[en] The result of a search for gaugino pair production with a trilepton signature is reinterpreted in the framework of minimal supergravity (MSUGRA) with R-parity violation via leptonic λ Yukawa couplings. The search used 95 pb^-1 of pp(bar sign) collisions at √(s)=1.8 TeV recorded by the DOe detector at the Fermilab Tevatron. A large domain of the MSUGRA parameter space is excluded for λ₁₂₁, λ₁₂₂≥10^-4. (c) 2000 The American Physical Society

[en] The cross section for Drell-Yan pair production in the limit of small x_target is derived in the rest frame of the target hadron. Our calculation is based on the fundamental quantity σ(ρ), the cross section for the scattering of a q bar q pair with fixed transverse separation ρ off a hadronic target. As in deep inelastic scattering the result can be given in terms of integrals of σ(ρ). This is consistent with well-known factorization theorems and also relates higher-twist terms in both processes. An analysis of the angular distribution of the produced lepton shows that additional integrals of σ(ρ) can be obtained in the Drell-Yan process, which are not measurable in inclusive deep inelastic scattering. copyright 1997 The American Physical Society

[en] We have searched for rare and forbidden decays of the η^' meson in hadronic events at the CLEO II detector. The search is conducted on 4.80 fb^-1 of e⁺e^- collisions at 10.6 GeV center-of-mass energy at the Cornell Electron Storage Ring. We find no signals, and set 90% confidence level upper limits of their branching fractions: B(η^'→e⁺e^- η)<2.4x10^-3 , B(η^'→e⁺e^- π⁰)<1.4x10^-3 , B(η^'→e⁺e^- γ)<0.9x10^-3 , and B(η^'→eμ)<4 .7x10^-4 . We also fit the matrix element of the η^'→π⁺π^-η Dalitz plot with the parametrization |M|²=A|1+αy|² , where y is a linear function of the kinetic energy of the η , and find Re(α)=-0.021±0.025 . (c) 1999 The American Physical Society

[en] We calculate the transverse momentum distribution for the production of massive lepton pairs in longitudinally polarized proton-proton reactions at collider energies within the context of perturbative quantum chromodynamics. For values of the transverse momentum Q_T greater than roughly half the pair mass Q, Q_T>Q/2, we show that the differential cross section is dominated by subprocesses initiated by incident gluons, provided that the polarized gluon density is not too small. Massive lepton-pair differential cross sections should be a good source of independent constraints on the polarized gluon density, free from the experimental and theoretical complications of photon isolation that beset studies of prompt photon production. We provide predictions for the spin-averaged and spin-dependent differential cross sections as a function of Q_T at energies relevant for the Relativistic Heavy Ion Collider at Brookhaven, and we compare these with predictions for real prompt photon production. (c) 2000 The American Physical Society

[en] Both dilepton and charm production at RHIC are considered to be important signatures for quark-gluon plasma production. Recently it was argued by Gavin et al. that the background from semileptonic correlated charm decays is so large that it makes dilepton measurements virtually impossible. We show that this conclusion is in fact reversed if the energy loss due to a secondary interaction of charmed quarks is included. copyright 1997 The American Physical Society

[en] The production of low mass dileptons from π-ρ interactions in a hot medium is studied. Applying finite temperature perturbation theory the dilepton rate is computed up to order g_ρ². For dilepton masses below the ρ the two-body reactions ππ→ργ^*, πρ→πγ^*, and the decay process ρ→ππγ^* give significant contributions. Nonequilibrium contributions to the thermal rate are estimated, including the modification of the particle distribution function with a nonzero pion chemical potential. A comparison of the dilepton rate with the recent data measured in nucleus-nucleus collisions at CERN SPS energy by the CERES Collaboration is also performed. It is shown that the additional thermal dileptons from π-ρ interactions can partially account for the excess of the soft dilepton yield seen experimentally. copyright 1997 The American Physical Society

[en] We calculate the nuclear enhancement in the angular distribution of Drell-Yan pairs produced in proton-nucleus reactions. Nuclear effects are encoded in universal twist-4 parton correlation functions. We find that the Lam-Tung relation for the angular coefficients of the lepton-pair distribution holds for the double-hard, but not for the soft-hard, contribution. We also predict that nuclear enhancement effects at the Relativistic Heavy Ion Collider energies can be large. (c) 1999 The American Physical Society

[en] In the present work we investigate the baryonic contributions to the dilepton yield in high energy heavy ion collisions. The production rate of dielectron pairs from the Dalitz decay of low lying baryonic excitation are estimated. It has been observed that most dominant contribution comes from the decay of N^*(1520) compared to other baryonic sources. (c) 1999 American Institute of Physics

[en] We present a calculation of two photon radiation in W and Z boson production in hadronic collisions, based on the complete matrix elements for the processes qq(bar sign)^'→l^±νγγ and qq(bar sign)→l⁺l^-γγ, including finite charged lepton masses. In order to achieve stable numerical results over the full phase space, multiconfiguration Monte Carlo techniques are used to map the peaks in the differential cross section. Numerical results are presented for the Fermilab Tevatron. (c) 2000 The American Physical Society

[en] We use the transverse momentum spectrum of leptons in the decay chain t→bW with W→lν to measure the helicity of the W bosons in the top quark rest frame. Our measurement uses a tt sample isolated in 106±4 pb^-1 of data collected in pp collisions at √(s)=1.8 TeV with the CDF detector at the Fermilab Tevatron. Assuming a standard V-A weak decay, we find that the fraction of W 's with zero helicity in the top rest frame is F₀=0.91±0.37(stat )±0.13(syst) , consistent with the standard model prediction of F₀=0.70 for a top mass of 175 GeV/c² . (c) 2000 The American Physical Society