In-medium $\omega$ mass from the $\gamma + Nb \to \pi^{0}\gamma + X$ reaction

TL;DR

This study re-analyzed photoproduction data of omega mesons on nuclei to investigate in-medium mass modifications, developing a new background subtraction method, and found no evidence of the previously claimed omega mass shift in nuclear matter.

Contribution

The paper introduces a new background subtraction method for analyzing omega meson photoproduction data, challenging previous claims of in-medium mass shifts.

Findings

No deviation in omega line shape on Nb target compared to hydrogen.

Re-analysis does not confirm earlier in-medium omega mass shift claim.

Developed a new method for background determination in meson photoproduction data.

Abstract

Data on the photoproduction of $\omega$ mesons on nuclei have been re-analyzed in a search for in-medium modifications. The data were taken with the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. First results from the analysis of the data set were published by D. Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 \cite{david}, claiming a lowering of the $\omega$ mass in the nuclear medium by 14$%$ at normal nuclear matter density. The extracted $\omega$ line shape was found to be sensitive to the background subtraction. For this reason a re-analysis of the same data set has been initiated and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the re-analysis and of the background determination are described. The $\omega$ signal on the $Nb$ target, extracted in the re-analysis, does not show a deviation from the corresponding line shape on a $LH_2$ target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the $\omega$ line shape to different in-medium modification scenarios is discussed.