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Measurements of Leptonic B Decays from BaBar

Measurements of Leptonic B Decays from BaBar. Gregory Dubois-Felsmann Caltech for the BaBar collaboration ICHEP 2004 Heavy Quark Mesons and Baryons session. Overview. Motivations for studying leptonic B decays SM and beyond-SM considerations B +  l + n l

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Measurements of Leptonic B Decays from BaBar

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  1. Measurements ofLeptonic B Decaysfrom BaBar Gregory Dubois-FelsmannCaltechfor the BaBar collaborationICHEP 2004Heavy Quark Mesons and Baryons session

  2. Overview • Motivations for studying leptonic B decays • SM and beyond-SM considerations B+l+nl B0l+l–, l+l'– , B0n n, n n g B+K+n n • Analyses • Decays to nX states: tag-recoil analyses • general methods • specific results • B0l+l– analysis • Conclusions and prospects All results are preliminary unless otherwise noted Leptonic B Decays - BaBar preliminary

  3. b l+ W+ W+ l+,n b B0 n l–,l’–,n u d W– Leptonic B decays to t+ n, l+l–, n n • Leptonic decays of heavy-quark mesons provide a laboratory • For testing straightfoward SM predictions: • For searching for non-SM effectsin highly suppressed processes. Some new-physics in loops (e.g., SUSY) can enhance these by orders of magnitude. Also LFV? Note helicitysuppression B+ Calculable in Lattice QCD In SM: B(B+t+nt ) ~ (9.3±3.9)×10-5(PDG’04 Vub, fB)(pred.)B(B+m+nm ) ~ (4.2±1.8)×10-7 l+,n b g,Z qu l–,l’–,n W– d l+,n b qu g,Z In SM: B(B0m+m–) ~ 8×10-11(pred.)B(B0nn) ~ zero W qu l–,l’–,n d (nnvia Z only) Leptonic B Decays - BaBar preliminary

  4. n n n l n B decays to K/p+n n • Motivation • The flavor-changing neutral current decays BK/p+nn occur in the Standard Model via one-loop radiative penguin and box diagrams: • SM expectation: B(B+K+nn) ~ (3.8 + 1.2) × 10-6, B(B+p+nn ) ~ 2.8 × 10-7 • 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. – 0.6 Leptonic B Decays - BaBar preliminary

  5. Tag-recoil analyses: B1 tag, B2t+n, K+nn, nn Neutrino analyses require extra kinematic and/or particle selection constraints. Use BB initial state to achieve this. • Explicitly identify 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 • Study the recoil for the decay of interest • Typically we require the recoil to have • Exact charged-particle content expectedfor signal • Number and total energy Eextra of neutrals observedless than an analysis-dependent threshold • Tagging efficiencies can be checked by“double-tagging” • All analyses apply anti-continuum shape cuts n•pORl + n•p Btag e- D(*) Brecoil e+ ν visiblecharged and neutralparticles Eextra = SEEMC neutrals Leptonic B Decays - BaBar preliminary

  6. 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 +Np final states, identified by inv. mass ±3s • D* mesons are identified by the mass difference Dm=mD*–mD • The semileptonic decay kinematics are enforcedby looking at the variable cos qB,D(*)l , defined as and requiring it to be in the physical region. Some of the analyses are more tolerant of missing/extra particles (higher D*(*) states), trading purity for statistics. Leptonic B Decays - BaBar preliminary

  7. Hadronic ( BD + X ) tags • An additional B–D(*)0 + X– hadronic tag sample is used for the K/p+nn and t+nt (I) analyses: • D mesons are reconstructed in D0K–p+, K–p+p+p–, K–p+p0, KS0p+p– • In the t+nt (I) analysis, D*0Dp0 candidates are formed as well. The X– system is composed of 1-5 p±, 0-2 K±, 0-2 p0, and 0-1 KS0. • In the K+nn analysis, only D0 are considered, andthe X– system is composed of 1-5 mesons, including p±, K±, and p0, with no more than two p0 used. • B mesons are identified with the kinematic variables DE=EB–Ebeam and the beam-energy substituted mass sideband signal t+nt (I) BaBarpreliminary K+n n BaBarpreliminary Leptonic B Decays - BaBar preliminary

  8. hep-ex/0407038submitted to PRL B+t+nt analysis I • Semileptonic tags: ML fit to Eextra • Consider t+e+nent and m+nmnt • Signal: 14.8 ± 6.3 eventsBackground: 115.2 ± 11.8 events • Significance as a signal: 2.3s (stat. only) • Limit (CLs method, incl. syst.): B(B+t+nt) < 6.7×10-4 (90%CL) • Hadronic tags: event counts • t+p+nt, p+p0nt, p+p–p+nt, e+nent, m+nmnt • Signal in kin./Eextra regions: 15 eventsBackground: 17.2 ± 2.1(stat) ± 1.3 (syst) events • Limit (L-ratio w/ null hypothesis): B(B+t+nt) < 4.2×10-4 (90%CL) • Combined limit: B(B+t+nt) < 4.2×10-4 (90%CL) BaBarpreliminary t branchingfraction eff’c’ygiven tag backgr’ndestimate Nobs BB pairs used: (88.9±1.0)×106 Leptonic B Decays - BaBar preliminary

  9. Conference paper:ICHEP: CONF-04/026 B+t+nt analysis II – D*l–n tags • Refinement of semileptonic part of analysis I • Tighten tag selection to require clean D* • Consider p+nt, p+p0nt, p+p–p+nt, e+nent, and m+nmnt • Mode-dependent kinematic selections • Determine signals by counting events in Eextra regions Using (124.1±1.4)×106BB pairs • Validations • Signal efficiency, Eextra: • Double tags • Backgrounds: • Control samples with extra tracks (Signal MC scaled to BF=10-3) Result: B(B+t+nt) < 4.3×10-4 (90%CL) Combined with hadronic tags: (independent, 88.9M BB) B(B+t+nt) < 3.3×10-4 (90%CL) Also recall (PRL 92, 221803, ’04)BaBarB(B+m+nm) < 6.6×10-6 Leptonic B Decays - BaBar preliminary

  10. hep-ex/0408086ICHEP: CONF-04/050 B+K/p+n n analysis Eextra hadronicsemileptonic • Uses tags of both types • Signal obtained from Eextra/pK region • Combined limit: B(B+K+nn) < 5.2×10-5 (90%CL) • Also possible to set limit on B+p+nn • Change PID requirement; use only hadronic analysis: B(B+p+nn) < 1.0 ×10-4 (90%CL) BaBarpreliminary BaBarpreliminary Points: data; solid: background MC; dashed: signal MC (arbitrary scale) pK BaBarpreliminary BaBarpreliminary BB pairs used: (88.9±1.0)×106 Leptonic B Decays - BaBar preliminary

  11. hep-ex/0405071accepted by PRL B0 invisible (n n ), n n g analysis • Semileptonic tags only: B0 D(*)+l–n (D*+  D0p+) • Require “nothing” in recoil: no charged tracks, limited neutrals • Signal obtained from ML fit to Eextra(KEYS shapes from MC): • n n : • Signal: 17 ± 9, background 19 + 10events • n n g: • Signal: –1.1 + 2.4, background 28 + 6 • Upper limits: • Systematics for n n ( n n g): • Additive: 7.4 ( 4.3 ) events • Multiplicative: 10.9% ( 11.1% ) • Frequentist limit-setting procedure • Systematics taken as Gaussian • B(B0 invisible) < 22 × 10-5 (90%CL)B(B0 n n g ) < 4.7 × 10-5 (90%CL)* BB pairs used: (88.5±1.0)×106 – 8 n n n n g – 5 – 1.9 * Depends on constituent quark model for Dalitz plot shape(Lu & Zhang, Phys. Lett. B 381, 349 (1996)) Leptonic B Decays - BaBar preliminary

  12. B0l+l– (e+e–,m+m–,e+m–) analysis overview • Very straightforward mES, DE reconstruction • Extremely clean monochromatic kinematics • Key features:Lepton identification, rejection of QED and qq backgrounds • Bremsstrahlung recovery applied to e’s • Strategy: define cuts for ID and purity, andsideband region, blind signal box in mES, DE • Selection variables • | cos qT |: qT is angle between thrust axes of l+l– candidate and rest of event • mROE: invariant mass of rest of event • R2: norm’d 2nd Fox-Wolfram moment • Ntrk + ½Ng: measure of multiplicity • EEMC < 11 GeV, rejects QED accept accept accept accept Signal MC (blue)vs. sideband data Leptonic B Decays - BaBar preliminary

  13. In prep. for PRL:ICHEP: PUB-04/032 B0l+l– (e+e–,m+m–,e+m–) results • Unblinded data for (122.5±1.2)×106BB: Signal regions Events observed e+e– m+m– • Systematics: • Dominated by multiplicative uncertainties on the efficiency,primarily from tracking and m ID:5.7%, 7.1%, 6.8% for e+e–, m+m–, e+m– • Limits: • Using the Barlow method: • B(B0 e+e–) < 6.1 × 10-8 (90%CL) • B(B0 m+m–) < 8.3 × 10-8 (90%CL) • B(B0 e+ m–) < 18 × 10-8 (90%CL) • A substantial improvement on previous measurements! e±m+ Leptonic B Decays - BaBar preliminary

  14. Finding additional details • B+ m+nm • Published in PRL 92, 221803 (2004); hep-ex/0401002 • B+ t+nt (88.9M BB) • Submitted to PRL: hep-ex/0407038, BABAR-PUB-04/021 • B+ t+nt (124.1M BB, new semileptonic analysis with D*0l–n tags) • ICHEP04 conference paper, abstract #11-0933, BABAR-CONF-04/026 • B+ K+ n n • ICHEP04 conference paper, abstract #11-0937, BABAR-CONF-04/050, hep-ex/0408086 • B0 nn,nng • Accepted for publication in PRL: hep-ex/0405071, BABAR-PUB-04/014; ICHEP abs. #11-0977 • B0 l+l– • About to be submitted to PRL; BABAR-PUB-04/032; ICHEP abstract #11-0956 • See also D0 l+l–: hep-ex/0408023, BABAR-PUB-04/027, submitted to PRL; ICHEP #11-0964 Leptonic B Decays - BaBar preliminary

  15. 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., fB), and • probes for new physics beyond the Standard Model • These decays have not yet come into sightbut some are now within an order of magnitude or less of the SM expectations… • 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… nn g nn (absent in SM) p+nn K+nn t+n m+n m+m– m+m–(e+e–, e+m– similar) 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 Leptonic B Decays - BaBar preliminary

  16. Additional slides

  17. The BaBar detector and dataset Asymmetric C.M.for Y(4S)  BB,bg ~ 0.56 Work presented today B+t+nt (D*ln tags) B0l+ l– B+t+nt,m+nm B+K+n n B0n n, n n g Leptonic B Decays - BaBar preliminary

  18. B+t+nt analysis I details • For semileptonic tags D0l–nlX, t+ decays e+nent and m+nmnt 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 qB,Dl < 1.1; admits excited D states with some unobserved particles • Signal determined from fit to shape of distribution of remaining neutral energy (Eextra) • Peaks near zero and at low values (~0.15 GeV) for signal, from X=(nil) and X=g, p0, respectively; rising at larger Eextra for principal backgrounds. • For hadronic tags, we consider p+nt, p+p0nt, p+p–p+nt, e+nent, and m+nmnt • Total charge 0. Positive PID is required for charged tracks. • Extra p0 or KS0 or high energy photons are rejected. • Characteristic kinematical and dynamical features of the t decay modes are required • Minimum missing momentum (calculable using the fully reconstructed tag) req’d per mode • Signal determined by counting events in the signal region for each t decay modemES > 5.27 GeV/c2, -0.09 < DE < 0.06 GeV • Background extrapolated from sideband 5.21 < mES < 5.26 GeV/c2, checked against simulation 168k tags Leptonic B Decays - BaBar preliminary

  19. B+t+ nt analysis II – D*0l–n tags – details • Refinement of semileptonic analysis, tightening tag selection • Require a D*0, in eitherof its decays (D0p0,D0g) • Tight kinematic cut rejects other DX:–1.1 < cos qB,D*l < 1.1 • Select best D, D* • Tag efficiency:(1.818±0.074)10-3 • Consider p+nt, p+p0nt, p+p–p+nt, e+nent, and m+nmnt • Require 1 or 3 charged tracks of good quality & matching PID& a good p0 (Eg > 50 MeV) for the p+p0nt mode (no p0 for p+nt) • Mode-dependent cuts on missing mass, energies of “t” daughters, n-p sub-resonance masses (r, a1) • Eextra from remaining neutrals identifies signal • Signal: Eextra < 300 MeV • Background: 350 < Eextra < 1000 MeV D0p0en D0gen DM (Gev/c2) Entries/0.067 GeV Eextra validationfrom double tags Leptonic B Decays - BaBar preliminary

  20. B+t+nt analysis II – event count breakdown • Expectations and results for (124.1±1.4)×106BB: Leptonic B Decays - BaBar preliminary

  21. B+K+n n analysis details • Semileptonic tags • D0 mesons are reconstructed in D0K–p+, K–p+p+p–, K–p+p0 • Leptons are identified as e, m • Require -2.5 < cosqB,Dl < 1.1 (accepts D*(*)0); reject tags with extra p± consistent w/D*+ • Hadronic tags • D mesons and D0X– combinations are reconstructed as stated earlier, selecting on mES and DE. • Signal determination • Require exactly one additional charged track, a well-identified kaon. Reject p0’s, >6 neutrals. • Compute Eextra as above • Count events: • Signal: Eextra < 250 MeV, pK* > 1.25 GeV/c • Continuum and combinatoric B background determinedfrom sidebands (of D0 and B); • “Peaking” B background determined from simuation forhadronic analysis, not subtracted in semileptonic BaBarpreliminary “D0” candidate mass (GeV/c2) continuum background subtracted BaBarpreliminary Leptonic B Decays - BaBar preliminary

  22. B0 invisible (n n ), n n g analysis overview • Semileptonic tags only: B0 D(*)+l–n (D*+  D0p+)with D+  K–p+p+ and D0K–p+, K–p+p+p–, K–p+p0 • Require -2.5 < cosqB,D(*)l < 1.1 (accepts some higher D*(*)’s) • Result: 126108 tags, purity 66% • Reconstruct “nothing” • No additional charged tracks • n n: N(“g-like” clusters in EMC) < 3, N(“K0L-like” clusters) < 3 • n n g: Exactly 1 EMC cluster with Eg* > 1.2 GeV • Again look at Eextra • Determine signals: • Construct PDFs in Eextra 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 n n n n g Eextra Leptonic B Decays - BaBar preliminary

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