiency LVL2/LVL1 efficiency Roberto Covarelli

  9. ATLAS low-luminosity trigger • “Improved” di-muon triggers: • Extended Region of Interest around a single LVL1 seed • Inside-out reconstruction of track pairs compatible with RoI  unbiased double trigger, can be used for efficiency calibration • Possibility to add selections on: • Invariant mass • Vertex c2 • Muon + Inner Detector triggers: • Full scan of ID possible at 1031 cm-2 s-1 • Possible to identify particular hadronic states with subsequent requirements on invariant masses (e.g. Bs→ Dsp, Ds → fp, f → KK) pp → X J/y → m+ m- “Improved” di-muon Roberto Covarelli

  10. The CMS B-physics trigger • Level 1 trigger (hardware – all types of muon detectors): • High-level trigger (HLT, software – muon stations + tracker): • all muon paths: • pT cut • pixel track seeds • partial track reconstruction up to 6 tracker hits or pT/pT < 0.02 • di-muon trigger paths: • opposite charge • possibility to add invariant mass cuts • “displaced” di-muon trigger paths: • add requirements on secondary vertex 2 and flight length (2 x 1033 cm-2 s-1) (2 x 1033 cm-2 s-1) no mass cut Roberto Covarelli

  11. CMS low-luminosity triggers • Muon thresholds slowly varying with luminosity  initial physics/efficiency studies can be done with the lowest single muon thresholds Main B-physics trigger rates vs. instantaneous luminosity Roberto Covarelli

  12. Bs→ J/y f : theory • “Golden channel” for time-dependent measurements in the Bs system: • weak phase, analogous of 2b for Bd system, at present tightly constrained by CKM fits: fs = -0.0366 ± 0.0015 • Room for New Physics considering recent Tevatron measurements (see Diego’s talk) • P → VV structure: different CP eigenvalues for longitudinal/parallel/transverse polarizations • Use of the transversity basis to describe the total amplitude: ■68% p.c. ■95% p.c. fNP SM CNP The apparent complexity w.r.t. P → VP modes improves sensitivity instead, since physics parameters enter each term differently  possibility of untagged analyses Roberto Covarelli

  13. Bs→ J/y f : event selection • Reconstruction channel: Bs→ J/y (m+m-) f (K+K-) • Similar in ATLAS and CMS • Both studies based on high-luminosity scenarios (1-2 x 1033 cm-2 s-1) • Vertexing: • Primary vertex reconstruction using highest sum of pT2 to discriminate from pile-up events • Kinematic fit to muons and K tracks determine secondary vertex • Selection: • Displaced di-muon trigger • J/y and f invariant mass requirements • Decay length significance and c2 of kinematic fit • cos apoint (angle between Bs transverse momentum and the vector pointing from primary to secondary vertex) • Main backgrounds: • Bd→ J/y (m+m-) K*0 (K+p-) • Prompt/non-prompt J/y Roberto Covarelli

  14. Bs→ J/y f : flavor tagging • OST: using charge of highest-pT lepton in the rest of the event • Only usable for a small fraction of events (other b → l) • SST: jet charge method • Charge ordering in fragmentation models implies that higher-momentum tracks from a b-quark fragmentation tend to have negative charge and viceversa • Jet charge defined as: Performances on the “self-tagging” mode Bd→ J/y K*0 w = (37.4 ± 0.5)% • k = 0.7 • pi , qi = momentum/charge of tracks (not associated to signal) within DR = 0.7 from the B meson direction Roberto Covarelli

  15. Bs→ J/y f : results • ATLAS and CMS: • Using unbinned maximum likelihood fit to pseudo-proper time distribution including experimental effects: • Acceptance • Mistag rate (ATLAS) • Resolution function Roberto Covarelli

  16. Bs→ m+m- : theory • Process highly suppressed in SM: • Effective FCNC, helicity suppression • BRtheor(Bs→ m+m-) = (3.86 ± 0.15) x 10-9 M. Artuso et al., Eur. Phys. J. C57, 309 (2008) • Sensitivity to New Physics models: • CMSSM: • 2HDM: Roberto Covarelli

  17. Bs→ m+m- : selection • Cut-based analyses (similar in ATLAS and CMS) using: • Di-muon angular separation: • cosapoint(pointing angle in 3-D) • Flight length in 2-D (both absolute value and significance) • c2 of secondary vertex fit • Bs-candidate isolation: trk varying over all tracks with pT > 0.9 GeV/c in a cone of semi-aperture R = 1.0 • Region of |mmm – m(Bs)| (asymmetric in ATLAS to reject Bd → mm) Lxy isolation Roberto Covarelli

  18. Bs→ m+m- : signal and backgrounds • Signal mass resolution • ATLAS: 70 (barrel)-120 (endcap) MeV/c2 • CMS: (53.0 ± 1.4) MeV/c2 (total) • Main surviving backgrounds: • Non-peaking: • independent bb semileptonic decays • True muons from same side [Bc→ J/y (mm) mn] • One true-one fake muon [B → mhX] • Peaking: • Two fake muons [Bs→ hh] Roberto Covarelli

  19. Bs→ m+m- : results • ATLAS (10 fb-1): nS = 5.7, nB = 14+13-10 • No upper limit extraction performed • CMS (1 fb-1): nS = 2.36 ± 0.07, nB = 6.5 ± 2.4 • Using Bu→ J/y K+as a normalization channel to extract BR: • Bayesian method: BR(Bs→ m+m-) < 1.5 x 10-8 @ 90% CL Bu yield at 1 fb-1 • CDF-DØ combined exclusion limit: BR(Bs→ m+m-) < 4.5 x 10-8 @ 95% CL Roberto Covarelli

  20. Bs→ fm+m- in CMS • Also a FCNC process  sensitivity to NP in: • Branching Ratio: • CDF: BR(Bs→ fm+m-)/BR(Bs→ J/y f) < 2.61 x 10-3 • SM: (1.3 ± 0.5) x 10-3 • FW-BW asymmetry: • hard to measure at ATLAS/CMS: requires tagging • Normalization to J/y f by selecting resonant/non-resonant regions of mm mass Vetoed regions y’ J/y PRELIMINARY • Trigger: low-luminosity double muon at 3 GeV/c • After selection: • Main background from tails of resonant decays • Observation after 150-200 pb-1 of collected data (statistical only) with “earliest data” systematics with systematics statistical only Roberto Covarelli

  21. Dms measurement at ATLAS • Signal modes: Bs→ Dsp, Bs→ Ds a1 (other b → m) • Accurate study of low-luminosity trigger strategy (1031 cm-2 s-1): • Muon LVL1 seed (pT > 4 GeV/c) • Triggering full scan of ID, to look for combinations of tracks satisfying loose invariant mass cuts according to the chain Ds → fp, f → KK • Background rate = 7.5 Hz • Timing = 91 ms/event (LVL2) - 470 ms/event (EF) • OS-flavor tagging with soft muon charge: • etagDtag2 (Bs→ Dsp) = 30% • etagDtag2 (Bs→ Ds a1) = 28% • Mistag dominated by mixing and b → c → m processes Roberto Covarelli

  22. Dms measurement at ATLAS • Event selection: • Combination of tracks with angular separation and invariant mass cuts • Maximum-likelihood trec fit: • 5 simultaneous PDFs for: 1-2) Unmixed-mixed Bs 3-4) Unmixed-mixed Bd 5) Non-oscillating background • Amplitude fit method on toy MC samples to determine sensitivity vs. integrated luminosity Main backgrounds s =(53.0 ± 0.7) MeV/c2 Bs, mixed Roberto Covarelli

  23. Conclusions • Perspectives for Bs-meson physics at ATLAS and CMS have been presented • Key element: developing dedicated trigger strategies to improve efficiency during low-luminosity LHC data-taking period (2009/10) • CP violation measurements in Bs → J/y f • ATLAS (CMS) with 30 (10) fb-1 of data will be able to confirm TeVatron (not SM!) fs values with a precision of ~0.05 (0.07) • Search for the rare decay Bs → m+ m- • CMS can reach upper limits competitive with TeVatron with 1 fb-1 • Search for the rare decay Bs → f m+ m- • CMS can reach 5s sensitivity with ~200 pb-1 (statistical errors only) • Measurement of Bs oscillations • TeVatron values of Dms accessible in ATLAS analysis with 5 fb-1 Roberto Covarelli

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