Antiproton-nucleus electromagnetic annihilation as a way to access the proton timelike form factors

TL;DR

This paper proposes using antiproton-deuteron annihilation to measure proton electromagnetic form factors near the (pbar p) threshold, providing a new method to access time-like region data with estimated cross sections around 1 pb/MeV.

Contribution

It introduces a novel approach to study proton form factors in the time-like region via antiproton-deuteron reactions near threshold, with feasibility analysis for experimental implementation.

Findings

Estimated cross section near threshold is about 1 pb/MeV.

Reactions allow access to proton form factors near the (pbar p) threshold.

Feasibility discussed for the Panda experiment.

Abstract

Contrary to the reaction pbar + p --> e+ e- with a high momentum incident antiproton on a free target proton at rest, in which the invariant mass M of the (e+ e-) pair is necessarily much larger than the (pbar p) mass, in the reaction pbar + d --> n e+ e- the value of M can take values near or below the (pbar p) mass. In the antiproton-deuteron electromagnetic annihilation, this allows to access the proton electromagnetic form factors in the time-like region of q^2 near the (pbar p) threshold. We estimate the cross section dsigma(pbar +d --> e+ e- n)/dM for an antiproton beam momentum of 1.5 GeV/c. We find that near the (pbar p) threshold this cross section is about 1 pb/MeV. The case of heavy nuclei target is also discussed. Elements of experimental feasibility are presented for the process pbar + d --> n e+ e- in the context of the Panda project.