Supersymmetric Particle Production at HERA

In the framework of the minimal supersymmetric standard model and the $R$-parity breaking model, we investigate various production processes of the supersymmetric partner at HERA energies. Our emphasis is paid upon the scalar top quark, the partner of top quark, characterized by its lighter mass than the top quark and other scalar quarks in a model. We propose experimentally feasible approaches to search for clean signals of the stop from either its production or decay processes.Comment: 30 pages, LaTeX, 21 figures available upon reques

Single scalar top production with polarized beams in ep collisions at HERA

From the point of view of the R-parity breaking supersymmetric model, we propose a scalar top (stop) search with longitudinally polarized electron (e-) and positron(e+) beams which will soon be available at the upgraded HERA. Fully polarized e- or e+ beams could produce the stop two times as much as unpolarized beams, while they increase background events due to the process of the standard model by about 30% in comparison with unpolarized ones. We show that right-handed e+ beams at HERA is efficient to produce the stop in the model. With 1 fb**(-1) of integrated luminosity we estimate reach in the coupling constant lambda'(131) for masses of the stop in the range 160-400 GeV. We can set a 95% confidence-level exclusion limit for lambda'(131) > 0.01-0.05 in the stop mass range of 240-280 GeV if no singal of the stop is observed. We also point out that y(=Q**2/sx) distributions of e+ coming from the stop shows the different behavior from those of the standard model.Comment: 12 pages, 6 eps figure

Z radiation off stops at a linear collider

We calculate e+e- --> stop/stop/Z at a linear collider. For large splitting between the two stops the cross-section is sensitive to the value of m(stop2) when this particle is too heavy to be directly produced. The results are compared to e+e- --> stop/stop/h.Comment: 19 pages, 8 figure

Spin and spin-spin correlations in chargino pair production at future linear e+e- colliders

A possibility to measure the spin and spin-spin correlations of a chargino pair is investigated in the process electron positron -> chargino_1 anti-chargino_1 -> (neutralino_1 quark anti-quark) (neutralino_1 quark anti-quark) at future linear-collider energies. The total and the differential cross sections are calculated by the GRACE system which allows for the full spin correlation. Experimental sensitivity of the measurements are examined by assuming the limited detector resolution, the initial state radiation and the beam-beam effect (beamstrahlung). It is found that generally the spin-spin correlation can only be measured with a lower sensitivity than the chargino spin itself. The dependence of the correlation measurements on the relevant SUSY parameters can be seen for a light sneutrino case, but the situation becomes worse for a heavier sneutrino.Comment: 23 pages, 7 figures, 6 tables; added reference for section

Possible Excess in Charged Current Events with High-Q^2 at HERA from Stop and Sbottom Production

We investigate a production process e^+p \to \st X \to \sb W^+ X at HERA, where we consider a decay mode \sb \to \bar{\nu}_e d of the sbottom in the framework of an R-parity breaking supersymmetric standard model. Both processes of the stop production e^+ d \to \st and the sbottom decay \sb \to \bar{\nu}_e d are originated from an R-parity breaking superpotential $\lambda'_{131} \hat{L}_1 \hat{Q}_3 \hat{D^c}_1$. One of signatures of the process should be a large missing transverse momentum plus multijet events corresponding to hadronic decays of the $W$. It is shown that the signal could appear as an event excess in the charged current (CC) processes $e^+p \to \nu X$ with the high $Q^2$ at HERA. We compare expected event distributions with the CC data recently reported by the H1 and ZEUS groups at HERA. Methods for extracting the signal from the standard CC processes are also discussed.Comment: 13 pages, LaTeX, 3 figure