Measurements of Leptonic B Decays from BaBar

Transcription

1 Measurements of Leptonic B Decays from BaBar Gregory Dubois-Felsmann Caltech for the BaBar collaboration ICHEP 2004 Heavy Quark Mesons and Baryons session

2 Overview Motivations for studying leptonic B decays SM and beyond-sm considerations B + l + ν l B 0 l + l B 0 ν ν, ν ν γ Analyses B + K + ν ν Tag-recoil analyses: general methods specific results B 0 l + l analysis Conclusions and prospects Leptonic B Decays - BaBar preliminary 2 All results are preliminary unless otherwise noted

3 Leptonic B decays to τ + ν, l + l, ν ν Leptonic decays of heavy-quark mesons provide a laboratory For testing straightfoward SM predictions: Note helicity suppression b l W B + G 2 2 ( + + F Vub m ν) = τ 1 l B B l f B BmBml 2 8π u ν mb In SM: B(B + τ + ν τ ) ~ (9.3±3.9) 10-5 (PDG 04 V ub, f B ) (pred.) B(B + µ + ν µ ) ~ (4.2±1.8) 10-7 Calculable in Lattice QCD For searching for non-sm effects in highly suppressed processes. Some newphysics loop diagrams (e.g., SUSY) can enhance these by orders of magnitude. b B 0 d W + l +,ν W l,ν In SM: B(B 0 µ + µ ) ~ (pred.) B(B 0 ν ν ) ~ zero Leptonic B Decays - BaBar preliminary 3 b d b d q u γ,z l +,ν W l,ν q u γ,z l +,ν W q u (νν via Z only) l,ν

4 B decays to K ( * ) ν ν Motivation The flavor-changing neutral current decays B K ( * ) νν occur in the Standard Model via one-loop radiative penguin and box diagrams: ν ν ν ν l SM expectation: B(B + K + νν) ~ ( ) 10-6, B(B + π + νν ) Their analysis is theoretically very clean; observation of these processes would be complementary to the observation of B K ( * ) l + l and will help in understanding the basic Standard Model physics of such diagrams. These also present another opportunity for the observation of new-physics effects in the loops. Leptonic B Decays - BaBar preliminary 4

5 The BaBar detector and dataset Asymmetric C.M. for Y(4S) BB, βγ ~ 0.56 Work presented today B + τ + ν τ (D*lν tags) B 0 l + l B + τ + ν τ, µ + ν µ B + K + ν ν B 0 ν ν, ν ν γ Leptonic B Decays - BaBar preliminary 5

6 Tag-recoil analyses: B 1 tag, B 2 τ + ν, K + νν, νν Neutrino analyses require extra kinematic and/or particle selection constraints These were here performed by: Explicitly identifying the decay of one tag B Fully reconstructed B D+X hadronic decays... full knowledge of kinematics b c semileptonic decays some ambiguity, more tags Studying the recoil for the decay of interest Typically we require the recoil to have The exact charged-particle content expected for signal Below an analysis-dependent threshold on the number and total energy of neutrals observed Tagging efficiencies can be checked by double-tagging All analyses apply anti-continuum shape cuts Leptonic B Decays - BaBar preliminary 6 e - B recoil B tag n p, l + n pn e + visible charged and neutral particles í D (*)

7 Semileptonic (b c + l + X) tags Selection varies somewhat with analysis The lepton must be either an electron or muon with positive PID and p*>~1gev/c Kaons are identified by the DIRC D mesons are reconstructed in several K +Nπ final states, identified by inv. mass ±3σ D* mesons are identified by the mass difference m=m D* m D The semileptonic decay kinematics are enforced by looking at the variable cos θ B,D(*)l, defined as and requiring it to be in the physical region. Some of the analyses are more tolerant of missing/extra particles, trading purity for statistics. Analysis τ + ν τ (I) τ + ν τ (II) K + ν ν νν, ννγ D selection D 0 Kπ, K3π, Kππ 0, K S 0 ππ D 0 Kπ, K3π, Kππ 0, K S 0 ππ D 0 Kπ, K3π, Kππ 0 D + Kππ D 0 Kπ, K3π, Kππ 0 Tag selection D 0 l ν l X X: γ, π 0, nothing D* 0 l ν l D* 0 D 0 π 0,D 0 γ D 0 l ν l X X unseen D* + l ν l, D + l ν l D* + D 0 π + Leptonic B Decays - BaBar preliminary 7

8 Hadronic (B D + X) tags An additional B D ( * )0 + X hadronic tag sample is used for the K + ν ν and τ + ν τ (I) analyses: D mesons are reconstructed in D 0 K π +, K π + π + π, K π + π 0, K S0 π + π In the τ + ν τ (I) analysis, D* 0 Dπ 0 candidates are formed as well. The X system is composed of 1-5 π ±, 0-2 K ±, 0-2 π 0, and 0-1 K S0. In the K + ν ν analysis, only D 0 are considered, and the X system is composed of 1-5 mesons, including π ±, K ±, and π 0, with no more than two π 0 used. B mesons are identified with the kinematic variables E=E B -E beam and the beam-energy substituted mass sideband τ + ν τ (I) BaBar preliminary K + ν ν BaBar preliminary signal Leptonic B Decays - BaBar preliminary 8

9 B + τ + ν τ analysis I overview For semileptonic tags D 0 l ν l X, τ + decays e + ν e ν τ and µ + ν µ ν τ are considered Exactly one charged track originating near the interaction point, Positively identified as an electron or muon, with p* < 1.2 GeV/c. Continuum quark and t + t events are rejected by means of event shape cuts and a minimum invariant-mass requirement on triplets of charged tracks. Require -2.5 < cos θ B,Dl < 1.1; admits excited D states with some unobserved particles Signal determined from fit to shape of distribution of remaining neutral energy (E extra ) Peaks near zero and at low values (~0.15 GeV) for signal, from X=(nil) and X=γ, π 0, respectively; rising at larger E extra for principal backgrounds. For hadronic tags, we consider π + ν τ, π + π 0 ν τ, π + π π + ν τ, e + ν e ν τ, and µ + ν µ ν τ Total charge 0. Positive PID is required for charged tracks. Extra π 0 or K S 0 or high energy photons are rejected. Characteristic kinematical and dynamical features of the τ decay modes are required 168k tags Minimum missing momentum (calculable using the fully reconstructed tag) req d per mode Signal determined by counting events in the signal region for each τ decay mode m ES > 5.27 GeV/c 2, < E < 0.06 GeV Background extrapolated from sideband 5.21 < m ES < 5.26 GeV/c 2, checked against simulation Leptonic B Decays - BaBar preliminary 9

10 B + τ + ν τ analysis I results Semileptonic-tag fit: Signal: 14.8 ± 6.3 events Background: ± 11.8 events Significance as a signal: 2.3σ (stat. only) Limit (CLs method): B(B + τ + ν τ ) < (90%CL) hep-ex/ submitted to PRL BaBar preliminary Hadronic-tag event counts: Signal: 15 events Background: 17.2 ± 2.1(stat) ± 1.3 (syst) events Limit (L-ratio w/ null hypothesis): B(B + τ + ν τ ) < (90%CL) τ branching fraction eff c y given tag backgr nd estimate N obs Combined limit: B(B + τ + ν τ ) < (90%CL) BB pairs used: (88.9±1.0) 10 6 Leptonic B Decays - BaBar preliminary 10

11 B + τ + ν τ analysis II overview D* 0 l ν tags Next step: refine semileptonic analysis, tightening tag selection Require a D* 0, in either of its decays (D 0 π 0,D 0 γ) Tight kinematic cut rejects other D X : 1.1 < cos θ B,D*l < 1.1 Select best D, D* Tag efficiency: (1.818±0.074)10-3 Consider π + ν τ, π + π 0 ν τ, π + π π + ν τ, e + ν e ν τ, and µ + ν µ ν τ Require 1 or 3 charged tracks of good quality & matching PID & a good π 0 (Eγ > 50 MeV) for the π + π 0 ν τ mode (no π 0 for π + ν τ ) Mode-dependent cuts on missing mass, energies of τ daughters, n-π sub-resonance masses (ρ, a 1 ) Entries/ E extra from remaining neutrals identifies signal GeV Signal: E extra < 300 MeV Background: 350 < E extra < 1000 MeV Leptonic B Decays - BaBar preliminary 11 D 0 π 0 eν M (Gev/c 2 ) E extra validation from double tags D 0 γeν

12 B + τ + ν τ analysis II results D*l ν tags hep-ex/? ICHEP paper Expectations and results for (124.1±1.4) 10 6 BB: Validations: Signal efficiency, E extra : Double tags Backgrounds: Control samples with extra tracks BG incl. systematics Observed 15.2± ± ± ± ± (Signal MC is for BF=10-3 ) Result: B(B + τ + ν τ ) < (90%CL) Leptonic B Decays - BaBar preliminary 12 Combined with hadronic tags: (independent, 88.9M BB) B(B + τ + ν τ ) < (90%CL) Also recall (PRL 92, , 04) BaBar B(B + µ + ν µ ) <

13 B + K + ν ν analysis overview Semileptonic tags D 0 mesons are reconstructed in D 0 K π +, K π + π + π, K π + π 0 Leptons are identified as e, µ Require -2.5 < cosθ B,Dl < 1.1 (accepts D* ( * )0 ); reject tags with extra π ± consistent w/d* + Hadronic tags D mesons and D 0 X combinations are reconstructed as stated earlier, selecting on m ES and E. Signal determination Require exactly one additional charged track, a well-identified kaon. Reject π 0 s, >6 neutrals. Compute E extra as above Count events: Signal: E extra < 250 MeV, p K * > 1.25 GeV/c Continuum and combinatoric B background determined from sidebands (of D 0 and B); Peaking B background determined from simuation for hadronic analysis, not subtracted in semileptonic Leptonic B Decays - BaBar preliminary 13 BaBar preliminary D 0 candidate mass (GeV/c 2 ) continuum background subtracted BaBar preliminary

14 B + K + ν ν analysis results Semileptonic: Observe 6 events with efficiency (0.115±0.009)% Non-peaking BG > 3.4±1.2 evt. (includes systematics) Limit: B(B + K + νν) < (90%CL) Hadronic Observe 3 events with efficiency (0.055±0.005)% Total background = 3.9±1.1 evt. (includes systematics) Limit: B(B + K + νν) < (90%CL) Combined limit: B(B + fi K + nn) < (90%CL) Also possible to set limit on B(B + π + νν) Change PID requirement; use only hadronic analysis: B(B + π + νν) < (90%CL) E extra p K Leptonic B Decays - BaBar preliminary 14 hadronic BaBar preliminary semileptonic Points: data; solid: background MC; dashed: signal MC (arbitrary scale) BaBar preliminary hep-ex/? to be submitted to PRL BaBar preliminary BaBar preliminary BB pairs used: (88.9±1.0) 10 6

15 B 0 invisible (ν ν), ν ν γ analysis overview Semileptonic tags only: B0 D ( * )+ l ν (D* + D 0 π + ) with D + K π + π + and D 0 K π +, K π + π + π, K π + π 0 Require -2.5 < cosθ B,D(*)l < 1.1 (accepts some higher D* ( * ) s) Result: tags, purity 66% Reconstruct nothing No additional charged tracks ν ν: N( γ-like clusters in EMC) < 3, N( K 0 L-like clusters) < 3 ν ν γ: Exactly 1 EMC cluster with E γ * > 1.2 GeV Again look at E extra Determine signals: Construct PDFs in E extra for signal, background Non-parametric KEYS PDFs used Perform unbinned ML fit to distribution in data Validations: Efficiency checked against double tags Analyses repeated in B + invisible control sample Leptonic B Decays - BaBar preliminary 15 ν ν ν ν γ E extra

16 B 0 invisible (ν ν), ν ν γ analysis results hep-ex/ accepted by PRL Results of ML fit for (88.5±1.0) 10 6 BB: ν ν: Signal: 17 ± 9, background ν ν γ: Signal: , background νν Upper limits: Systematics for ν ν ( ν ν γ ): Additive: 7.4 ( 4.3 ) events Multiplicative: 10.9% ( 11.1% ) Frequentist limit-setting procedure Systematics taken as Gaussian ννγ B(B 0 invisible) < (90%CL) B(B 0 ν ν γ ) < (90%CL)* Leptonic B Decays - BaBar preliminary 16 * Depends on constituent quark model for Dalitz plot shape (Lu & Zhang, Phys. Lett. B 381, 349 (1996))

17 B 0 l + l (e + e, µ + µ, e + µ ) analysis overview Very straightforward m ES, E reconstruction Extremely clean monochromatic kinematics Key features: Lepton identification, rejection of QED and qq backgrounds Strategy: define cuts for ID and purity, and sideband region, blind signal box in m ES, E Selection variables accept cos θ T : θ T is angle between thrust axes of l + l candidate and rest of event m ROE : invariant mass of rest of event R 2 : norm d 2 nd Fox-Wolfram moment N trk + ½N γ : measure of multiplicity accept accept accept Bremsstrahlung recovery applied to e s E EMC < 11 GeV, rejects QED Leptonic B Decays - BaBar preliminary 17 Signal MC (blue) vs. sideband data

18 B 0 l + l (e + e, µ + µ, e + µ ) results hep-ex/? to be submitted to PRL Unblinded data for (122.5±1.2) 10 6 BB: Signal regions Events observed e + e µ + µ e ± µ+ Systematics: Dominated by multiplicative uncertainties on the efficiency, primarily from tracking and µ ID: 5.7%, 7.1%, 6.8% for ee,µµ,eµ Limits: Using the Barlow method: B(B 0 e + e ) < (90%CL) B(B 0 µ + µ ) < (90%CL) B(B 0 e + µ ) < (90%CL) A substantial improvement on previous measurements! Leptonic B Decays - BaBar preliminary 18

19 Finding additional details B + fi m + n m PRL 92, (2004), hep-ex/ B + fi t + n t (88.9M BB) Submitted to PRL, hep-ex/ , BABAR-PUB-04/021 B + fi t + n t (124.1M BB, new semileptonic analysis with D*0 l ν tags) ICHEP04 conference paper, BABAR-CONF-04/026 B + fi K + n n [expected to be by then] submitted to PRL, hep-ex/?, BABAR-PUB-04/036 B 0 fi nn Accepted for publication in PRL, hep-ex/ , BABAR-PUB-04/014 B 0 fi l + l [expected to be by then] submitted to PRL, hep-ex/?, BABAR-PUB-04/032 See also D 0 l + l in hep-ex/?, BABAR-PUB-04/027, [to be] submitted to PRL Leptonic B Decays - BaBar preliminary 19

20 Summary We have set upper limits on branching fractions of several rare leptonic and/or FCNC decays of B mesons useful as checks of basic predictions for weak decays of heavy-quark mesons (e.g., f B ), and probes for new physics beyond the Standard Model These decays have not yet come into sight [insert HFAG-style graph of all results in this talk: including new BaBar numbers, PDG 04, and SM expectations] but some are now within less than an order of magnitude of the expected branching fractions in the SM We have 2-3 times the data available already (>240M BB) and being analyzed, with another factor of two to come in the next few years Some of these will very likely be observed one or two ICHEPs from now Leptonic B Decays - BaBar preliminary 20