We report the observation of the rare charm decay D0 → K-π+e+e- , based on 468 fb-1 of e+e- annihilation data collected at or close to the center-of-mass energy of the γ ( 4 S ) resonance with the ...BABAR detector at the SLAC National Accelerator Laboratory. We find the branching fraction in the invariant mass range 0.675<m ( e+e- ) <0.875 GeV / c2 of the electron-positron pair to be B ( D0 → K-π+e+e- ) = ( 4.0±0.5±0.2±0.1 ) ×10-6 , where the first uncertainty is statistical, the second systematic, and the third due to the uncertainty in the branching fraction of the decay D0 → K-π+π+π- used as a normalization mode. The significance of the observation corresponds to 9.7 standard deviations including systematic uncertainties. This result is consistent with the recently reported D0 → K-π+μ+μ- branching fraction, measured in the same invariant mass range, and with the value expected in the standard model. In a set of regions of m ( e+e- ) , where long-distance effects are potentially small, we determine a 90% confidence level upper limit on the branching fraction B ( D0 → K-π+e+e- ) <3.1×10-6 .
Using a sample of (470.9±2.8)×106 BB¯ pairs, we measure the decay branching fraction B(B0→D*−π+π−π+)=(7.26±0.11±0.31)×10−3, where the first uncertainty is statistical and the second is systematic. ...Our measurement will be helpful in studies of lepton universality by measuring B(B0→D*−τ+ντ) using τ+→π+π−π+ν¯τ decays, normalized to B(B0→D*−π+π−π+).
We search for the rare flavor-changing neutral current process $B^- \to \Lambda {\overline p} \nu{\overline{\nu}}$ using data from the B\textsc{a}B\textsc{ar} experiment. A total of 424 fb$^{-1}$ of ...$e^+e^-$ collision data collected at the center-of-mass energy of the $\Upsilon$(4S) resonance is used in this study, corresponding to a sample of ${(471 \pm 3) \times 10^{6}}$ $B\overline{B}$ pairs. Signal $B^- \to \Lambda {\overline p} \nu{\overline{\nu}}$ candidates are identified by first fully reconstructing a $B^+$ decay in one of many possible exclusive decays to hadronic final states, then examining detector activity that is not associated with this reconstructed $B^+$ decay for evidence of a signal $B^- \to \Lambda {\overline p} \nu{\overline{\nu}}$ decay. The data yield is found to be consistent with the expected background contribution under a null signal hypothesis, resulting in an upper limit of ${{\cal B} (B^- \to \Lambda {\overline p} \nu{\overline{\nu}}) < 3.0\times 10^{-5}}$ at the $90\%$ confidence level.
An angular analysis of the decay $\bar{B}$ → D*ℓ ¯$\bar{ν}_ℓ$, ℓϵ {e,μ}, is reported using the full e+e- collision data set collected by the BABAR experiment at the Υ(4S) resonance. One B meson from ...the Υ(4S) → $B\bar{B}$ decay is fully reconstructed in a hadronic decay mode, which constrains the kinematics and provides a determination of the neutrino momentum vector. The kinematics of the semileptonic decay is described by the dilepton mass squared, q2, and three angles. The first unbinned fit to the full four-dimensional decay rate in the standard model is performed in the so-called Boyd-Grinstein-Lebed approach, which employs a generic q2 parametrization of the underlying form factors based on crossing symmetry, analyticity, and QCD dispersion relations for the amplitudes. A fit using the more model-dependent Caprini-Lellouch-Neubert (CLN) approach is performed as well. Our form factor shapes show deviations from previous fits based on the CLN parametrization. The latest form factors also provide an updated prediction for the branching fraction ratio $\mathscr{R}$(D*) ≡ $\mathscr{B}$($\bar{B}$ → D* τ¯ $\bar{ν}_τ$)/$\mathscr{B}$($\bar{B}$ → D*ℓ¯ $\bar{ν}_ℓ$) = 0.253 ± 0.005 . Finally, using the well-measured branching fraction for the $\bar{B}$ → D*ℓ¯ $\bar{ν}_ℓ$ decay, a value of |Vcb| = (38.36 ± 0.90) × 10⁻3 is obtained that is consistent with the current world average for exclusive $\bar{B}$ → D (*)ℓ⁻$\bar{ν}_ℓ$ decays and remains in tension with the determination from inclusive semileptonic B decays to final states with charm.
The photon spectrum in the inclusive electromagnetic radiative decays of the B meson, B → X s γ plus B → X d γ , is studied using a data sample of ( 382.8 ± 4.2 ) × 10 6 Υ ( 4 S ) → B ¯¯¯ B decays ...collected by the BABAR experiment at SLAC. The spectrum is used to extract the branching fraction B ( B → X s γ ) = ( 3.21 ± 0.33 ) × 10 − 4 for E γ > 1.8 GeV and the direct C P asymmetry A C P ( B → X s + d γ ) = 0.057 ± 0.063 . The effects of detector resolution and Doppler smearing are unfolded to measure the photon energy spectrum in the B meson rest frame.
We present a measurement of the asymmetry ACP between same-sign inclusive dilepton samples scriptl+scriptl+ and scriptl-scriptl- (scriptl=e, mu ) from semileptonic B decays in Y(4S) arrow right BB- ...events, using the complete data set recorded by the BABAR experiment near the Y(4S) resonance, corresponding to 471x106 BB- pairs. The asymmetry ACP allows comparison between the mixing probabilities P(B-0 arrow right B0) and P(B0 arrow right B-0), and therefore probes CP and T violation. The result, ACP=-3.9+ or -3.5(stat) + or -1.9(syst)x10-3, is consistent with the standard model expectation.
Here, we study the processes e+e– → π+π–π0π0π0γ and π+π–π0π0ηγ in which an energetic photon is radiated from the initial state. The data are collected with the BABAR detector at SLAC. About 14 000 ...and 4700 events, respectively, are selected from a data sample corresponding to an integrated luminosity of 469 fb–1. The invariant mass of the hadronic final state defines the effective e+e– center-of-mass energy. From the mass spectra, the first precise measurement of the e+e– → π+π–π0π0π0 cross section and the first measurement ever of the e+e– → π+π–π0π0η cross section are performed. The center-of-mass energies range from threshold to 4.35 GeV. The systematic uncertainty is typically between 10% and 13%. The contributions from ωπ0π0, ηπ+π–, and other intermediate states are presented. We observe the J/ψ and ψ(2S) in most of these final states and measure the corresponding branching fractions, many of them for the first time.
Recent investigations have suggested that the six-quark combination uuddss could be a deeply bound state (S) that has eluded detection so far, and a potential dark matter candidate. We report the ...first search for a stable, doubly strange six-quark state in Υ→S$\bar{Λ}\bar{Λ}$ decays based on a sample of 90×10$^{6}$Υ(2S) and 110×106Υ(3S) decays collected by the BABAR experiment. No signal is observed, and 90% confidence level limits on the combined Υ(2S,3S) → S$\bar{Λ}\bar{Λ}$ branching fraction in the range (1.2-1.4) × 10-7 are derived for mS < 2.05 GeV. These bounds set stringent limits on the existence of such exotic particles.
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