${\rho }^0$ photoproduction in ultraperipheral relativistic heavy ion collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ${\rho }^0$ and direct ${\pi }^{+}{\pi }^{-}$ photoproduction in ultraperipheral relativistic heavy ion collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of $\sigma (\mathrm{AuAu}\to {\mathrm{Au}}^{*}{\mathrm{Au}}^{*}{\rho }^0)=530\pm 19(\mathrm{stat}.)\pm 57(\mathrm{syst}.)$ mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ${\rho }^0$ transverse momentum spectrum ($p_T^2$) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find ${\sigma }_{\mathrm{inc}}/{\sigma }_{\mathrm{coh}}=0.29\pm 0.03(\mathrm{stat}.)\pm 0.08(\mathrm{syst}.)$. The ratio of direct ${\pi }^{+}{\pi }^{-}$ to ${\rho }^0$ production is comparable to that observed in $\gamma p$ collisions at HERA and appears to be independent of photon energy. Finally, the measured ${\rho }^0$ spin helicity matrix elements agree within errors with the expected $s$-channel helicity conservation.

Figure 1

The diagram for ${\rho }^0$ photoproduction accompanied by mutual Coulomb excitation. The latter process proceeds by mutual photon exchange; the vertical dashed line shows how the photoproduction factorizes from the mutual Coulomb excitation.

Figure 2

ZDC spectra obtained with the minimum bias sample after the ${\rho }^0$ selection cuts are applied and fit with three Gaussians. The east ZDC is shown on the left and the west ZDC is shown on the right. The ratio of numbers of candidates in the west ZDC of $1n:2n:3n$ is $1:0.48\pm 0.07:0.42\pm 0.04$, whereas in the east ZDC, we find that $1n:2n:3n$ is $1:0.46\pm 0.08:0.42\pm 0.04$.

Figure 3

Top: The invariant mass distribution for the coherently produced ${\rho }^0$ candidates from the minimum bias sample with the cut on the ${\rho }^0$ transverse momentum $p_T<$ 150 MeV/$c$. Bottom: The invariant mass distribution for the coherently produced ${\rho }^0$ candidates obtained from the topology sample with the cut on the ${\rho }^0$ transverse momentum $p_T<$ 150 MeV/$c$. The hatched area is the contribution from the combinatorial background. The solid line corresponds to Eq. (3), which encompasses the Breit-Wigner part (dashed), the mass-independent contribution from the direct ${\pi }^{+}{\pi }^{-}$ production (dash-dotted), and the interference term (dotted).

Figure 4

The ratio $|B/A|$ as a function of $y_{{\rho }^0}$ for the minimum bias data, obtained by fitting Eq. (3) to the invariant mass distributions in bins of $y_{{\rho }^0}$.

Figure 5

Coherent ${\rho }^0$ production cross section for the minimum bias data set as a function of $y_{{\rho }^0}$ (black dots) overlaid by the $d\sigma /\mathit{dy}$ distribution predicted by the KN model [23] (solid line).

Figure 6

${\rho }^0$ production cross section as a function of the momentum transfer squared, $t$, together with the fit of Eq. (5). The fit parameters are shown in Table 1.

Figure 7

Comparison of theoretical predictions to the measured differential cross section for coherent ${\rho }^0$ production. The statistical errors are shown by the solid vertical line at each data point. The sum of the statistical and systematic error bars is shown by the gray band.

Figure 8

Projections of the two-dimensional efficiency-corrected ${\Phi }_h$ vs $\cos ({\Theta }_h)$ distributions obtained with the minimum bias data set. The solid line shows the result of the two-dimensional fit to the data with Eq. (6) and the coefficients given in Table 3.