Freeze-out dynamics via charged kaon femtoscopy in $\sqrt{s_{NN}}=200$ GeV central Au + Au collisions

We present measurements of three-dimensional correlation functions of like-sign, low-transverse-momentum kaon pairs from $\sqrt{s_{NN}}=200$ GeV $\mathrm{Au}+\mathrm{Au}$ collisions. A Cartesian surface-spherical harmonic decomposition technique was used to extract the kaon source function. The latter was found to have a three-dimensional Gaussian shape and can be adequately reproduced by Therminator event-generator simulations with resonance contributions taken into account. Compared to the pion one, the kaon source function is generally narrower and does not have the long tail along the pair transverse momentum direction. The kaon Gaussian radii display a monotonic decrease with increasing transverse mass $m_{\mathrm{T}}$ over the interval of $0.55\le m_{\mathrm{T}}\le 1.15$ GeV/$c^2$. While the kaon radii are adequately described by the $m_{\mathrm{T}}$ -scaling in the outward and sideward directions, in the longitudinal direction the lowest $m_{\mathrm{T}}$ value exceeds the expectations from a pure hydrodynamical model prediction.

Figure 1

Independent correlation moments $R^l\left(q\right)$ for orders $l=0,2,4$ for midrapidity, low-transverse-momentum kaon pairs from the 20% most central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV. Panel (a) also shows a comparison between $R^0\left(q\right)$ and $R\left(q\right)$. The error bars are statistical. The solid curves represent results of the Gaussian fit.

Figure 2

Kaon correlation function profiles (circles) for midrapidity, low-transverse-momentum kaon pairs from the 20% most central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV (a) $C\left(q_x\right)\equiv C(q_x,0,0)$, (b) $C\left(q_y\right)\equiv C(0,q_y,0)$, and (c) $C\left(q_z\right)\equiv C(0,0,q_z)$ in the $x$, $y$, and $z$ directions. The curves denote the Gaussian fit profiles.

Figure 3

Kaon source function profiles extracted from the data (solid circles with error band) and 3D pion source function (squares) from PHENIX [9] together with Therminator model calculation for kaons with indicated parameter values (triangles).

Figure 4

Transverse mass dependence of Gaussian radii (a) $R_{\mathrm{out}}$, (b) $R_{\mathrm{side}}$, and (c) $R_{\mathrm{long}}$ for midrapidity kaon pairs from the 30% most central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV. STAR data are shown as solid stars; PHENIX data [10] are shown as solid circles (error bars include both statistical and systematic uncertainties). Hydrokinetic model [23] with initial Glauber condition and Buda-Lund model [22] calculations are shown by solid squares and solid curves, respectively. The dotted line between the solid squares is to guide the eye. For comparison purposes, we also plot the result from the 20% most central $\mathrm{Au}+\mathrm{Au}$ collisions as open stars. Panel (d) shows corresponding experimental values of the Gaussian fit parameter $\lambda$.