The intense photon fluxes from relativistic nuclei provide an opportunity to study photonuclear interactions in ultraperipheral collisions. The measurement of coherently photoproduced $\pi^+\pi^-\pi^+\pi^-$ final states in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is presented for the first time. The cross section, d$\sigma$/d$y$, times the branching ratio ($\rho\rightarrow \pi^+ \pi^+ \pi^- \pi^-$) is found to be $47.8\pm2.3~\rm{(stat.)}\pm7.7~\rm{(syst.)}$ mb in the rapidity interval $|y| <~ 0.5$. The invariant mass distribution is not well described with a single Breit-Wigner resonance. The production of two interfering resonances, $\rho(1450)$ and $\rho(1700)$, provides a good description of the data. The values of the masses ($m$) and widths ($\Gamma$) of the resonances extracted from the fit are $m_{1}=1385\pm14~\rm{(stat.)}\pm3~\rm{(syst.)}$ MeV/$c^2$, $\Gamma_{1}=431\pm36~\rm{(stat.)}\pm82~\rm{(syst.)}$ MeV/$c^2$, $m_{2}=1663\pm13~\rm{(stat.)}\pm22~\rm{(syst.)}$ MeV/$c^2$ and $\Gamma_{2}=357 \pm31~\rm{(stat.)}\pm49~\rm{(syst.)}$ MeV/$c^2$, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.
Submitted to: PLB
e-Print: arXiv:2404.07542 | PDF | inSPIRE
CERN-EP-2024-104
Figure group
Figure 1
Distribution of the event $\pt$ in events with zero net charge in the invariant mass range $0.8 < m < 2.5$ GeV/$c^2$ and the rapidity interval $|y| < 0.5$. The data (red circles) are shown together with the fitted function (solid black line) and its three components as described in the text. |  |
Figure 2
Corrected invariant mass spectrum for the coherent four pion photoproduction fitted with one resonance (top) and two resonance with interference (bottom) models, as described in the text. Black error bars represent the statistical error, the orange band shows the uncorrelated systematic uncertainty, and the boxes show them added in quadrature. "Prob" reflects the probability of having the given or a higher $\chi^2/\rm{ndf}$ of the fit. |   |
Figure 3
Cross sections for the one-resonance and two-resonances scenarios (full red circles) compared to the theoretical calculations from Ref. [39] (open black circles). Horizontal error bars on the data points show the statistical uncertainty and the boxes represent the statistical and systematic uncertainties added in quadrature. |  |