[Paper Review] Parton energy loss or nuclear absorption: What quenches hadron spectra at HERA ?

François Arleo|ArXiv.org|Sep 10, 2003

High-Energy Particle Collisions Research3 citations

TL;DR

This paper proposes two observables to distinguish between parton energy loss and nuclear absorption as the origin of hadron spectrum quenching in deep inelastic scattering on nuclei. By comparing semi-inclusive hadron production on large nuclei like xenon and krypton, and analyzing the $Q^2$ dependence of kaon isospin effects, the study shows that parton energy loss predicts a weak nuclear size dependence and a $Q^2$-dependent $K^{-}/K^{+}$ suppression ratio, while nuclear absorption predicts stronger size dependence and no such $Q^2$ trend.

ABSTRACT

We suggest and explore two observables which may clarify the origin of the attenuation of semi-inclusive hadron production reported in DIS on nuclear targets.

Motivation & Objective

• To resolve the ambiguity between parton energy loss and nuclear absorption as the dominant mechanism for hadron spectrum attenuation in DIS on nuclei.
• To identify observables that can discriminate between partonic-level energy loss and final-state hadronic absorption effects.
• To test whether the $Q^2$ dependence of kaon isospin effects can serve as a probe of partonic medium interactions.
• To predict the behavior of hadron production ratios on large nuclei under the parton energy loss model to contrast with absorption-based models.

Proposed method

• Model parton energy loss using the transport coefficient $\hat{q}$, which quantifies medium-induced radiation and depends on the in-medium pathlength $L$.
• Calculate $L$ using a sharp nuclear density profile, with $L \approx t_f$ for large nuclei where $R \gg t_f$, leading to $\omega_c \propto t_f^2$.
• Predict the ratio of semi-inclusive hadron production on xenon and krypton targets under the parton energy loss model, expecting near-unity ratio for large nuclei.
• Analyze the $Q^2$ dependence of $K^{-}/K^{+}$ suppression ratios, leveraging the fact that at small $x$ (large $Q^2$), sea quark fragmentation dominates and isospin effects vanish.
• Use the $z$-dependence of $u \to K^{-}$ fragmentation functions to predict stronger $K^{-}$ suppression at small $x$, linked to valence quark dominance.
• Compare theoretical predictions with experimental data, particularly the $Q^2$ evolution of $R^{K^{-}}/R^{K^{+}}$ on lead, to test for partonic-scale dynamics.

Experimental results

Research questions

• RQ1 Does the ratio of semi-inclusive hadron production on two large nuclei (e.g., xenon and krypton) differ significantly under parton energy loss versus nuclear absorption?
• RQ2 How does the $Q^2$ dependence of $K^{-}/K^{+}$ suppression ratio reflect the transition from valence to sea quark fragmentation in nuclear DIS?
• RQ3 Can the $Q^2$-dependent isospin effect in kaon production serve as a signature of partonic energy loss in the nuclear medium?
• RQ4 What is the expected nuclear size dependence of hadron suppression in the large-$A$ limit under the parton energy loss model?
• RQ5 How does the hadron formation time $t_f$ influence the pathlength $L$ and thus the magnitude of parton energy loss in different nuclei?

Key findings

• The ratio of semi-inclusive charged pion production on xenon over krypton is predicted to be very close to one for large nuclei, due to $L \approx t_f$ and independence of $R$ in the large-$A$ regime.
• The $Q^2$ dependence of the $K^{-}/K^{+}$ suppression ratio on lead is predicted to decrease smoothly with increasing $Q^2$, indicating a transition from valence to sea quark dominance.
• At $\nu = 15$ GeV and $z = 0.6$, the $K^{-}/K^{+}$ suppression ratio decreases with increasing $Q^2$, signaling the onset of isospin effects at larger $x$.
• The observed $Q^2$-dependence of isospin effects is inconsistent with a purely hadronic absorption model, which would not predict such a smooth $Q^2$ evolution.
• The weak dependence of hadron production ratios on nuclear size in large nuclei strongly disfavors nuclear absorption as the dominant mechanism.
• The $Q^2$-dependent isospin effect provides a clear, quantitative signature of partonic energy loss, as it arises from the $z$-dependence of valence quark fragmentation into $K^{-}$.