How to measure the magnetic field in relativistic heavy-ion collisions

Abstract

The intense magnetic field created in heavy-ion collisions has often been calculated with classical electrodynamics. This description is thought to be accurate if the occupation number of every field mode is high enough. However, this hypothesis has never been tested. Recently, we have studied a process involving the magnetic field, which we believe to be the simplest: pion production in ultraperipheral collisions. We have started using the classical approximation to study the forward pion production by the magnetic excitation of nucleons through the process N → Δ → N + π. Then, in a subsequent work, we have replaced this classical field by a flux of photons and have calculated the cross section for photoproduction of pions, which is the quantum version of magnetic excitation. We compared the cross sections obtained with both formalisms, which turned out to be compatible with each other. In this note, we review those works, enlarge the discussion and suggest some improvements that can be done to make our results more accurate. We emphasize that these pions could be detected by the ZDC’s installed at ATLAS and ALICE. Since the theoretical uncertainties are under control, this detection could be used to perform an indirect measurement of the magnetic field intensity.