Real-Time Simulation of Large Open Quantum Spin Systems driven by Measurements

We consider a large quantum system with spins $\frac{1}{2}$ whose dynamics is driven entirely by measurements of the total spin of spin pairs. This gives rise to a dissipative coupling to the environment. When one averages over the measurement results, the corresponding real-time path integral does not suffer from a sign problem. Using an efficient cluster algorithm, we study the real-time evolution of a 2-d Heisenberg antiferromagnet, which is driven to a disordered phase, either by sporadic measurements or by continuous monitoring described by Lindblad evolution.Comment: 5 pages, 7 figure

Supersymmetry and Charged Current Events at HERA

A light stop, with an R-parity-violating coupling $\lambda'_{131}$, has been suggested as an explanation of the excess in high-$Q^2$ neutral current events observed at the HERA collider. We show that in this scheme a corresponding excess in charged current events --- such as that reported by the H1 Collaboration --- can appear naturally, without calling for the presence of light sleptons or additional R-parity-violating couplings, if there exists a chargino lighter than the stop. The predicted event shapes agree well with the data. The relevant region of parameter space is identified, taking into account constraints coming from precision electroweak measurements, atomic parity violation and recent searches for first-generation leptoquarks at the Tevatron collider.Comment: 18 pages, LaTeX, 4 embedded figure

GRACE/SUSY Automatic Generation of Tree Amplitudes in the Minimal Supersymmetric Standard Model

GRACE/SUSY is a program package for generating the tree-level amplitude and evaluating the corresponding cross section of processes of the minimal supersymmetric extension of the standard model (MSSM). The Higgs potential adopted in the system, however, is assumed to have a more general form indicated by the two-Higgs-doublet model. This system is an extension of GRACE for the standard model(SM) of the electroweak and strong interactions. For a given MSSM process the Feynman graphs and amplitudes at tree-level are automatically created. The Monte-Carlo phase space integration by means of BASES gives the total and differential cross sections. When combined with SPRING, an event generator, the program package provides us with the simulation of the SUSY particle productions.Comment: 39 page, 1 figur

Automatic one-loop calculation of MSSM processes with GRACE

We have developed the system for the automatic computation of cross-sections, {\tt GRACE/SUSY}, including the one-loop calculations for processes of the minimal supersymmetric extension of the the standard model. For an application, we investigate the process $e^+ e^- \to Z^0 h^0$ .Comment: 4 pages, 1 figure, Talk presented by Jimbo, M. at ACAT-0

Automatic Computation of Cross Sections in HEP

For the study of reactions in High Energy Physics (HEP) automatic computation systems have been developed and are widely used nowadays. GRACE is one of such systems and it has achieved much success in analyzing experimental data. Since we deal with the cross section whose value can be given by calculating hundreds of Feynman diagrams, we manage the large scale calculation, so that effective symbolic manipulation, the treat of singularity in the numerical integration are required. The talk will describe the software design of GRACE system and computational techniques in the GRACE.Comment: 6 pages, Latex, ICCP