A report from the ALICE experiment

An ultra-relativistic electromagnetically charged projectile carries a strongly contracted field that can be thought of as a flux of quasi-real photons. This is known as the equivalent-photon approximation, and was proposed by Fermi and later developed by Weizsäcker and Williams. In practice, this means that the proton or lead (Pb) beams of the LHC, moving at ultra-relativistic energies, also carry a quasi-real photon beam, which can be used to look inside protons or nuclei. The ALICE collaboration is in this way using the LHC as a photon–hadron collider, shining light inside lead nuclei to measure the photoproduction of charmonia and provide constraints on nuclear shadowing.

The intensity of the electromagnetic field, and the corresponding photon flux, is proportional to the square of the electric charge. This type of interaction is therefore greatly enhanced in the collisions of lead ions (Z = 82). Ultra-peripheral collisions (UPCs), in which the impact parameter is larger than the sum of the radii of two Pb nuclei, are a particularly useful way to study photonuclear collisions. Here, purely hadronic interactions are suppressed, due to the short range of the strong force, and photonuclear interactions dominate. The photoproduction of vector mesons in these reactions has a clean experimental signature: the decay products of the vector meson are the only signals in an otherwise empty detector.

Nuclear shadowing was first observed by the European Muon Collaboration at CERN in 1982

Coherent heavy-vector–meson photoproduction, wherein the photon interacts consistently with all the nucleons in a nucleus, is of particular interest because of its connection with gluon distribution functions (PDFs) in protons and nuclei. At low Bjorken-x values, gluon PDFs are significantly suppressed in the nucleus relative to free proton PDFs – a phenomenon known as nuclear shadowing that was first observed by the European Muon Collaboration at CERN in 1982 by comparing the structure functions of iron and deuterium in the deep inelastic scattering of muons.

Heavy-vector–meson photoproduction measurements provide a powerful tool to study poorly known gluon-shadowing effects at low x. The scale of the four-momentum transfer of the interaction corresponds to the perturbative regime of QCD in the case of heavy charmonium states. The gluon shadowing factor – the ratio of the nuclear PDF to the proton PDF – can be evaluated by measuring the nuclear suppression factor, defined to be the square root of the ratio of the coherent vector–meson photonuclear production cross section on nuclei to the photonuclear cross-section in the impulse approximation that is based on the exclusive photoproduction measurements with a proton target.

### Ultra-peripheral collisions

The ALICE collaboration recently submitted for publication the measurement of the coherent photoproduction of J/ψ and ψ′ at midrapidity |y| < 0.8 in Pb–Pb UPCs at 5.02 TeV. The J/ψ is reconstructed using the dilepton (ℓ^{+}ℓ^{–}) and proton–antiproton decay channels, while for the ψ′, the dilepton and the ℓ^{+}ℓ^{–} π^{+}π^{–} decay channels are studied. These data complement the ALICE measurement of the coherent J/ψ cross-section at forward rapidity, –4 < y < –2.5, providing stringent constraints on nuclear gluon shadowing.

The nuclear gluon shadowing factor of about 0.65 at Bjorken-x between 0.3 × 10^{–3} and 1.4 × 10^{–3} is estimated from the comparison of the measured coherent J/ψ cross-section with the impulse approximation at midrapidity, which implies moderate nuclear shadowing. The measured rapidity dependence of the coherent cross-section is not completely reproduced by models in the full rapidity range. The leading twist approximation of the Glauber–Gribov shadowing (LTA-GKZ) and the energy-dependent hot-spot model (GG-HS (CCK)) gives the best overall description of the rapidity dependence but shows tension with data at semi-forward rapidities 2.5 < |y| < 3.5 (figure 1). The data might be better explained with a model where shadowing has a smaller effect at Bjorken x ~ 10^{–2} or x ~ 5 ∙ 10^{–5}, corresponding to this rapidity range.

The ratio of the ψ′ to J/ψ cross-sections at midrapidity is consistent with the ratio of photoproduction cross sections measured by the H1 and LHCb collaborations, with the leading twist approximation predictions for Pb–Pb UPCs as well as with the ALICE measurement at forward rapidities. This leads to the conclusion that shadowing effects are similar for 2S (ψ′) and 1S (J/ψ) states.

In LHC Run 3 and 4, ALICE expects to collect a 10-times-larger data sample than in Run 2, taking data in a continuous mode, and thus with higher efficiency. UPC physics will profit from this by large integrated luminosity as well as lower systematic uncertainty connected to the measurement and will be able to provide the shadowing factor differentially in wide Bjorken-x intervals.

### Further reading

ALICE Collab. 2021 arXiv:2101.04577.