Wide acceptance measurement of the K$^-$/K$^+$ ratio from Ni+Ni collisions at 1.91A GeV

TL;DR

This study measures the K$^-$ to K$^+$ ratio in Ni+Ni collisions at 1.91A GeV, providing wider acceptance data to investigate in-medium kaon modifications and the impact of $ ext{phi}$ meson decays.

Contribution

It offers the first wide acceptance measurement of the K$^-$ to K$^+$ ratio at this energy, including the effects of $ ext{phi}$ meson decay contributions.

Findings

The K$^-$ to K$^+$ ratio shows a decreasing trend with kinetic energy after $ ext{phi}$ subtraction.

Inclusion of $ ext{phi}$ decay reduces differences between K$^-$ and K$^+$ spectra.

$ ext{Lambda}(1520)$ decay contribution is of moderate relevance.

Abstract

The FOPI Collaboration at the GSI SIS-18 synchrotron measured charged kaons from central and semi-central collisions of Ni+Ni at a beam energy of 1.91A GeV. We present the distribution of the K$^-$\K$^+$ ratio on the energy vs polar angle plane in the nucleon-nucleon center-of-mass frame, with and without subtraction of the contribution of $\phi$(1020) meson decays to the K$^-$ yield. The acceptance of the current experiment is substantially wider compared to the previous measurement of the same colliding system. The ratio of K$^-$ to K$^+$ energy spectra is expected to be sensitive to the in-medium modifications of basic kaon properties like mass. Recent results obtained by the HADES Collaboration at 1.23A and 1.76A GeV indicate that after inclusion of the $\phi$ meson decay contribution to the K$^-$ production no difference between the slopes of the K$^-$ and K$^+$ energy spectra is observed within uncertainties. For our data a linear fit to this ratio obtained after subtraction of the $\phi$ meson contribution still shows a decrease with kinetic energy, although a constant value cannot be rejected. The contribution of $\Lambda(1520) \rightarrow p\text{K}^-$ decays estimated from fitting the thermal model to the experimental yields appears to be another factor of moderate relevance.