Particle reaction at HERA

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We have studied particle reactions realized at the world first electron-proton collider HERA in DESY,Hamburg in April 1992.1. Search for the scalar top quark(stop)(1) Stop production in the minimal supersymmetric model(MSSM) :We investigated the light scalar top quark via boson-gluon fusion at HERA, based on the MSSM ; e<@D1*@>D1p*e<@D1*@>D1t<@D4-@>D4<@D21@>D2t<@D4-@>D4<@D21<@D4・@>D4@>D2X. The calculations have been carried out both an exact tree level and the Weizsacker-Williams approximation. It has been shown that the total cross section is larger than 0.1 pb if the mass of the stop m<@D2t<@D4・@>D4<@D21@>D2@>D2, (] SY.ltorsim.[) 50 GeV and that the photon-gluon contribution to the subprocess is much larger than that of the Z boson-gluon. Since the former does not depend upon the stop mixing angle theta<@D2t@>D2, the cross section is not sensitive to theta<@D2t@>D2, but only depends upon the stop mass m<@D2t<@D4・@>D4<@D21@>D2@>D2. This enables us to search a possible region of parame … More ters for the existence of light scalar top quark at HERA in a wider parameter space (m<@D2t<@D4-@>D4@>D2,theta<@D2t@>D2) than LEP.(2) Stop resonances in R-parity breaking model(RPBM) :In the RBM there could exist the exotic Yukawa couplings of the stop t^^-_1 which violate the R-parity defined by (-1)^<3B+L+2S>, L, B and S being the lepton number, baryon number and spin, respectively. This induces the s-channel resonances in the electron-quark subprocesses, e^*p*(t^^-_1X)*e^*d^^<(-)>X. A sharp peak could be expected in the Bjorken parameter distribution as a clear signal for this process. We can conclude that HERA has a potential ability to discover the stop resonances or to impose a very severe upper bound for the R-parity breaking coupling through the detailed analyses of inclusive neutral current processes.2. Radiative corrections to deep inelastic electron-proton scattering at HERAWe have calculated one-loop electromagnetic radiative corrections delta_<NC> to neutral current deep inelastic electron-proton scattering. We found that (1) delta_<NC> is really independent of the cut-off energy of the photon, (2) delta_<NC> is almost independent of the choice of the quark-parton distribution functions. Less