Latest results on B 0 s. mesons from LHCb. Greig Cowan. May 28th 2013 Oxford

Transcription

1 Latest results on B s mesons from LHCb Greig Cowan May 28th 213 Oxford

2 Motivation for flavour physics The LHCb experiment Measuring CP violation in the Bs Rare Bs decays system 2 / 64

3 Symmetries Charge-symmetry converts particles antiparticles. Parity-symmetry converts x x. C and P are conserved in strong/em interactions, violated by weak interaction. Combined CP is also violated by weak interaction! Matter-antimatter asymmetry CP violation one of Sakharov conditions BUT......Standard Model only predicts small amount. must be some other source! 3 / 64

4 Searching for New Physics DIRECT Cannot produce particles with mc 2 > E INDIRECT Higher energy particles can appear virtually in quantum loops flavour physics 4 / 64

5 CP violation in the Standard Model q' = {d,s,b} Coupling of charged current interaction to up, down-type quarks given by CKM matrix: q = {u,c,t} Vqq' W V ud V us V ub 1 λ 2 /2 λ Aλ 3 ( ρ i η) V CKM = V cd V cs V cb = λ 1 λ 2 /2 Aλ 2 +O(λ 4 ) V td V ts V tb Aλ 3 (1 ρ i η) Aλ generations + 1 phase η is only source of CP violation in SM. A =.8 ±.2, λ =.225 ±.1 ρ =.14 ±.27, η =.343 ±.15 5 / 64

6 Experimental and theoretical success V ud V ub + V cdv cb + V tdv tb = Measurements using K, D, B, B s systems. CKM picture confirmed up to 1%. New Physics should have flavour structure similar to SM. The NP scale is very very large. Need more precision measurements to look for small deviations. 6 / 64

7 Experimental and theoretical success V us V ub + V csv cb + V tsv tb = Measurements using K, D, B, B s systems. CKM picture confirmed up to 1%. New Physics should have flavour structure similar to SM. The NP scale is very very large. Need more precision measurements to look for small deviations. 7 / 64

8 The Large Hadron Collider (LHC) Setting the scale 27km tunnel. pp collisions at s = 7, 8TeV. O(1k) bb pairs/sec. 8 / 64

9 The LHCb detector 28 JINST 3 S85 9 / 64

10 A typical LHCb event 28 JINST 3 S85 p ~1cm b b p npvs 2.3 ntracks 2 1 / 64

11 The vertex locator 28 JINST 3 S85 IP X Resolution Vs 1/p s = 7 TeV 211 Data T µm 5 21 silicon strip detectors, 8mm from beam line LHCb VELO Preliminary 1 σ = /p µm T /p [c/gev] T Operates in vacuum, separated from primary vacuum by 3µm Al foil. Primary vertex resolution 13, 13, 69µm in x, y, z. IP resolution of tracks with p T > 2 GeV/c 2 is 2µm. 11 / 64

12 Particle ID arxiv: different gas radiators (C 4 F 1, CF 4 ) + aerogel. Photomultiplier tubes to detect Cerenkov light. No PID with PID 12 / 64

13 Tracking 28 JINST 3 S85 Silicon microstrip detectors closest to beam pipe. Straw tubes cover larger area. Aligned to 14µm using large samples of J/ψ µµ, D Kπ. σ 15 MeV/c 2 Tag-and-probe J/ψ 13 / 64

14 The trigger arxiv: Use trigger which has high efficiency for interesting events. 4 MHz bunch crossing rate L Hardware Trigger : 1 MHZ readout, high ET/PT signatures 45 khz h ± 4 khz μ/μμ 15 khz e/γ Software High Level Trigger 29 Logical CPU cores Offline reconstruction tuned to trigger time constraints Mixture of exclusive and inclusive selection algorithms DiMuon trigger LHCb-PUB khz Rate to storage 2 khz Inclusive Topological 2 khz Inclusive/ Exclusive Charm 1 khz Muon and DiMuon Lower efficiency for multi-body final states 14 / 64

15 Recorded luminosity 212 Efficiency > 93% LHCb designed to run at lower luminosity than ATLAS/CMS. L cm 2 s 1 in / 64

16 Recorded luminosity LHCb lumi LHCb designed to run at lower luminosity than ATLAS/CMS. L cm 2 s 1 in / 64

17 ) 1st obs. of CP violation in B s meson decay arxiv: A CP = Γ(B (s) f) Γ(B (s) f) A CP (B K + π ) =.8 ±.7 ±.3 Γ(B (s) f) + Γ(B (s) f) A CP (B s K π + ) =.27 ±.4 ±.1 2 Candidates / ( 1 MeV/c LHCb (a) (b) (c) B s (d) B Kπ B s Kπ B ππ B s KK B 3-body Comb. bkg B s K + π invariant mass [GeV/c ] K π + invariant mass [GeV/c ] 17 / 64

18 Brief introduction to B s meson mixing and decay i t ( B s (t) B s (t) ) = ([ ] M11 M 12 M i [ ]) ( Γ11 Γ 12 B s () 12 M 22 2 Γ 12 Γ 22 B s () ) B s,l = p B s + q B s B s,h = p B s q B s NP? We want to measure... M B s = M H + M L GeV/c 2, Γ s = Γ L + Γ H 2.7 ps 1 m s = M H M L 2 M ps 1, Γ s = Γ L Γ H 2 Γ 12 cos ϕ 12.1 ps 1 18 / 64

19 The B s oscillation frequency, m s decay time = Length m B p B Number of B s f decays osc. period 35 fs c.f. B 12 ps decay time [ps] B s Need good understanding: Backgrounds Decay time resolution 45 fs Tagging the flavour of the B s /B s Efficiencies P(t σ t ) [ Γ s e Γst 1 2 [cosh( Γ st/2) + D cos( m s t)] ] G(t; S σt, σ t )ε(t) 19 / 64

20 B s D s π event selection arxiv: Use flavour specific decay modes Allowed Forbidden Bs D s π + B s D+ s π B s D s π + Bs D+ s π 5 different D s decay modes: D s (K + K )π, D s (K π + )π, D s π + π π 2, 3, 4 track displaced vertex trigger. 34k events 1 large IP track, p T > 1.7 GeV/c. 2 / 64

21 Flavour tagging Specialised tagging algorithms to analyse event to determine initial flavour b or b. Opposite-side Use charge of leptons/hadrons from other B meson decay Same-side Use charge of kaon produced from fragmentation of signal B 21 / 64

22 Flavour tagging Opposite-side ω = p + p 1 (η η ) Effective tagging efficiency 2.6 ±.4% ω B + J/ψK + LHCb Preliminary s = 7 TeV Data η c B s D s π Same-side kaon Effective tagging efficiency 1.2 ±.3% 22 / 64

23 Fitting the B s decay time arxiv: m SM s = 17.3 ± 2.6 ps 1 m s = ±.23(stat) ±.6(syst) ps 1 Systematic from length and momentum scales. 23 / 64

24 CP violation in B s meson mixing/decay φ mix = 2 arg(v ts V tb ) φ dec = arg(v cs V cb ) Use interference between mixing and decay to measure CP-violating phase φ s = φ mix 2φ dec φ SM s =.36 ±.2 rad 24 / 64

25 CP violation in B s meson mixing/decay A CP = Γ(B s f) Γ(B s f) Γ(B s f) + Γ(B s f) = η f sin φ s sin( m s t) Number of B s f decays amplitude sin φ s decay time [ps] B s Experimental considerations: Backgrounds Decay time resolution 45 fs Tagging the flavour of the B s /B s. Efficiencies. 25 / 64

26 The GOLDEN mode: B s J/ψφ arxiv: ~1cm } } ) 2 Candidates / (2 MeV/c LHCb m(µ + µ ) [MeV/c ] ) 2 Candidates / (2.5 MeV/c 45 4 LHCb Double-Gaussian fit N sig = ± 115 σ m = 7. MeV/c m(j/ψ K K ) [MeV/c ] ) 2 Candidates / (1 MeV/c LHCb φ dominates 2% S-wave m(k K ) [MeV/c 2 ] 26 / 64

27 Decay time resolution arxiv: Use prescaled sample of prompt-j/ψ events to extract resolution scale factor. } } Candidates / (5 fs) LHCb Decay time [ps] Candidates / (5 fs) 7 LHCb Sσ eff 45 fs Decay time [ps] Use σ t, per-event decay time error, scaled by S = 1.45 ±.6. If Sσ eff 45 fs D.73 If Sσ eff 9 fs D / 64

28 Decay time efficiency arxiv: Acceptance 1.8 Acceptance Unbiased LHCb.2.1 Biased LHCb 1 1 Decay time [ps] 1 1 Decay time [ps] Use sample of unbiased events to understand trigger efficiency. Additional efficiency effect at large decay times: ε(t) 1 + βt, β 1 2 ps 1. Understand this using data: B + J/ψ K / 64

29 Separating CP-odd and CP-even arxiv: Spin- particle (B s ) decaying to two spin-1 particles (J/ψ, φ). Final state is ad-mixture of CP-odd and CP-even. Perform complex fit to the angular distributions of the final state particles. CP J/ψφ l = ( 1) l J/ψφ l y K x µ + ϕ θ K K + K B s µ + µ θ µ z K + µ 29 / 64

30 Time, angular PDF arxiv: Rich system to understand. Gives access to many physics parameters. 3 / 64

31 Angular efficiency arxiv: Scaled acceptance integral 1.15 LHCb simulation cosθ µ Scaled acceptance integral LHCb simulation cosθ K Scaled acceptance integral LHCb simulation -2 2 ϕ h Detector geometry and implicit momentum cuts cause majority of effect. Knowledge of acceptance is dominant source of systematic error. [rad] Tagging the Bs flavour Tagger ε eff OS 2.29 ±.6% SSK.89 ±.17% Overall 3.13 ±.2% 31 / 64

32 Projection of time-dependent angular fit arxiv: Candidates / (.274 ps) Background subtracted fit using sweights. S-wave LHCb 5 1 Decay time [ps] Candidates / CP-even CP-odd LHCb cosθ µ Candidates / LHCb Candidates / (.67 π rad) LHCb cosθ K -2 2 ϕ [rad] h 32 / 64

33 Result and systematics arxiv: φ s =.7 ±.9 (stat) ±.1 (syst) rad Γ s (Γ L + Γ H )/2 =.663 ±.5 (stat) ±.6 (syst) ps 1 Γ s Γ L Γ H =.1 ±.16 (stat) ±.3 (syst) ps 1 Source Γ s Γ s A 2 A 2 δ δ φ s λ [ps 1 ] [ps 1 ] [rad] [rad] [rad] +.13 Stat. uncertainty Background subtraction B J/ψ K background Ang. acc. reweighting Ang. acc. statistical Lower decay time acc. model.23.2 Upper decay time acc. model.4 Length and mom. scales.2 Fit bias.1 Decay time resolution offset.4.6 Quadratic sum of syst Total uncertainties Dominant systematics come from angular acceptance, decay time efficiency and background. 33 / 64

34 Resolving the ambiguity arxiv: Expressions are invariant under the transformation, giving rise to a two-fold ambiguity. (φ s, Γ s, δ, δ, δ, δ S ) (π φ s, Γ s, δ, δ, π δ, δ S ) Physical solution: Γ s > the heavy Bs eigenstate lives longer than the light one! P-wave phase S-wave phase [rad] - δ 4 LHCb 3 δ S 2 δ S δ 1-1 Γ s > m(k K ) [MeV/c 2 ] 34 / 64

35 φ s from B s J/ψπ + π arxiv: v3, PLB B s J/ψπ + π is another b cc s. π + π is > 97.7% 95% Conf. Level. Events / 5 MeV B J/ψKπ B J/ψππ LHCb 7421 ± 15 cands Events / 15 MeV LHCb S-wave dominates m( J/ψπ + π - ) (MeV) m(π π ) (MeV) φ s = ±.1 rad 35 / 64

36 Precision measurement of φ s Only now, with LHCb, are we reaching precision that is required to look for small deviations from the SM. ] -1 Γ s [ps LHCb 1fb + CDF 9.6fb + D 8fb + ATLAS 4.9fb D SM LHCb HFAG April % CL contours ( logl = 1.15) Combined.5 CDF ATLAS -1 1 φ s [rad] φ s =.1 ±.7 (stat) ±.1 (syst) rad, Γ s (Γ L + Γ H )/2 =.661 ±.4 (stat) ±.6 (syst) ps 1, Γ s Γ L Γ H =.16 ±.11 (stat) ±.7 (syst) ps 1, 36 / 64

37 Impact on New Physics Lenz et al. arxiv: v2 [hep-ph] M NP,s 12 = M SM,s 12 s LHCb + CDF m s s = s e iφ s SM s = 1 NP contribution to Bs mixing is limited to < 3% at 3σ. D A SL LHCb φ s Next step Use full LHCb dataset (factor 3 more data). Precision measurement. Control of systematic uncertainties is essential. Extend physics reach by including rarer modes: B s ψ(2s)φ, B s J/ψη( ) / 64

38 Precision B s lifetime measurements arxiv: , arxiv: Precise measurements of B s -meson lifetimes are another important input for constraining φ s, Γ s, Γ s. τ = 1.7 ±.4 ±.26ps B s J/ψ f (98) τ = ±.46 ±.6 Bs J/ψ KK 38 / 64

39 avour changing interactions are governed by the same CP violation using charmless Bs CKM matrix and that the one ndependent phase in the CKM matrix is the only source of decays CP violation[1]. arxiv: In general, he MSSM and models involving MFV are plagued by the SUSY CP problem 4. A class of odels that Bs does φφ: not b in general s penguin suffer decays from the sensitive SUSY CP to NP problem in the and loops. does not involve FV is given by supersymmetric flavour models (the interested reader is directed to the ork of Altmannshofer φ KK: 5 different et al. (21)[11]). polarisation amplitudes angular analysis. Decay time resolution: 4fs..3 CP Violation in B Tagging power: ε(1 s 2ω) 2 φφ = 3.29 ±.48% he decay Efficiencies Bs φφfrom is an MC. example of a flavour changing neutral current (FCNC) and ence, is forbidden at tree level in the standard model. The decay is only permitted hrough penguin diagrams (shown in figure 5)[12]. SM: φ s <.2 rad s b u,c,t W + s s s b N sig = 88 ± 31 W + s s s s s s (a) (b) s W + 39 / 64

40 CP violation using charmless B s decays arxiv: Background subtracted fit using sweights. 4 / 64

41 First measurement of CP phase using penguin decay arxiv: φ s [ 68%C.L. (defined using Feldman-Cousins). p-value of SM prediction is 16%. Dominant systematic from understanding of KKKK S-wave 41 / 64

42 CP violation in B s mixing LHCb-CONF A CP = Prob( B B, t) Prob(B B, t) Prob( B B, t) + Prob(B B, t) = Γ 12 M 12 sin ϕ 12. Experimentally, measure asymmetry in semileptonic Bs decays (between D+ s Xµ ν µ and D s Xµ + ν µ) A measured CP = Γ[D s µ + ] Γ[D + [ ] s µ ] Γ[D s µ + ] + Γ[D + s µ ] = as sl 2 + a p as sl e Γ st cos( m st)ε(t)dt 2 e Γ st cosh( Γ s/2t)ε(t)dt a p is production asymmetry - difference in number of b b at LHC. Fast Bs mixing dilutes second term below precision of this measurement. Untagged and time-integrated analysis. Detection asymmetries: cancel using flip of magnetic field. Systematics are important Asymmetries can bias result. Use D s φ(k + K )π +, so worry about π and µ detection/reconstruction asymmetries. Swap magnetic field to help cancel effects. Obtain any corrections from data/control samples. 42 / 64

43 Semileptonic asymmetries LHCb-CONF k Bs signal candidates in magnetic up and down. Background asymmetries from K, π µ mis-id. Small systematic. Candidatess / 3 MeV LHCb 4 1 Preliminary (a) D s φπ + LHCb D + s φπ Candidatess / 3 MeV 4 1 Preliminary (b) Pull m(k K π + ) (MeV) 4 2 (%) A raw LHCb Preliminary MAG UP MAG DOWN m(k K 2 π + ) (MeV) Pull (%) A raw m(k + K - π - ) (MeV) LHCb Preliminary Magnet UP Magnet DOWN m(k + 2 K - π - ) (MeV) -5-1 Cross-checks Run Block p ( μ) [GeV] T 43 / 64

44 Semileptonic asymmetries LHCb-CONF a s sl = [.24 ±.54(stat) ±.33(syst)]% Ian Bertram, DIS a d sl Dominant systematic is from limited statistics in control sample. 3σ tension with SM in the D result, not confirmed or excluded by LHCb. 44 / 64

45 Rare decays Jose Lazo-Flores, FPCP Heavy suppressed decay in SM. BR(B s µµ) = (3.23 ±.27) 1 9 BR(B µµ) = (1.7 ±.1) 1 9 Requires FCNC transition. Helicity suppressed by factor (m µ /m B ) 2. Sensitive to new physics. i.e., MSSM BR (tan β) / 64

46 B s µµ: event selection µ - Two muons from long-lived B s. B µ + µ - Two independent semileptonic B decays. One or two hadronic B decays with mis-id hadron. B B µ + 46 / 64

47 PRL 11 (213) mµ+µ [MeV/c2] Probability Signal selection LHCb LHCb fb (8TeV) Signal Background BDT Single and di-muon trigger. Classify signal using 2D discriminant: 1 m(µµ) 2 BDT containing: Bs impact parameter, pt ; µ pt, χ2ip... Train using MC (Bs µµ and b b µµx ). Calibrate BDT on data: Background: m(µµ) sidebands Signal: B hh which has same topology.8 1 BDT 47 / 64

48 Calibrating the signal models PRL 11 (213) 2181 Candidates / (1 MeV/c 2 ) 1 5 LHCb [MeV/c 2 ] σ µ + µ 6 4 LHCb m [MeV/c 2 KK ] 2 B B s m [MeV/c 2 µ + µ ] Signal shape is Crystal Ball: Mean determined from B hh. Resolution from interpolation between charmonium/bottomium resonances (σ m = 25. ±.4 MeV/c 2 ). Radiative tail transition point from B s µµ MC. 48 / 64

49 Simultaneous fit to all bins PRL 11 (213) 2181 Candidates / (11 MeV/c 2 ) LHCb BDT.25 Candidates / (11 MeV/c 2 ) LHCb.25<BDT.4 Candidates / (11 MeV/c 2 ) LHCb.4<BDT.5 Candidates / (11 MeV/c 2 ) LHCb.5<BDT m [MeV/c 2 µ + µ ] m [MeV/c 2 µ + µ ] m [MeV/c 2 µ + µ ] m [MeV/c 2 µ + µ ] - Candidates / (11 MeV/c 2 ) LHCb.6<BDT.7 Candidates / (11 MeV/c 2 ) LHCb.7<BDT.8 Candidates / (11 MeV/c 2 ) LHCb.8<BDT m [MeV/c 2 µ + µ ] m [MeV/c 2 µ + µ ] m [MeV/c 2 µ + µ ] - Combine 211 (1. fb 1 ) (1.1 fb 1 ) data. Float yield of background, B, Bs in fit. Observe excess of events over bkg-only hypothesis (p-value = ). 49 / 64

50 First evidence for B s µ + µ! PRL 11 (213) 2181 BR(B s µµ) = Candidates / (5 MeV/c 2 ) LHCb fb (7TeV) +1.1 fb (8TeV) BDT > [MeV/c 2 ] m µ + µ Normalise to BR(B + J/ψK + ) (424k events). 3.5σ excess of Bs µ + µ signal! Systematic comes from modelling of the background in the fit. Now working on update using full 212 dataset (2 fb 1 ). 5 / 64

51 B s µ + µ implications arxiv: v2 Disfavours SUSY models with large (tan β) 6. Correlation between Bs and B BR allows to distinguish between different NP models. 51 / 64

52 B s µ + µ µ + µ LHCb-PAPER In SM, non-resonant BR(B (s) µ+ µ γ( µ + µ )) < 1 1 [PRD7 (24) 11428] Resonant BR(B s J/ψ φ) = Normalise to BR(B d J/ψ K (892)), main systematic uncertainty. BR(B s 4µ) < % CL BR(B 4µ) < % CL 52 / 64

53 Studies of excited B s mesons PRL 11, (213) First observation of B s2 (584) B + K Help understand heavy quark effective theory, used for calculating B meson properties. 53 / 64

54 And there s more... Lots of physics in decays of B +, B and B c + mesons and Λ b baryons. Huge samples of charm meson decays (Oxford). Quarkonia production and exotic states (X (3872), Z(443)... ). QCD, electroweak and exotica. 54 / 64

55 Looking forward: LHCb upgrade LHC will be upgraded to run at higher luminosity from 218. LHCb will run at L 1 33 cm 2 s 1. Upgraded detector will be read out at 4MHz. Factor-1 increase signal yields. Existing design will saturate at higher luminosities. Current σ(φ s) Upgrade 5 fb 1 σ(φ s) SM prediction φ s.9 rad.8 rad.36 ±.2 rad 55 / 64

56 Summary Flavour physics provides access to high energy scales where new physics may exist. LHCb provides a unique laboratory for the precision study of heavy flavour CP violation in Bs decays. Rare decays of Bs mesons. And more... LHCb upgrade will move forward the precision frontier. 56 / 64

57 Backup. 57 / 64

58 m s zoom # candidates /.2 ps 15 1 Tagged mixed Tagged unmixed Fit mixed Fit unmixed decay time [ps] 58 / 64

59 B s φφ S-wave 59 / 64

60 B s φφ S-wave 6 / 64

61 B s µµ normalisation Candidates / (2 MeV/c 2 ) LHCb [MeV/c 2 ] m J/ψ K 61 / 64

62 A close-up of a B s J/ψ φ decay 62 / 64

63 Bs 4µ 63 / 64

64 Experimental and theoretical success Measurements using K, D, B, Bs systems. CKM picture confirmed up to 1% level. New Physics should have flavour structure similar to SM. The NP scale is very very large. Need more precision measurements to look for small deviations. 64 / 64