Speaker
Description
The neutron skin, in which the neutron distribution extends farther out than the proton distribution in neutron-rich nuclei, is characterized by the radius $R$ and the diffuseness $a$ of each distribution, and is closely connected to the nuclear equation of state. At present, parity-violating electron scattering is essentially the only direct measurement of this quantity. We investigate a method using forward direct photons produced in high-energy lead+proton (Pb+p) collisions. By selecting the most peripheral events, in which the incident proton collides with a single nucleon of the Pb nucleus ($N_{\rm coll}=1$), the neutron skin makes the struck nucleon more likely to be a neutron. Direct photons are dominantly produced by QCD Compton scattering ($qg \to q\gamma$), for which the difference of the quark charges makes the cross section larger for a proton target than for a neutron target. Since the kinematics is governed by the high-$x$ valence quarks of the nucleon in the lead involved in the collision, this proton/neutron discrimination improves toward the direction of motion of the lead, i.e., the forward direction. Combining this geometrical effect with the isospin dependence of the hard scattering, we expect the forward photon yield ratio to be sensitive to the neutron skin thickness. Based on nucleon configurations generated with a Monte Carlo Glauber model, we determined, event by event, the species of the struck nucleon in the lead for each peripheral event, and generated direct photons in proton-proton and proton-neutron scattering with PYTHIA8, predicting the dependence of $R_\gamma(p_T) = \sigma_{\rm Pb+p}/\sigma_{pp}$ on the neutron skin thickness. In this talk, in view of future measurements with the ALICE forward calorimeter FoCal, we show that $R_\gamma$ decreases with increasing neutron skin thickness, and discuss its statistical discriminating power.
