Measurement of forward jet suppression in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$$ TeV with the ATLAS detector

Technical Summary: Measurement of Forward Jet Suppression in Pb+Pb Collisions at $\sqrt{s_{NN}} = 5.02$ TeV

Problem and Motivation
The formation of a quark–gluon plasma (QGP) in ultrarelativistic heavy-ion collisions allows for the study of parton energy loss, known as jet quenching. While inclusive jet suppression has been extensively measured at mid-rapidity ($|y| \lesssim 2.8$), the forward rapidity region remains largely unexplored in heavy-ion collisions. Theoretical considerations suggest that the forward region offers a distinct kinematic environment: the fraction of quark-initiated jets is expected to increase with rapidity, while the initial parton spectra become steeper. Furthermore, the longitudinal expansion of the QGP may result in different effective medium densities and path-length dependencies compared to mid-rapidity. Previous measurements at mid-rapidity have shown that jet suppression depends on the initial parton flavor (quark vs. gluon) and the steepness of the spectrum. However, disentangling these factors requires measurements in kinematic regions with different flavor compositions. This paper addresses the lack of data on inclusive jet suppression in the forward rapidity region to provide new constraints on the flavor, geometric, and medium-density dependence of parton energy loss.

Methodology
The analysis utilizes data collected by the ATLAS detector at the LHC during Run 2. The dataset includes Pb+Pb collisions recorded in 2018 with an integrated luminosity of 1.72 nb$^{-1}$ and $pp$ collisions from 2017 with 255 pb$^{-1}$, both at a nucleon–nucleon center-of-mass energy of $\sqrt{s_{NN}} = 5.02$ TeV.

• Jet Reconstruction: Jets are reconstructed using the anti-$k_t$ algorithm with a radius parameter $R=0.4$. The analysis focuses on two forward rapidity intervals: $2.8 < |y| < 3.2$ and $3.2 < |y| < 3.6$. A transverse momentum threshold of $p_T > 74$ GeV is applied to ensure full trigger efficiency and to suppress contributions from underlying event (UE) fluctuations.
• Centrality Determination: Event centrality in Pb+Pb collisions is estimated using the total transverse energy in the forward calorimeters ($\Sigma E_{FCal}^T$), correlated with a Monte Carlo Glauber simulation. Four centrality intervals are defined: 0–10%, 10–20%, 20–50%, and 50–80%.
• Background Subtraction and Calibration: The contribution of the underlying event to jet reconstruction is subtracted on an event-by-event basis, accounting for harmonic flow modulation. Jet energy scale (JES) and jet energy resolution (JER) corrections are applied. The JES includes components specific to the heavy-ion environment to account for potential modifications in jet fragmentation patterns.
• Unfolding: To correct for detector resolution effects and migration between bins, a one-dimensional Bayesian unfolding method is employed. Response matrices are constructed from simulated dijet events overlaid with minimum-bias Pb+Pb data to realistically model the UE.
• Nuclear Modification Factor ($R_{AA}$): The $R_{AA}$ is calculated as the ratio of the $\langle T_{AA} \rangle$-normalized jet yield in Pb+Pb collisions to the jet cross-section in $pp$ collisions.

Key Contributions and Results
This work presents the first measurement of inclusive jet suppression at forward rapidity in heavy-ion collisions.

• Suppression Observation: Significant suppression of jet yields is observed in Pb+Pb collisions relative to $pp$ collisions across all measured centrality intervals and rapidity regions.
• Centrality and $p_T$ Dependence: The magnitude of $R_{AA}$ increases monotonically from central (0–10%) to peripheral (50–80%) collisions. The suppression exhibits a mild dependence on jet $p_T$, with $R_{AA}$ values generally increasing with $p_T$. The $p_T$ dependence remains comparable across different centrality and rapidity intervals.
• Rapidity Comparison:
  • In the $2.8 < |y| < 3.2$ region, the measured $R_{AA}$ values show a tendency to be lower than those measured in the mid-rapidity region ($|y| < 2.8$). This is consistent with trends suggested by previous mid-rapidity measurements and may reflect the changing flavor composition (increasing quark fraction) and steeper spectra at forward rapidity.
  • In the more forward region of $3.2 < |y| < 3.6$, the statistical precision of the measurement is insufficient to draw a conclusive comparison with the $|y| < 2.8$ region.
• Theoretical Comparison: The measured $R_{AA}$ values are compared with three theoretical models: the Linear Boltzmann Transport (LBT) model, the Hybrid model, and a Parametric Model.
  • The LBT model reproduces the shape of the $R_{AA}$ but underpredicts the magnitude of the suppression.
  • The Parametric Model overpredicts the suppression at high $p_T$.
  • The Hybrid model provides the best agreement with the data when both Molière scatterings and the medium wake (the response of the QGP to deposited energy) are included.

Significance
The paper claims that these results constitute the first measurement of inclusive jet suppression in the forward-rapidity region in heavy-ion collisions. By probing a kinematic regime with different admixtures of quark- and gluon-initiated jets, potentially different effective medium densities, and different levels of selection bias compared to mid-rapidity, these measurements provide new constraints on theoretical descriptions of jet quenching. Specifically, the data helps to disentangle the flavor dependence of parton energy loss and its dependence on the geometry and density of the expanding QGP. The results serve as a benchmark for models attempting to describe the interplay between perturbative and non-perturbative mechanisms in jet-medium interactions.