Upper limit of the total cross section for the $\mathit{pn}\to \mathit{pn}{\eta }^{\prime }$ reaction

The upper limit of the total cross section for the $\mathit{pn}\to \mathit{pn}{\eta }^{\prime }$ reaction has been determined near the kinematical threshold in the excess energy range from 0 to 24 MeV. The measurement was performed using the COSY-11 detector setup, a deuteron cluster target, and the proton beam of COSY with a momentum of 3.35 GeV$/c$. The energy dependence of the upper limit of the cross section was extracted exploiting the Fermi momenta of nucleons inside the deuteron. Comparison of the determined upper limit of the ratio $R_{{\eta }^{\prime }}=\sigma (\mathit{pn}\to \mathit{pn}{\eta }^{\prime })/\sigma (\mathit{pp}\to \mathit{pp}{\eta }^{\prime })$ with the corresponding ratio for $\eta$-meson production does not favor the dominance of the $N^{*}(1535)$ resonance in the production process of the ${\eta }^{\prime }$ meson and suggests nonidentical production mechanisms for $\eta$ and ${\eta }^{\prime }$ mesons.

Figure 1

Scheme of the COSY-11 detector system with superimposed tracks from the $\mathit{pd}\to p_{\mathrm{spec}}(\mathit{pn}{\eta }^{\prime })$ reaction. Protons are registered in two drift chambers, D1 and D2, and in the scintillator hodoscopes, S1 and S3. The S1 detector is built of 16 vertically arranged modules, whereas the S3 hodoscope is a nonsegmented scintillator wall viewed by a matrix of 217 photomultipliers. An array of silicon pad detectors (spectator detector) is used for registration of spectator protons. Neutrons are registered in the neutral particle detector, consisting of 24 independent detection units. To distinguish neutrons from charged particles, a veto detector is used. Elastically scattered protons are measured in the scintillator detector, S4, and the position-sensitive silicon detector, ${\mathrm{Si}}_{\mathrm{mon}}$. The sizes of the detectors and their relative distances are not to scale.

Figure 2

Momentum distribution of the spectator proton as reconstructed in the experiment (crosses) in comparison with simulations taking into account the Fermi momentum distribution of nucleons inside the deuteron according to the Paris potential [24] (solid histogram). Simulated events were analyzed in the same way as the experimental data.

Figure 3

(a, c, e) Experimental missing mass distributions of the $\mathit{pn}\to \mathit{pnX}$ reaction (solid lines). Distributions were obtained for ranges of excess energy $Q$ from 0 to 8 MeV (a), from 8 to 16 MeV (c), and from 16 to 24 MeV (e). Corresponding background spectra are shown as dashed lines. (b, d, f) Experimental missing mass distributions of the $\mathit{pn}\to \mathit{pn}{\eta }^{\prime }$ reaction for excess energy ranges from 0 to 8 MeV (b), from 8 to 16 MeV (d), and from to 24 MeV (f) determined after background subtraction. Vertical bars indicate statistical errors. The arrow depicts the nominal value of the ${\eta }^{\prime }$ mass.

Figure 4

Total cross sections for the $\mathit{pp}\to \mathit{pp}{\eta }^{\prime }$ reaction as a function of the excess energy (open squares). Upper limit of the total cross section for the $\mathit{pn}\to \mathit{pn}{\eta }^{\prime }$ reaction as a function of the excess energy (filled circles).

Figure 5

Upper limit of the ratio $(R_{{\eta }^{\prime }})$ of the total cross sections for the $\mathit{pn}\to \mathit{pn}{\eta }^{\prime }$ and $\mathit{pp}\to \mathit{pp}{\eta }^{\prime }$ reactions (arrows) in comparison with the ratio $(R_{\eta })$ determined for the $\eta$ meson [16, 17] (open squares). The superimposed solid line indicates the result of a fit to the $R_{\eta }$ data taking into account the final-state interaction of nucleons [17]. The dotted line presents the result of calculations performed under the assumption of dominance of the ${\mathrm{S}}_{11}$(1535) resonance in the production process [13], and the dashed line denotes the result obtained within a covariant effective meson-nucleon theory including meson and nucleon currents with nucleon resonances ${\mathrm{S}}_{11}$(1650), ${\mathrm{P}}_{11}$(1710), and ${\mathrm{P}}_{13}$(1720) [12].