First measurement of coherent $ρ^{0}$ photoproduction in ultra-peripheral Xe-Xe collisions at $\mathbf{\sqrt{s_{\rm {\scriptscriptstyle \mathbf{NN}}}} = 5.44}$ TeV

The first measurement of the coherent photoproduction of $\rho^{0}$ vector mesons in ultra-peripheral Xe-Xe collisions at $\sqrt{s_{\mathrm{\rm NN}}}= 5.44$ TeV is presented. This result, together with previous HERA $\gamma$p data and $\gamma$-Pb measurements from ALICE, describes the atomic number ($A$) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the ${\rm Xe}+{\rm Xe} \to \rho^{0} + {\rm Xe}+{\rm Xe}$ process, measured at midrapidity through the decay channel $\rho^{0}\to\pi^+\pi^-$, is found to be ${\rm d}\sigma/{\rm d}y=131.5\pm 5.6 {\rm (stat.)} ^{+17.5}_{-16.9} {\rm (syst.)}$ mb. The ratio of the continuum to resonant contributions for the production of pion pairs is also measured. In addition, the fraction of events accompanied by electromagnetic dissociation of either one or both colliding nuclei is reported. The dependence on $A$ of cross section for the coherent $\rho^{0}$ photoproduction at a centre-of-mass energy per nucleon of the $\gamma A$ system of $W_{\gamma A,n}= 65$ GeV is found to be consistent with a power-law behaviour $\sigma (\gamma A\rightarrow \rho^{0} A) \propto A^{\alpha}$ with a slope $\alpha = 0.96 \pm 0.02 {\rm (syst.)}$. This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit.


Phys. Lett. B 820 (2021) 136481
HEP Data
e-Print: arXiv:2101.02581 | PDF | inSPIRE
Figure group

Figure 1

Uncorrected invariant mass (left) and transverse momentum (right) distribution of dipion candidates. Also shown are track pairs that have the same electric charge and fulfil all other requirements. The STARlight templates for coherent and incoherent production---shown in the right panel with the green and magenta lines, respectively---are normalised to the corresponding luminosity of data. An example of a fit to obtain the incoherent contribution (see text for details) is also shown (black line)

Figure 2

Invariant mass distribution of pion pairs with the different components of the fit represented by lines. The number of $\Rz$ candidates is integrated over the resonant Breit--Wigner part (green, full line), the interference term between $A$ and $B$ of Eq.~(\ref{eq:Soeding}) is shown by the dash-dotted blue line and the muon template $M$ (red, dashed line) is taken from STARlight. See text for more details.

Figure 3

Cross section for the coherent photoproduction of $\Rz$ vector mesons in Xe--Xe UPC. The lines show the predictions of the different models described in the text

Figure 4

$A$ dependence of the $\gamma A$ cross section for the coherent production of a $\Rz$ meson and the corresponding power-law fit shown as a band. The data are from this analysis and from~. The general expectations for three extreme cases are represented by the dashed, dotted-dashed, and dotted lines, respectively. The red band corresponds to the GKZ predictions when varying the parameters of the model. A power-law fit to the CCKT model is shown by the blue band. See text for details.