David Lopes Pegna
We present recent results on exclusive Bbar -> Xc l- nubar_l decays and measurements of the CKM matrix element |Vcb| based on data collected at the Upsilon(4S) resonance with the babar detector at the PEP-II e+e- storage rings.
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We present a measurement of the branching fraction for the semileptonic B decay Bbar -> D+l-nubar, where l- can be either an electron or a muon. We find Gamma(Bbar -> D+l-nubar)= (13.79+/-0.76+/-2.51) ns^{-1}, and the resulting branching fraction Br(Bbar -> D+l-nubar)= (2.13 +/- 0.12 +/- 0.39)%, where the first error is statistical and the second systematic. We also investigate the Bbar -> D+l-nubar form factor and the implications of the result for |Vcb|. From a fit to the differential decay distribution we obtain the rate normalization |Vcb|F_D(1)= (4.11+/-0.44+/-0.52)x10^{-2}. Using a theor…
We present a measurement of the Cabibbo-Kobayashi-Maskawa matrix element |Vcb| based on a sample of about 53,700 B0bar --> D*+ l- nubar_l decays observed by the BaBar detector. We obtain the branching fraction averaged over l = e, mu, BR(B0bar --> D*+ l- nubar_l) = (4.90 +- 0.07(stat.) +0.36-0.35(syst.))%. We measure the differential decay rate as a function of w, the relativistic boost gamma of the D*+ in the B0bar rest frame. By extrapolating dGamma/dw to the kinematic limit w --> 1, we extract the product of |Vcb| and the axial form factor A_1(w=1). We combine this measurement with a lattic…
We present the measurements of the CKM matrix parameters Vub, Vcb and of the B mixing oscillation frequency with the BaBar experiment at the asymmetric B-factory PEPII. Data were collected in the years 2000-2002 and the total available statistics corresponds to 91 fb-1. The Vub, Vcb measurements utilize both inclusive and exclusive semileptonic decays of the $B$ meson. The Delta(md) parameter is measured by using the time evolution of the B, determined from the flight length difference between the two B mesons.
We present the recent BaBar searches for the leptonic decay B+ --> tau+ nu and for the rare decay B --> K(*) nu nubar, using a data sample of about 470 x 10^6 BB pairs. Both searches use a technique in which one B meson is hadronically reconstructed before searching for evidence of the signal decay in the rest of the event. BaBar measures B(B+ --> tau+ nu) = (1.83 ^{+0.53}_{-0.49} +/- 0.24) x 10^{-4}, and preliminary upper limits at 90% confidence level of B(B --> K nu nubar) < 3.2 x 10^{-5} and B(B --> K* nu nubar) < 7.9 x 10^{-5}. We also discuss the expected observations and precision impro…
BaBar measurements involving semileptonic decays of B mesons are reviewed. Attention is focused on the extraction of |Vub| and |Vcb| elements of the Cabibbo-Kobayashi-Maskawa quark mixing matrix. Recent results of inclusive and exclusive approaches are presented.
We present measurements of the semileptonic decays B- --> D0 tau- nubar, B- --> D*0 tau- nubar, B0bar --> D+ tau- nubar, and B0bar --> D*+ tau- nubar, which are potentially sensitive to non--Standard Model amplitudes. The data sample comprises 232x10^6 Upsilon(4S) --> BBbar decays collected with the BaBar detector. From a combined fit to B- and B0bar channels, we obtain the branching fractions B(B --> D tau- nubar) = (0.86 +/- 0.24 +/- 0.11 +/- 0.06)% and B(B --> D* tau- nubar) = (1.62 +/- 0.31 +/- 0.10 +/- 0.05)% (normalized for the B0bar), where the uncertainties are statistical, systematic,…
A review of recent analyses on semileptonic decays of B mesons into charmed final states is given. |Vcb| is extracted both by the Babar and the Belle collaboration from their datasets using inclusively and exclusively reconstructed final states. In addition there are recent results on the determination of exclusive branching fractions to the ground states D and D*, as well as to excited D** states. Those play an important role in understanding the composition of the total decay width. They represent also a sizable fraction of the backgrounds for exclusive analyses and are presented here as wel…
We present preliminary measurements of branching fractions for the semileptonic decays B --> D tau nubar and B --> D* tau nubar, which are potentially sensitive to non--Standard Model amplitudes. The data sample comprises 232x10^6 Upsilon(4S) --> BBbar decays collected with the BaBar detector at the PEP-II e+e- storage ring. We obtain B(B- --> D0 tau nubar) = (0.63 +/- 0.38 +/- 0.10 +/- 0.06)%, B(B- --> D*0 tau nubar) = (2.35 +/- 0.49 +/- 0.22 +/- 0.18)%, B(B0bar --> D+ tau nubar) = (1.03 +/- 0.35 +/- 0.14 +/- 0.10)%, and B(B0bar --> D*+ tau nubar) = (1.15 +/- 0.33 +/- 0.04 +/- 0.04)%, where t…
We determine the inclusive B --> Xc l nu branching fraction, the CKM matrix element |Vcb|, and other heavy-quark parameters from a simultaneous fit to moments of the hadronic-mass and lepton-energy distributions in semileptonic B-meson decays, measured as a function of the lower limit on the lepton energy, using data recorded with the BABAR detector. Using Heavy Quark Expansions (HQEs) to order 1/mb^3, we extract BR_cenu=(10.61 +- 0.16(exp) +- 0.06(HQE))% and |Vcb| = (41.4 +- 0.4(exp) +- 0.4(HQE) +- 0.6(th)) 10^-3. The stated errors refer to the experimental, HQE, and additional theoretical un…
We present some recent BABAR measurements of the magnitudes of the elements Vub and Vcb of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, and of the angles alpha and gamma of the unitary triangle of the standard model of the electroweak interactions. Most of the measurements presented here are based on the full BABAR Y(4S) dataset, consisting of about 467 million B anti-B pairs.
We report a measurement of the branching fractions for Bbar -> D(*)(pi)l-nubar decays based on 341.1 fb-1 of data collected at the Upsilon(4S) resonance with the babar detector at the PEP-II e+e- storage rings. Events are tagged by fully reconstructing one of the B mesons in a hadronic decay mode. We obtain B(B- -> D0l-nubar) = (2.33 +/- 0.09(stat.) +/- 0.09(syst.))%, B(B- -> D*0l-nubar) = (5.83 +/- 0.15(stat.) +/- 0.30(syst.))%, B(B0bar -> D+l-nubar) = (2.21 +/- 0.11(stat.) +/- 0.12(syst.))%, B(B0bar -> D*+l-nubar) = (5.49 +/- 0.16(stat.) +/- 0.25(syst.))%, B(B- -> D+pi-l-nubar) = (0.42 +/- 0…
We report a preliminary branching fraction of (1.80 +/- 0.37(stat.) +/- 0.23(syst.)) x 10^-4 for the charmless exclusive semileptonic B+ -> pi0 l+ nu decay, where l can be either a muon or an electron. This result is based on data corresponding to an integrated luminosity of 81 fb^-1 collected at the Upsilon(4S) resonance with the BABAR detector. The analysis uses BBbar events that are tagged by a B meson reconstructed in the semileptonic B- -> D0 l- nubar (X) decays, where X can be either a gamma or a pi0 from a D* decay.
We present a search for the rare decays B+ -> K+ nu nubar and B0 -> K0 nu nubar using 459 million BBbar pairs collected with the BaBar detector at the SLAC National Accelerator Laboratory. Flavor-changing neutral-current decays such as these are forbidden at tree level but can occur through one-loop diagrams in the Standard Model (SM), with possible contributions from new physics at the same order. The presence of two neutrinos in the final state makes identification of signal events challenging, so reconstruction in the semileptonic decay channels B -> D(*) l nu of the B meson recoiling from …
We report a measurement of the branching fractions of Bbar -> D** l^- nubar_l decays based on 417 fb-1 of data collected at the Upsilon(4S) resonance with the BaBar detector at the PEP-II e+e- storage rings. Events are selected by fully reconstructing one of the B mesons in a hadronic decay mode. A fit to the invariant mass differences m(D(*)pi)-m(D(*)) is performed to extract the signal yields of the different D** states. We observe the Bbar -> D** l^- nubar_l decay modes corresponding to the four D** states predicted by Heavy Quark Symmetry with a significance greater than six standard devia…
We search for the flavor-changing neutral-current decays B->K(*)nu nubar, and the invisible decays J/psi->nu nubar and psi(2S)->nu nubar via B->K(*)J/psi and B->K(*)psi(2S) respectively, using a data sample of 471 x10^6 BB pairs collected by the BaBar experiment. We fully reconstruct the hadronic decay of one of the B mesons in the Upsilon(4S)->BB decay, and search for the B->K(*)nu nubar decay in the rest of the event. We observe no significant excess of signal decays over background and report branching fraction upper limits of BR(B+->K+nu nubar)<3.7 x10^-5, BR(B0->K0nu nubar)< 8.1 x10^-5, B…
We present a measurement of the Cabibbo-Kobayashi-Maskawa matrix element |V_cb| and the form-factor slope rho^2 in Bbar -> Dl- nubar decays based on 460 million BBar events recorded at the Upsilon(4S) resonance with the BaBar detector. BBar -> Dl- nubar decays are selected in events in which a hadronic decay of the second B meson is fully reconstructed. We measure the differential decay rate and determine G(1) |V_cb|= (43.0 \pm 1.9 \pm 1.4)\times 10^{-3} and rho^2 = 1.20 \pm 0.09 \pm 0.04, where G(1) is the the hadronic form factor at the point of zero recoil. We also determine the exclusive b…
We establish upper limits on branching fractions for B0 decays to final states where the decay products are purely invisible (i.e., no observable final state particles) and for B0 decays to nu nubar gamma. Within the Standard Model, these decays have branching fractions that are below current experimental sensitivity, but various models of physics beyond the Standard Model predict significant contributions from these channels. Using 88.5 million B Bbar pairs collected at the Y(4S) resonance by the BaBar experiment at the PEP-II e+e- storage ring at the Stanford Linear Accelerator Center, we es…
We describe in detail a previously published measurement of CP violation induced by B0-anti B0 oscillations, based on an integrated luminosity of 425.7 fb-1 collected by the BABAR experiment at the PEPII collider. We apply a novel technique to a sample of about 6 million B0bar -> D*+ l- nubar decays selected with partial reconstruction of the D*+ meson. The charged lepton identifies the flavor of one B meson at its decay time, the flavor of the other B is determined by kaon tagging. We determine a CP violating asymmetry ACP = (0.0006 +- 0.0017 + 0.0038 - 0.0032). This measurement is consistent…
We present measurements of the semileptonic decays B- --> D0 tau- nubar_tau, B- --> D*0 tau- nubar_tau, B0bar --> D+ tau- nubar_tau, and B0bar --> D*+ tau- nubar_tau, which are sensitive to non--Standard Model amplitudes in certain scenarios. The data sample consists of 232x10^6 Upsilon(4S) --> BBbar decays collected with the BaBar detector at the PEP-II e+e- collider. We select events with a D or D* meson and a light lepton (ell=e or mu) recoiling against a fully reconstructed B meson. We perform a fit to the joint distribution of lepton momentum and missing mass squared to distinguish signal…
Using 226 million BBbar events recorded on the Upsilon(4S) resonance with the BABAR detector at the SLAC e+e- PEPII storage rings, we reconstruct B- -> D*0 e- nubar decays using the decay chain D*0 -> D0 pi0 and D0 -> K pi. From the dependence of their differential rate on w, the dot product of the four-velocities of B and D*0, and using the form factor description by Caprini et al. with the parameters F(1) and rho_A1^2, we obtain the results rho_A1^2 = 1.16 +- 0.06 +- 0.08, F(1)|V_cb| = (35.9 +- 0.6 +- 1.4) 10^-3, and BF(B- -> D*0 e- nubar) = (5.56 +- 0.08 +- 0.41)%.
We present a search for the decay $B^{-} \to τ^{-} \barν_τ$ in a sample of $88.9 \times 10^{6}$ $B\bar{B}$ pairs recorded with the BaBar detector at the SLAC $B$-Factory. One of the two $B$ mesons from the $Υ(4S)$ is reconstructed in a hadronic or a semileptonic final state and the decay products of the other $B$ in the event are analyzed for consistency with a $B^{-} \to τ^{-} \barν_τ$ decay. We find no evidence of a signal and set an upper limit on the branching fraction of $\mathcal{B}(B^{-} \to τ^{-} \barν_τ) < 4.2 \times 10^{-4}$ at the 90% confidence level.
A preliminary measurement of |Vcb| and the branching fraction BR(B0 -> D* l nu_l) has been performed based on a sample of about 55,700 B0 -> D* l nu_l decays recorded with the BaBar detector. The decays are identified in the D* -> D0 pi final state, with the D0 reconstructed in three different decay modes. The differential decay rate is measured as a function of the relativistic boost of the D* in the B0 rest frame. The value of the differential decay rate at `zero recoil', namely the point at which the D* is at rest in the B0 frame, is predicted in Heavy Quark Effective Theory as a kinematic …
We present a search for the rare flavor-changing neutral-current decay $B^+ \to K^+ ν\barν$ based on data sample of 82 fb$^{-1}$ collected with the BABAR detector at the PEP-II B-factory. Signal events are selected by examining the properties of the system recoiling against either a reconstructed hadronic or semileptonic charged B decay. Using these two independent samples, a combined limit of ${\mathcal B}(B^+ \to K^+ ν\barν)<5.2 \times 10^{-5}$ is obtained at the 90% confidence level. In addition, by modifying the particle identification criteria, a limit of ${\mathcal B} (B^+\toπ^+ν\barν)<1…
We present a measurement of the CKM matrix element |V_cb| and the form-factor slope rho^2 for Bbar -> D l^- nubar_l decays based on 417 fb-1 of data collected at the Upsilon(4S) resonance with the BaBar detector. The semileptonic decays are selected in BBar events in which the hadronic decay of the second B meson is fully reconstructed. From the measured differential decay rate of the signal decay we determine G(1) |V_cb|= (43.0 +/- 1.9 +/- 1.4) x 10^-3, rho^2 = 1.20 +/- 0.09 +/- 0.04, where G(1) is the hadronic form factor at the point of zero recoil. Using a lattice calculation for G(1) we e…
We present results for the moments of observed spectra in inclusive semileptonic B-meson decays to charm hadrons Bbar --> X_c l- nubar. Moments of the hadronic-mass and the combined mass-and-energy spectra for different minimum electron or muon momenta between 0.8 and 1.9 GeV/c are obtained from a sample of 232x10^{6} Y(4S) --> B Bbar events, collected with the BABAR detector at the PEP-II asymmetric-energy B-meson factory at SLAC. We also present a re-evaluation of the moments of electron-energy spectra and partial decay fractions BR(Bbar --> X_c e- nubar for minimum electron momenta between …
We report preliminary results from a study of the charmless exclusive semileptonic decay B0 -> pi- l+ nu based on the data collected at the Upsilon(4S) resonance using the BABAR detector at SLAC. The analysis uses events in which the signal B meson recoils against a B meson that has been reconstructed in a semileptonic decay B0bar -> D(*)+ l- nubar. We extract the total branching fraction BF(B0 -> pi- l+ nu) = (1.03 +/- 0.25(stat.) +/- 0.13(syst.)) x 10^-4 and the partial branching fractions in three bins of q^2, the invariant mass squared of the lepton-neutrino system. From the partial branch…
Using 226 million $B\bar{B}$ events recorded on the $Υ(4S)$ resonance with the BABAR detector at the SLAC $e^+e^-$ storage rings PEPII, we reconstruct $B^-\to D^{*0} e^-\barν_e$ decays using the decay chain $D^{*0}\to{}D^0{}π^0$ and $D^{0}\to{}K^-π^+$. From the dependence of their differential rate on $w$, the product of the four-velocities of $B^-${} and $D^{*0}$, and using the description of the form factor $F(w)$ by Caprini et al., we obtain the preliminary results $ρ^2_{A_1} = 1.15\pm 0.06 \pm 0.08$, $F(1)\cdot|V_{cb}| = (36.3\pm 0.6 \pm 1.4)\cdot 10^{-3}$, and ${\cal B}(B^-\to D^{*0} e^-\…
We report updates on two analyses of inclusive semileptonic B decays based on a dataset of 89 million BB events recorded with the BABAR detector at the Y(4S) resonance. Events are selected by fully reconstructing the decay of one B meson and identifying a charged lepton from the decay of the other B meson. In the first analysis, the measurement of the first and second moment of the hadronic mass distribution in Cabibbo-favored B ->X_c l nu decays allows for the determination of the nonperturbative parameters $\barΛ$ and $λ_1$ of Heavy Quark Effective Theory (HQET) and |Vcb|. In the second anal…
We present results on semileptonic B decays obtained with the BABAR detector. The large data set accumulated at the PEP-II asymmetric-energy B-Factory allows a new measurement technique, where the hadronic decay of one B meson is fully reconstructed and the semileptonic decay of the recoiling B meson is studied. Traditional analysis techniques of inclusive and exclusive B decays complement this approach with very high statistics data samples. These measurements play an important role in the tests of the description of CP violation in the Standard Model: The determinations of the Cabibbo-Kobaya…
Several hundred million tau decays have been studied with the BaBar detector at the PEP-II e+e- collider at the SLAC National Accelerator Laboratory. Recent results on Charged Current Lepton Universality and two independent measurements of |V_{us}| using tau- -> e- nubar nu, mu- nubar nu, pi- nu, K- nu and KS0 pi- nu decays, and a search for Second Class Current in tau- -> pi- omega nu decays are presented, where the charge conjugate decay modes are also implied.
We present a combined measurement of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|$ and of the parameters $ρ^2$, $R_1$, and $R_2$, which fully characterize the form factors of the $B^0 \to D^{*-}\ell^{+}ν_\ell$ decay in the framework of HQET, based on a sample of about 52,800 $B^0 \to D^{*-}\ell^{+}ν_\ell$ decays recorded by the BABAR detector. The kinematical information of the fully reconstructed decay is used to extract the following values for the parameters (where the first errors are statistical and the second systematic): $ρ^2 = 1.156 \pm 0.094 \pm 0.028$, $R_1 = 1.329 \pm 0.13…
We present partial branching fractions for inclusive charmless semileptonic B decays Bbar --> X_u ell nubar, and the determination of the CKM matrix element |V_{ub}|. The analysis is based on a sample of 383 million Y(4S) decays into B Bbar pairs collected with the BaBar detector at the PEP-II e+ e- storage rings. We select events using either the invariant mass M_X of the hadronic system, the invariant mass squared, q^2, of the lepton and neutrino pair, the kinematic variable P_+ or one of their combinations. We then determine partial branching fractions in limited regions of phase space: Del…
We present a search for the decay $B^{+} \to τ^{+} ν_τ$ using 288 fb-1 of data collected at the $Υ(4S)$ resonance with the BaBar detector at the SLAC PEP-II $B$-Factory. A sample of events with one reconstructed semileptonic B decay is selected, and in the recoil a search for $B^{+} \to τ^{+} ν_τ$ signal is performed. The $τ$ is identified in the following channels: {τ^+ \to e^+ ν_e \nutb}, {τ^+ \to μ^+ ν_μ \nutb}, {τ^+ \to π^+ \nutb} and {τ^+ \to π^+ π^0 \nutb}. We measure a branching fraction of $\mathcal{B}(B^{+} \to τ^{+} ν_τ)=(0.88^{+0.68}_{-0.67}({stat.}) \pm 0.11 ({syst.})) \times 10^{-…
We report measurements of partial branching fractions for inclusive charmless semileptonic B decays Bbar --> Xu l nubar, and the determination of the CKM matrix element |V_{ub}|. The analysis is based on a sample of 467 million Upsilon(4S) --> BBar decays recorded with the BaBar detector at the PEP-II e^+ e^- storage rings. We select events in which the decay of one of the B mesons is fully reconstructed and an electron or a muon signals the semileptonic decay of the other B meson. We measure partial branching fractions DeltaB in several restricted regions of phase space and determine the CKM …
We present a search for the decay B+ -> tau+ nu using 383x10^6 BBbar pairs collected at the Y(4S) resonance with the BABAR detector at the SLAC PEP-II B-Factory. A sample of events with one reconstructed semileptonic B decay (B- -> D0 l nu X)) is selected, and in the recoil a search for B+ -> tau+ nu is performed. The tau is identified in the following channels: tau+ -> e nu nubar, tau+ -> mu nu nubar, tau+ -> pi+ nu, and tau+ -> pi+ pi0 nu. We measure a branching fraction of B(B+ -> tau+ nu)=(0.9 +- 0.6(stat.) +- 0.1(syst.)) x 10^-4. In the absence of a significant signal, we calculate an upp…
We present the first evidence for B semileptonic decays into the charmed baryon Lambda_c+ based on 420 fb-1 of data collected at the Upsilon(4S) resonance with the BABAR detector at the PEP-II e+e- storage rings. Events are tagged by fully reconstructing one of the B mesons in a hadronic decay mode. We measure the relative branching fraction BF(Bbar -> Lambda_c+ X l- nubar_l)/BF(Bbar -> Lambda_c+/Lambda_c- X) = (3.2 +/- 0.9_stat +/- 0.9_syst)%. The significance of the signal including the systematic uncertainty is 4.9 standard deviations.
We present a search for semileptonic B decays to the charmed baryon Lambda_c+ based on 420 fb-1 of data collected at the Upsilon(4S) resonance with the BABAR detector at the PEP-II $e^+e^-$ storage rings. By fully reconstructing the recoiling B in a hadronic decay mode, we reduce non-B backgrounds and determine the flavor of the signal B. We statistically correct the flavor for the effect of the B0 mixing. We obtain a 90% confidence level upper limit of BF(Bbar --> Lambda_c+ X l- nubar)/BF(Bbar --> Lambda_c+ X) < 3.5 %.
The interplay of experiment and theory is explored in the context of current data on b and c decay. Measurements of |Vcb| and |Vub| are extracted from existing data. Conservative estimates give |Vcb|=(42.4 +/- 1.2{exp} +/- 2.3{thy})x10^{-3} and |Vub|=(3.90 +/- 0.16{exp} +/- 0.53{thy})x10^{-3}. Using these values along with data on Bd, Bs mixing and CP violation in the K_{Long} system, the allowed region of the CKM parameters rho and eta is derived. Tests of factorization in two-body hadronic B decays to one heavy and one light meson are shown and compared with modern theories which are also us…
We present a fit to measured moments of inclusive distributions in B->Xc lv and B-> Xs gamma decays to extract values for the CKM matrix element |Vcb|, the b- and c-quark masses, and higher order parameters that appear in the Heavy Quark Expansion. The fit is carried out using theoretical calculations in the kinetic scheme and includes moment measurements of the Babar, Belle, CDF, CLEO and DELPHI collaborations for which correlation matrices have been published. We find |Vcb| = (41.96 +- 0.23(exp) +- 0.35(HQE) +- 0.59(Gamma_SL)) 10^-3 and m_b = 4.590 +- 0.025(exp) +- 0.30(HQE) GeV where errors…
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