${\cal O}(α_s^3)$ analysis of inclusive jet and di-jet production in heavy ion reactions at the Large Hadron Collider

TL;DR

The paper tackles jet production and modification in heavy-ion collisions at the LHC by deriving first-principles ${\cal O}(\alpha_s^3)$ predictions for inclusive jets and di-jets in pp, then embedding in-medium energy loss at ${\cal O}(\alpha_s^2 \alpha_s^{\rm rad})$ with cold nuclear matter effects and hadronization corrections to Pb+Pb at ${\sqrt{s_{NN}}=2.76}$ TeV, enabling quantitative comparisons to ATLAS/CMS measurements. This yields Pb+Pb predictions and illuminates the roles of final-state energy loss, CNM effects, and hadronization in shaping jet observables, including the di-jet asymmetry $A_J$. It finds that radiative energy loss largely drives jet suppression while background subtraction and possible collisional diffusion can significantly alter observed jet and di-jet signatures, underscoring the importance of jet radius and background treatment. The framework provides a unified perturbative approach linking pp baselines to heavy-ion jet quenching signals and motivates new observables to disentangle radiative versus collisional energy loss in dense QCD matter.

Abstract

Jets physics in heavy ion reactions is an important new area of active research at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) that paves the way for novel tests of QCD multi-parton dynamics in dense nuclear matter. At present, perturbative QCD calculations of hard probes in elementary nucleon-nucleon reactions can be consistently combined with the effects of the nuclear medium up to $ {\cal O}(α_s^3) $. While such accuracy is desirable but not necessary for leading particle tomography, it is absolutely essential for the new jet observables. With this motivation, we present first results and predictions to $ {\cal O}(α_s^3) $ for the recent LHC lead-lead (Pb+Pb) run at a center-of-mass energy of 2.76 TeV per nucleon-nucleon pair. Specifically, we focus on the suppression of the single and double inclusive jet cross sections. Our analysis includes not only final-state inelastic parton interactions in the QGP, but also initial-state cold nuclear matter effects and an estimate of the non-perturbative hadronization corrections. We demonstrate how an enhanced di-jet asymmetry in central Pb+Pb reactions at the LHC, recently measured by the ATLAS and CMS experiments, can be derived from these results. We show quantitatively that a fraction of this enhancement may be related to the ambiguity in the separation between the jet and the soft background medium and/or the diffusion of the parton shower energy away from the jet axis through collisional processes. We point to a suite of measurements that can help build a consistent picture of parton shower modification in heavy ion collisions at the LHC.

Figures (9)

• Figure 1: Comparison between the $R=0.4$ inclusive jet cross section measured by the ATLAS Collaboration in p+p collisions at $\sqrt{s}=7$ TeV and the ${\cal O}( \alpha_s^3 )$ QCD theory which includes non-perturbative corrections. Experimental error bars are statistical only.
• Figure 2: A three-dimensional representation of the ${\cal O}( \alpha_s^3 )$ di-jet cross section $\tilde{\sigma}$, defined in Eq. (\ref{['brief']}), in $\sqrt{s} = 7$ TeV p+p collisions at the LHC versus $E_{T\, 1}$ and $E_{T\, 2}$. We have used a jet radius $R=0.4$ and a rapidity interval $|y|<2.8$.
• Figure 3: The normalized differential distribution of the di-jet asymmetry $A_J$, defined in Eq. (\ref{['ajdef']}), is evaluated at ${\cal O}( \alpha_s^3 )$ and compared to the ATLAS experimental measurement at $\sqrt{s}=7$ TeV. Solid (dashed) lines include (do not include) non-perturbative corrections, respectively.
• Figure 4: Top panel: the $E_T$ dependence of the nuclear modification factor for different jet cone sizes $R=0,2, \, 0.6$ is calculated in central Pb+Pb collisions at the LHC $\sqrt{s_{NN}}=2.76$ TeV. Bands represent the variation in the coupling strength between the jet and the medium. Bottom panel: the relative contribution of cold nuclear matter effects to $R_{AA}$ is illustrated for $R=0.2, \, 0.4, \, 0.6, \, 0.8$. ALICE experimental data on charged hadron suppression in central Pb+Pb collision is shown for reference. Preliminary ATLAS $R_{CP}$ data is also shown for both $R= 0.2,\, 0.4$.
• Figure 5: Ratio of the inclusive jet cross sections in central Pb+Pb collisions at LHC at $\sqrt{s_{NN}}=2.76$ TeV for two different radii $R_1 =0.2$ and $R_2=0.4$. The bands show results with different extrapolation of non-perturbative corrections to small radii. The lines show effect of different coupling strength between the jet and the medium.