Tadashi Kon

Automatic Computation of Cross Sections in HEP Status of GRACE System

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.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.

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.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.

QCD corrections to Higgs boson decays into squarks in the minimal supersymmetric standard model

Abstract We calculate the supersymmetric O(αs) QCD corrections to the widths of the Higgs boson decays H + → t b and H 0 , A 0 → t t , b b in the on-shell scheme within the Minimal Supersymmetric Standard Model. We find that the QCD corrections are significant, but that the squark pair decay modes are still dominant in a wide parameter region.

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.

Stop resonances at HERA in R-parity breaking models

Abstract The possible detection of the supersymmetric partner of the top quark (stop) at the ep collider HERA is investigated in the framework of R -parity breaking models. Since there exist the R -parity breaking coupling ted, the s -channel process e − p → t → e − d X would occur and therefore a sharp peak could be expected in the Bjorken parameter x distribution as a clear signal for this process.

QCD corrections to the decay H+ → tb in the minimal supersymmetric standard model

Abstract We present a complete calculation of the O ( α s ) QCD corrections to the width of the decay H + → t b within the Minimal Supersymmetric Standard Model. We find that the QCD corrections are quite important, and that the supersymmetric QCD corrections (due to gluino, t and b exchange) can be comparable to or even larger than the standard QCD corrections in a large region of the supersymmetric parameter space. This is mainly due to the effect of large left-right mixings of stop ( t ) and sbottom ( b ). This could significantly affect the phenomennology of the H + search.

Bosonic decays of and

Abstract We perform a detailed study of the decays of the heavier top and bottom squarks ( t 2 and b 2 ) in the Minimal Supersymmetric Standard Model (MSSM). We show that the decays into Higgs or gauge bosons, i.e. t 2 → t 1 +(h 0 ,H 0 ,A 0 or Z 0 ) , t 2 → b 1,2 +(H + or W + ) , and the analogous b 2 decays, can be dominant in a wide range of the model parameters due to the large Yukawa couplings and mixings of t and b. Compared to the conventional decays into fermions, such as t 2 →t+( χ 0 i or g ) and t 2 →b+ χ + j , these bosonic decay modes can have significantly different decay structures and distributions. This could have an important impact on the search for t 2 and b 2 and the determination of the MSSM parameters at future colliders.We perform a detailed study of the decays of the heavier top and bottom squarks (stop_2 and sbottom_2) in the Minimal Supersymmetric Standard Model (MSSM). We show that the decays into Higgs or gauge bosons can be dominant in a wide range of the model parameters due to the large Yukawa couplings and mixings of stops and sbottoms. Compared to the conventional decays into fermions these bosonic decay modes can have significantly different decay structures and distributions. This could have an important impact on the search for stop_2 and sbottom_2 and the determination of the MSSM parameters at future colliders.

THE GRACE SYSTEM FOR THE MINIMAL SUPERSYMMETRIC STANDARD MODEL

Abstract The algorithm of constructing the Feynman amplitudes for the GRACE system is extended to processes involving supersymmetric particles. New vertex amplitude subroutines needed to compute these processes are now part of the CHANEL library.

Impact of squark pair decay modes on search for supersymmetric neutral Higgs bosons

Abstract We present a detailed study of the decays of the neutral Higgs bosons H 0 and A 0 within the Minimal Supersymmetric Standard Model. We find that the supersymmetric modes tt and bb can dominate the H 0 and A 0 decays in a wide range of the model parameters due to large Yukawa couplings and mixings of t and b . Compared to the conventional modes t t and b b , these modes have very distinctive signatures. This could have a decisive impact on the neutral Higgs boson searches at future colliders.

Signatures of scalar top with R-parity breaking coupling at HERA

Abstract In the framework of the minimal supersymmetric standard model with an R-parity breaking coupling of the scalar top quark (stop) we investigate production processes and decay properties of the stop at HERA energies. The model is characterized by a light stop possibly lighter than the other squarks. We show that the stop could be singly produced not only in the neutral current processes but also in associated processes whose final states contain some heavy flavor quarks, bottom and top quarks. These signatures would be useful to discriminate the stop from leptoquarks.