Charmless Semileptonic Decay Research Articles

Search for the rare decay of B+→ℓ+νℓγ with improved hadronic tagging

We present the result of the search for the rare $B$ meson decay of $B^+ \to \ell^{\,+} \nu_{\ell} \gamma$ with $\ell =e,\mu$. For the search the full data set recorded by the Belle experiment of $711 \, \mathrm{fb}^{-1}$ integrated luminosity near the $\Upsilon (4S)$ resonance is used. Signal candidates are reconstructed for photon energies $E_{\gamma}$ larger than $1 \, \mathrm{GeV}$ using a novel multivariate tagging algorithm. The novel algorithm fully reconstructs the second $B$ meson produced in the collision using hadronic modes and was specifically trained to recognize the signal signature in combination with hadronic tag-side $B$ meson decays. This approach greatly enhances the performance. Background processes that can mimic this signature, mainly charmless semileptonic decays and continuum processes, are suppressed using multivariate methods. The number of signal candidates is determined by analyzing the missing mass squared distribution as inferred from the signal side particles and the kinematic properties of the tag-side $B$ meson. No significant excess over the background-only hypothesis is observed and upper limits on the partial branching fraction $ \Delta \mathcal{B} $ with $E_{\gamma}> 1 \, \mathrm{GeV}$ individually for electron and muon final states as well as for the average branching fraction of both lepton final states are reported. We find a Bayesian upper limit of $\Delta \mathcal{B}( B^{+} \to \ell^{\, +} \nu_{\ell} \gamma) < 3.0 \times 10^{-6}$ at 90% CL and also report an upper limit on the first inverse moment of the light-cone distribution amplitude of the $B$ meson of $\lambda_B$ at 90% CL.

Measurement of theB+→ωℓ+νbranching fraction with semileptonically taggedBmesons

We report a measurement of the branching fraction of the exclusive charmless semileptonic decay B+ --> omega l+ nu, where l is either an electron or a muon. We use samples of B+ mesons tagged by a reconstructed charmed semileptonic decay of the other B meson in the event. The measurement is based on a dataset of 426.1 fb-1 of e+e- collisions at a center-of-mass energy of 10.58 GeV recorded with the BABAR detector at the PEP-II asymmetric-energy e+e- storage rings. We measure a branching fraction of BF(B+ --> omega l+ nu) = (1.35 +/- 0.21 +/- 0.11) x 10(-4), where the uncertainties are statistical and systematic, respectively. We also present measurements of the partial branching fractions in three bins of q2, the invariant-mass squared of the lepton-neutrino system, and we compare them to theoretical predictions of the form factors.

Charmless weakBsdecays in the relativistic quark model

The form factors of the weak ${B}_{s}$ transitions to ground-state and orbitally excited strange mesons are calculated in the framework of the QCD-motivated relativistic quark model based on the quasipotential approach. These form factors are expressed through the overlap integrals of meson wave functions found in their mass spectrum evaluations. The momentum dependence of the form factors is determined in the whole accessible kinematical range without any additional assumptions and extrapolations. Relativistic effects, including the wave function transformation from rest to a moving reference frame as well as the contributions of the intermediate negative-energy states, are consistently taken into account. The calculated form factors are used for the evaluation of the charmless semileptonic decay rates and two-body nonleptonic ${B}_{s}$ decays in the factorization approximation. The obtained results are confronted with previous predictions and available experimental data.

Measurement of the decayB0→π−ℓ+νand determination of|Vub|

We present a measurement of the charmless semileptonic decay B0→π−ℓ+ν using a data sample containing 657×106 BB¯ events collected with the Belle detector at the KEKB asymmetric-energy e+e− collider operating near the Υ(4S) resonance. We determine the total branching fraction of the decay, B(B0→π−ℓ+ν)=(1.49±0.04(stat)±0.07(syst))×10−4. We also report a new precise measurement of the differential decay rate and extract the Cabibbo-Kobayashi-Maskawa matrix element |Vub| using model-independent and model-dependent approaches. From a simultaneous fit to the measured differential decay rate and lattice QCD results, we obtain |Vub|=(3.43±0.33)×10−3, where the error includes both experimental and theoretical uncertainties.Received 30 November 2010DOI:https://doi.org/10.1103/PhysRevD.83.071101© 2011 American Physical Society

Measurements of branching fractions and q2 distributions for B→πℓν and B→ρℓν decays with B→D(∗)ℓν decay tagging

We report measurements of the charmless semileptonic decays $B^0 \to \pi^- / \rho^- \ell^{+} \nu$ and $B^+ \to \pi^0 / \rho^0 \ell^{+} \nu$, based on a sample of $2.75 \times 10^8$ $B \bar{B}$ events collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+e^-$ asymmetric collider. In this analysis, the accompanying $B$ meson is reconstructed in the semileptonic mode $B \to D^{(*)} \ell \nu$, enabling detection of the signal modes with high purity. We measure the branching fractions ${\mathcal B}(B^0 \to \pi^- \ell^+ \nu) = (1.38\pm 0.19\pm 0.14\pm 0.03) \times 10^{-4}$, ${\mathcal B}(B^0 \to \rho^- \ell^+ \nu) = (2.17\pm 0.54\pm 0.31\pm 0.08) \times 10^{-4}$, ${\mathcal B}(B^+ \to \pi^0 \ell^+ \nu) = (0.77\pm 0.14\pm 0.08\pm 0.00) \times 10^{-4}$ and ${\mathcal B}(B^+ \to \rho^0 \ell^+ \nu) = (1.33\pm 0.23\pm 0.17\pm 0.05) \times 10^{-4}$, where the errors are statistical, experimental systematic, and systematic due to form-factor uncertainties, respectively. For each mode we also present the partial branching fractions in three $q^2$ intervals: $q^2 < 8$, $8 \leq q^2 < 16$, and $q^2 \geq 16$ GeV$^2/c^2$. From our partial branching fractions for $B \to \pi \ell \nu$ and recent results for the form factor from unquenched Lattice QCD calculations, we obtain values of the CKM matrix element $|V_{ub}|$.

Measurement of theB0→π−l+νForm-Factor Shape and Branching Fraction, and Determination of|Vub|with a Loose Neutrino Reconstruction Technique

We report the results of a study of the exclusive charmless semileptonic decay, B0-->pi(-l)(+nu), undertaken with approximately 227 x 10(6) BB pairs collected at the Upsilon(4S) resonance with the BABAR detector. The analysis uses events in which the signal B decays are reconstructed with an innovative loose neutrino reconstruction technique. We obtain partial branching fractions in 12 bins of q2, the momentum transfer squared, from which we extract the f + (q2) form-factor shape and the total branching fraction B(B0-->pi(-l)(+nu))=(1.46+/-0.07stat+/-0.08syst) x 10(-4). Based on a recent unquenched lattice QCD calculation of the form factor in the range q2>16 GeV2, we find the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element /Vub/ to be (4.1+/-0.2stat +/- 0.2syst(+0.6)-0.4_{FF}) x 10(-3), where the last uncertainty is due to the normalization of the form factor.

Running-coupling effects in the triple-differential charmless semileptonic decay width

We compute the fully-differential B -> X_u l nu decay width to all orders in perturbation theory in the large-beta0 limit. Each of the five structure functions that build the hadronic tensor is expressed as a Borel integral, summing up O(CF beta0^{n-1} alpha_s^n) corrections for any n. We derive analytic expressions for the Borel transforms of both real and virtual diagrams with a single dressed gluon, and perform an all-order infrared subtraction, where the Borel parameter serves also as an infrared regulator. Expanding the result we recover the known triple-differential NLO coefficient, and obtain an explicit expression for the O(CF beta0 alpha_s^2) triple-differential NNLO correction. This result can be used to improve the determination of |V_{ub}| from inclusive B -> X_u l nu measurements at the B factories with a variety of kinematic cuts.

Determinations of | V ub| with inclusive techniques at LEP

The charmless semileptonic decay B branching fraction has been measured, using inclusive techniques, by the Aleph, Delphi and L3 experiments at LEP. The average is ▪ From this result the value of the | V ub | element in the CKM mixing matrix has been derived, using OPE predictions, obtaining: ▪

Extracting | Vub| from the inclusive charmless semileptonic branching ratio of b hadrons

We calculate the inclusive charmless semileptonic decay width of the B meson using a QCD-based approach. This approach is able to account for both dynamic and kinematic effects of nonperturbative QCD. The charmless semileptonic decay width is found to be enhanced by long-distance strong interactions by +(7±5)% with respect to the free quark decay width. Using the resulting theoretical value for the charmless semileptonic decay width, an extraction of | V ub | is made from the measured lifetime and inclusive charmless semileptonic branching ratio of b hadrons.

V from the hadronic invariant mass spectrum in semileptonic B decay

The hadronic invariant mass spectrum for the inclusive charmless semileptonic decay $B\to X_u e \bar\nu_e$ is studied. Particular attention is paid to the region $s_H<m_D^2$, which may be useful for extracting the value of $|V_{ub}|$. The sensitivity of the spectrum to the parameter $\bar\Lambda \equiv m_B - m_b$ is explored. Perturbative QCD corrections to $d\Gamma/ds_H$ of order $\alpha_s^2\beta_0$ are calculated. For $s_H \sim \bar\Lambda m_b$ nonperturbative QCD effects are important and the shape of the invariant mass spectrum is controlled by the B meson matrix element of an infinite sum of local operators. The utility of the hadronic mass spectrum for extracting $|V_{ub}|$ is explored.

Measurement of charmless semileptonic decays of B mesons.

Using the CLEO II detector and a sample of 955 000 Υ(4S) decays we have confirmed charmless semileptonic decays of B mesons. In the momentum interval 2.3–2.6 GeV/c we observe an excess of 107±15±11 leptons, which we attribute to b→ulν. This result yields a model-dependent range of values for ‖Vub/Vcb‖ that is lower than has been obtained in previous studies. For the inclusive spectator model of Altarelli et al. we find ‖Vub/Vcb‖=0.076±0.008. Models that describe b→ulν with a limited set of exclusive final states give ‖Vub/Vcb‖=0.06-0.10.Received 7 September 1993DOI:https://doi.org/10.1103/PhysRevLett.71.4111©1993 American Physical Society

Extracting Vbu/Vbc from semileptonic B decays.

Recent CLEO and ARGUS results for charmless semileptonic decays of $B$ mesons are compared to several models for exclusive channels under the assumption that the $\ensuremath{\pi}$, $\ensuremath{\rho}$, $D$, and ${D}^{*}$ saturate the region of high electron momentum of interest in determining $\frac{{V}_{\mathrm{bu}}}{{V}_{\mathrm{bc}}}$. The effect of current-algebra-inspired $\ensuremath{\pi}\ensuremath{\pi}$ backgrounds and threshold constraints in the $\ensuremath{\rho}$ channel, as well as the error made by the narrow-width approximation, are examined with a view to estimating the model dependence inherent in extracting $\frac{{V}_{\mathrm{bu}}}{{V}_{\mathrm{bc}}}$ from the data. Total semileptonic rates to $\ensuremath{\pi}$ and $\ensuremath{\rho}$ differ widely, model to model, while rates to $D$ and ${D}^{*}$ final states have much less model dependence. Model dependence is somewhat smaller when only high lepton energies are concerned. ${V}_{\mathrm{bc}}$ calculated from exclusive semileptonic decay to $D$ and ${D}^{*}$ final states vary about 25% and 10%, respectively, model to model. The model dependence of $\frac{{V}_{\mathrm{bu}}}{{V}_{\mathrm{bc}}}$ is also considerable. The narrow-width approximation in the $\ensuremath{\rho}$ channel can be a 20% effect in the determination of this ratio because of the sensitivity of the high-energy region to the lowest mass of the final hadronic system. If current-algebra-inspired backgrounds are present, $\frac{{V}_{\mathrm{bu}}}{{V}_{\mathrm{bc}}}$ may be considerably smaller than suggested by many models. If the ${D}^{**}$ component of $b$ to $c$ is as large as recent CLEO data suggest, $\frac{{V}_{\mathrm{bu}}}{{V}_{\mathrm{bc}}}$ may be 16% larger than expected by models that ignore this contribution. Electron spectra for various models are presented, illustrating the high variation of shape features in the end-point region.