Person:

Sciolla, Gabriella

We search for single-photon decays of the (Υ(1S)) resonance, (Υ→γ+) invisible, where the invisible state is either a particle of definite mass, such as a light Higgs boson (A^0), or a pair of dark matter particles, (χ\overline{χ}). Both (A^0) and χ are assumed to have zero spin. We tag (Υ(1S)) decays with a dipion transition (Υ(2S)→π^{+}π^{−}Υ(1S)) and look for events with a single energetic photon and significant missing energy. We find no evidence for such processes in the mass range (m_{A^{0}}≤9.2 GeV) and (m_{χ}≤4.5 GeV) in the sample of (98×10^{6} Υ(2S)) decays collected with the BABAR detector and set stringent limits on new physics models that contain light dark matter states.

We report on analyses of tau lepton decays (τ^{−}→ηK^{−}ν_{τ}) and (τ^{−}→ηπ^{−}ν_{τ}), with (η→π^{+}π^{−}π^{0}), using (470 fb^{−1}) of data from the BABAR experiment at PEP-II, collected at center-of-mass energies at and near the (Υ(4S)) resonance. We measure the branching fraction for the (τ^{−}→ηK^{−}ν_{τ}) decay mode, (ℬ(τ^{−}→ηK^{−}ν_{τ})=(1.42±0.11(stat)±0.07(syst))×10^{−4}), and report a 95% confidence level upper limit for the second-class current process (τ^{−}→ηπ^{−}ν_{τ}), (ℬ(τ^{−}→ηπ^{−}ν_{τ})<9.9×10^{−5}).

We present an analysis of exclusive charmless semileptonic B-meson decays based on (377×10^{6} B\overline{B}) pairs recorded with the BABAR detector at the (Υ(4S)) resonance. We select four event samples corresponding to the decay modes (B^{0}→π^{−}ℓ^{+}ν), (B^{+}→π^{0}ℓ^{+}ν), (B^{0}→ρ^{−}ℓ^{+}ν), and (B^{+}→ρ^{0}ℓ^{+}ν) and find the measured branching fractions to be consistent with isospin symmetry. Assuming isospin symmetry, we combine the two (B→πℓν) samples, and similarly the two (B→ρℓν) samples, and measure the branching fractions (ℬ(B^{0}→π^{−}ℓ^{+}ν)=(1.41±0.05±0.07)×10^{−4}) and (ℬ(B^{0}→ρ^{−}ℓ^{+}ν)=(1.75±0.15±0.27)×10^{−4}), where the errors are statistical and systematic. We compare the measured distribution in (q^2), the momentum transfer squared, with predictions for the form factors from QCD calculations and determine the Cabibbo-Kobayashi-Maskawa matrix element (|V_{ub}|). Based on the measured partial branching fraction for (B→πℓν) in the range (q^{2}<12 GeV^{2}) and the most recent QCD light-cone sum-rule calculations, we obtain (|V_{ub}|=(3.78±0.13^{+0.55}{−0.40})×10^{−3}), where the errors refer to the experimental and theoretical uncertainties. From a simultaneous fit to the data over the full (q^2) range and the FNAL/MILC lattice QCD results, we obtain (|V{ub}|=(2.95±0.31)×10^{−3}) from (B→πℓν), where the error is the combined experimental and theoretical uncertainty.

We present a measurement of the branching fractions of the 22 decay channels of the (B^0) and (B^+) mesons to (\overline{D}^{()}D^{()}K), where the (D^{()}) and (\overline{D}^{()}) mesons are fully reconstructed. Summing the 10 neutral modes and the 12 charged modes, the branching fractions are found to be (ℬ(B^{0}→\overline{D}^{()}D^{()}K)=(3.68±0.10±0.24)%) and (ℬ(B^{+}→\overline{D}^{()}D^{()}K)=(4.05±0.11±0.28)%), where the first uncertainties are statistical and the second systematic. The results are based on (429 fb^{−1}) of data containing (471×10^{6}B\overline{B}) pairs collected at the (Υ(4S)) resonance with the BABAR detector at the SLAC National Accelerator Laboratory.