[Paper Review] The Rapidity Dependence of Jet Quenching

Thorsten Renk|arXiv (Cornell University)|Jun 26, 2014

High-Energy Particle Collisions Research3 citations

TL;DR

This paper investigates how jet quenching, measured by the nuclear suppression factor $R_{AA}$, varies with rapidity in Pb-Pb collisions at $\sqrt{s}=2.76$ TeV using the YaJEM Monte Carlo framework. It shows that $R_{AA}$ decreases at forward rapidity not due to medium opacity, but primarily due to kinematic effects: a steeper parton spectrum and a shift toward quark-dominated jets, which reduces energy loss. The key finding is that hadrons are more sensitive to the kinematic limit than jets, despite jets being closer to the energy threshold.

ABSTRACT

The suppression of high transverse momentum (P_T) jets and hadrons in ultrarelativistic heavy-ion collisions with respect to a p-p baseline in terms of the nuclear suppression factor R_AA is one of the key observables to gauge the density of a hot and dense QCD medium. However, the suppression measured by R_AA is not a straightforward measure of the medium properties, the value of the observable also depends on the ratio of quark to gluon jets and on the slope of the hard parton spectrum, which explains why R_AA is found to be fairly similar at RHIC and LHC despite the very different dynamics. Measuring high P_T jets and hadrons at forward rapidity offers the same possibility of varying medium density, parton mixture and spectral slope without the need to compare across different sqrt(s) and experiments. In this work, the well-tested jet quenching Monte-Carlo (MC) framework YaJEM is utilized to compute the rapidity dependence of R_AA for three test cases.

Motivation & Objective

• To investigate the rapidity dependence of jet quenching $R_{AA}$ in heavy-ion collisions at LHC energies, independent of center-of-mass energy changes.
• To disentangle the contributions of parton spectrum slope, quark-to-gluon jet ratio, and medium opacity to $R_{AA}$ variations across rapidity.
• To assess whether forward rapidity measurements can serve as a controlled probe of parton-medium interactions by varying kinematics and parton composition.
• To evaluate the relative importance of kinematic effects versus medium-induced energy loss in shaping $R_{AA}(y)$.

Proposed method

• The YaJEM Monte Carlo framework is used to simulate in-medium shower evolution and jet quenching for partons in Pb-Pb collisions at $\sqrt{s}=2.76$ TeV.
• Three test cases are simulated: hadron $R_{AA}$ at 80 GeV, jet $R_{AA}$ at 100 GeV, and a case assuming all partons are quarks in the jet sample.
• The parton spectrum is modeled using perturbative QCD (pQCD), and the kinematic limit at forward rapidity ($y=3$) is explicitly accounted for via $E_{\text{max}} = \sqrt{s}/2$.
• The suppression factor $R_{AA}(P_T, y)$ is computed as the ratio of A-A to p-p yields, normalized by the number of binary collisions.
• The role of parton type (quark vs. gluon) is analyzed by tracking the fraction of gluon jets contributing to the final hadron and jet observables.
• The impact of spectral steepening and parton composition is isolated by comparing results at midrapidity ($y=0$) and forward rapidity ($y=3$).

Experimental results

Research questions

• RQ1 How does the rapidity dependence of $R_{AA}$ for high-$P_T$ jets and hadrons arise from kinematic effects rather than medium properties?
• RQ2 To what extent does the steepening of the parton spectrum at forward rapidity affect $R_{AA}$?
• RQ3 How does the shift from gluon-rich to quark-rich parton spectra at forward rapidity influence energy loss and $R_{AA}$?
• RQ4 Why is $R_{AA}$ for hadrons more sensitive to the kinematic limit than for jets, despite jets being closer to the energy threshold?
• RQ5 Can forward rapidity measurements serve as a controlled probe of parton spectrum and composition effects in jet quenching?

Key findings

• At forward rapidity ($y=3$), the parton spectrum steepens significantly, increasing the power-law index $n$ in a local $1/p_T^n$ fit, which enhances suppression for a fixed energy loss.
• The fraction of gluon jets drops sharply at forward rapidity, shifting the parton mixture toward quarks, which reduces the average energy loss per parton due to their lower color charge coupling.
• Despite the stronger suppression expected from a steeper spectrum, $R_{AA}$ decreases at forward rapidity primarily due to the kinematic restriction of parton $p_T$ to below 137 GeV at $y=3$, which limits the phase space of high-energy partons.
• Hadrons at 80 GeV are more strongly suppressed at $y=3$ than jets at 100 GeV because the phase space for high-$p_T$ partons contributing to hadrons is drastically reduced, while jets retain more energy due to efficient clustering.
• The contribution of gluon jets to the final hadron yield is significantly reduced due to their softer fragmentation patterns, so even a large gluon fraction in the parton spectrum results in a minor effect on $R_{AA}$ for hadrons.
• The observed downward trend in $R_{AA}$ at forward rapidity is not due to increased medium opacity but is dominated by kinematic effects, with parton spectrum steepening and phase space restriction being the primary drivers.