Large effects from small QCD instantons: making soft bombs at hadron colliders

Valentin V Khoze,Matthias Schott,Frank Krauss

doi:10.1007/jhep04(2020)201

Abstract

It is a common belief that the last missing piece of the Standard Model of particles physics was found with the discovery of the Higgs boson at the Large Hadron Collider. However, there remains a major prediction of quantum tunnelling processes mediated by instanton solutions in the Yang-Mills theory, that is still untested in the Standard Model. The direct experimental observation of instanton-induced processes, which are a consequence of the non-trivial vacuum structure of the Standard Model and of quantum tunnelling in QFT, would be a major breakthrough in modern particle physics. In this paper, we present for the first time a full calculation of QCD instanton-induced processes in proton-proton collisions accounting for quantum corrections due to both initial and final state gluon interactions, a first implementation in an MC event generator as well as a basic strategy how to observe these effects experimentally.

Highlights

An obvious way to reduce the semiclassical ’t Hooft suppression instanton factor is to consider QCD instantons since the suppression is exponential, e−4π/α, and the strong coupling constant is αs αw
We present for the first time a full calculation of QCD instanton-induced processes in proton-proton collisions accounting for quantum corrections due to both initial and final state gluon interactions, a first implementation in an MC event generator as well as a basic strategy how to observe these effects experimentally
It is verified that the initial state interactions quantum effect provides an exponential cut-off of the large instanton/anti-instanton sizes; the cut-off scale is set by the energy scale s log s of the scattering process, and further it contains a factor of αs, as it should in the radiative corrections

Summary

QCD instanton preliminaries

Instanton [5] is the solution of the classical equations of motion in Euclidean spacetime; for QCD the instanton field configuration involves the gluon component Aiμnst as well as the fermion components — the fermion zero modes ψ(0). Nf light quark flavours; they correspond to the Weyl fermions qLf and qRf. The QCD instanton of topological charge Q = 1 has two fermion zero modes for each of the f = 1, . Taking all momenta on-shell and performing the LSZ reduction for all external legs of the Green’s function G The outcome of this procedure is that the instanton contribution to the n-point amplitude at the leading order is recast as an effective n-point vertex involving ng + 2 gluons and 2Nf quarks,. The instanton production cross-section σfor the process (2.1) can be obtained in the usual way by squaring the scattering amplitude and integrating over the (ng + 2Nf )particle phase space including the relevant symmetry factors. This program was developed and implemented in the classic high-energy instanton papers [14, 15, 17, 18] (for reviews see [40, 48]) in the context of the electroweak theory for (B + L)-violating processes

The optical theorem on the instanton-anti-instanton configuration

The saddle-point solution and the instanton cross-section

Implementation in the SHERPA event generator

Experimental signatures

Findings

Conclusions