Study of the process $e^{+}{e}^{-}\to n\overline{n}$ at the VEPP-2000 $e^{+}{e}^{-}$ collider with the SND detector

The process $e^{+}{e}^{-}\to n\overline{n}$ has been studied at the VEPP-2000 $e^{+}{e}^{-}$ collider with the SND detector in the energy range from threshold up to 2 GeV. As a result of the experiment, the $e^{+}{e}^{-}\to n\overline{n}$ cross section and effective neutron form factor have been measured.

Figure 1

Schematic view of the SND detector. The collider vacuum pipe (1) is surrounded by the tracking detector (2) based on a nine-layer drift chamber. The aerogel Cherenkov counter (3) provides $K$ meson identification. The spherical electromagnetic calorimeter consists of 1680 NaI(Tl) crystals (4) with phototriode (5) readout. The muon detector (7)–(9) located after the iron absorber (6) provides muon identification and suppression of cosmic-ray background.

Figure 2

Neutron and antineutron interaction lengths in NaI(Tl) as a function of the particle energy.

Figure 3

The energy dependence of the detection efficiency for $e^{+}{e}^{-}\to n\overline{n}$ events determined using MC simulation. The filled circles show the efficiency for the 2011 data set, and the triangles for 2012.

Figure 4

The detection efficiency for $e^{+}{e}^{-}\to n\overline{n}$ events as a function of $\cos \theta$. The variable $\cos \theta$ bin size is used, corresponding to $\mathrm{\Delta }\theta =9°$.

Figure 5

(a) The distribution of the longitudinal position (number of EMC layer) of the crystal with maximum energy deposition in an $e^{+}{e}^{-}\to n\overline{n}$ event. (b) The distribution of the total energy deposition in the EMC for $e^{+}{e}^{-}\to n\overline{n}$ events. The points with error bars represent data. The histogram is the simulated distribution normalized to the area of the data distribution.

Figure 6

The distribution of the difference between the true and measured antineutron polar angles at $E_b=960\mathrm{MeV}$.

Figure 7

(a) The $\cos \theta$ distribution for simulated $e^{+}{e}^{-}\to n\overline{n}$ events generated with $G_E=0$ and $G_M=0$. (b) The $\cos \theta$ distribution for data $e^{+}{e}^{-}\to n\overline{n}$ events.

Figure 8

The energy dependence of the radiative correction for the $e^{+}{e}^{-}\to n\overline{n}$ process. The vertical line indicates the $n\overline{n}$ threshold.

Figure 9

The $e^{+}{e}^{-}\to n\overline{n}$ cross section measured in this paper. The filled triangles represent 2011 data, while the filled circles correspond to 2012 data. The FENICE results [4] are shown by the empty squares. The lines below and above the $n\overline{n}$ threshold indicate the average levels of the cross section. The quoted errors are statistical.

Figure 10

A comparison of the neutron effective form factor measured in this paper (SND) and in Ref. [4] (FENICE) and the proton effective form factor measured in the BABAR experiment [2].