Coherent J/$ψ$ photoproduction and polarization in peripheral Pb-Pb collisions with ALICE (2407.09707v1)

Abstract: Photonuclear reactions are induced by the strong electromagnetic field generated by ultrarelativistic heavy ions. These processes have been extensively studied in ultraperipheral collisions. In recent years, the observation of coherent J/$\psi$ photoproduction has been claimed in nucleus-nucleus (A-A) collisions with nuclear overlap, based on the measurement of an excess in the very low transverse momentum ($p_{\rm T}$) J/$\psi$ yield. Such quarkonium measurements can help constraining the nuclear gluon distribution at low Bjorken-$x$ and can shed light on the theory behind photon induced reactions in A-A collisions with nuclear overlap. In order to confirm the photoproduction origin of the very low-$p_{\rm T}$ J/$\psi$ yield excess, polarization measurement is a golden observable. It is indeed expected that the produced quarkonium would keep the polarization of the incoming photon due to $s$-channel helicity conservation. ALICE can measure inclusive and exclusive quarkonium production down to zero transverse momentum, at forward rapidity (2.5 $< y <$ 4) and midrapidity ($|y| <$ 0.9). In this contribution, we will report on the new preliminary measurement of the $y$-differential cross section and the first polarization analysis at LHC of coherently photoproduced J/$\psi$ in peripheral Pb-Pb collisions. Both measurements are conducted at forward rapidity in the dimuon decay channel.