BRIDGE: Branching Ratio Inquiry/Decay Generated Events

TL;DR

BRIDGE tackles the bottlenecks in beyond-Standard-Model collider simulations by providing a general width calculator (BRI) and a model-agnostic decay generator (DGE) that operate on arbitrary matrix-element models and output Les Houches–formatted events. It relies on Vegas integration to compute $2$- and $3$-body widths and uses HELAS amplitudes to preserve helicity information, enabling efficient long decay chains and partial spin correlations, including MSSM scenarios via MadGraph usrmod. The authors validate BRIDGE against full matrix-element calculations and SUSY-HIT in various MSSM contexts, showing good agreement in decay widths and meaningful retention of angular correlations, while also enabling loop-induced decays like $h\rightarrow gg$. They discuss batch modes, SLHA-based MSSM interfaces, and future enhancements such as finite-width effects and broader model support, highlighting BRIDGE’s practical impact for rapid phenomenology studies and integration with existing event-generation workflows.

Abstract

We present the manual for the program BRIDGE: Branching Ratio Inquiry/Decay Generated Events. The program is designed to operate with arbitrary models defined within matrix element generators, so that one can simulate events with small final-state multiplicities, decay them with BRIDGE, and then pass them to showering and hadronization programs. BRI can automatically calculate widths of two and three body decays. DGE can decay unstable particles in any Les Houches formatted event file. DGE is useful for the generation of event files with long decay chains, replacing large matrix elements by small matrix elements followed by sequences of decays. BRIDGE is currently designed to work with the MadGraph/MadEvent programs for implementing and simulating new physics models. In particular, it can operate with the MadGraph implementation of the MSSM. In this manual we describe how to use BRIDGE, and present a number of sample results to demonstrate its accuracy.

Figures (9)

• Figure 1: We have generated $t\bar{t}$ events in MadGraph and decayed them with BRIDGE, and also generated $e^+ \nu_e b e^- \bar{\nu}_e \bar{b}$ events in MadGraph. Here we plot the $p_T$ histogram for the $b$ quark in the decayed events versus the full matrix element. In this and other figures, the histograms are normalized to have the same area. In this figure we also show $t\bar{t}$ events from MadGraph decayed with BRIDGE with the amplitude set to 1, so that the decay is governed by the phase space volume.
• Figure 2: The $p_T$ histogram for the $e^+$ in the decayed events versus the full matrix element.
• Figure 3: The $M^2(b\bar{b})$ histogram in the decayed events versus the full matrix element.
• Figure 4: The $M^2(e^+e^-)$ histogram in the decayed events versus the full matrix element.
• Figure 5: Histogram of $M_{e^+ b}$ computed from MadGraph $t\bar{t}$ events decayed with BRIDGE compared to MadGraph $e^+\nu_e b \bar{t}$ events. In both cases the $V-A$ and $V+A$ structures for the $W^+\bar{t}b$ vertex are compared.