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

The collaboration
Phys.Rev.C 77 (2008) 034910, 2008.

Abstract (data abstract)
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 ultra-peripheral relativistic heavy ion collisions at $\sqrt{s_{NN}}$=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross-section of $\sigma(AuAu \rightarrow Au^{*}Au^{*} \rho^{0})$ = 530 $\pm$ 19 (stat.) $\pm$ 57 (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 $\sigma_{inc}/\sigma_{coh}$ = 0.29 $\pm$ 0.03 (stat.) $pm$ 0.08 (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 2 a

Data from Figure 2 a

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ZDC spectra obtained with the minimum bias sample after the $\rho^{0}$ selection cuts are applied, and fit with three Gaussians....

• #### Figure 2 b

Data from Figure 2 b

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ZDC spectra obtained with the minimum bias sample after the $\rho^{0}$ selection cuts are applied, and fit with three Gaussians....

• #### Figure 3 a

Data from Figure 3 a

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The invariant mass distribution for the coherently produced $\rho^{0}$ candidates from the minimum bias sample with the cut on the...

• #### Figure 3 b

Data from Figure 3 b

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The invariant mass distribution for the coherently produced $\rho^{0}$ candidates obtained from the topology sample with the cut on the...

• #### Figure 4

Data from Figure 4

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

• #### Figure 5

Data from Figure 5

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Coherent $\rho^{0}$ production cross-section for the minimum bias data set as a function of $y_{\rho^{0}}$ (black dots) overlaid by the...

• #### Figure 6

Data from Figure 6

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$\rho^{0}$ production cross-section as a function of the momentum transfer squared $t$, together with the fit of Eq. 5. The...

• #### Figure 7

Data from Figure 7

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Comparison of theoretical predictions to the measured differential cross-section for coherent $\rho^{0}$ production. The statistical errors are shown by the...

• #### Figure 8 a

Data from Figure 8 a

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Projections of the two dimensional efficiency corrected $\Phi_{h}$ vs $cos(\Theta_{h})$ distributions obtained with the minimum bias data set. The solid...

• #### Figure 8 b

Data from Figure 8 b

10.17182/hepdata.98962.v1/t10

Projections of the two dimensional efficiency corrected $\Phi_{h}$ vs $cos(\Theta_{h})$ distributions obtained with the minimum bias data set. The solid...