In a recent paper^{1} published in Phys. Rev. C, NSD Senior Scientist Spencer Klein pointed out an intriguing puzzle associated with the observation of coherent photonuclear reactions.

The concept of coherence is well known to most physicists, from lasers, coherent light sources, and diffraction patterns. One adds the amplitudes for the identical process to occur on different targets and then squares; the cross-section scales as the square of the number of targets. This squaring has an important consequence for relativistic heavy-ion collisions: the cross-sections are large. Coupled with the charge-squared photon flux, the cross-sections for coherent photonuclear interactions in ultra-peripheral collisions (UPCs), interactions without hadronic interactions, are large – 7 barns for ^{0 }photoproduction on lead at the LHC, similar to the total hadronic cross-section. UPCs have become an important technique for studying high-energy photoproduction. At the LHC, UPCs reach higher -p energies than the HERA ep collider, and orders of magnitude higher -Pb energies than fixed-target experiments. Measurements of J/ production in Pb collisions, like the event in Fig. 1, show that gluon saturation is moderate at Bjorken-x 10^{-3}, comparable to the predictions from the leading-twist approximation and near the mid-point of nuclear parton distribution parameterizations.

The new ALICE Run-3 streaming data acquisition system will greatly boost UPC data collection by eliminating the previous bottleneck – a trigger to collect low-multiplicity UPC events, while rejecting most backgrounds. For STAR, the main UPC triggers had to require neutrons in their zero-degree calorimeters in addition to low–multiplicity in the central detector. The neutrons come from nuclear excitation via the exchange of additional photons, as shown in Fig. 2. Multiphoton exchange is expected at relatively small impact parameters.

And therein lies the puzzle highlighted by Klein. Quantum mechanically, coherence requires that the target nucleus remain in the ground state – the Good-Walker paradigm. Nuclear excitation violates the ground state requirement, but a coherent enhancement is still observed. Coherence can be explained semi-classically, by adding the amplitudes to interact with each target, with the usual propagator, but not in a fully quantum approach. This puzzle remains unresolved, but might be explained with a higher-order calculation.

^{1 }The arXiv preprint of the article can be found here.