Probing Late-Stage Hadronic Interactions at High Baryon Density via $K^{*0}$ Production in the RHIC Beam Energy Scan Program

Abstract

A precision measurement of the $K^{*0}$ meson yield is reported in Au+Au collisions at $\sqrt{s_{NN}} = 7.7,\; 11.5,\; 14.6,\; 19.6,$ and $27~\mathrm{GeV}$ using the high-statistics data sample collected by the STAR experiment during the Beam Energy Scan II (BES-II) program at RHIC. The transeverse momentum ($p_{T}$)-integrated yield ratios $(K^{*0} + \overline{K^{*0}})/(K^{+} + K^{-})$ in central collisions show a suppression relative to peripheral collisions at the $(1.7\text{-}3.6)\,σ$ level, while a thermal model without final-stage rescattering overpredicts this ratio with a deviation of $(6.9\text{-}8.2)\,σ$. These results indicate a loss of the measured $K^{*0}$ signal in central collisions due to re-scattering of its hadronic decay products in the hadronic phase. The $p_{T}$-integrated yield of charged kaons exhibits an approximate scaling with charged-particle multiplicity, independent of collision energy and system size. A similar trend is observed for the short-lived $K^{*0}$ resonance, although significant deviations emerge at lower energies. At BES energies, the $K^{*0}/K$ ratio shows stronger suppression than at the highest RHIC and LHC energies within a given multiplicity bin, particularly in central and mid-central collisions. This behavior is consistent with changes in the effective hadronic interaction cross section and is supported by transport model calculations, which indicate dominant meson-baryon interactions at lower energies and meson-meson interactions at higher energies.

Figures (5)

• Figure 1: (Left panels) $K^{*0}$ signal from unlike-sign pairs (red markers) and background estimated from the track rotation method (black markers). (Right panels) Invariant mass distribution of $K\pi$ pairs after subtraction of the estimated background using the track rotation method. The blue line denotes the Breit Wigner fit and the dashed red line represents the residual background.
• Figure 2: $K^{*0}$$p_{T}$ spectra at mid rapidity ($|y| < 1.0$) in various centralities in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.6, 19.6 and 27 GeV. The data-points are placed to the mean position in each $p_T$ bin. The statistical and systematic uncertainties are shown as bars and boxes, respectively.
• Figure 3: The $p_{T}$-integrated yield of $K^{*0}$ (left panel) and charged kaons (right panel) at mid-rapidity as a function of $(dN_{ch}/dy)^{1/3}$ for various collision systems and collision energies STAR:2008medSTAR:2017salSTAR:2019vcpBRAHMS:2016cucu.For the $K^{*0}$ measurement, a rapidity window of $|y|<1.0$ is used at BES energies and $|y|<0.5$ at $62.4$ and $200$ GeV. In contrast, kaon measurements are done within $|y|<0.1$ for all collision systems and energies. The error bars shown here are the quadrature sum of statistical and systematic uncertainties. The dotted line shown on both the panels are the fit to 200 GeV data with a polynomial function.
• Figure 4: $K^{*0}/K$ ratio as a function of $(dN_{ch}/dy)^{1/3}$ in Au+Au collisions. The measurements are compared with previous experimental results. The data points with closed symbols are generated from this analysis. The model calculation (blue solid line) is from LEROUX2021136284. UrQMD results for $\sqrt{s_{NN}}$ = 7.7 and 200 GeV Au+Au collisions are shown in colored bands and taken from Sahoo:2024urqmd. The $K^{*0}(\overline{K^{*0}})$ and $K^{\pm}$ are combined and denoted by $K^{*0}$ and $K$, respectively, for BES energies.
• Figure 5: $K^{*0}/K$ ratios as a function of $\sqrt{s_{NN}}$ in Au+Au collisions in most-central (left panel) and most-peripheral (right panel) collisions. The measurements are compared with a thermal model Sahoo:2023thermal (dashed line) and the UrQMD model Sahoo:2024urqmd (blue band). The central collisions correspond to 0-10% centrality (0-20% for 62.4 GeV) and the peripheral collisions correspond to 60-80% centrality for all RHIC energies. The $K^{*0}(\overline{K^{*0}})$ and $K^{\pm}$ are combined and denoted by $K^{*0}$ and $K$, respectively, for BES energies.