We report recent results from the BABAR experiment on semileptonic charmless
B-meson decays and electroweak penguin processes. Semileptonic charmless decays
are used to determine |Vub| and the exclusive modes considered here also begin
to constrain QCD-lattice form factor calculations. Radiative penguin decays are
both sensitive to physics beyond the Standard Model and can be used to extract
Heavy Quark parameters related to the b-quark mass and its motion inside the
hadron.Comment: To be submitted to the proceedings of the LVth Rencontres de Moriond
  session devoted to "QCD and High Energy Hadronic Interactions", La
  Thuile(Italy), 12-19th March 200

Walsh, John

English

arXiv

UNT Digital Library

Work supported in part by Department of Energy contract DE-AC02-76SF00515SEMILEPTONIC AND ELECTROWEAK PENGUIN RESULTS FROM BABARJOHN WALSH a(on behalf of the BaBar Collaboration)INFN, Sezione di Pisa, Largo Pontecorvo 3,56127 Pisa, ItaliaWe report recent results from the BaBar experiment on semileptonic charmless B-meson de-cays and electroweak penguin processes. Semileptonic charmless decays are used to determine|Vub| and the exclusive modes considered here also begin to constrain QCD-lattice form factorcalculations. Radiative penguin decays are both sensitive to physics beyond the StandardModel and can be used to extract Heavy Quark parameters related to the b-quark mass andits motion inside the hadron.1 IntroductionAn important goal of the study of semileptonic charmless B-meson decays is the measurementof |Vub|, which essentially measures one side of the Unitarity Triangle of the CKM matrix. Bothinclusive and exclusive analyses have been used to measure |Vub|, here we report on recentresults using exclusive decays. The measurement of exclusive branching fractions is also usefulfor distinguishing among theoretical calculations of the form factors, as we shall see.The the radiative decay B → Xsγ is studied as a probe of New Physics. Since this decayoccurs at the one-loop level, the branching fraction is sensitive to models with additional heavyparticles that can participate in the loop. In contrast, the shape of the photon energy spectrumis quite insensitive to contributions from New Physics, but it is rather sensitive to two importantparameters of Heavy Quark (HQ) theory: the b-quark mass mb and the quantity µ2pi, which isrelated to the Fermi motion of the b-quark inside the hadron.In this report, we present recent results from the BaBar experiment1 on semileptonic charm-less B-meson decays to exclusive states and on the B → Xsγ process. Beyond providing informa-tion on the CKM matrix and probing the possibility of New Physics, these analyses also provideinsight into B-meson decay dynamics and QCD. All results presented herein are preliminary.2 Exclusive semileptonic charmless B decaysSemileptonic charmless B-meson decays to exclusive final states can be used to measure |Vub| byexploiting the dependence of the branching fraction on the CKM matrix element. In the case ofB0 → π−ℓ+ν, we have:dΓ(B0 → π−ℓ+ν)dq2=G2F24π3|Vub|2p3pi|f+(q2)|2 (1)ajohn.walsh@pi.infn.itSLAC-PUB-11347SLAC, Stanford Univesity, Stanford, CA  94309hep-ex/0506043Figure 1: Projected mES (top) and ∆E (bottom) distributions in five bins of q2 for the B → πℓν modes. The datais represented by points with (statistical) error bars, while the simulation is shown as a histogram. The simulatedcomponents are: signal (white), combinatoric signal (dashed), crossfeed (hatched), b → cℓν (light shaded) andcontinuum (dark shaded).Here GF is the Fermi coupling constant, ppi is the pion momentum in the center-of-mass frame, qis the invariant mass of the lepton-neutrino pair and f+(q2) is the form factor, which is calculatedtheoretically. The goal is to measure the branching fraction in bins of q2, which allows one todistinguish among form factor calculations as well as extract the value of |Vub|.Experimentally, the branching fractions of exclusive b→ u decays are small and backgroundsfrom b → c transitions are substantial. BaBar has used two different methods for overcomingthe experimental difficulties: 1) an “untagged” analysis, based on 83 million BB¯ pairs, wherea premium is placed on high quality neutrino reconstruction using the missing momentum inthe event; and 2) a “tagged” analysis (232 million BB¯ pairs for the B0 → π−ℓ+ν state, 88million BB¯ pairs for B+ → π0ℓ+ν state), where backgrounds are reduced by requiring the otherB-meson in the event be “tagged” via a D(∗)ℓν decay.The untagged analysis relies on good neutrino reconstruction to perform its measurementof the branching fractions of B → πℓν and B → ρℓν. The neutrino momentum is inferredfrom the event missing momentum and strict requirements are placed to ensure good neutrinoreconstruction. For example, the event missing mass is required to be compatible with zero:since its resolution broadens linearly with missing energy, we require |m2miss/2Emiss| < 0.4GeV. The variables used to distinguish signal from background are mES =√s/4− |~p ∗B |2 and∆E = E∗B −√s/2, where√s is the total energy in the Υ(4S) center-of-mass frame. Figure 1shows the distribution of these two variables for the B → πℓν modes in five bins of q2. Branchingfraction measurements from both the tagged and untagged analyses are reported in Table 1.The statistics of the untagged sample permits the study of the q2 dependence of the branchingfraction and an investigation of several form factor calculations. Figure 2 shows the differentialdecay rates along with the predictions of four theoretical calculations: LCSR1 2, LQCD1 3,LQCD24 and ISGW II5. The χ2 probabilities are good (∼ 50%) for the first three calculations,while it is marginal (3%) for the ISGW II prediction. We extract the value of |Vub| usingthe B → πℓν data and the LQCD2 calculation over the full q2 range 0 − 25 GeV2. TheBK parametrization 6 is used to extrapolate the LQCD2 form factor calculation to low q2. Weobtain |Vub| = (3.82±0.14±0.24±0.11+0.88−0.52)×10−3, where the uncertainties are due to statistics,systematics, form factor shape and form factor normalization, respectively.Table 1: Branching fractions to charmless semileptonic ex-clusive states from the tagged (labeled “T”) and untagged(“U”) analyses. The uncertainties shown are statistical, sys-tematic and (for the untagged analysis) due to form factoruncertainties. The untagged analysis assumes the isposinrelations: B(B0 → π−(ρ−)ℓ+ν) = 2B(B+ → π0(ρ0)ℓ+ν)Mode (Technique) Branching Fraction (10−4)B0 → π−ℓ+ν (T) 1.03± 0.25 ± 0.13B+ → π0ℓ+ν (T) 1.80± 0.37 ± 0.23B0 → π−ℓ+ν (U) 1.38 ± 0.10 ± 0.18± 0.08B0 → ρ−ℓ+ν (U) 2.14 ± 0.21 ± 0.53± 0.28Figure 2: Comparison of the differential decay ratefrom the untagged analysis for B → πℓν (left) andB → ρℓν (right) together with several theoreticalform factor calculations. (GeV)γReconstructed E*2 2.2 2.4 2.6 2.8 / 100 MeV)-4Partial Branching Fraction (1000.20.40.60.8BABARpreliminary (GeV)γE1.9 2 2.1 2.2 2.3 2.4 2.5 2.6Branching Fraction / 100 MeV-0.0500.050.10.150.2-310×Figure 3: The photon energy spectrum, background-subtracted and efficiency-corrected, for the fully inclusiveanalysis (left) and the semi-inclusive analysis (right). The fully inclusive spectrum is measured in the Υ(4S) restframe, while the semi-inclusive spectrum is measured in the B-meson rest frame. The peak due to B → K∗γdecays at high photon energy of the semi-inclusive spectrum is visible due to the good resolution (see text).3 B → XsγBaBar has performed two analyses of the B → Xsγ channel: a fully inclusive measurement,where no requirements are made on the hadronic state (Xs) and a semi-inclusive analysis, whichaims to reconstruct a large part of the total B → Xsγ rate by summing many exclusively recon-structed modes. The two approaches are complementary: the fully inclusive method requiresa lepton tag to reduce continuum background, but nevertheless suffers from significant back-grounds from BB¯ events. The semi-inclusive analysis, which sums 38 exclusive decay modes,has the advantage of reduced backgrounds due to the kinematic handles provided by fully re-constructed B candidates. This analysis however, has a significant systematic uncertainty dueto the missing fraction, the part of the B → Xsγ rate that it does not reconstruct. Both ofthese analysis are based on approximately 89 million BB¯ pairs.Figure 3 shows the resulting photon energy spectra for the two analyses. The semi-inclusiveanalysis has better photon energy resolution for two reasons: 1) the energy is measured in theB-meson rest frame and 2) the photon energy is actually inferred from the hadronic invariantmass, which has quite good resolution. We present in Table 3 the energy moments of the photonspectrum calculated above a certain energy threshold, measured in the B-meson rest frame. Acorrection is applied to the fully-inclusive values to bring them into this frame. These momentsmay be directly compared to theoretical calculations to give information on HQ parameters. ATable 2: Photon energy spectrum moments and partial branching fractions (PBF) from B → Xsγ. The first fourrows are from the fully inclusive analysis, the remaining rows are from the semi-inclusive analysis. Values aregiven for the B-meson rest frame, including the minimum photon energy. The uncertainties shown are statistical,systematic and model-dependent (for the fully inclusive analysis), respectively.Min Eγ PBF (10−4) 1st Moment (GeV) 2nd Moment (GeV2) 3rd Moment (GeV3)1.9 3.67 ± 0.29 ± 0.34± 0.29 2.288 ± 0.025 ± 0.017 ± 0.012 - -2.0 3.41 ± 0.27 ± 0.29± 0.23 2.316 ± 0.016 ± 0.010 ± 0.012 - -2.1 2.97 ± 0.24 ± 0.25± 0.17 2.355 ± 0.014 ± 0.007 ± 0.010 - -2.2 2.42 ± 0.21 ± 0.20± 0.13 2.407 ± 0.012 ± 0.005 ± 0.008 - -1.897 - 2.321 ± 0.044+0.037−0.026 0.0253 ± 0.0116+0.0049−0.0042 0.0006 ± 0.0085+0.0041−0.00321.999 - 2.314 ± 0.025+0.026−0.027 0.0273 ± 0.0039+0.0042−0.0037 0.0009 ± 0.0036+0.0036−0.00222.094 - 2.357 ± 0.018+0.014−0.016 0.0183 ± 0.0023+0.0021−0.0012 0.0005 ± 0.0017+0.0016−0.00092.181 - 2.396 ± 0.013+0.008−0.004 0.0115 ± 0.0014+0.0010−0.0006 0.0001 ± 0.0008+0.0006−0.00032.261 - 2.425 ± 0.009+0.004−0.002 0.0075 ± 0.0007+0.0003−0.0002 −0.0001 ± 0.0003+0.0002−0.0001fit to the semi-inclusive spectrum was performed to extract the HQ parameters mb and µ2pi. Twotheoretical schemes were used to perform the fits: the kinetic scheme 7, which gives:mb = 4.69+0.05−0.04 GeV and µ2pi = 0.30+0.07−0.05 GeV2; (2)and the shape function scheme 8, which yields:mb = 4.65± 0.04 GeV and µ2pi = 0.19+0.06−0.05 GeV2; (3)where the errors are the sum of statistical and systematic, but do not include theoretical un-certainties. We note that the parameter µ2pi is not defined the same way in the two schemes.The spectrum fit also yields the total inclusive branching fraction down to Eγ > 1.6 GeV.Averaging the results from the two theoretical schemes gives: B(b → sγ,Eγ > 1.6 GeV) =(3.38±0.19+0.64+0.07−0.41−0.08)×10−4. We note that the branching fraction result is compatible with theStandard Model calculation 9 and with the experimental world average 10.AcknowledgmentsA heartfelt thanks the organizers for a stimulating and enjoyable conference. Thanks also go tomy BaBar colleagues for their assistance in preparing these results.References1. BaBar Collaboration, B. Aubert et al., Nucl. Instrum. Methods A 479, 1 (2002).2. P. Ball, R. Zwicky, Phys. Rev. D 71, 014015 (2005).3. J. Shigemitsu et al. hep-lat/0408019.4. M. Okamoto et al., hep-lat/0409116; see also proceedings from this Conference.5. D. Scora, N. Isgur, Phys. Rev. D 52, 2783 (1999).6. D. Becirevic, A.B. Kaidalov, Phys. Lett. B 478, 417 (2000).7. D. Benson, I.I. Bigi, N. Uraltsev, hep-ph/0410080.8. M. Neubert, hep-ph/0412241.9. P. Gambino, M. Misiak, Nucl. Phys. B 611, 338 (2001); A.J. Buras, A. Czarnecki, M.Misiak, J. Urban, Nucl. Phys. B 631, 219 (2002).10. Heavy Flavor Averaging Group (HFAG), hep-ex/0412073.

Semileptonic and Electroweak Penguin Results from BABAR

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Semileptonic and Electroweak Penguin Results from BABAR

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