The fundamental structure of nucleons is described by the properties and dynamics of their constituent quarks and gluons, as described by QCD. The gluon’s self-interaction complicates this picture considerably. Non-linear recombination reactions, where two gluons fuse, are predicted to lead to a saturation of the gluon density. This largely unexplored phenomenon is expected to occur when the gluons in a hadron overlap transversally, and is enhanced for hadrons with high atomic numbers. Gluon saturation may be studied in lead-proton collisions at the LHC in the kinematic region where the gluon density is high and the gluons have sizable transverse dimensions.
Gluon saturation has been at the focal point of the heavy-ion community for decades. Precision measurements at HERA, RHIC and previously at the LHC agree with the predictions made by saturation models, however, the measurements do not allow an unambiguous interpretation of whether gluon saturation occurs in nature. This is a strong motivation both for the LHC experiments and for the planned Electron Ion Collider (CERN Courier October 2018 p31).
The CMS collaboration recently submitted a paper on gluon saturation in proton-lead collisions to the Journal of High Energy Physics (JHEP). The collisions that were used for this analysis occurred in 2013 at a centre-of-mass energy of 5 TeV and were detected using the CMS experiment’s CASTOR calorimeter. This is a very forward calorimeter of CMS, where “forward” refers to regions of the detector that are close to the beam pipe. Therefore, unlike any other LHC experiment, CMS can measure jets at very forward angles (–6.6<|η|<–5.2) and with transverse momenta (pT) as low as 3 GeV. This is the first time that a jet-energy spectrum measurement from the CASTOR calorimeter has been submitted to a journal.
Forward jets with a small pT can target high-density-regime gluons with ample transverse dimensions, making CASTOR ideal for a study of gluon saturation. By colliding protons with lead ions, the sensitivity of the CASTOR jet spectra to saturation effects was further enhanced. This enabled CASTOR to overcome the ambiguity associated with the interpretation of the previous measurements.
The jet-energy spectrum obtained using CASTOR was compared to two saturation models (figure 1, left). These were the “Katie KS” model and predictions from the AAMQS collaboration; the latter are based on the colour-glass-condensate model. In the Katie KS model, the strength of the non-linear gluon recombination reactions can be varied. Upon comparison with the model, it was seen that the linear and non-linear predictions differed by an order of magnitude for the lowest energy bins of the spectrum, which correspond to low-pT jets. Meanwhile, they converged at the highest energies, confirming the high sensitivity of the measurement to gluon saturation. The AAMQS predictions underestimated the data progressively, up to an order of magnitude, in the region most strongly affected by saturation. Overall, neither model described the spectrum correctly.
The spectrum was also compared to two cosmic ray models (EPOS-LHC and QGSJetII-04) and to the HIJING event generator (figure 1, right). The former models underestimated the data by over two orders of magnitude while HIJING, which incorporates an implementation of nuclear shadowing, agreed well with the data. Nuclear shadowing is an interference effect between the nucleons of a heavy ion. Like gluon saturation, it is expected to lead to a decrease in the probability for a proton-lead collision to occur, however further data analysis is required for more definite conclusions on nuclear shadowing.
These results establish CASTOR jets as an experimental reality and their sensitivity to saturation effects is encouragement for further, more refined CASTOR jet studies.
CMS Collaboration 2018 arXiv:1812.01691 (submitted to JHEP).