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

M. N. Achasov, A. E. Alizzi, A. Yu. Barnyakov, E. V. Bedarev, K. I. Beloborodov, A. V. Berdyugin, A. G. Bogdanchikov, A. A. Botov, T. V. Dimova, V. P. Druzhinin, R. A. Efremov, V. N. Zhabin, V. V. Zhulanov, P. V. Zhulanova, L. V. Kardapoltsev, A. S. Kasaev, A. A. Kattsin, D. P. Kovrizhin, I. A. Koop, A. A. Korol, A. S. Kupich, A. P. Kryukov, N. A. Melnikova, N. Yu. Muchnoi, A. E. Obrazovsky, A. A. Oorzhak, I. V. Ovtin, E. V. Pakhtusova, I. A. Polomoshnov, K. V. Pugachev, S. A. Rastigeev, Yu. A. Rogovsky, A. I. Senchenko, S. I. Serednyakov, Z. K. Silagadze, K. D. Sungurov, I. K. Surin, Yu. V. Usov, A. G. Kharlamov, D. E. Chistyakov, D. A. Shtol

TL;DR

The paper addresses the neutron timelike form-factor ratio in the process $e^+e^- \to n\bar{n}$ near threshold. It employs the SND detector at VEPP-2000 to extract $|G_E|/|G_M|$ from the angular distribution of the produced antineutron, using MC-derived templates for the magnetic and electric contributions and a time-spectrum analysis to separate signal from background. Across eight energy points in the range $E_{cm} \approx 1.89$–$2.00$ GeV, the measured $|G_E|/|G_M|$ values lie between $1.0$ and $1.5$, with a weighted average of $1.21 \pm 0.13$, consistent with prior SND and BESIII results and with theoretical expectations near threshold. The study also reports a small but nonzero charge asymmetry in the $\cos\theta$ distributions, discusses possible detector or higher-order QED origins, and provides a robust assessment of systematic uncertainties, reinforcing the reliability of the measured form-factor ratio for nucleon structure studies in the timelike region.

Abstract

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.

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

TL;DR

The paper addresses the neutron timelike form-factor ratio in the process near threshold. It employs the SND detector at VEPP-2000 to extract from the angular distribution of the produced antineutron, using MC-derived templates for the magnetic and electric contributions and a time-spectrum analysis to separate signal from background. Across eight energy points in the range – GeV, the measured values lie between and , with a weighted average of , consistent with prior SND and BESIII results and with theoretical expectations near threshold. The study also reports a small but nonzero charge asymmetry in the distributions, discusses possible detector or higher-order QED origins, and provides a robust assessment of systematic uncertainties, reinforcing the reliability of the measured form-factor ratio for nucleon structure studies in the timelike region.

Abstract

In the experiment to study the e+e->n+anti n process at the VEPP-2000 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.
Paper Structure (5 sections, 6 equations, 8 figures, 1 table)

This paper contains 5 sections, 6 equations, 8 figures, 1 table.

Table of Contents

  1. Collider, detector, experiment
  2. Event selection
  3. Distribution of $\cos\theta$
  4. Measurement of the neutron $|G_E|/|G_M|$ ratio
  5. Summary

Figures (8)

  • Figure 1: SND detector, section along the beams: (1) beam pipe, (2) tracking system, (3) aerogel Cherenkov counters, (4) NaI (Tl) crystals, (5) vacuum phototriodes, (6) iron absorber, (7) proportional tubes, (8) iron absorber, (9) scintillation counters, (10) VEPP-2000 focusing solenoids.
  • Figure 2: The time distribution for selected data events at $E_b=950$ MeV (points with error bars). The wide peak to the right is formed by delayed $n\bar{n}$ events. The vertical line at $t=0$ shows the time of beam collision. The red curve is the fit of the time spectrum described in the text.
  • Figure 3: The MC detection efficiency $\varepsilon$ for $e^+e^-\to n\bar{n}$ process versus the beam energy.
  • Figure 4: The MC detection efficiency $\varepsilon$ versus $\cos\theta$. The vertical lines at $\cos\theta=\pm 0.8$ show the area used in analysis.
  • Figure 5: The $\cos\theta$ distribution of selected $n\bar{n}$ events at the beam energy 970 MeV (points with error bars). The solid (red) curve is the result of the fit described in the text. The histograms labeled $G_E$ and $G_M$ show the contributions of the electric and magnetic form factors, respectively. The vertical lines at $\cos\theta=\pm 0.8$ delimit the area for analysis.
  • ...and 3 more figures