Measurement of the neutron timelike electric and magnetic form factors ratio at the VEPP-2000 $e^+e^-$ collider

In the experiment to study the e+e->n+anti n process at the VEPP-2000 $e^+e^-$ collider, the ratio |GE|/|GM| of the neutron timelike electric and magnetic form factors has been measured. The experiment was carried out with the SND detector in the center-of-mass energy range 1890-2000 MeV in 8 energy points with an integrated luminosity of 83 inv.pb. The |GE|/|GM| ratio is determined by the analyzing the distribution of the polar angle of the produced antineutron. The measured |GE|/|GM| value in the energy range under study is between 1.0 and 1.5 with an average value of 1.21+-0.13.

💡 Research Summary

The paper reports the first precise measurement of the ratio of the neutron timelike electric (|G_E|) and magnetic (|G_M|) form factors, |G_E|/|G_M|, using the SND detector at the VEPP‑2000 e⁺e⁻ collider. Data were collected at eight centre‑of‑mass energies between 1.89 GeV and 2.00 GeV, corresponding to beam energies 945 MeV–1003 MeV, with a total integrated luminosity of 83 pb⁻¹. The process under study is e⁺e⁻ → n n̄, which proceeds via a virtual photon; its differential cross‑section depends on both form factors according to

dσ/dΩ = (α²β)/(4s)