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  • Letter
  • Open Access

Timelike nucleon electromagnetic form factors: All about interference of isospin amplitudes

Xu Cao1,2,*, Jian-Ping Dai3,†, and Horst Lenske4,‡

  • 1Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
  • 2University of Chinese Academy of Sciences, Beijing 100049, China
  • 3Department of Physics, Yunnan University, Kunming 650091, China
  • 4Institut für Theoretische Physik, Universität Giessen, D-35392 Giessen, Germany

  • *caoxu@impcas.ac.cn
  • †Corresponding author. daijianping@ynu.edu.cn
  • ‡horst.lenske@physik.uni-giessen.de
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Phys. Rev. D 105, L071503 – Published 25 April, 2022

DOI: https://doi.org/10.1103/PhysRevD.105.L071503

Abstract

A striking feature of the timelike nucleon electromagnetic form factors, investigated in e+e−→NN¯ annihilation reactions, is the modulation by local structures of small magnitude and oscillatory form, showing up above the NN¯ threshold. Starting from an isospin decomposition of the proton and neutron form factors it is shown that such structures are the natural consequence of the interference of a large and a small amplitude, resulting in a sinusoidal behavior as a function of the “invariant energy” if the relative phase shift varies with energy. Thus, periodic oscillations superimposed on a smooth background will be observed. In this scenario, the equal size of the modulation for neutrons and protons (discovered by recent BESIII data) implies the particular isoscalar or isovector nature of these local structures, or their orthogonal interference. Hence, it specifies their origin as excited vector mesons whose widths are tied to the modulation frequency. We clarify that the phase difference of the modulation between neutrons and protons as BESIII data found, but not the modulation itself, is the evidence of an imaginary part of the timelike nucleon electromagnetic form factors, which is associated with the rescattering processes.

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Phys. Rev. D 105, L071503– Published 25 April, 2022
  • Received 22 October 2021
  • Accepted 24 March 2022
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