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Rare Decays at LHCb

Rare Decays at LHCb. Mitesh Patel (CERN) CERN Theory Institute LHC b Focus week Wednesday 28 th May 2008. From G. Hiller [ hep-ph/0308180 ]. Introduction. New physics can give effective Hamiltonian, H , new operators O i or modified Wilson coefficients C i

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Rare Decays at LHCb

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  1. Rare Decays at LHCb Mitesh Patel (CERN) CERN Theory Institute LHC b Focus week Wednesday 28th May 2008

  2. From G. Hiller [hep-ph/0308180] Introduction • New physics can give effective Hamiltonian, H, new operators Oi or modified Wilson coefficients Ci • Rare B decays give a number of opportunities to constrain these contributions:

  3. Bs→mm

  4. Bs→mm • Bs→mm helicity suppressed • Well predicted in SM: • BR(Bs→mm) = (3.35±0.32)×10-9 [1] • Sensitive to (pseudo) scalar operators • MSSM: tan6β/MA4 enhancement • NUHM: favours large tanb (~30) • Current limits from Tevatron: • CDF BR < 4.7×10-8 90% CL [2] • D0 BR < 7.5×10-8 90% CL [3] NUHM • J. Ellis et al., arXiv:0709.0098v1 [hep-ph] [1] hep-ph/06040507v5 [2] arXiv:0712.1708v1 [hep-ex] [3] arXiv:0705.300v1 [hep-ex]

  5. Bs  μμ Bs KK • Mass (MeV) Bs→mm at LHCb • Searching for Bs→mm with LHCb: • Large prodn x-secn for b’s at high h, low pT → At L=2x1032 cm2s-1, 1012 bb pairs in 107s • Trigger has m pT threshold >~1GeV →~1.5kHz inclusive μ, di-μ • Small event size →Can write this rate out, open analysis – can retain max. efficiency • High precision magnetic spectrometer → Bs mass resolution ~20MeV (c.f. CMS ~40MeV, ATLAS ~80MeV) • Vertex detector very close to LHC beams → Excellent vtx, impact parameter resolution 100 mb 230 mb

  6. Invariant Mass PID Geometry Bs→mm at LHCb • Events classified according to geometrical likelihood, PID and Bs invariant mass: • Geometric likelihood: • Bs Lifetime • m SIPS: Mu Impact Parameter Significance • DOCA: Distance of closest approach • Bs IP: Bs impact parameter to prim. vtx • Isolation: No. of good secondary vtx that can be made with m candidates • PID: • Calibration muons (MIPs in calorimeter, J/y muons) • Bs Invariant Mass Arbitrary normalisation Red: signal Blue: bb inc. Black: b μ b μ Green: Bc+  J/Ψμν

  7. Bs→mm at LHCb • Analysis: • Signal description: B→hh (~200k events/2fb-1) • Background estimation from mass sidebands • Normalisation: B+→J/yK+ (2M events/2fb-1) • Dominant uncertainty on BR from relative Bs, B+ hadronisation fraction ~13% Red: signal Blue: bb inc. Black: b μ b μ Green: Bc+  J/Ψμν

  8. Bs→mm at LHCb • Background: • Dominated by b→μ, b→μ, b→μ, b→c→μ also contributes • Mis-id (B→hh), insignificant • Dominant exclusive bkgrd Bc+→J/Ymn, tiny cf. b→μ, b→μ • Drell-yan insignificant at these masses • Total efficiency for all geometric likelihood values ~10% • Taking events with GL>0.5, assuming SM BR, with 2fb-1: • Signal ~30 events • Bkgrd ~83 events Background Signal Geometric likelihood

  9. 90% CL limit on BR (only bkgrd is observed) 2x10-8 (~0.05 fb-1) 10-7 BR (x10–9) Expected final CDF+D0 limit Uncertainty in background prediction 5x10-9 (~ 0.4 fb-1) Integrated luminosity (fb–1) Exclusion: 0.1 fb–1 BR < 10-8 0.5 fb–1 < SM With SM Branching ratio 2 fb–1 3 evidence 6 fb–1 5 observation Bs→mm at LHCb SM prediction • J. Ellis et al., arXiv:0709.0098v1 [hep-ph] With 0.1fb-1 can measure BR 9 (15)×10-9 at 3 (5)s With 0.5fb-1 can measure BR 5 (9)×10-9 at 3 (5)s

  10. Bd→K*mm

  11. Bd→K*mm • BR measured at B-factories, in agreement with SM: BR(Bd→K*μμ)= (1.22+0.38-0.32)×10-6 [1] • Decay described by three angles (ql, f, qK*) • Angular distributions as function of q2gives sensitivity to NP contributions • Forward-backward asymmetry AFB in ql angle has received particular theoretical attention – predicted in a number of different models Ali et al, Phys. Rev. D61:074024, 2000 [1] PDG 2006

  12. AFB BELLE ’06 Mmm2 (GeV2) Bd→K*mm at LHCb • B-factories each collected O(100) signal events • CDF has ~35 signal events • Given projected total datasets these experiments, a total of <1000 events might be observed at all facilities • With L=2x1032 cm2s-1, LHCb will observe this no. of events with ~0.25fb-1 integrated luminosity AFB BABAR ’08 SM Mmm2 (GeV2) Wrong sign C9,C10 excluded ?

  13. Bd→K*mm at LHCb An example 0.1fb-1 experiment AFB • Signal selection: • Total selection efficiency ~1% → 7200 signal events /2fb-1 (~50% below mJ/Y) • Full AFB spectrum of interest but zero-crossing point often computed: • s0SM = 4.39+0.38-0.35 GeV2 [1] (older value used in model →) • Simple linear fit suggests precision: • Looking at extended beyond linear fit Mmm2 (GeV2) An example 0.5fb-1 experiment AFB Mmm2 (GeV2) [1] arXiv:0106067v2 [hep-ph]

  14. Bd→K*mm at LHCb bkgrd from: b→mb→m • Background: • b→μ, b→μ dominant contribution, symmetric distribution in ql– scales AFBobserved • b→μ, b→c→μ significant contribution, asymmetric qldistribution – effect on AFBdepends on ql shape • As for Bs→μμ, don’t observe any significant background from m mis-id • Non-resonant Kpmm events not yet observed • Bkgrd rejection dependent on Bd mass resoln: s(mBd) ~15MeV (c.f. ATLAS 50MeV) • B/S ~0.5 • Analysis issues: • In order to correct AFB value measured, require knowledge relative angular efficiency: • pT cuts on muons (in e.g. trigger), remove events with ql ~0,p • muon reconstruction requirements distort momentum spectrum No. of Events ql / rad bkgrd from: b→μ, b→c→μ No. of Events ql / rad

  15. FL AT(2) Bd→K*mm at LHCb 2fb-1 • Decays contain much more information than ql, AFBdistributions • Fitting projections of ql, f, qK* angular distributions: →fraction of longitudinal polarization, FL, and transverse asymmetry AT2 Mmm (GeV2) 2fb-1 Kruger & Matias, Phys. Rev. D71: 094009, 2500 Mmm (GeV2)

  16. Bd→K*mm at LHCb • Full angular fit also under investigation: • Parameterised in terms of transversity amplitudes • A0L,R, A┴L,R, A║L,R, 6 complex numbers • Correlations give access to helicities • Probe chiral structure NP operators • Once have enough events in each q2 bin for fit to converge → better precision on AFB, FL, and AT2 (but require full acceptance correction) • Can form any observable once have fitted all amplitudes – new theoretically clean observables with good NP sensitivity sought! AFB (SM, 2.2 – 4.1 GeV2) Full ang. fit (AFB) = 0.02 AFB (SM, 2.2 – 4.1 GeV2) Fit of proj. angles (AFB) = 0.04

  17. Bs→fg

  18. Bd→K*γ, Bs→fg • BR(Bd→Xsg) measured by B-factories, rate in agreement with SM • B-factories measured CP asymmetry ACPin Bd→K*(Ksπ0)γ : → C=-0.03±0.14, S=-0.19±0.23 [HFAG] • LHCb can perform analogous measurement in Bs→fg • As DGs≠0, Bs→fg decay probesAΔas well as C and S In SM, C=0 (direct CPV) S=sin 2y sin f AΔ=sin 2y cos f wherey fraction of “wrong” polarization

  19. Bs→fg at LHCb • Signal Selection: • ET > 2.7GeV • Mass resoln ~90MeV • Proper time resoln ~80fs (not critical for measuring AD) • Total Efficiency ~0.3% • Yield: • Bs→fg • 11k / 2fb-1 with B/S<0.55 • Bd→K*(K+p-)g • 68k / 2fb-1 with B/S~0.60 Equivalent of 13mins of simulated BB events – already see a peak! True K*g signal events Combinatorial background

  20. Bs→fg at LHCb • Analysis issues: • Acceptance function a(t) (Bd→K*g) • s(t) as function of topology • Precision on ACP parameters with Bs→fgdecays from 0.5fb-1 • s(AD) = 0.3 (no tagging required) • s(S, C) = 0.2 (require tagging) • With 2fb-1: • s(AD) = 0.22 • s(S, C) = 0.11

  21. Conclusions • Rare B decays in LHCb will find NP or constrain extensions of SM • With the first data: • Bs→μμ excluded at SM value with 0.5fb-1 • Bd→K*μμ measure AFBspectrum, σ(s0) ~0.8GeV2 with 0.5fb-1 • With 2fb-1 integrated luminosity: • Bs→μμ evidence if SM BR (observation with 6fb-1 data) • Bd→K*μμ measure AFBspectrum,σ(s0) ~0.5GeV2, new observables with more complex fits (A(2)T, …) • Bs →fγ CP asymmetry ACP→ fraction of “wrong” polarization • Host of other channels will be accessible: • Radiative : Lb→Lγ, Lb→ L*γ,B →r0g, B→wγ, mmg • b→sll :B+→K+ll (RK), Bs→fmm • LFV : Bq→ll’ • …

  22. Other Channels • Preliminary study of Bs→fmm: • Expect ~1000 signal events from 2fb-1 data with B/S<0.9 @ 90% CL • Factor 4 reduction in production rate Bs cf. Bd • The f does not tag the B → need flavour tagging, factor ~15 reduction → expect √60 worse resolution than Bd→K*mm • Can make CP-averaged measurement of AFB (if non-zero CPV) • Study of b→d transition Bs→K*mm also planned: • Again, factor 4 reduction in production rate Bs cf. Bd • Rate reduced by |Vtd/Vts|2=0.2082 ~ 1/25 • Given these reductions, expect will have to work harder to reduce background → ~< 700 events/ 2fb-1

  23. BsKK 1.0 fb -1 2 evt in s.r PID “Robustness” of mis-id bkg estimation: bb inclusive above GL = 0.2: 19 b  dimuon 3 other muons 2 muon + mis-id single mis-id probability needs to increase a factor ~10 to be of the same order as di-muon bkgrd • Double mis-id: • dominated by Bhh • ~4 evts/fb-1 a factor ~50 less than dimuon • Mis-id needs to increase by a factor ~7 to be of the same order as dimuon bkgrd

  24. RK in B+→K+ll • RKtheoretically well controlled in SM : • Effect of extensions to SM can be O(10%) e.g. from neutral Higgs boson exchange • Related to BR(Bs→mm) • LHCb sensitivity with B+→K+ll has been investigated – can also be done with the K* decay BaBar/Belle 95% CLlimit on RK CDF 95% CL limit on Bs  mm

  25. RK in B+→K+ll (cont’d) • From 10 fb-1 data : • Bd eeK ~ 10k • Bd mmK ~ 19k • Gives RK = 1 (fixed) ± 0.043 • Possible status with 10fb-1 data : • BR(Bs→mm) ~ 3×10-9 • RK ~ 1 – compatible with MSSM with small tan b • RK ≠ 1 – NP : right handed currents or broken lepton universality • BR(Bs→mm) ≠ 3×10-9 • RK ~ 1 – as above • RK = 1+e – MFV

  26. Bd→K*mm –non-resonant bkgrd Drawn for Mmm= 2 GeV • Region I: soft pion, energetic kaon • Shifts zero of AFB and larger theory errors • Region II: energetic Kp pair • Can be treated as B  Xmm and X  Kp • Region III: soft kaon, energetic pion • Amplitude suppressed so very few events… Defined by kinematics • Presently neglecting non-resonant background • Limit can crudely be derived from BaBar data → expect ~2000 events/2fb-1 (→ B/S=0.5±0.2) • Has been suggested that, under certain kinematic conditions, these can be treated as signal [Grinstein, Pirjol, hep-ph/0505155] :

  27. Bd→K*mm –non-resonant bkgrd (cont’d) Drawn for Mmm= 2 GeV • Region I: soft pion, energetic kaon • Shifts zero of AFB and larger theory errors • Region II: energetic Kp pair • Can be treated as B  Xmm and X  Kp • Region III: soft kaon, energetic pion • Amplitude suppressed so very few events… Defined by kinematics • Presently neglecting non-resonant background • Limit can crudely be derived from BaBar data → expect ~2000 events/2fb-1 (→ B/S=0.5±0.2) • Has been suggested that, under certain kinematic conditions, these can be treated as signal [Grinstein, Pirjol, hep-ph/0505155] :

  28. Bd→K*mm –non-resonant bkgrd (cont’d) Drawn for Mmm= 2 GeV • Region I: soft pion, energetic kaon • Shifts zero of AFB and larger theory errors • Region II: energetic Kp pair • Can be treated as B  Xmm and X  Kp • Region III: soft kaon, energetic pion • Amplitude suppressed so very few events… Defined by kinematics • Presently neglecting non-resonant background • Limit can crudely be derived from BaBar data → expect ~2000 events/2fb-1 (→ B/S=0.5±0.2) • Has been suggested that, under certain kinematic conditions, these can be treated as signal [Grinstein, Pirjol, hep-ph/0505155] :

  29. Kpmm (non-resonant) K*mm signal Mmm ~ 2GeV Mmm ~ 2GeV Bd→K*mm –non-resonant bkgrd (cont’d) • However, isolating region II has a large effect on the signal yield: Have relaxed the K* mass cut for signal and NR events • E.g. separating regions at Ep=600MeV : find 27% signal events and 44% NR events in region II

  30. Kpmm (non-resonant) K*mm signal Mmm ~ 2GeV Mmm ~ 2GeV Bd→K*mm –non-resonant bkgrd (cont’d) • However, isolating region II has a large effect on the signal yield: Have relaxed the K* mass cut for signal and NR events • E.g. separating regions at Ep=600MeV : find 27% signal events and 44% NR events in region II

  31. Kpmm (non-resonant) K*mm signal Whole Mmm range Whole Mmm range Bd→K*mm –non-resonant bkgrd (cont’d) • However, isolating region II has a large effect on the signal yield: Have relaxed the K* mass cut for signal and NR events • E.g. separating regions at Ep=600MeV : find 27% signal events and 44% NR events in region II • Plan to measure dG/dmKp

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