Near-threshold production of the $\eta$ meson via the quasifree $\mathit{pn}\to \mathit{pn}\eta$ reaction

Total cross sections for the quasifree $\mathit{pn}\to \mathit{pn}\eta$ reaction in the range from the kinematical threshold up to 20 MeV excess energy have been determined. At threshold they exceed corresponding cross sections for the $\mathit{pp}\to \mathit{pp}\eta$ reaction by a factor of about 3 in contrast to the factor of 6 established for higher excess energies. To large extent, the observed decrease of the ratio $\sigma (\mathit{pn}\to \mathit{pn}\eta )/\sigma (\mathit{pp}\to \mathit{pp}\eta )$ toward threshold may be assigned to the different energy dependence of the proton-proton and proton-neutron final-state interactions. The experiment was conducted using a proton beam of the cooler synchrotron COSY and a cluster jet deuteron target. The proton-neutron reactions were tagged by the spectator proton whose momentum was measured for each event. Protons and neutron outgoing from the $\mathit{pn}\to \mathit{pn}\eta$ reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.

Figure 1

(a) Schematic view of the COSY-11 detection setup [26]. D1 and D2 represent drift chambers. S1, S3, and S4 denote scintillator counters. N stands for the neutral particle detector and V for the veto counter. ${\mathrm{Si}}_{\mathrm{mon}}$ and ${\mathrm{Si}}_{\mathrm{spec}}$ are silicon strip detectors used for the detection of the elastically scattered protons and spectator protons, respectively. Superimposed solid lines indicate correspondingly spectator proton, neutron, and forward-scattered protons from the quasifree $\mathit{pd}\to p_{\mathrm{sp}}pn\eta$ reaction and the dashed lines show an example of the quasifree elastically scattered protons used for the monitoring of the luminosity. The size of detectors and their relative distances are not to scale. (b) Projection of the arrangement of the spectator detector as seen along the beam axis. The beam is moving in the direction perpendicular to the plane of the projection, while the target deuterons are moving from the bottom to the top.

Figure 2

Distribution of the time of flight measured between the target and the neutral particle detector.

Figure 3

dE–E plot for events registered in the spectator silicon detector for two pairs placed under different angles with respect to the interaction region. Signals from deuterons are not observed since deuterons cannot be emitted toward the detector (upward to the proton beam) due to the kinematics.

Figure 4

(a) Points with error bars denote experimental distribution of the momentum of the spectator protons. Full line represents the Monte Carlo spectrum of the spectator momentum resulting from the Fermi momentum distribution [36] of nucleons inside the deuteron convoluted with the acceptance and efficiency of the COSY-11 detector setup. (b) Experimental distribution of the excess energy $Q$ with respect to the $\mathit{pn}\eta$ system determined for the $\mathit{pn}\to \mathrm{pnX}$ reaction (solid line) and for the $\mathit{pn}\to \mathit{pn}\eta$ events (shaded histogram).

Figure 5

(a) The solid line denotes the missing mass distribution of the $\mathit{pn}\to \mathrm{pnX}$ reaction determined for the excess energy range from 10 to 15 MeV, and the dashed line shows the background distribution constructed from events with $Q<0$ according to the method described in Ref. [38]. (b) Missing mass distribution of the $\mathit{pn}\to \mathit{pn}\eta$ reaction determined by subtraction of histograms from the left panel. Vertical bars indicate statistical errors only. Superimposed dashed line denotes result of simulations normalized to the data in amplitude.

Figure 6

(a) Excitation functions for the near-threshold $\eta$-meson production via reactions $\mathit{pn}\to \mathit{pn}\eta$ (filled symbols) and $\mathit{pp}\to \mathit{pp}\eta$ (open symbols) [1, 2, 3, 4, 5, 6]. Results of this article, presented as full circles, are consistent with values of cross sections determined by the CELSIUS/WASA group (filled squares) [15]. (b) Ratio of the total cross sections for the $\mathit{pn}\to \mathit{pn}\eta$ and $\mathit{pp}\to \mathit{pp}\eta$ reactions. A superimposed line indicates a result of the fit taking into account the final-state interaction of nucleons.