$K^{*0}$ production in Cu $+$ Cu and Au $+$ Au collisions at $\sqrt{s_{NN}}=62.4$ GeV and 200 GeV

We report on $K^{*0}$ production at midrapidity in Au $+$ Au and Cu $+$ Cu collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV collected by the Solenoid Tracker at the Relativistic Heavy Ion Collider detector. The $K^{*0}$ is reconstructed via the hadronic decays $K^{*0}\to K^{+}{\pi }^{-}$ and $\overline{K^{*0}}\to K^{-}{\pi }^{+}$. Transverse momentum, $p_T$, spectra are measured over a range of $p_T$ extending from 0.2 GeV$/c$ up to 5 GeV$/c$. The center-of-mass energy and system size dependence of the rapidity density, $dN/dy$, and the average transverse momentum, $\langle p_T\rangle$, are presented. The measured $N\left(K^{*0}\right)/N\left(K\right)$ and $N\left(\phi \right)/N\left(K^{*0}\right)$ ratios favor the dominance of rescattering of decay daughters of $K^{*0}$ over the hadronic regeneration for the $K^{*0}$ production. In the intermediate $p_T$ region ($2.0<p_T<4.0$ GeV$/c$), the elliptic flow parameter, $v_2$, and the nuclear modification factor, $R_{\mathrm{CP}}$, agree with the expectations from the quark coalescence model of particle production.

Figure 1

$dE/dx$ for charged particles versus momentum divided by charge of the particle as measured in STAR TPC for Au $+$ Au collisions at $\sqrt{s_{NN}}=$ 200 GeV. The curves are the Bethe-Bloch predictions for different particle species.

Figure 2

The $K\pi$ pair invariant mass distribution integrated over the $K^{*0}$ $p_T$ for minimum bias Au $+$ Au and Cu $+$ Cu collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV after the mixed-event background subtraction. The solid curve is the signal fit to a Breit-Wigner function Eq. (2) plus linear function Eq. (3) while the dashed line is the linear function representing the residual background. The statitical errors are small and are within the symbol size.

Figure 3

The $K\pi$ pair invariant mass distribution for various $p_T$ bins [(top left) $p_T=0.4$–0.6 GeV$/c$; (top right) $p_T=0.6$–0.8 GeV$/c$; (bottom left) $p_T=0.8$–1.0 GeV$/c$; and (bottom right) $p_T=1.0$–1.2 GeV$/c$] in Au $+$ Au collisions at $\sqrt{s_{NN}}=62.4$ GeV after the mixed-event background subtraction. The solid curve is the signal fit to a Breit-Wigner function Eq. (2) plus linear function Eq. (3) while the dashed line is the linear function representing the residual background. The errors shown are statistical.

Figure 4

The signal-to-background ratio for $K^{*0}$ measurements as a function of $p_T$ for different collision centrality bins in Au $+$ Au collisions at 200 GeV.

Figure 5

$K^{*0}$ (a) mass and (b) width as a function of $p_T$ for minimum bias Au $+$ Au and Cu $+$ Cu collisions at $\sqrt{s_{NN}}=$ 62.4 and 200 GeV. The dashed line represents the PDG values of 896.0 MeV$/c^2$ and 50.3 MeV$/c^2$ for mass and width, respectively.

Figure 6

The $K^{*0}$ reconstruction efficiency multiplied by the detector acceptance as a function of $p_T$ in (a) Au $+$ Au ($\left|\eta \right|<0.8$) and (b) Cu $+$ Cu ($\left|\eta \right|<1.0$) collisions at 200 GeV for different collision centrality bins.

Figure 7

Midrapidity $K^{*0}$ $p_T$ spectra for various collision centrality bins in Au $+$ Au and Cu $+$ Cu collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV. The dashed lines represent the exponential fit to data. The errors shown are quadratic sums of statistical and systematic uncertainties.

Figure 8

The midrapidity yields $dN/dy$ of $K^{*0}$ as a function of the average number of participating nucleons, $\langle N_{\mathrm{part}}\rangle$, for Au $+$ Au and Cu $+$ Cu collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV. The boxes represent the systematic uncertainties.

Figure 9

The midrapidity $K^{*0}$ $\langle p_T\rangle$ as a function $\langle N_{\mathrm{part}}\rangle$ for Au + Au and Cu $+$ Cu collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV. The boxes represent the systematic uncertainties.

Figure 10

The midrapidity $\langle p_T\rangle$ of $\pi$, $K$, $p$, and $K^{*0}$ as a function of $\langle N_{\mathrm{part}}\rangle$ for Au $+$ Au collisions at $\sqrt{s_{NN}}=62.4$ GeV.

Figure 11

(a) Midrapidity $N\left(K^{*0}\right)/N\left(K^{-}\right)$ ratio for Au $+$ Au, Cu $+$ Cu, and $p+p$ collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV versus $\langle N_{\mathrm{part}}\rangle$. (b) Midrapidity $N\left(K^{*0}\right)/N\left(K^{-}\right)$ in Au $+$ Au, Cu $+$ Cu, and $d$ $+$ Au collisions divided by $N\left(K^{*0}\right)/N\left(K^{-}\right)$ ratio in $p+p$ collisions at $\sqrt{s_{NN}}=$ 200 GeV as a function of $\langle N_{\mathrm{part}}\rangle$. (c) Midrapidity $N\left(K^{*0}\right)/N\left(K^{-}\right)$ ratio in minimum bias Au $+$ Au, Cu $+$ Cu, $p+p$ collisions as a function of $\sqrt{s_{NN}}$. The boxes represents systematic uncertainties. The value of $N\left(K^{*0}\right)/N\left(K^{-}\right)$ ratio in $p+p$ at 63 GeV is from ISR [38].

Figure 12

(a) Midrapidity $N\left(\phi \right)/N\left(K^{*0}\right)$ ratio for Au $+$ Au, Cu $+$ Cu, and $p+p$ collisions at $\sqrt{s_{NN}}=$ 62.4 and 200 GeV versus $\langle N_{\mathrm{part}}\rangle$. (b) Midrapidity $N\left(\phi \right)/N\left(K^{*0}\right)$ in Au $+$ Au, Cu $+$ Cu, and $d$ $+$ Au collisions divided by $N\left(\phi \right)/N\left(K^{*0}\right)$ ratio in $p+p$ collisions at $\sqrt{s_{NN}}=$ 200 GeV as a function of $\langle N_{\mathrm{part}}\rangle$. (c) Midrapidity $N\left(\phi \right)/N\left(K^{*0}\right)$ ratio in minimum bias Au $+$ Au, Cu $+$ Cu, and $p+p$ collisions as a function of $\sqrt{s_{NN}}$. The boxes represents systematic uncertainties. The values of $N\left(\phi \right)/N\left(K^{*0}\right)$ ratio in $p+p$ at 63 GeV is from ISR [38].

Figure 13

The $K^{*0}$ $v_2$ as a function of $p_T$ in minimum bias Au $+$ Au collisions at $\sqrt{s_{NN}}=$ 200 GeV. Only statistical uncertainties are shown. The dashed lines represent the $v_2$ of hadrons with different number of constituent quarks.

Figure 14

The $K^{*0}$ $R_{\mathrm{CP}}$ as a function of $p_T$ in Au $+$ Au collisions at 200 and 62.4 GeV compared to the $R_{\mathrm{CP}}$ of $K_S^0$ and $\Lambda$ at 200 GeV. The brackets around Au $+$ Au 200 GeV data points are the systematic errors.