Saturation and forward jets in proton-lead collisions at the LHC

TL;DR

This study analyzes forward jet production in proton–lead collisions at LHC energies within the Color Glass Condensate framework, focusing on the CASTOR calorimeter's very forward rapidities to probe small-x nuclear gluon saturation. Using a hybrid CGC approach with BK evolution and a Glauber-based nucleus initial condition, the authors include multi-parton interactions and jet merging to model the CASTOR jet-energy spectrum. They find substantial nuclear suppression (RpA ~ 0.5) at low jet energies, with two- and three-jet merging contributing significantly at higher energies, achieving good agreement with CMS CASTOR data. The work highlights the importance of saturation and MPI effects in forward-forward jet observables and discusses limitations due to LO treatment and fragmentation neglect, suggesting avenues for refinement.

Abstract

We investigate the forward-jet energy spectrum within the Color Glass Condensate framework at 5 TeV center-of-mass energy. In particular, we focus on the kinematic range covered by the CMS-CASTOR calorimeter. We show that our saturation-model calculations are compatible with the CASTOR measurements and that to optimally reproduce the data, effects of multi-parton interactions need to be included. We predict a significant nuclear suppression - reaching down to 50% at the lowest considered jet energies $E_{\rm jet} \sim 500 \, {\rm GeV}$.

Figures (9)

• Figure 1: Jet energy spectra in proton-proton collisions at $\sqrt{s}=13$ TeV in the cases where one or two jets are merged. No rapidity shift is applied here. The thin lines refer to the "naive" approximation where no angular constraints are implemented on the jets that are not merged.
• Figure 2: Contribution to the jet production cross section in proton-nucleus collisions at $\sqrt{s}=5.02$ TeV from the cases where $n$ jets are merged into one. The results are normalized by the corresponding cross section in single jet production (with no merging) in proton-nucleus collisions. The thick lines are obtained with our standard choice for the infrared cutoff $p_{T,\text{min}}=1.0\ \textrm{GeV}$. Upper thin lines correspond to $p_{T,\text{min}}=0.5\ \textrm{GeV}$ and lower thin lines $p_{T,\text{min}}=1.5\ \textrm{GeV}$.
• Figure 3: Jet energy spectrum in proton-nucleus collisions compared with the CASTOR data Sirunyan:2018ffo at $\sqrt{s}=5.02\ \textrm{TeV}$ including one, two and three merged jet contributions. The scaled proton-proton result includes two- and three merged jets contributions.
• Figure 4: Nuclear modification factor for the jet energy spectrum in CASTOR kinematics at $\sqrt{s}=5.02\ \textrm{TeV}$ including contributions from up to three merged jets. The proton-proton reference is computed with the same kinematical shift as applied in the proton-nucleus scattering.
• Figure 5: Jet production cross section ratio $R_\text{shift}$ at $\sqrt{s}=5.02\ \textrm{TeV}$ defined in Eq. \ref{['eq:rshift']} in case of no rapdity shift in the proton-proton baseline with and without nuclear effects (see text for details). Thick lines include contribution from one, two or three merged jets, and thin lines have no merged jets.