Unveiling atomic electron motion effects in $e^+e^-$ high energy collisions at NA64

TL;DR

This paper proposes that atomic electron motion effects can be directly observed in high-energy fixed-target experiments like NA64 at CERN, by detecting muon pairs that indicate increased center-of-mass energy due to electron momentum.

Contribution

It introduces a novel method to observe atomic electron motion effects in high-energy collisions through muon pair detection at NA64, providing potential experimental evidence of this phenomenon.

Findings

Detection of muon pairs would confirm increased center-of-mass energy from electron motion.

Predicted muon pair rates vary with beam energy, showing reductions at higher energies.

Proposes specific experimental signatures for upcoming NA64 runs.

Abstract

Atomic electron motion is responsible for well-studied effects observed in low-energy (MeV-scale) processes. Recently, interest in this phenomenon has also emerged within the high-energy physics community, due to its potential to increase significantly the center-of-mass energy in fixed-target experiments. However, direct experimental evidence of this effect in high energy collisions has yet to be observed. We argue that a striking manifestation of atomic electron momenta could be revealed by the NA64 experiment at CERN during the proposed run with a 40 GeV positron beam. At this energy, $\mu^+\mu^-$ production via positron annihilation on electrons at rest is kinematically forbidden. The detection of $\mu^+\mu^-$ pairs from the annihilation channel would thus constitute direct evidence of an increase in the center-of-mass energy resulting from atomic electron motion. We also investigate the expected signatures for the proposed 60 GeV run, as well as for the data already collected at 70 GeV. Intriguingly, in both these cases, the predicted number of $\mu^+\mu^-$ pairs from positron annihilation is reduced compared to the electron-at-rest approximation.