Determination of the Total Width of the ${\eta }^{\prime }$ Meson

Taking advantage of both the low-emittance proton beam of the cooler synchrotron COSY and the high momentum precision of the COSY-11 detector system, the mass distribution of the ${\eta }^{\prime }$ meson was measured with a resolution of $0.33\mathrm{MeV}/c^2$ (FWHM), improving the experimental mass resolution by almost an order of magnitude with respect to previous results. Based on the sample of more than 2300 reconstructed $pp\to pp{\eta }^{\prime }$ events, the total width of the ${\eta }^{\prime }$ meson was determined to be ${\Gamma }_{{\eta }^{\prime }}=0.226\pm 0.017(\mathrm{stat})\pm 0.014(\mathrm{syst})\mathrm{MeV}/c^2$.

Figure 1

Schematic view of the COSY-11 detector setup (top view). $S1$, $S2$, $S3$, and $S4$ denote scintillator detectors, $D1$ and $D2$ indicate drift chambers, and Si stands for the silicon-pad detector. Left corner: Momentum spectrum for the measurement with the nominal beam momentum of $3211\mathrm{MeV}/c$. As an example, the effective spread of the beam momentum due to the dispersion is shown as for a target width of 1 mm (dashed line) and 1 cm (solid line).

Figure 2

Missing-mass spectra for the $pp\to ppX$ reaction determined at beam momenta of $3211$ (open points) and $3224\mathrm{MeV}/c$ (filled points). The filled points were shifted to the kinematic limit and normalized to the open points.

Figure 3

The missing-mass spectra for the $pp\to ppX$ reaction. The ${\eta }^{\prime }$ meson signal is clearly visible. The experimental data are presented as points, while in each plot the line corresponds to the sum of the Monte Carlo generated signal for the $pp\to pp{\eta }^{\prime }$ reaction with ${\Gamma }_{{\eta }^{\prime }}=0.226\mathrm{MeV}/c^2$ and the background obtained from another energy. The plot at the bottom right of the figure presents ${\chi }^2$ as a function of the ${\Gamma }_{{\eta }^{\prime }}$. The minimum value of the ${\chi }^2$ divided by the number of degrees of freedom amounts to 0.96.

Figure 4

Comparison of available values of ${\Gamma }_{{\eta }^{\prime }}$.