[Paper Review] Can we distinguish energy loss from hadron absoprtion?

Alberto Accardi|ArXiv.org|Oct 31, 2005

High-Energy Particle Collisions Research1 references4 citations

TL;DR

This paper investigates whether energy loss or hadron absorption better explains jet quenching in nuclear collisions by analyzing semi-inclusive deep inelastic scattering (nDIS) data. It demonstrates that both models produce similar $A^{2/3}$ scaling in hadron suppression, making standard $A$-dependence analysis insufficient to distinguish them, and shows that refined $(c, \alpha)$ fits also fail to clearly differentiate the two mechanisms.

ABSTRACT

Knowing whether a hadron is formed inside or outside the nuclear medium is very important for correctly interpreting jet quenching in heavy-ion collisions. The cleanest experimental environment to study the space-time evolution of hadronization is semi-inclusive DIS on nuclear targets. Two frameworks are presently competing to explain the observed attenuation of hadron production: quark energy loss, with hadron formation outside the nucleus, and nuclear absorption with hadronization starting inside the nucleus. I demonstrate that the observed approximate A^(2/3) scaling of experimental data cannot conclusively establish the correctness of either energy loss or absorption.

Motivation & Objective

• To determine whether experimental $A$-dependence of hadron suppression in nDIS can distinguish between quark energy loss and nuclear absorption models.
• To assess the validity of the widely used $A^{2/3}$ scaling argument as a discriminator between energy loss and absorption mechanisms.
• To evaluate whether refined $R_M = cA^\alpha$ fits can resolve the ambiguity between the two models.
• To identify more exclusive observables capable of distinguishing the true mechanism of hadronization in nuclear matter.

Proposed method

• Uses the HERMES nDIS data on $\pi^+$ production in semi-inclusive deep inelastic scattering on light nuclei (He, N, Ne, Kr, Pb) to compare theoretical models.
• Applies two competing frameworks: (1) quark energy loss with medium-induced gluon radiation, and (2) hadron absorption via prehadron formation and interaction with nuclear matter.
• Models prehadron and hadron formation lengths using the Lund fragmentation model, with $\langle l^*\rangle$ and $\langle l^h\rangle$ as key parameters.
• Computes survival probabilities $S^{A}_{f,h}(z,\nu)$ via transport equations integrating over nuclear density $\rho_A(b,y)$ and path lengths.
• Fits the attenuation ratio $R_M^h(z)$ to $cA^\alpha$ across multiple $A$-values and $z$-bins to extract $c(z)$ and $\alpha(z)$ as fit parameters.
• Compares model predictions with experimental data and assesses model distinguishability through $2\sigma$ confidence contours in the $(c,\alpha)$ plane.

Experimental results

Research questions

• RQ1 Can the $A$-dependence of hadron suppression in nDIS uniquely distinguish between quark energy loss and hadron absorption models?
• RQ2 Does the observed $A^{2/3}$ scaling in $R_M^h$ truly indicate energy loss, or can absorption models also reproduce this scaling?
• RQ3 Can the $cA^\alpha$ fit to $R_M^h$ resolve the ambiguity between energy loss and absorption mechanisms?
• RQ4 What exclusive observables, such as $p_T$-broadening or dihadron correlations, are needed to definitively distinguish the two models?
• RQ5 How does a finite prehadron formation length $\langle l^*\rangle$ affect the $A$-scaling behavior in absorption models?

Key findings

• Both energy loss and hadron absorption models reproduce the observed $A^{2/3}$ scaling of $R_M^h$ in nDIS data, invalidating the common assumption that this scaling proves energy loss.
• The $A^{2/3}$ scaling in absorption models arises not from $\langle L_A\rangle \propto A^{1/3}$, but from the interplay between finite prehadron formation length $\langle l^*\rangle$ and nuclear radius $R_A$, leading to $R_M^h \propto A^{2/3} + O(A)$.
• The $cA^\alpha$ fit to $R_M^h$ fails to clearly distinguish between the two models, as both yield compatible $(c,\alpha)$ parameters within $2\sigma$ confidence contours.
• Nonlinear breaking of the $A^{2/3}$ law is observed at $A \gtrsim 80$, indicating that higher-$A$ targets could reveal differences, but only if systematic uncertainties are controlled.
• The study concludes that single hadron suppression data alone cannot resolve whether hadronization begins inside or outside the nucleus.
• More exclusive observables—such as $z$-dependent $p_T$-broadening, the Cronin effect, or dihadron correlations—are required to definitively test the mechanism of hadronization.