Name: Coherent diffractive photoproduction of $\rho^{0}$ mesons on gold nuclei at RHIC
Creator: STAR Collaboration
Published: 2021

Abstract

The STAR Collaboration reports on the photoproduction of $\pi^+\pi^-$ pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real photon emitted by one ion scatters from the other ion. We fit the $\pi^+\pi^-$ invariant mass spectrum with a combination of $\rho$ and $\omega$ resonances and a direct $\pi^+\pi^-$ continuum. This is the first observation of the $\omega$ in ultra-peripheral collisions, and the first measurement of $\rho-\omega$ interference at energies where photoproduction is dominated by Pomeron exchange. The $\omega$ amplitude is consistent with the measured $\gamma p\rightarrow \omega p$ cross section, a classical Glauber calculation and the $\omega\rightarrow\pi^+\pi^-$ branching ratio. The $\omega$ phase angle is similar to that observed at much lower energies, showing that the $\rho-\omega$ phase difference does not depend significantly on photon energy. The $\rho^0$ differential cross section $d\sigma/dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with 2 minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.

Figure 4

Data from Figure 4

10.17182/hepdata.101354.v1/t1

The $\pi^+\pi^-$ invariant-mass distribution for all selected $\pi\pi$ candidates with $p_T~<~100~\textrm{MeV}/c$.
Figure 5 a

Data from Figure 5 a

10.17182/hepdata.101354.v1/t2

The ratio $|B/A|$ of amplitudes of nonresonant $\pi^+\pi^-$ and $\rho^0$ mesons in the present STAR analysis.
Figure 5 b

Data from Figure 5 b

10.17182/hepdata.101354.v1/t3

The ratio $|B/A|$ of amplitudes of nonresonant $\pi^+\pi^-$ and $\rho^0$ mesons in the previous STAR analysis, Phys. Rev. C 77...
Figure 5 c

Data from Figure 5 c

10.17182/hepdata.101354.v1/t4

The ratio $|C/A|$ of amplitudes of $\omega$ and $\rho^0$ mesons.
Figure 6 a

Data from Figure 6 a

10.17182/hepdata.101354.v1/t5

$d\sigma/dy$ for exclusively photoproduced $\rho^0$ mesons in $Xn\,Xn$ events.
Figure 6 b

Data from Figure 6 b

10.17182/hepdata.101354.v1/t6

$d\sigma/dy$ for exclusively photoproduced $\rho^0$ mesons in the previous STAR analysis, Phys. Rev. C 77 034910 (2008).
Figure 6 c

Data from Figure 6 c

10.17182/hepdata.101354.v1/t7

$d\sigma/dy$ for exclusively photoproduced $\rho^0$ mesons in $1n\,1n$ events.
Figure 7 a

Data from Figure 7 a

10.17182/hepdata.101354.v1/t8

The $-t$ distribution for exclusive $\rho^0$ mesons in events with $Xn\,Xn$ mutual event dissociation.
Figure 7 b

Data from Figure 7 b

10.17182/hepdata.101354.v1/t9

The $-t$ distribution for exclusive $\rho^0$ mesons in events with $1n\,1n$ mutual event dissociation.
Figure 8 a

Data from Figure 8 a

10.17182/hepdata.101354.v1/t10

$d\sigma/dt$ for coherent $\rho^0$ photoproduction in $Xn\,Xn$ events
Figure 8 b

Data from Figure 8 b

10.17182/hepdata.101354.v1/t11

$d\sigma/dt$ for coherent $\rho^0$ photoproduction in $1n\,1n$ events
Figure 9 a

Data from Figure 9 a

10.17182/hepdata.101354.v1/t12

The target distribution in the transverse plane, the result of a two-dimensional Fourier transform (Hankel transform) of the $1n\, 1n$...
Figure 9 b

Data from Figure 9 b

10.17182/hepdata.101354.v1/t13

The target distribution in the transverse plane, the result of a two-dimensional Fourier transform (Hankel transform) of the $Xn\, Xn$...

Figure 4

Figure 5 a

Figure 5 b

Figure 5 c

Figure 6 a

Figure 6 b

Figure 6 c

Figure 7 a

Figure 7 b

Figure 8 a

Figure 8 b

Figure 9 a

Figure 9 b

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