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

Fernando Arias-Aragón, Giovanni Grilli di Cortona, Enrico Nardi

TL;DR

This work addresses whether atomic electron motion can raise the effective center-of-mass energy in high-energy $e^+e^-$ fixed-target collisions. The authors incorporate the atomic-electron momentum distribution $n(k_A)$, derived from Compton profiles, into the di-muon production cross section and evaluate yields for NA64 at beam energies of $E_B=40$, 60, and 70 GeV, using $s=2 m_e (E_B + m_e)$ and the FEAR baseline cross section $\sigma_\text{FEAR}$. They find that at $40$ GeV, di-muon production is possible only due to bound-electron motion, yielding about $1.4\times 10^3$ events, while at $60$ and $70$ GeV the atomic motion reduces yields by roughly $30\%$ relative to FEAR. Theoretical uncertainties are controlled by multiple $n(k_A)$ modeling approaches and independent Monte Carlo integrators, with total uncertainties at the $\mathcal{O}(10\%)$ level. If confirmed experimentally, these results would provide direct high-energy evidence of atomic electron momentum effects and demonstrate a novel way to effectively extend the collision energy in high-$Z$ materials, bridging atomic physics and high-energy phenomenology.

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, $μ^+μ^-$ production via positron annihilation on electrons at rest is kinematically forbidden. The detection of $μ^+μ^-$ 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 $μ^+μ^-$ pairs from positron annihilation is reduced compared to the electron-at-rest approximation.

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

TL;DR

This work addresses whether atomic electron motion can raise the effective center-of-mass energy in high-energy fixed-target collisions. The authors incorporate the atomic-electron momentum distribution , derived from Compton profiles, into the di-muon production cross section and evaluate yields for NA64 at beam energies of , 60, and 70 GeV, using and the FEAR baseline cross section . They find that at GeV, di-muon production is possible only due to bound-electron motion, yielding about events, while at and GeV the atomic motion reduces yields by roughly relative to FEAR. Theoretical uncertainties are controlled by multiple modeling approaches and independent Monte Carlo integrators, with total uncertainties at the level. If confirmed experimentally, these results would provide direct high-energy evidence of atomic electron momentum effects and demonstrate a novel way to effectively extend the collision energy in high- materials, bridging atomic physics and high-energy phenomenology.

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, production via positron annihilation on electrons at rest is kinematically forbidden. The detection of 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 pairs from positron annihilation is reduced compared to the electron-at-rest approximation.

Paper Structure

This paper contains 6 sections, 9 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Overview of the experimental setup
  3. Di-muon production
  4. Theoretical uncertainties
  5. Results
  6. Conclusions

Figures (2)

  • Figure 1: Cross section for the $e^+e^- \to \mu^+\mu^-$ process in the FEAR approximation as a function of the beam energy $E_B$.
  • Figure 2: Number of $\mu^+\mu^-$ events produced at NA64 via $e^+e^-$ annihilation for positron beam energies of $E_B = 40$, 60, and 70 GeV (shown as blue, green, and red solid lines, respectively) compared to the expected yields in the FEAR approximation (green and red dashed lines). In this approximation, for $E_B = 40\,\mathrm{GeV}$ the process remains below the c.m. energy threshold.