[Paper Review] ISAJET 7.37: A Monte Carlo Event Generator for $pp$, $\bar pp$, and $e^+e^-$ Interactions

Howard Baer, F. Paige+2 more|ArXiv.org|Apr 17, 1998

Particle physics theoretical and experimental studies322 citations

TL;DR

ISAJET 7.37 is a Monte Carlo event generator for high-energy $pp$, $\bar{p}p$, and $e^+e^-$ collisions, implementing perturbative QCD, QCD radiative corrections, parton fragmentation, and beam jet modeling. It supports hard scattering processes including QCD, Drell-Yan, W-pair, Higgs, SUSY, and technicolor, with full decay chain simulations via ISASUSY for the MSSM, enabling detailed phenomenological studies of new physics at hadron and $e^+e^-$ colliders.

ABSTRACT

ISAJET is a Monte Carlo event generator for $pp$, $\bar pp$, and $e^+e^-$ interactions at high energies. This document summarizes the physics underlying the program and describes how to use it.

Motivation & Objective

• To provide a comprehensive Monte Carlo event generator for simulating high-energy hadron and $e^+e^-$ collisions across multiple energy regimes.
• To incorporate perturbative QCD cross sections and leading-logarithmic QCD radiative corrections for initial and final states.
• To model parton fragmentation into hadrons and beam jet contributions using phenomenological tuning to minimum bias data.
• To enable detailed simulation of supersymmetric particle production and decay chains via the ISASUSY module for the Minimal Supersymmetric Standard Model (MSSM).
• To support $e^+e^-$ processes with polarized beams and cross sections for squarks, sleptons, gauginos, and Higgs bosons in both MSSM and SUGRA models.

Proposed method

• Generate primary hard scattering events using QCD cross sections with parton distribution functions (e.g., CTEQ3L, EHLQ, Duke-Owens) and $Q^2$-dependent structure functions.
• Apply QCD radiative corrections in the leading-log approximation for initial and final states using perturbative QCD evolution.
• Simulate parton fragmentation into hadrons using independent fragmentation models, with lifetimes below $10^{-12}$ s treated as prompt decays.
• Model beam jets as minimum-bias events at remaining energy, ensuring total cross section consistency.
• Integrate ISASUSY to compute branching ratios for MSSM particles, including cascade decays and R-parity conserving modes.
• Support $e^+e^-$ processes via dedicated cross sections for SM and SUSY particles, with polarized beam options and SUGRA parameterization via RGEs.

Experimental results

Research questions

• RQ1 How can a unified Monte Carlo framework simulate diverse high-energy processes across $pp$, $\bar{p}p$, and $e^+e^-$ colliders with consistent QCD and electroweak dynamics?
• RQ2 What is the impact of including full cascade decays and ISASUSY branching ratios on the phenomenology of supersymmetric particle signals?
• RQ3 How accurately can beam jet modeling reproduce the total inelastic cross section when combined with two-jet events?
• RQ4 To what extent does the inclusion of polarized $e^+e^-$ beams improve sensitivity to SUSY particle production and spin correlations?
• RQ5 How do SUGRA parameter constraints (e.g., $m_0$, $m_{1/2}$, $A_0$, $\tan\beta$, $\text{sgn}(\mu)$) affect the predicted spectrum and decay patterns?

Key findings

• ISAJET 7.37 supports a wide range of hard scattering processes, including QCD, Drell-Yan, W-pair, Higgs, SUSY, and technicolor, with full matrix element calculations for W decays.
• The inclusion of ISASUSY enables accurate simulation of MSSM decay chains, including cascade decays, with branching ratios derived from the model's parameter space.
• The SUGRA model implementation allows solving renormalization group equations iteratively using Runge-Kutta integration with correct thresholds, reducing the parameter space to four key inputs.
• Polarized $e^+e^-$ processes are supported with dedicated cross sections for squarks, sleptons, gauginos, and Higgs bosons, enhancing sensitivity to SUSY signals.
• The generator correctly models $g + t \to g + t$ and $g + g \to t + \bar{t}$ processes with full top quark mass effects, crucial for top physics.
• Several bugs related to beam jet generation, gluino decay, and $\tilde{g} \to \tilde{q}\bar{q}$ branching ratios were corrected in versions 7.13 and later, improving reliability.