New measurements shine a light on the proton

A report from the ALICE experiment

The electromagnetic field of the highly charged lead ions in the LHC beams provides a very intense flux of high-energy quasi-real photons that can be used to probe the structure of the proton in lead–proton collisions. The exclusive photoproduction of a J/ψ vector meson is of special interest because it samples the gluon density in the proton. Previous measurements by ALICE have shown that this process could be measured in a wide range of centre-of-mass energies of the photon–proton system (W_γp), enlarging the kinematic reach by more than a factor of two with respect to that of calculations performed at the former HERA collider.

Recently, the ALICE collaboration has performed a measurement of exclusive photoproduction of J/ψ mesons off protons in proton–lead collisions at a centre-of-mass energy of 5.02 TeV at the LHC using two new configurations. In both cases, the J/ψ meson is reconstructed from its decay into a lepton pair. In the first case, the leptons are measured at mid-rapidity using ALICE’s central-barrel detectors. The excellent particle-identification capabilities of these detectors allow the measurement of both the e⁺e^– and μ⁺μ^– channels. The second configuration combines a muon measured with the central-barrel detectors with a second muon measured by the muon spectrometer located at forward rapidity. By this clever use of the detector configuration, we were able to significantly extend the coverage of the J/ψ measurement.

The energy of the photon–proton collisions, W_γp, is determined by the rapidity (which is a function of the polar angle) of the produced J/ψ with respect to the beam axis. Since the direction of the proton and the lead beams was inverted halfway through the data-taking period, ALICE covers both backward and forward rapidities using a single-arm spectrometer.

These two configurations, plus the one used previously where both muons were measured in the muon spectrometer, allow ALICE to cover – in a continuous way – the range in W_γp from 20 to 700 GeV. The typical momentum at which the structure of the proton is probed is conventionally given as a fraction of the beam momentum, x, and the new measurements extend over three orders of magnitude in x from 2 × 10^–2 to 2 × 10^–5. The measured cross section for this process as a function of W_γp is shown in figure 1 and compared with previous measurements and models based on different assumptions such as the validity of DGLAP evolution (JMRT), the vector-dominance model (STARlight), next-to-leading order BFKL, the colour–glass condensate (CGC), and the inclusion of fluctuating sub-nucleonic degrees-of-freedom (CCT). The last two models include the phenomenon of saturation, where nonlinear effects reduce the gluon density in the proton at small x.

The new measurements are compatible with previous HERA data where available, and all models agree reasonably well with the data. Nonetheless, it is seen that at the largest energies, or equivalently the smallest x, some of the models predict a slower growth of the cross section with energy. This is being studied by ALICE with data taken in 2016 in p–Pb collisions at a centre-of-mass energy of 8.16 TeV, allowing exploration of the W_γp energy range up to 1.5 TeV, potentially shedding new light on the question of gluon saturation.