Cross sections for multi-particle final states at a linear collider

TL;DR

The study benchmarks two independent multi-purpose matrix-element generators, HELAC/PHEGAS and AMEGIC++, by computing total cross sections for numerous six-particle final states in $e^+e^-$ collisions. It details the complementary algorithms (Dyson-Schwinger recursion with color-flow in HELAC/PHEGAS vs diagrammatic helicity amplitudes with color grouping in AMEGIC++) and uses adaptive multichannel phase-space integration. The results show strong agreement between the codes across 86 processes, including top-quark channels, vector-boson fusion, Higgs-strahlung, and triple-Higgs scenarios, and quantify the impact of QCD vs electroweak contributions and Higgs effects. This work validates the precision of these tools for planning and interpreting high-precision measurements at a future linear collider, where six-particle final states will be central to testing the Higgs potential and electroweak dynamics.

Abstract

In this paper total cross sections for signals and backgrounds of top- and Higgs-production channels in electron-positron collisions at a future linear collider are presented. All channels considered are characterized by the emergence of six-particle final states. The calculation takes into account the full set of tree-level amplitudes in each process. Two multi-purpose parton level generators, HELAC/PHEGAS and AMEGIC++ are used, their results are found to be in perfect agreement.

Figures (3)

• Figure 1: Factoring out common pieces of amplitudes with identical colour structure. In the example above, the parts within the boxes are identical, hence the two amplitudes can be added and the terms inside the box can be factored out.
• Figure 2: Translation of a Feynman diagram into a phase-space parametrization. $D_{s,a}$ denote symmetric or asymmetric decays ; the latter ones reproduce the typical feature of collinear emission of particles notorious for gauge theories with massless spin-1 bosons. The propagator terms for massless particles $P_0$ peak at the minimal allowed invariant mass.
• Figure 3: The distribution of deviations $s^{(i)}$, given by Eq.\ref{['deviationformula']}, for the eighty-six total cross sections $i$ presented in this paper. The average value is $\bar{s} = -0.065$, their variance is $\sigma_s \approx 1$ .