Search results
28,129 records were found.
We study the reaction e+e-→e+e-π0 in the single tag mode and measure the differential cross section dσ/dQ2 and the γγ*→π0 transition form factor in the momentum transfer range from 4 to 40 GeV2. At Q2>10 GeV2 the measured form factor exceeds the asymptotic limit predicted by perturbative QCD. The analysis is based on 442 fb-1 of integrated luminosity collected at PEP-II with the BABAR detector at e+e- center-of-mass energies near 10.6 GeV.
The branching fraction of the τ-→3h-2h+ντ decay (h=π,K) is measured with the BABAR detector to be (8.56±0.05±0.42)×10-4, where the first error is statistical and the second systematic. The observed structure of this decay is significantly different from the phase space prediction, with the ρ resonance playing a strong role. The decay τ-→f1(1285)π-ντ, with the f1(1285) meson decaying to four charged pions, is observed and the branching fraction is measured to be (3.9±0.7±0.5)×10-4.
We study the processes e+e-→3(π+π-)γ, 2(π+π-π0)γ and K+K-2(π+π-)γ, with the photon radiated from the initial state. About 20 000, 33 000 and 4000 fully reconstructed events, respectively, have been selected from 232 fb-1 of BABAR data. The invariant mass of the hadronic final state defines the effective e+e- center-of-mass energy, so that these data can be compared with the corresponding direct e+e- measurements. From the 3(π+π-), 2(π+π-π0) and K+K-2(π+π-) mass spectra, the cross sections for the processes e+e-→3(π+π-), e+e-→2(π+π-π0) and e+e-→K+K-2(π+π-) are measured for center-of-mass energies from production threshold to 4.5 GeV. The uncertainty in the cross section measurement is typically 6%–15%. We observe a structure at 1.9 GeV in both cross sections and a resonance structure with mass 1645±0.008 GeV/c2 and width 0.114±0.014 ...
Measurements of the double-differential π± production cross section in the range of momentum 0.5⩽p⩽8.0 GeV/c and angle 0.025⩽θ⩽0.25 rad in collisions of protons on beryllium, carbon, nitrogen, oxygen, aluminum, copper, tin, tantalum, and lead are presented. The data were taken with the large-acceptance HAdRon Production (HARP) detector in the T9 beamline of the CERN Proton Synchrotron. Incident particles were identified by an elaborate system of beam detectors. Thin targets of 5% of a nuclear interaction length were used. The tracking and identification of the produced particles were performed using the forward system of the HARP experiment. Results are obtained for the double-differential cross sections d2σ/dp dΩ mainly at four incident proton beam momenta (3, 5, 8, and 12 GeV/c). Measurements are compared with the GEANT4 and MARS Mon...
We report the result of a search for the rare decay B0→γγ in 426 fb-1 of data, corresponding to 226×106 B0B̅ 0 pairs, collected on the Υ(4S) resonance at the PEP-II asymmetric-energy e+e- collider using the BABAR detector. We use a maximum likelihood fit to extract the signal yield and observe 21-12+13 signal events with a statistical significance of 1.8σ. This corresponds to a branching fraction B(B0→γγ)=(1.7±1.1(stat.)±0.2(syst.))×10-7. Based on this result, we set a 90% confidence level upper limit of B(B0→γγ)<3.2×10-7.
Charged-lepton flavor-violating processes are unobservable in the standard model, but they are predicted to be enhanced in several extensions to the standard model, including supersymmetry and models with leptoquarks or compositeness. We present a search for such processes in a sample of 99×106Υ(2S) decays and 117×106Υ(3S) decays collected with the BABAR detector. We place upper limits on the branching fractions B(Υ(nS)→e±τ∓) and B(Υ(nS)→μ±τ∓) (n=2,3) at the 10-6 level and use these results to place lower limits of order 1 TeV on the mass scale of charged-lepton flavor-violating effective operators.
