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Prospects for the measurement of mixing induced CP violation in B s J/ at LHCb

VI th Meeting on B physics, Ferrara, Italy, March 19-20, 2009. Prospects for the measurement of mixing induced CP violation in B s J/ at LHCb. Olivier Leroy. Introduction, phenomenology The LHCb detector at LHC T rigger and selection Flavour tagging Fit and s ystematics

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Prospects for the measurement of mixing induced CP violation in B s J/ at LHCb

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  1. VIth Meeting on B physics, Ferrara, Italy, March 19-20, 2009 Prospects for the measurement of mixing induced CP violation in BsJ/ at LHCb Olivier Leroy • Introduction, phenomenology • The LHCb detector at LHC • Trigger and selection • Flavour tagging • Fit and systematics • Conclusion and prospects CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France

  2. Many thanks to my LHCb colleagues, in particular to: A. Carbone, M. Calvi, P. Clarke, T. du Pree, S. Hansmann-Menzemer, B. Khanji, G. Lanfranchi, C. Langenbruch, C. Linn, M. Musy, S. Poss, U. Uwer, S. Vecchi and Y. Xie. Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  3. Introduction (1) • The interference between Bs decay to J/yfeither directly or via mixing gives rise to a CP violating phase F • F is the sensitive probe of New Physics because: • It is well predicted in the SM: FSM=-2s+dP = (-0.0370.002+dP) rad • New particles can contribute to the Bs-Bs box diagrams and significantly modify the SM prediction Bs f =J/yf Bs Fis observable, FD and FM are not Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  4. Introduction (2) • In the SM, BsJ/yf decay is dominated by a single weak phase: • Various penguin pollution estimates: • dP~10-4 [H. Boos et al., Phys.Rev. D70 (2004) 036006] • dP~10-3 [M. Gronau et al., arXiv:0812.4796] • dP up to ~0.1 [S. Faller et al., arXiv:0810.4248v1] Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  5. Notations/conventions • bs = the smallest b-s unitarity triangle angle • F = the observable weak phase in BsJ/ • = -2bs + dP • No penguin pollution AND no NP  F = -2bs • = the possible new physics phase in Bs-Bs mixing • The NP phase modify other measurements: • ~10 times smaller than 2bs Standard Model: 2s=(0.0370.002) rad Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  6. PVV decays (1) • Bs is a pseudo scalar (spin=0), f and J/y are vectors mesons (JPC = 1—) • Total angular momentum conservation => in the Bs rest frame, f and J/y have relative orbital momentum ℓ =0,1,2 • Since CP|J/>= CP|J/> CP|> (-1)ℓ, final state is mixture of CP even (ℓ=0,2) and CP odd (ℓ=1) • Decompose decay amplitudes in term of linear polarization, when J/y and f are: • A0: longitudinally polarized (CP-even) • A┴: transversely polarized and ┴ to each other (CP-odd) • A||: transversely polarized and ║ to each other (CP-even) J/ ψ Φ ℓ = 0 B0s μ+μ-K+K- ℓ = 1 B0s ℓ = 2 Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  7. PVV decays (2) • 3 angles q, ϕ, ψ describe directions of final decay products J/y→mm and →K+K • Allow to separate statistically the 3 decay amplitudes • J/y rest frame (where the  meson moves in the x direction, the z axis is perpendicular to the decay plane of →K+K−, and py(K+) ≥ 0), (θ, ϕ) are the polar and azimuthal angles of the μ+. • In the  rest frame, ψ is the angle between p(K+) and −p(J/y) Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  8. Differential decay rates Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  9. Differential decay rates Time dependent part Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  10. Differential decay rates Angular dependent part Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  11. Time dependent Bs decay amplitudes Moduli2 Interferences • Dependends on 8 physics parameters: • F, Gs, DGs, Dms, R┴, R, δ┴, δ • Conventions: • d0=0 • |A┴(0)|2+|A(0)|2+|A0(0)|2 = 1 Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  12. Time dependent Bs decay amplitudes • Tagged analyses sensitive to both cosF and sinF • Exact symmetry of the decay rates (2-fold ambiguity): Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  13. Road map to measure Fat LHCb • The decay rates give the road map: • Trigger and select BsJ/ events • Measure proper time • Measure 3 transversity angles • Tag initial Bs flavour • Likelihood fit of time and angular decay rates • 6 observables: proper time, 3 angles, q (=0,-1,+1 for untagged, Bs, Bs) and mass • 8 physics parameters: F, Gs, DGs, Dms, R┴, R, δ┴, δ • + many detector parameters (resolutions, acceptances, tagging, …) • We will increase sensitivity and complexity step by step: • Take detector param. from MC, fit only phys. param. • Measure detector param. in control channels, fit only phys. param. • Fit all param. from BsJ/ alone • Control channels: • B+J/K+, BdJ/K*, BsDsp, … Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  14. LHCb detector at LHC Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  15. LHCb detector at LHC • Designed to maximize B-acceptance (within cost and space constraints) • Forward spectrometer, 1.9<<4.9 • More b-hadrons produced at low angles • Single arm is OK as b-quarks are correlated • Rely low pT triggers, efficient also for purely hadronic decays • Choose to run at L~2x1032cm-2s-1 • Clean environment: easier event reconstruction • Less radiation damage • bb cross section at 14TeV = 500b 40% (Pythia) •  1012 bb produced / 2fb-1 (nominal year) (Pythia) Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  16. The LHCb detector Muon System Trigger and PID RICHES: PID: K, separation VELO: primary vertex impact parameter displaced vertex Interaction region Calorimeters: Trigger and PID: h,e,, 0 Trigger Tracker: p for trigger and Ks reconstruction Tracking Stations: p of charged particles Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  17. Trigger and selection Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  18. BJ/(mm)X unified trigger and selection (X= , K+, K*) • Coherent selection of the BsJ/f and two control channels: B+J/K+ and BdJ/K* in order to extract as much as possible information from data (rather than MC) • Selection criteria to obtain a reasonable statistical uncertainty on bs while keeping systematics under control: • Maximize signal yield while keeping background < few tens of millions of events per nominal year (computing limit) • Miminize proper time and angular acceptance distortions. Do not cut on variables which bias the proper time • b-hadrons have similar 4-momenta such that tagging properties can be extracted from control channels and exported so signal channel without big corrections.Proper time resolution model can be cross-checked amongst channels • Other selections currently under investigation Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  19. BsJ/() trigger selection • Trigger uses lifetime-unbiased di-muons: • L0: pT>1.5GeV, e~94% • HLT1: Distance of closest approach (mm)<0.5mm and mmm>2.5GeV, e~80% • HLT2: pT(both m)>0.5GeV and |m(mm)-m (J/)|<3s and c2(Vtx)<7, e~95% • Selection: • J/mm, • PID, c2(Vtx)<6, pT(J/)>1GeV, |m(mm)-m(J/)|<3s • fKK • PID, c2(Vtx)<20, pT(f)>1GeV, |m(mm)-m(f)|<3s • c2(Bs Vtx)<5, |m(mmKK)-m(Bs)|<3s • Event yield after trigger and selection (2fb-1) ~ 117k Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  20. BJ/X selection: backgrounds • Main background is prompt J/ but “easy” to isolate because t~0ps • Background studied: • pp  bb inclusive • pp  Buds J/ X • Prompt pp  J/ X • Minimum bias (pp  X) Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  21. BsJ/ mass <s>~16MeV/c Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  22. BsJ/ proper time resolution and acceptance • Average proper time resolution ~ 39 fs • Acceptance nearly flat Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  23. BsJ/ angular acceptance • Acceptance distortions at ~10% level, come from: • Kinematical cuts on the Bs decay products • LHCb geometry (10-400 mrad) • Single particle reconstruction efficiency = function of momentum • Above plots are zero-suppressed and made with 17fb-1 Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  24. Angular resolutions • Resolution “good enough” to be ignored in likelihood fit (2fb-1) • If resolutions two times worse: • |A┴,|||2 get biased • bs remains unbiased Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  25. Prompt background mass and proper time • Main background is prompt J/y but “easy” to isolate because t~0ps Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  26. Prompt background angles • Flat ? More statistics needed ! Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  27. Long lived background mass and proper time • Average proper time smaller than signal (72% of events have t~0.18 ps) Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  28. Long lived background angles • Flat ? More statistics needed ! Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  29. Flavour tagging Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  30. Flavour tagging at LHCb (1) same side kaon tagger b Bs s b K+ s K- K+ u m+ m- u Same side primary vertex signal Bs proton proton vertex-charge taggerfrom inclusive vertexing Opposite side opposite B opposite kaon tagger (K-) positive lepton taggers from bclcascade negative lepton taggers (e-, m-) from b-quark BsJ/: efficiency e~ 55.7%, mistag rate w~33.3%, e(1-2w)2~6.2% Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  31. Flavour tagging (2) • Main control channels • B+J/y K+ , B0J/y K*0 for opposite side taggers • BsDsp for same side kaon • First results will probably come with opposite side taggers (higher statistics in Bu,d channels and proper time resolution less crucial) • Unified selection  true opposite-side mistag compatible amongst BJ/yX channels: True performance (MC truth) after L0 Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  32. Opposite-side mistag (wOS) in BdJ/yK* • BRvis[BsJ/y(mm)f(K+K-)]= (2.710.96)x10-5 • Measure wOS from mixing asymmetry inBdJ/yK* • Toy MC including backgrounds • Fit with 17 free parameters, successful • s(wOS/wOS)~0.3% with 2fb-1 Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  33. Sensitivity studies Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  34. Baseline BsJ/ parameters for toy MC sensitivity studies, extracted from full MC G means Gaussian: first parameter is the mean, second is the sigma.d(t) is the Dirac function. “Flat Acc”, means a flat acceptance, “No Res” means a perfect resolution.The quoted yield is after L0, HLT and offline selection. Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  35. Baseline sensitivity studies (2fb-1) • Perform hundreds of toy MC, maximum likelihood fit with: • 6 observables m, t, cosq, j, cosy, q • 7 free parameters F, DGs, Gs, R┴, R0, δ1, δ2(Dms fixed, can be floated, no changes) • All “detector parameters” fixed • Pulls of fitted quantities normally distributed • With 2fb-1 (one nominal year),s(2bs)~0.03 • 5s NP discovery, if 2bstrue>0.2 ! Standard Model: 2s=0.0370.002 Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  36. Baseline sensitivity studies (2fb-1) • Sensitivity to other parameters is also good: • Similar results when all detector parameters are free (see backup) Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  37. Fit BsJ/ in full MC • Angular and proper time acceptance included • Angles and proper time well described Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  38. BsJ/KK S-wave (1) • The “simple” previous decay rates consider only the f->K+K- (P-wave) in the vicinity of the KK and ignored any other resonant or non-resonant contributions in this region. Is this a problem ? • BJ/KK is a P → VPP process • In principle the KK system can have any zero or positive integer angular momentum • A 5...10% S-wave contribution to Bs→J/yK+K- cannot be excluded a priori[arXiv:0812.2832] • The S wave has a time-dependent angular distribution different from that of any P-wave • CP eigenvalue: -1 • angular distribution • Its interferences with P waves introduce new terms • Including S-wave contribution and its interference with P waves in the signal pdf is necessary for correct extraction of bs Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  39. BsJ/KK S-wave (2) • Toy MC studies with 5-10% S-wave: • If ignored, leads to 2bs ~10-20% biased toward zero • Including it in the fit adds 2 parameters and leads to a reduction in 2bs resolution of ~20% • If KK S-wave contribution large enough, it is promising to measure the S-P phase difference as a funtion of KK mass in order to measure sign of cos(2bs); like BaBar in Bd->J/yK* Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  40. Preliminary systematics studies • bs measurement dominated by statistics at least up to 2fb-1 ? • Some systematics biases increases with bs • Work still ongoing Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  41. Sensitivity versus integrated luminosity Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

  42. Conclusions and prospects • BsJ/ hot topic and promising way to discover NP at LHCb • But also one of the most challenging ! • Critical points: • Need all LHCb detectors working and calibrated • Flavour tagging • Angular / proper time acceptance • Proper time resolution • Fit stability with low statistics • Tevatron expects s(2bs)~0.13 by the end of 2010 • LHCb with 2fb-1 • 117k BsJ/(mm), s(2bs)~0.03 • Could expect some deteriorations with more realistic MC and real data However, if true 2bs value is the current Tevatron one ~0.8 (NP-like), we should measure it with ~0.2fb-1 ! Standard Model: 2s=0.0370.002 Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

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