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Measurement of dijet $k_{\rm T}$ in p-Pb collisions at ${\sqrt{s_{\rm NN}}=5.02}$ TeV

ALICE Collaboration

TL;DR

This work probes whether cold nuclear matter in p-Pb modifies dijet acoplanarity by measuring the dijet transverse momentum component $|k_T|$ using full and charged jets at midrapidity. The analysis employs anti-$k_T$ jets with $R=0.4$, event-by-event underlying-event corrections, random-cone background fluctuations, and detector-response corrections, comparing the results to PYTHIA8 with and without ISR. The key finding is that the $|k_T|$ distributions and their mean agree with ISR-inclusive PYTHIA and show no significant dependence on event multiplicity, indicating minimal nuclear $k_T$ broadening in this kinematic regime. This supports the interpretation that the large jet $p_T$ imbalance observed in heavy-ion collisions arises from hot, dense medium effects rather than initial-state cold nuclear matter in p-Pb, within the explored range. The results provide a robust pp/PYTHIA baseline for interpreting dijet acoplanarity in nucleus-nucleus collisions.

Abstract

A measurement of dijet correlations in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV with the ALICE detector is presented. Jets are reconstructed from charged particles measured in the central tracking detectors and neutral energy deposited in the electromagnetic calorimeter. The transverse momentum of the full jet (clustered from charged and neutral constituents) and charged jet (clustered from charged particles only) is corrected event-by-event for the contribution of the underlying event, while corrections for underlying event fluctuations and finite detector resolution are applied on an inclusive basis. A projection of the dijet transverse momentum, $k_{\rm Ty} = p_\rm{T,jet}^\rm{ch+ne} \; \rm{sin}(Δ\varphi_{\rm{dijet}})$ with $Δ\varphi_{\rm{dijet}}$ the azimuthal angle between a full and charged jet and $p_\rm{T,jet}^\rm{ch+ne}$ the transverse momentum of the full jet, is used to study nuclear matter effects in p-Pb collisions. This observable is sensitive to the acoplanarity of dijet production and its potential modification in p-Pb collisions with respect to pp collisions. Measurements of the dijet $k_{\rm Ty}$ as a function of the transverse momentum of the full and recoil charged jet, and the event multiplicity are presented. No significant modification of $k_{\rm Ty}$ due to nuclear matter effects in p-Pb collisions with respect to the event multiplicity or a PYTHIA8 reference is observed.

Measurement of dijet $k_{\rm T}$ in p-Pb collisions at ${\sqrt{s_{\rm NN}}=5.02}$ TeV

TL;DR

This work probes whether cold nuclear matter in p-Pb modifies dijet acoplanarity by measuring the dijet transverse momentum component using full and charged jets at midrapidity. The analysis employs anti- jets with , event-by-event underlying-event corrections, random-cone background fluctuations, and detector-response corrections, comparing the results to PYTHIA8 with and without ISR. The key finding is that the distributions and their mean agree with ISR-inclusive PYTHIA and show no significant dependence on event multiplicity, indicating minimal nuclear broadening in this kinematic regime. This supports the interpretation that the large jet imbalance observed in heavy-ion collisions arises from hot, dense medium effects rather than initial-state cold nuclear matter in p-Pb, within the explored range. The results provide a robust pp/PYTHIA baseline for interpreting dijet acoplanarity in nucleus-nucleus collisions.

Abstract

A measurement of dijet correlations in p-Pb collisions at TeV with the ALICE detector is presented. Jets are reconstructed from charged particles measured in the central tracking detectors and neutral energy deposited in the electromagnetic calorimeter. The transverse momentum of the full jet (clustered from charged and neutral constituents) and charged jet (clustered from charged particles only) is corrected event-by-event for the contribution of the underlying event, while corrections for underlying event fluctuations and finite detector resolution are applied on an inclusive basis. A projection of the dijet transverse momentum, with the azimuthal angle between a full and charged jet and the transverse momentum of the full jet, is used to study nuclear matter effects in p-Pb collisions. This observable is sensitive to the acoplanarity of dijet production and its potential modification in p-Pb collisions with respect to pp collisions. Measurements of the dijet as a function of the transverse momentum of the full and recoil charged jet, and the event multiplicity are presented. No significant modification of due to nuclear matter effects in p-Pb collisions with respect to the event multiplicity or a PYTHIA8 reference is observed.

Paper Structure

This paper contains 12 sections, 1 equation, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experimental Setup and Data Sample
  3. Jet reconstruction and dijet $\mathbf{{\it k}_{Ty}}$
  4. Jet Reconstuction
  5. Dijet $\mathbf{{\it k}_{Ty}}$
  6. Corrections and Systematic Uncertainties
  7. Results
  8. Evolution with full-jet transverse momentum
  9. Evolution with event multiplicity and $\mathbf{{\it p}_{T,assoc\;jet}^{ch}}$
  10. Evolution and characterization via $\mathbf{\langle |{\it k}_{Ty}| \rangle}$
  11. Conclusion
  12. The ALICE Collaboration

Figures (3)

  • Figure 1: Dijet $|k_{\rm Ty}|$ distributions in p--Pb collisions in the 0-40% V0A multiplicity event class for several kinematic intervals of the full jet ($p_{\mathrm{T,jet}}^{\mathrm{ch+ne}}$). The measurement is compared to PYTHIA8 (tune 4C, $K=0.7$) with and without initial state radiation. The lower panels show the ratio between the measurement and PYTHIA8 including initial state radiation.
  • Figure 2: Distributions of $|k_{\rm Ty}|$ for two V0A event classes (left panel) and three $p_{\mathrm{T,assoc\;jet}}^{\mathrm{ch}}$ ranges (right panel).
  • Figure 3: Mean of the $|k_{\rm Ty}|$ distributions as a function of the full jet transverse momentum $p_{\mathrm{T,jet}}^{\mathrm{ch+ne}}$ (left) and the associated charged jet transverse momentum $p_{\mathrm{T,assoc\;jet}}^{\mathrm{ch}}$ (right) compared to PYTHIA8.