Simulation of Beyond Standard Model Physics in Herwig++

TL;DR

Addresses the problem of simulating varied BSM physics within a general-purpose event generator by introducing a modular, factorized approach in Herwig++ that uses Vertex classes and HELAS-like matrix elements for $2\rightarrow 2$ production and subsequent decays. It preserves spin correlations through spin-density matrices and allows QCD radiation to occur between production and decay. Implementations include the MSSM with CP, R-parity and flavour conservation and a Randall–Sundrum KK-model, with validations against full matrix-element calculations. This framework reduces modeling effort for new BSM scenarios while delivering consistent, realistic collider simulations with spin effects and radiation.

Abstract

We present a new approach for the simulation of Beyond Standard Model (BSM) physics within the Herwig++ event generator. Our approach is more generic than previous methods with the aim of minimising the effort of implementing further new physics models. Spin correlations, which are important for BSM models due to new heavy fermions and bosons, are discussed and their effects demonstrated for the Minimal Supersymmetric Standard Model (MSSM) and Randall-Sundrum Model using our new framework.

Figures (4)

• Figure 1: Angle between the beam and the outgoing lepton in $e^+e^-\rightarrow t\bar{t}\rightarrow b\bar{b} l^{+}\nu_l l^{-}\bar{\nu_l}$ in the lab frame for a centre-of-mass energy of 500 GeV with (a) unpolarised incoming beams, (b) negatively polarised electrons and positively polarised positrons and (c) positively polarised electrons and negatively polarised electrons.
• Figure 2: Angle between the lepton and the top quark in $e^+e^-\rightarrow t\bar{t}\rightarrow b\bar{b} l^{+}\nu_l l^{-}\bar{\nu_l}$ in the lab frame for a centre-of-mass energy of 500 GeV with (a) unpolarised incoming beams, (b) negatively polarised electrons and positively polarised positrons and (c) positively polarised electrons and negatively polarised electrons.
• Figure 3: Angle between the outgoing lepton and anti-lepton in $e^+e^-\rightarrow t\bar{t}\rightarrow b\bar{b} l^{+}\nu_l l^{-}\bar{\nu_l}$ in the lab frame for a centre-of-mass energy of 500 GeV with (a) unpolarised incoming beams, (b) negatively polarised electrons and positively polarised positrons and (c) positively polarised electrons and negatively polarised electrons.
• Figure 4: Tree-level topologies for a $2\rightarrow2$ process. The arrows denote the flow of momenta.