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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/ 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 ₓ) J/ 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 ₓ J/ 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/ in peripheral Pb-Pb collisions. Both measurements are conducted at forward rapidity in the dimuon decay channel.
L. Massacrier (Fri,) studied this question.