Supersymmetry Simulations with Off-Shell Effects for LHC and ILC

TL;DR

This paper addresses the need for accurate SUSY collider phenomenology by incorporating off-shell effects and multi-particle final states into MSSM event generation. It implements the MSSM Lagrangian in three independent tools (MADGRAPH II/MADEVENT, O’MEGA/WHIZARD, AMEGIC++/SHERPA) and performs extensive cross-checks across hundreds of 2→2 processes to ensure gauge invariance, unitarity, and consistent SLHA-based inputs. The authors then apply these methods to sbottom production at the LHC and an ILC, demonstrating that off-shell contributions and initial-state radiation can substantially modify cross sections and kinematic distributions, often beyond simple Breit-Wigner or narrow-width estimates. They provide a comprehensive set of cross sections as a reference for validating MSSM implementations and emphasize that full off-shell calculations are essential for reliable precision measurements and signal isolation at future colliders.

Abstract

At the LHC and at an ILC, serious studies of new physics benefit from a proper simulation of signals and backgrounds. Using supersymmetric sbottom pair production as an example, we show how multi-particle final states are necessary to properly describe off-shell effects induced by QCD, photon radiation, or by intermediate on-shell states. To ensure the correctness of our findings we compare in detail the implementation of the supersymmetric Lagrangian in MadGraph, Sherpa and Whizard. As a future reference we give the numerical results for several hundred cross sections for the production of supersymmetric particles, checked with all three codes.

Figures (8)

• Figure 1: The $p_{T,b}^{\rm max}$ (left) and $p_T\space/\space$ (right) distributions for the signal process $gg\to b\bar{b}\tilde{\chi}^0_1\tilde{\chi}^0_1$ and the main SM background $pp\to b\bar{b}\nu\bar{\nu}$, at the LHC. The missing transverse momentum $p_T\space/\space$ is defined as the transverse momentum of the $\tilde{\chi}^0_1\tilde{\chi}^0_1$ or $\nu\bar{\nu}$ pair and does not include $b$ decay products. Both processes are evaluated including all off-shell diagrams.
• Figure 2: The $p_{T,b}$ (left) and $\eta_b$ (right) distributions for $gg\to b\bar{b}\tilde{\chi}^0_1\tilde{\chi}^0_1$ at the LHC. The blue (red) curves correspond to the harder (softer) of the two $b$ jets. The dashed lines show the Breit-Wigner approximation for sbottoms; solid lines include all off-shell effects.
• Figure 3: The $p_{T,b}$ distributions for the LHC process $gg\to b\bar{b}b\bar{b}\tilde{\chi}^0_1\tilde{\chi}^0_1$. The left panel orders the jets according to their $p_{T,b}$, while in the right panel they are ordered by $|\eta_b|$. These peaks from left to right corresond to more central jets.
• Figure 4: The $p_{T,b}^{\rm max}$ (left) and $p_T\space/\space$ (right) distributions for $gg\to b\bar{b}b\bar{b}\tilde{\chi}^0_1\tilde{\chi}^0_1$ (red) and $gg\to b\bar{b}\tilde{\chi}^0_1\tilde{\chi}^0_1$ (blue) at the LHC.
• Figure 5: The $b\bar{b}$ invariant mass distribution for the $\tilde{\chi}^0_1\tilde{\chi}^0_3$ contribution to $e^+e^-\to b\bar{b}\tilde{\chi}^0_1\tilde{\chi}^0_1$.