Coherent ${\rho }^0$ Production in Ultraperipheral Heavy-Ion Collisions

The STAR Collaboration reports the first observation of exclusive ${\rho }^0$ photoproduction, $\mathrm{A}\mathrm{u}\mathrm{A}\mathrm{u}\to \mathrm{A}\mathrm{u}\mathrm{A}\mathrm{u}{\rho }^0$, and ${\rho }^0$ production accompanied by mutual nuclear Coulomb excitation, $\mathrm{A}\mathrm{u}\mathrm{A}\mathrm{u}\to \mathrm{A}{\mathrm{u}}^{\mathrm{\star }}\mathrm{A}{\mathrm{u}}^{\mathrm{\star }}{\rho }^0$, in ultraperipheral heavy-ion collisions. The ${\rho }^0$ have low transverse momenta, consistent with coherent coupling to both nuclei. The cross sections at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ agree with theoretical predictions treating ${\rho }^0$ production and Coulomb excitation as independent processes.

Figure 1

Diagram for (a) exclusive ${\rho }^0$ production in ultraperipheral heavy-ion collisions, and (b) ${\rho }^0$ production with nuclear excitation. The dashed lines indicate factorization.

Figure 2

The $p_T$ spectra of pion pairs for the two-track events selected by (a) the topology trigger $(0n,0n$) and (b) the minimum bias trigger $(xn,xn)$. Points are oppositely charged pairs, and the shaded histograms are the normalized like-sign combinatorial background. The open histograms are simulated ${\rho }^0$ superimposed onto the background.

Figure 3

Rapidity distribution (a) of ${\rho }^0$ candidates $(xn,xn)$ for the minimum bias data (points) compared to the normalized reconstructed (shaded histogram) and generated (open histogram) events from the Monte Carlo simulation. The differential cross section (b) $d\sigma (\gamma Au\to \rho Au)/dt$ for the same data set; the line indicates the exponential fit.

Figure 4

The $d\sigma (\mathrm{A}\mathrm{u}\mathrm{A}\mathrm{u}\to \mathrm{A}{\mathrm{u}}^{\mathrm{*}}\mathrm{A}{\mathrm{u}}^{\mathrm{*}}\rho )/d{M}_{\pi \pi }$ spectrum for two-track $(xn,xn)$ events with pair-$p_T<150\mathrm{M}\mathrm{e}\mathrm{V}/c$ in the minimum bias data. The shaded histogram is the combinatorial background, and the hatched histogram contains an additional contribution from coherent $e^{+}{e}^{-}$ pairs. The fits correspond to Eq. (2): the sum (solid) of a Breit-Wigner, a mass-independent contribution from direct ${\pi }^{+}{\pi }^{-}$ production and their interference (dashed line), and a second order polynomial for the residual background (dash-dotted line).