$\Lambda -p$ femtoscopy in collisions of $\mathrm{Ar}+\mathrm{KCl}$ at $1.76A$ GeV

Results on $\Lambda p$ femtoscopy are reported at the lowest energy so far. At a beam energy of $1.76A$ GeV, the reaction $\mathrm{Ar}+\mathrm{KCl}$ was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $\Lambda$ sample was collected, allowing for the investigation of $\Lambda p$ correlations at low relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the $\Lambda p$ emission source. The $\Lambda p$ source radius is found to be slightly smaller than the $\mathit{pp}$ correlation radius for a similar collision system. The present $\Lambda p$ radius is significantly smaller than that found for $\mathrm{Au}+\mathrm{Au}/\mathrm{Pb}+\mathrm{Pb}$ collisions in the AGS, SPS, and RHIC energy domains but larger than that observed for electroproduction from He. Taking into account all available data, we find the $\Lambda p$ source radius to increase almost linearly with the number of participants to the power of one-third.

Figure 1

Top: $p{\pi }^{-}$ invariant-mass distribution. Light- and dark-hatched areas represent the statistics for the $\Lambda$ signal and the corresponding $\Lambda$ impurity, respectively, both contributing to the $\Lambda p$ correlation analysis. Bottom: Uncorrected $\Lambda p$ correlation function.

Figure 2

The same as Fig. 1, but for simulated $\Lambda$ hyperons with their decay products embedded in true experimental data. The solid curve in the lower panel is a fit with a Fermi-type function (see text) used to correct for reconstruction losses (owing to close tracks) of the experimental $\Lambda p$ correlation function displayed in Fig. 1.

Figure 3

Left: $\Lambda p$ correlation function after close-track and purity corrections. The solid curve represents the best fit (see text) with the analytical model of Lednicky and Lyuboshitz [9]. Right: Gaussian radius of the $\Lambda p$ emission source as a function of system size. Symbols indicate data taken with HADES at SIS, CLAS [6] at JLab, E895 [3] at AGS, NA49 [4] at SPS, and STAR [5] at RHIC. The dashed line is a linear regression to the data.