Search for gluon saturation in proton-lead collisions at a centre-of-mass energy of 5.02 TeV with the very forward CASTOR calorimeter at the CMS experiment

Date: 9 March 2018

Venue: Stadscampus, Promotiezaal van de Grauwzusters - Lange Sint-Annastraat 7 - 2000 Antwerpen (route: UAntwerpen, Stadscampus)

Time: 4:00 PM

Organization / co-organization: Department of Physics

PhD candidate: Merijn van de Klundert

Principal investigator: Pierre Van Mechelen

Short description: PhD defence Merijn van de Klundert - Faculty of Science, Department of Physics


The core of a hydrogen atom consists of a proton. A proton itself  consists of quarks and gluons. In a collider experiment, each constituent of the proton carries a fraction x of the momentum of the proton. From previous experiments, it is known that at low x-values the number of gluons in a proton rises dramatically. It is expected that in certain kinematical regimes, the density of gluons becomes very high.

It is hypothesised that this lead to recombination reactions between the gluons, which leads to a saturation of the parton densities. Gluon saturation has been investigated for over decades. Various analyses of experimental data found hints for this phenomena, but the interpretation of the key results was obscured by various effects. The effects of gluon saturation are expected to be stronger in the core of a heavy atom (for example lead), which is called a heavy ion. Furthermore, forward jets constitute a highly sensitive observable to
saturation effects.

The CMS experiment at the Large Hadron Collider collected data of proton-lead collisions at unprecedented energies. CMS is equipped with the CASTOR calorimeter. CASTOR can measure jets in a unique part of the forward phase-space, which is not accessed by any other experiment. Therefore, jets in CASTOR in proton-lead collisions provide us with an excellent and unprecedented observable to study effects of gluon saturation, which may overcome the afflictions that compromised the interpretation of the previous measurements.

We analyse the forward jet-energy spectrum in proton-lead collisions at a centre-of-mass energy of 5 TeV, and outline that indeed the models estimate that this observable is highly sensitive to saturation. We compare the results to, amongst others, two specific saturation models. None of these models is capable of describing all aspects of the data, and both models underestimate the data by an order of magnitude in the region that is most sensitive to saturation effects.

We conclude that either the models at hand do not correctly model the effects of saturation, or that gluon saturation appears not to be realised by Nature in this part of the phase-space.