One of the hot topics in hadron physics is the study of the new exotic charmonium states and the determination of their internal structure. Another important topic is the study of the magnetic field produced in relativistic heavy-ion collisions and its effects on observables. In this note, we show that we can use ultra-peripheral collisions to address both topics. We compute the cross section for the production of the D⁺D⁻ molecular bound state in photon-photon collisions and also the cross section for π⁰ production in the target induced by the magnetic field of the projectile. Both cross sections are sizeable, and their measurement would be very useful to elucidate the above-mentioned questions.

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.

It is believed that in noncentral relativistic heavy-ion collisions a very strong magnetic field is formed. There are several studies of the effects of this field, where (B) over bar is calculated with the expressions of classical electrodynamics. A quantum field may be approximated by a classical one when the number of field quanta in each field mode is sufficiently high. This may happen if the field sources are intense enough. In heavy-ion physics the validity of the classical treatment was not investigated. In this work we propose a test of the quality of the classical approximation. We calculate an observable quantity using the classical magnetic field and also using photons as input. If the results of both approaches coincide, this will be an indication that the classical approximation is valid. More precisely, we focus on the process in which a nucleon is converted into a delta resonance, which then decays into another nucleon and a pion, i.e., N -> Delta -> N' pi. In ultraperipheral relativistic heavy-ion collisions this conversion can be induced by the classical magnetic field of one of the ions acting on the other ion. Alternatively, we can replace the classical magnetic field by a flux of equivalent photons, which are absorbed by the target nucleons. We calculate the cross sections in these two independent ways and find that they differ from each other by similar or equal to 10 % in the considered collision energy range. This suggests that the two formalisms are equivalent and that the classical approximation for the magnetic field is reasonable.

In this note we study the conversion of nucleons into deltas induced by a strong magnetic field in ultraperipheral relativistic heavy ion collisions. The interaction Hamiltonian couples the magnetic field to the spin operator, which, acting on the spin part of the wave function, converts a spin 1/2 into a spin 3/2 state. We estimate this transition probability and calculate the cross section for delta production. This process can in principle be measured, since the delta moves close to the beam and decays almost exclusively into pions. Forward pions may be detected by forward calorimeters.