Momentum dependence of the imaginary part of the $\omega$- and $\eta^\prime$-nucleus optical potential

TL;DR

This study measures the momentum dependence of the imaginary part of the optical potential for $\omega$ and $\eta^\prime$ mesons in nuclei, providing insights into their in-medium properties and potential for bound state searches.

Contribution

It provides the first detailed experimental analysis of the energy dependence of the imaginary part of the optical potential for these mesons, with implications for mesic state detection.

Findings

The imaginary part of the $\eta^\prime$ potential is about three times smaller than its real part.

The imaginary part of the $\omega$ potential near threshold is comparable to its real part.

Results support the feasibility of $\eta^\prime$ mesic state searches, but suggest broad $\omega$ states are difficult to observe.

Abstract

The photoproduction of $\omega$ and $\eta^\prime$ mesons off carbon and niobium nuclei has been measured as a function of the meson momentum for incident photon energies of 1.2-2.9 GeV at the electron accelerator ELSA. The mesons have been identified via the $\omega \rightarrow \pi^0 \gamma \rightarrow 3 \gamma$ and $\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6 \gamma$ decays, respectively, registered with the CBELSA/TAPS detector system. From the measured meson momentum distributions the momentum dependence of the transparency ratio has been determined for both mesons. Within a Glauber analysis the in-medium $\omega$ and $\eta^\prime$ widths and the corresponding absorption cross sections have been deduced as a function of the meson momentum. The results are compared to recent theoretical predictions for the in-medium $\omega$ width and $\eta^\prime$-N absorption cross sections. The energy dependence of the imaginary part of the $\omega$- and $\eta^\prime$-nucleus optical potential has been extracted. The finer binning of the present data compared to the existing data allows a more reliable extrapolation towards the production threshold. The modulus of the imaginary part of the $\eta^\prime$ nucleus potential is found to be about three times smaller than recently determined values of the real part of the $\eta^\prime$-nucleus potential, which makes the $\eta^\prime$ meson a suitable candidate for the search for meson-nucleus bound states. For the $\omega$ meson, the modulus of the imaginary part near threshold is comparable to the modulus of the real part of the potential. As a consequence, only broad structures can be expected which makes the observation of $\omega$ mesic states very difficult experimentally.