[Paper Review] High-pT paradigms revisited

B. Z. Kopeliovich, J. Nemchik|arXiv (Cornell University)|Sep 6, 2010

High-Energy Particle Collisions Research3 citations

TL;DR

This paper critically re-evaluates high-𝑝_𝑇 paradigms in nuclear collisions, proposing that pre-hadron production and rapid absorption in dense matter—rather than prolonged energy loss—explain jet quenching. It introduces a parameter-free model predicting the nuclear suppression factor 𝑅_𝐴𝐴 via dipole size growth and short mean free paths, with strong agreement to RHIC data for both light and heavy flavors, including bottom quarks.

ABSTRACT

We present an attempt at a critical overview of the current status of modeling for high-pT processes in nuclei. The paper covers several topics including coherence phenomena, in particular gluon shadowing and CGC; nuclear effects related to the restrictions imposed by energy conservation at large xL and xT ; space-time development of hadronization of highly virtual light and heavy partons and the related time scales; and the role of early production and subsequent attenuation of pre-hadrons in a dense medium. We identify several intriguing problems in the current paradigms for high-pT processes and propose solutions for some of them.

Motivation & Objective

• Challenge the standard energy loss scenario for jet quenching, which assumes long-time hadronization outside the medium.
• Address inconsistencies in modeling high-𝑝_𝑇 hadron suppression, especially for bottom quarks.
• Reconcile observed 𝑅_𝐴𝐴 values with a mechanism based on early pre-hadron production and strong absorption in dense matter.
• Provide a parameter-free prediction of 𝑅_𝐴𝐴 by modeling dipole expansion and mean free path in the medium.
• Reconcile data on light and heavy flavor suppression under a unified framework, resolving discrepancies in the energy loss model.

Proposed method

• Model the space-time evolution of pre-hadron production using dipole phenomenology derived from DIS data.
• Estimate the initial transverse size of pre-hadrons as 𝑟_𝑇 ≈ 0.5 fm at the mean production length, with faster expansion for heavy quarks.
• Apply reciprocity equations to show mutual boosting of saturation scales in 𝐴𝐴 collisions, enhancing opacity compared to 𝑝𝐴.
• Use 𝑝_𝑇 broadening measurements to directly access the saturation scale in nuclei, validating the model.
• Assess energy conservation constraints at large 𝑥_𝐿 and 𝑥_𝑇, showing suppression from kinematic limits.
• Compare 𝑝𝐴 and 𝐴𝐴 collisions to test boosted saturation scale effects via broadening magnitudes.

Experimental results

Research questions

• RQ1 Can the suppression of high-𝑝_𝑇 hadrons in 𝐴𝐴 collisions be explained without assuming long-time hadronization outside the medium?
• RQ2 Why is bottom quark suppression stronger than predicted by standard energy loss models, and can this be resolved within a new framework?
• RQ3 To what extent do energy conservation constraints at large 𝑥_𝐿 and 𝑥_𝑇 contribute to the observed suppression in forward rapidities?
• RQ4 Can the nuclear suppression factor 𝑅_𝐴𝐴 be predicted without free parameters using pre-hadron absorption and expansion dynamics?
• RQ5 How does the mutual boosting of saturation scales in 𝐴𝐴 collisions affect the opacity to colorless dipoles compared to 𝑝𝐴?

Key findings

• The initial transverse size of pre-hadron dipoles is estimated at 𝑟_𝑇 ≈ 0.5 fm, increasing rapidly with production length, leading to strong absorption in dense matter.
• The model predicts the 𝑅_𝐴𝐴 suppression factor for high-𝑝_𝑇 hadrons in a parameter-free way, with excellent agreement to RHIC data for both light and heavy flavors.
• The saturation scale in 𝐴𝐴 collisions is significantly boosted compared to 𝑝𝐴 due to reciprocity equations, enhancing medium opacity.
• Energy conservation at large 𝑥_𝐿 and 𝑥_𝑇 leads to a deficit of available energy, contributing to suppression in forward rapidities, as supported by RHIC data.
• Vacuum radiation from highly virtual partons is flavor-independent due to the dead-cone effect, implying similar suppression for charm and light quarks—challenging the energy loss model.
• The transport coefficient inferred from energy loss models is too large compared to independent probes like 𝐽/Ψ suppression, indicating overestimation of medium density if pre-hadron absorption is neglected.