Hard bremsstrahlung photons in thet\bar t production anddecay: A background for the intermediate Higgs search

We discuss the radiation of hard photons in the $t\bar t$ production anddecay as a background for the intermediate Higgs search at LHC/SSC experiments. For the purpose of experimental simulation of this process we propose to use the approximate Monte Carlo algorithm together with any physics Monte Carlo not including QED corrections. We verify, by comparison with the exact matrix element Monte Carlo TOPKI, that this method can be used to generate single- and double-photon bremsstrahlung in the $t\bar t$ production process with a precision of 10% for experimental cuts. The predictions for the ψψl background from photons radiated in theproduction of $t\bar t$ pair and their cascadedecays, within simplified experimental cuts, are presented for the first time. We show that, even after additional cuts on the minimal lepton-photon mass, photons radiated in the productionand decay of a $t\bar t$ pair increase this background by a factor of 2 with respect to previous estimates from radiation in the $t\bar t$ production alone.     

Spin structure of hadrons in supersymmetric quantum chromodynamics at short distances

We study the short-distance spin structure of nucleons in supersymmetric QCD. We calculate the Altarelli-Parisi spin transition coefficients ΔP _ba and corresponding anomalous Δ γ _ab ⁿ fora, b=quarks, gluons, gluinos and squarks and present the supersymmetric relations between them. Further analysis is divided into two parts: the parity conserving case and parity violating case, both following from the mass mixing in the squark sector. Solving theQ ²-evolution equations we determine the quark, gluon, gluino and squark spin distribution at variousQ ² above the threshold for production of supersymmetric partners. We also show the analytic solutions for the first moments in the nonsinglet and singlet sector. In particular the spin carried by the valence quarks turn out to vanish asymptotically except of one particular case of squark mixing.