Title: Diffraction in coherent J/psi photoproduction in Au-Au collisions at sqrt(s_NN) = 200 GeV

Target journal: PRL

Abstract:

The STAR collaboration at RHIC has measured coherent J/psi photoproduction
at mid-rapidity in ultra-peripheral Au+Au collisions at sqrt(s_NN) = 200 GeV.
The J/psi mesons are reconstructed via their dielectron decays. The coherent
cross section is given as a function of |t|. It shows a clear diffractive
pattern. The experimental results are compared with theoretical calculations
based on dipole and Glauber models.

Summary and conclusions:

We present the first measurement of coherent J/psi photoproduction off
the Au nuclei in Au+Au collisions at sqrt(s_NN) = 200 GeV. The photoproduction
cross section is measured as a function of |t|, the square momentum
transfer at the target nucleus.

The data show a characteristic diffractive patter with position of the first
diffractive minimum consistent with the previous measurement of coherent
rho0 photoproduction.

Experimental results are compared to several theoretical calculations.
Models which use dipole approach are consistent with the data. The Glauber
approach and photon pT smearing as implemented in Starlight is not describing
the data.

The measurement is posing constraints to theoretical calculations and has
a potential to be a part of the baseline for the future facilities like the EIC.

Paper figures:

Figure 1: Invariant mass distribution for e+e- pairs satisfying the event selection. Coherent enriched sample of unlike-sing pairs
and like-sign pairs (red bars) is shown.

Figure 2: Dielectron pT distribution for events satisfying the event selection in the mass interval [2.9, 3.2] GeV.
Contributions from coherent J/psi (blue), incoherent J/psi (red) and gamma-gamma -> e+e- (green) processes
are shown.

Figure 3: Dielectron log10(pT^2) distribution for events satisfying the event selection in the mass interval
[2.9, 3.2] GeV. Parametrized fit is performed to the region of incoherent J/psi (red). Contributions from coherent
J/psi (blue) and gamma-gamma -> e+e- (green) processes are shown.

Figure 4: Differential cross section of coherent J/psi photoproduction at mid-rapidity and in intervals of |t|.
The errors are quadratical sum of the statistical and systematical errors. Theoretical calculations are also shown.