${\rho }^0$ photoproduction in AuAu collisions at $\sqrt{s_{NN}}=62.4$ GeV measured with the STAR detector

Vector mesons may be photoproduced in relativistic heavy-ion collisions when a virtual photon emitted by one nucleus scatters from the other nucleus, emerging as a vector meson. The STAR Collaboration has previously presented measurements of coherent ${\rho }^0$ photoproduction at center of mass energies of 130 GeV and 200 GeV in AuAu collisions. Here, we present a measurement of the cross section at 62.4 GeV; we find that the cross section for coherent ${\rho }^0$ photoproduction with nuclear breakup is $10.5\pm 1.5\pm 1.6$ mb at 62.4 GeV. The cross-section ratio between 200 GeV and 62.4 GeV is $4.4\pm 0.6$, less than is predicted by most theoretical models. It is, however, proportionally much larger than the previously observed $15\%\pm 55\%$ increase between 130 GeV and 200 GeV.

Figure 1

Acceptance-corrected invariant mass distributions for the coherently produced ${\rho }^0$ candidates collected with trigger A (left) and B (right). The fit function (solid) encompasses the Breit-Wigner (dashed), the mass-independent contribution from direct ${\pi }^{+}{\pi }^{-}$ production (dash-dotted), and the interference term (dotted). The hatched area is the contribution from the combinatorial background. The statistical errors are shown.

Figure 2

Transverse momentum distribution of the ${\rho }^0$ candidates (open distribution) overlaid by the combinatorial background estimated with like-sign pairs (not corrected to the acceptance and reconstruction efficiency) and scaled to match in the high transverse momentum region, $p_T$ $⩾$ 250 MeV/$c$ (hatched distribution). The plot is based on the data set collected with trigger B.

Figure 3

${\rho }^0$ production cross section determined with the data set collected with trigger B, as a function of momentum transfer $t$, fitted with a double-exponential fit function in the range $t<$ 0.1. The statistical errors are shown. The ratio of incoherent to coherent cross sections is measured to be 0.20 $\pm$ 0.8. The fit parameters are shown in Table 1.

Figure 4

Comparison of theoretical predictions to the measured total cross section for coherent ${\rho }^0$ production as a function of $\sqrt{s_{NN}}$. The measured cross section is based on the data set collected with trigger B. The error bars show the sum of the statistical and systematic uncertainties. See text for details.