Studies of rare B meson decays with the CMS detector

Transcription

1 Studies of rare B meson decays with the detector J.Pazzini on behalf of the Collaboration Padova University & INFN - August - 3 J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

2 Introduction and outline Rare B mesons decays are a crucial probe of physics Beyond the Standard Model FCNC, higly suppressed decays Virtual New Physics particles can contribute in loops Tiny effects so NP can be at the same level of SM High precision predictions of SM observables If observed, deviations from SM will be a indirect evidence of NP Rare B decays potential smoking gun for physics BSM In this talk: Angular analysis and branching ratio measurement of the decay B K µ µ Measurement of the B s µ µ branching fraction and search for B µ µ J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

3 Angular analysis and branching ratio measurement of the decay B K µ µ μ ϕ K θk θl B μ π -PAS-BPH--9 arxiv:3.39 Submitted to PLB J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 3

4 The B K µ µ decay b s` ` transition, suppressed in the standard model: FCNC I Tree-level suppression I Box/penguin diagrams Robust calculations, especially at low q = mµµ Four-particles final states (K πµµ) angular observables Decay fully described by q and three angles (θk,θl,φ) measurements: Differential branching fraction, db/dq Dimuon forward-backward asymmetry, AFB Longitudinal K polarization, FL ϕ μ K d d W B K u, c, t b s γ, Z d B b u, c, t W µ µ d K s W νµ µ µ θk θl μ B π J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

5 Analysis overview Full set: 5 fb, Unbinned extended maximum likelihood fits in q bins: signal q bins ( 9 GeV ) normalization q bin (B K J/ψ) control q bin (B K ψ ) Signal and control channels are selected/treated identically A FB and F L are extracted from fits to the two angular variables: cos θ K, and cos θ L. {[ d 3 Γ Γ d cos θ K d cos θ L dq = 9 ) ( F S ) [F L cos θ K ( cos θ L 3 F S 3 A S cos θ K ] ( cos θ L ) ) ) ( F L) ( cos θ K ( cos θ L ) ] } ( cos θ K cos θ L 3 A FB Residual contributions determined by the fit to the normalization sample B K J/ψ: Spinless (S-wave) contribution to the Kπ combination, F S S-wave/P-wave interference, A S db/dq is measured relative to the normalization channel B K J/ψ from fit to the B cand. invariant mass: ) db (B ) K µ µ dq ( = Y S ɛ B N Y N ɛ S B K J/ψ J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 5 dq

6 Fit on normalization channel Fit to the B K J/ψ channel (. < q <.9 GeV bin) Events / (. GeV ) 3 q :..9 GeV Signal yield: 7 ± L = 5. fb 7 Data Total fit Signal Comb. bkg µ ) M(Κ π µ (GeV) Events / (. ) q :..9 GeV L = 5. fb Data Total fit Signal Comb. bkg cos(θ l) Events / (. ) q :. Vector (P-wave) and scalar (S-wave) contributions obtained from the fit Direct estimation of the non-resonant Kπ component from.9 GeV L = 5. fb Data Total fit Signal Comb. bkg cos(θ Κ) results F L =.55 ±. A FB =. ±. Expectations F L =.57 ±. [PDG] A FB compatible with zero Fit performed neglecting the scalar Kπ contributions (F S,A S ) J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

7 Fitting the signal A total of more than signal events Clear yields from all the q bins Events / (. GeV ) L = 5. fb q :.. GeV Signal yield: 3 Data Total fit Signal Comb. bkg µ ) M(Κ π µ (GeV) ± Events / (. GeV ) L = 5. fb q :..3 GeV Signal yield: 5 Data Total fit Signal Comb. bkg µ ) M(Κ π µ (GeV) ± 9 Events / (. GeV ) L = 5. fb q :.3 Signal yield: 9 Data Total fit Signal Comb. bkg Peak. bkg. GeV µ ) M(Κ π µ (GeV) ± 7 Events / (. GeV ) 5 3 q :.9 Signal yield: 9. GeV ± L = 5. fb Data Total fit Signal Comb. bkg Peak. bkg Events / (. GeV ) L = 5. fb q :. Signal yield: 5 Data Total fit Signal Comb. bkg Peak. bkg. GeV ± Events / (. GeV ) L = 5. fb q :. Signal yield: 3 Data Total fit Signal Comb. bkg 9. GeV ± µ ) M(Κ π µ (GeV) µ ) M(Κ π µ (GeV) µ ) M(Κ π µ (GeV) Signal Combinatorial background: randomly associated hadron-muon pairs Peaking background: feed-through from dimuon resonant channels J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 7

8 Results B K µ µ angular analysis F L. L = 5. fb Data SM <SM> A FB Data SM <SM>.5 L = 5. fb q (GeV ) - q (GeV ) ) GeV 7 ( dbf/dq. Data SM <SM>... L = 5. fb Short error bars: statistical uncertainty Long error bars: total uncertainty Red band: standard model predictions Blue boxes: SM predictions averaged over the q bin [Bobeth et al. PRD 7, 3]. q (GeV ) No deviations from the standard model are found Results still dominated by the statistical uncertainty J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

9 Measurement of the Bs µ µ branching fraction and search for B µ µ -PAS-BPH-3- arxiv:37.55 Accepted by PRL J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 9

10 The Bs ( B ) µ µ decay ( B ) µ µ decays highly suppressed in standard Bs model: FCNC Tree-level suppression Box/penguin diagrams Elicity suppression (m µ/m B ) Internal quark annihilation (f B /m B ) b B s,d s, d b B s,d s, d t W Z µ H t H, h µ W χ µ t, c, u q l ν χ W µ SM ( predictions (time-integrated) B Bs µ µ ) = (3.57 ±.3) 9 ( B B µ µ ) = (.7 ±.) [De Bruyn et al. PRL 9, ] [A. Buras et al. arxiv:33.3] ( ( ) B Bs B µ µ ) probe for physics BSM B (tan β) in MSSM B (tan β), m H in HDM (Very) Small theoretical uncertainties High sensitivity to new physics Comparable to µ eγ, B Xνν J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

11 Analysis overview Complete re-analysis of entire (5 fb, ) ( fb, s = TeV) sets Blind analysis ( set re-blinded) Better muonid and fake suppression (BDT based) New (and improved) analysis variables BDT based discriminant Simultaneous maximum likelihood fit to dimuon mass in several BDT bins and two rapidity ranges B ( B s µ µ ) = Ns N B obs tot f u ε B f s ε tot B ( B J/ψ K ) ( ( ) B Bs B µ µ ) is measured relative to the B J/ψ K normalization channel B independent from the luminosity measurement Avoid uncertainties related to b quark production cross section Mitigate the effects of uncertainties in the efficiencies Control sample Bs J/ψ φ (KK) to compare and validate B s mesons in and MC simulations J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

12 MVA selection - L = fb ( s = TeV) The events are selected by a Boosted Decision Tree (BDT) Signal: Bs µ µ MC simulation Background: dimuon mass sidebands Data randomly split in 3 subset: Train on, test on, apply on 3 Avoid selection bias Studies on BDT output: insensitive to invariant mass insensitive to pileup Use the same BDT for normalization and control samples differences between Data and MC BDT output as systematic uncertainties A (very short) selection of the most significant variables: ε(b>.) B s J/ψ φ.. -. MC simulation BDT - L = fb ( s = TeV) (n > ) min p =.7 ±.3 χ /dof = 9.7/ (pol) p =. ±.9 p =. ±. χ /dof =./ 3 (pol) N PV L = fb ( s = TeV) Dimuon sidebands - B µ s µ Dimuon L = fb ( s = TeV) sidebands - B µ s µ - L = fb ( s = TeV) 9 Dimuon sidebands - B µ µ 7 s α 3D 5 l 3D /σ(l ) 3D.5 isolation J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

13 BR extraction approaches: Categorized-BDT to extract B ( B s µ µ ) UML fit to mass distributions in BDT bins split in Barrel/Endcap BDT binning chosen to equalize the expected number of signal events D-BDT to extract CL s limits on B ( B µ µ ) Optimized cut on BDT output and event counting in mass windows min. bin edges 3 barrel endcap barrel endcap b > barrel endcap S/(SB) Weighted Events / (. GeV) L = 5 fb -, L = fb s = TeV full PDF Bs µ µ - - B µ µ combinatorial bkg semileptonic bkg peaking bkg S/(SB) Weighted Events / (. GeV) - - L = 5 fb -, L = fb s = TeV full PDF Bs µ µ - - B µ µ combinatorial bkg semileptonic bkg peaking bkg m µµ (GeV) m µµ (GeV) J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 3

14 Results B s ( B ) µ µ ) - µ µ BF(B.... Observation of Bs µ µ decay ( B B s µ µ ) ) = 9 ( σ significance (expected. σ median) -9 lnl - - s=7 TeV, L=5 fb s= TeV, L= fb.3σ σ - BF(B µ µ )= s. -9 (3. ) σ lnl σ σ SM SM σ σ BF(B 3 µ µ ) BF(B µ µ ) s BF(B µ µ.σ - )= σ. - (3.5 ) -. 3 σ CL s.. No significant excess of B µ µ ( B B µ µ ) =. σ significance ) (3.5.. ( B B µ µ ) <. 9, 95% CL - - L = 5fb (7TeV) fb (TeV) CL s observed Expected SM CL ± σ s Expected SM CL ± σ s Expected SM CL s median... 3 σ σ. σ σ.. SM BF(B µ µ ) s BF(B µ µ - ) -9 J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

15 Conclusions and Outlook First angular analysis of B K µ µ, using set (5 fb ) More than signal decays No deviations from the SM predictions are found Results with are statistically limited Analysis of the is starting ( ( ) B Bs B µ µ ) analysis, using sets (5 fb ) Substantial improvements to previous analysis Muon identification with BDT Analysis selection with BDT UML fit to mass distributions.3 σ significance of Bs µ µ observation Consistent with SM predictions No significant excess is observed for B µ µ decay Upper limit using CLs method Future plans Looking forward for LHC Run All public BPH results from the collaboration J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 5

16 A 3 years quest J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

17 BACKUP J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 7

18 B K µ µ Signal vs Background Signal: B K µ µ Two well reconstructed/identified muons with invariant mass far from the J/ψ and ψ (q bins) K (9) reconstructed through the decay in Kπ (both tracks must fail muonid) -track vertex candidate identified as B ( B ) if Background: Peaking K π ( K π ) is the closest to the nominal K mass Feed-through from B K J/ψ and B K ψ decays not removed by the q cut Dimuon mass reconstructed far from the true J/ψ (ψ ) mass Kπµµ mass shifted from the true B mass Shape obtained from simulation. Combinatorial Unavoidable fraction of mis-reconstructed B from randomly associated muons to hadrons Shape described by a single exponential in mass, and by polynomials for the angular variables. J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

19 B K µ µ Event selection Muon selection events recorded with dimuon low-mass, vertex displaced trigger µ µ vertex transverse flight length significance cut (in trigger) < m µµ <. GeV (in trigger) p T (µ) > 3,,.5, 5 GeV (depending on trigger) Hadron selection hadron must fail muon ID p T (h) >.75 GeV cut on transverse distance of closest approach m(kπ) m PDG (K ) < MeV B selection B vertex CL > 9% B vertex transverse flight length significance cut cos(α) >.999, α being the angle in transverse plane between B momentum and line-of-flight CP state assignment tag the CP state based on closest distance from K PDG mass reject event if both K and K masses are within 5MeV of PDG mass ( Γ) mistag rate : % on MC; % on J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 9

20 B K µ µ Low q region ( GeV ) Special bin very precise theoretical predictions Events / (. GeV ) 5 3 L = 5. fb q :. Data Total fit Signal Comb. bkg. GeV Signal yield: 7 ± Events / (. ) 5 3 q :.. GeV L = 5. fb Data Total fit Signal Comb. bkg Events / (. ) 5 3 q :.. GeV L = 5. fb Data Total fit Signal Comb. bkg µ ) M(Κ π µ (GeV) cos(θ l) cos(θ Κ) results F L =. ±. (stat.) ±. (syst.) A FB =.7 ±. (stat.) ±. (syst.) db/dq = (. ±. (stat.) ±.7 (syst.)) GeV SM predictions (Bobeth et al. PRD 7 3) F L =.7..7 A FB =. ±.3 db/dq = (.9.. ) GeV J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

21 B K µ µ Comparison with other experiments F L <SM> LHCb Belle CDF L = 5. fb q (GeV ) A FB L = 5. fb <SM> LHCb Belle CDF q (GeV ) ) GeV 7 (. <SM> LHCb BaBar Belle. CDF L = 5. fb dbf/dq... q (GeV ) J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

22 Bs ( B ) µ µ Signal vs Background Signal Two muons from one decay (secondary) vertex Well reconstructed secondary vertex Dimuon momentum aligned with SV flight direction m µµ close to the nominal m B s (B ) value PV/SV/muon isolation µ _ µ B B Background Combinatorial (estimated from sidebands) One semileptonic B decay and one hadron (K/π) misidentified as a muon Two semileptonic B decays Rare single B decays (estimated from MC simulation) Peaking: B s Kπ, B s KK, B ππ,... Non-peaking: B s Kµν, Λ b pµν,... _ B B µ µ Crucial aspects of the analysis muonid and h µ suppression vertexing variables isolation variables J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

23 Bs ( B ) µ µ Muon Identification Improved muon identification to cope with peaking and semileptonic backgrounds Use a MVA based on Silicon Tracker and Muon Detector information distribution of hits in the tracker as compared to expected kink identification muon segment compatibility χ /ndof muon p T and η Muon MC efficiency validated with Tag&Probe methods in Muon fake rate reduced by a factor of compared to previous analysis, at the expenses of % muon efficiency reduction ε(µ π) <.5% ε(µ K) <.% ε(µ p) <.% Analysis uses MC p T -dependent fake rate, validated with (5% uncertainty) D D π, K s ππ, Λ pπ J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 3

24 Bs ( B ) µ µ Most discriminating variables Vertexing: Angle between PV ad SV SV flight length significance Dimuon impact parameter significance L = fb ( s = TeV) Dimuon sidebands - B µ s µ Dimuon L = fb ( s = TeV) sidebands - B µ s µ - L = fb ( s = TeV) Dimuon sidebands - B µ s µ.5. α 3D Isolation: 5 l 3D /σ(l ) 3D Primary Vertex isolation (dimuon isolation) Secondary Vertex isolation (B-cand. isolation) Muon isolation - L = fb ( s = TeV) 9 Dimuon sidebands - B µ µ 7 s L = fb ( s = TeV) 7 Dimuon sidebands - B µ s µ δ 3D /σ(δ ) 3D - L = fb ( s = TeV) Dimuon sidebands - B µ s µ isolation d ca [cm] m isolation J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

25 Bs ( B ) µ µ Data vs MC simulation comparison Comparison of sidebands subtracted distributions Good agreement - L = fb ( s = TeV) B s J/ψ φ MC simulation 35 B s J/ψ φ L = fb ( s = TeV) MC simulation - L = fb ( s = TeV) B J/ψ φ s MC simulation L = fb ( s = TeV) 3 B s J/ψ φ MC simulation.5. α 3D 5 l 3D /σ(l ) 3D.5 isolation p (B) [GeV] T 5 B J/ψ K 3 - L = fb ( s = TeV) MC simulation - L = fb ( s = TeV) B J/ψ K MC simulation 3 - L = fb ( s = TeV) B J/ψ K MC simulation - L = fb ( s = TeV) B J/ψ K MC simulation.5. α 3D 5 l 3D /σ(l ) 3D.5 isolation p (B) [GeV] T J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 5

26 Bs ( B ) µ µ Pileup independence Average number of interactions per bunch crossing : 9 : Pileup independence checked Signal MC event samples with pileup every single variable used in BDT is shown to be pileup independent Data studies with BDT output distribution vs. N PV mean and RMS, efficiency of BDT requirement No significant pileup dependence observed mean(b). - L = fb ( s = TeV) (n min > ) rms(b). - L = fb ( s = TeV) (n min > ) ε(b>.). - L = fb ( s = TeV) (n min > ) p =.7 ±.5 χ /dof = 7.7/ 5 (pol) p =.73 ±.3 p =. ±. χ -. /dof = 3./ (pol) N PV -. p =.3 ±.3 χ /dof = 3.9/ 5 (pol) p =.59 ±.9 p =.3 ±. χ -. /dof =.9/ (pol) N PV -. p =.7 ±.3 χ /dof = 9.7/ (pol) p =. ±.9 p =. ±. χ -. /dof =./ 3 (pol) N PV J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

27 Bs ( B ) µ µ Rare decays Normalized to B J/ψ K yield in N(X) = ( B (Y X) B B J/ψ K ) f Y ε X tot f u ε B N obs (B ) tot Candidates/.5 GeV L = fb ( s = TeV) Barrel simulation - B K K - B K π - B π π - B s K K - B π s K - B π s π - Λ b p K - Λ b p π m µ µ [GeV] - Candidates/.5 GeV L = fb ( s = TeV) Barrel simulation - B K µ µ - B µ µ γ - B π µ µ B π - µ ν - B K µ s ν - Bs µ µ γ - B K µ µ - B π µ µ - Λb pµ ν m µ µ [GeV] - J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 7

28 Bs ( B ) µ µ Categorized BDT fits L = - - L = 5 fb s = 7 TeV - Endcap Barrel. < BDT < L - = L 5 = fb fbs - = s7 = TeV TeV - Barrel - Endcap.3 < BDT <. 5 L - = L 5 = fb - fb s - = 7 s TeV = TeV - Endcap - Endcap. < BDT <. 7 L - = L 5 = fb - fb s - = 7 s TeV = TeV - Endcap - Endcap. < BDT <. Events / (. GeV ) Events / (. GeV ) L - = L = fb fb - s = s = TeV TeV - Barrel - Endcap. < BDT <.3 full PDF Bs µ µ - B µ µ - combinatorial bkg semileptonic bkg peaking bkg L - = L = fb fb - s = s = TeV TeV - Barrel - Endcap m µ µ (GeV) 3.3 < BDT < L = fb L = fb s = s = TeV TeV - Barrel - Endcap m µ µ (GeV) L - = L = fb fb - s = s = TeV TeV - Barrel - Endcap m µ µ (GeV).33 < BDT <.. < BDT <. Events / (. GeV ) - - L = - L = fb fbs = s = TeV TeV - Endcap - Endcap. < BDT < L = - L = fb fbs = s = TeV TeV - Endcap - Endcap m µ µ (GeV) L = - L = fb fbs = s = TeV TeV - Endcap - Endcap m µ µ (GeV). < BDT < < BDT < L = - L = fb fbs = s = TeV TeV - Endcap - Endcap m µ µ (GeV) 5.5 < BDT < (GeV) (GeV) (GeV) (GeV) mµ µ mµ µ mµ µ J.Pazzini Studies of rare B meson decays with the detector (SUSY3)

29 Bs ( B ) µ µ D-BDT fits Events / (. GeV ) L = 5 fb s = 7 TeV - Barrel full PDF - Bs µ µ - B µ µ combinatorial bkg semileptonic bkg peaking bkg Events / (. GeV ) L = 5 fb s = 7 TeV - Endcap full PDF - Bs µ µ - B µ µ combinatorial bkg semileptonic bkg peaking bkg m µµ (GeV) m µµ (GeV) Events / (. GeV ) L = fb s = TeV - Barrel full PDF - Bs µ µ - B µ µ combinatorial bkg semileptonic bkg peaking bkg Events / (. GeV ) L = fb s = TeV - Endcap full PDF - Bs µ µ - B µ µ combinatorial bkg semileptonic bkg peaking bkg m µµ (GeV) m µµ (GeV) J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 9

30 Bs ( B ) µ µ LHCb combination -PAS-BPH-3-7 CERN-LHCb-CONF-3- LHCb 3fb LHCb 3fb SM SM 5fb 5fb LHCb preliminary B(Bs µ µ ) [ ] B ( B s µ µ ) = (.9 ±.7) 9 LHCb preliminary B(B µ µ ) [ ] ( B B µ µ ) ( ) = 3... J.Pazzini Studies of rare B meson decays with the detector (SUSY3) 3