The investigation of light hadrons in UPCs is of great interest for QCD studies. ALICE is a superb detector for studying these processes because of its excellent particle identification and tracking capabilities. The measured cross section of coherent $\rho^{0}$ mesons in photon-lead interactions has been found to be about 40% smaller than what is predicted by the Glauber model, and...
Relativistic heavy-ion collisions generate extremely strong electromagnetic fields, providing an ideal environment to study the electromagnetic excitation of the vacuum. Furthermore, the electromagnetic fields are sensitive to the charge distributions of the colliding nuclei which can be used to study the nuclear structure. The Breit-Wheeler process, the lowest-order decay mode of the QED...
One of the most pressing questions in both hot and cold QCD communities is what the physics mechanism responsible for modified parton densities in heavy nuclei is. One promising channel to address this question is the photoproduction of vector mesons, which is considered a clean probe to the nuclear parton structures.
We present a measurement of $\pi^+\pi^-\pi^+\pi^-$ photonuclear...
Ultraperipheral nucleus-nucleus collisions produce very large photon fluxes such that fundamental quantum-mechanical processes can be observed and studied in a novel way. In this presentation, an observation of the $\tau$ lepton photoproduction at LHC is reported, using ultraperipheral lead-lead collision data collected by CMS. This measurement paves the way for a precise determination of the...
Central exclusive production (CEP) is a diffractive process in which the colliding particles remain intact. Three different processes are involved: photon-photon exchange, photon-pomeron exchange and double-pomeron exchange. Each process produces distinguishable states with specific sets of quantum numbers, making CEP measurements a unique tool for searching for exotic resonances. Furthermore,...
In ultra-relativistic heavy-ion collisions, strong electromagnetic fields arising from the Lorentz-contracted, highly charged nuclei can be approximated as a large flux of high-energy quasi-real photons that can interact via the Breit-Wheeler process to produce $e^{+}e^{-}$ pairs. The collision energy dependence of the cross section and the transverse momentum distribution of dielectrons from...
Angular correlations and polarization studies provide valuable insights into the vector meson production mechanism, including interference effects as well as information on the nuclear geometry of the target. In this talk, we present two new results. We will report the first measurement of the polarization of both coherent and incoherent J/psi photoproduction in ultra-peripheral Pb--Pb...
