[Paper Review] Probing the Frontiers of QCD

W. A. Horowitz|arXiv (Cornell University)|Nov 18, 2010

High-Energy Particle Collisions Research331 references8 citations

TL;DR

This thesis investigates jet quenching and azimuthal anisotropy in heavy-ion collisions to probe the quark-gluon plasma (sQGP). It proposes a focusing mechanism for partonic jets to explain the large $v_2$ despite strong suppression ($R_{AA}$), and demonstrates that including collisional energy loss and path-length fluctuations improves agreement with data. A key result is a robust experimental test: the momentum-dependent ratio of charm to bottom quark suppression $R^{cb}(p_T)$ distinguishes perturbative QCD from AdS/CFT-based energy loss models.

ABSTRACT

With the energy scales opened up by RHIC and LHC the age of high-pT physics is upon us. This has created new opportunities and novel mysteries, both of which will be explored in this thesis. The possibility now exists experimentally to exploit these high momentum particles to uniquely probe the unprecedented state of matter produced in heavy ion collisions. At the same time naive theoretical expectations have been dashed by data. The first puzzle we confront is that of the enormous intermediate-pT azimuthal anisotropy, or v2, of jets observed at RHIC. The second puzzle is the surprisingly similar suppression of light mesons and nonphotonic electrons, which precludes perturbative predictions predicated on gluon bremsstrahlung radiation as the dominant energy loss channel. Near qualitative agreement results from including collisional energy loss and integrating over the fluctuating jet pathlengths. Another conjecture for heavy quark energy loss comes via explicit construction using the AdS/CFT correspondence; the momentum loss of a hanging dragging string moving through the deconfined plasma leads to qualitative agreement with heavy quark decay data. We propose a robust test to experimentally differentiate these two competing ideas: the ratio of charm to bottom suppression rapidly approaches 1 for pQCD but is independent of momentum and well below 1 for AdS/CFT. Finally as a warmup problem to calculating the photon bremsstrahlung associated with jet energy loss we quantify improvements to the perturbative estimates of the Ter-Mikayelian effect.

Motivation & Objective

• To resolve the puzzle of large intermediate-$p_T$ azimuthal anisotropy ($v_2$) in jets despite strong suppression ($R_{AA}$), contradicting standard energy loss models.
• To reconcile the similar suppression of light mesons and nonphotonic electrons, challenging perturbative QCD predictions based on gluon bremsstrahlung.
• To test competing energy loss mechanisms—pQCD versus AdS/CFT—using heavy quark $R_{AA}$ data and propose a robust experimental observable.
• To improve perturbative estimates of photon bremsstrahlung by including interference from away-side jets and regulating low-momentum divergences.
• To assess theoretical uncertainties in jet tomography and identify critical approximation regimes for future modeling.

Proposed method

• Proposes a focusing mechanism in the deconfined quark-gluon plasma to enhance $v_2$ while reducing jet energy loss, explaining the observed $v_2$-$R_{AA}$ pattern.
• Integrates collisional energy loss and fluctuating jet pathlengths into a model that reproduces the similar $R_{AA}$ values for light mesons and nonphotonic electrons.
• Uses the AdS/CFT correspondence to model heavy quark energy loss via a dragging string in a thermal plasma background, yielding momentum-dependent $R_{AA}$.
• Derives photon bremsstrahlung radiation from heavy quarks using both classical and quantum field theory methods, including interference terms from the away-side jet.
• Compares results from full recoil calculations with simplified approximations, highlighting the importance of neglected terms in prior derivations.
• Proposes the double ratio $R^{cb}(p_T) = R^{c}_{AA}(p_T)/R^{b}_{AA}(p_T)$ as a momentum-dependent observable to distinguish pQCD (tending to 1) from AdS/CFT (constant and ~0.2) predictions.

Experimental results

Research questions

• RQ1 Why is the intermediate-$p_T$ azimuthal anisotropy ($v_2$) of jets anomalously large despite strong suppression ($R_{AA}$), contradicting standard energy loss models?
• RQ2 How can the similar suppression of light mesons and nonphotonic electrons be explained if gluon bremsstrahlung is the dominant energy loss mechanism?
• RQ3 What observable can experimentally distinguish between perturbative QCD and AdS/CFT-based energy loss mechanisms for heavy quarks?
• RQ4 How do interference effects and low-momentum contributions affect the Ter-Mikayelian effect in in-medium photon bremsstrahlung?
• RQ5 What are the theoretical uncertainties in current jet tomography models, and how can they be minimized for future precision studies?

Key findings

• The observed $v_2$-$R_{AA}$ pattern cannot be explained by standard energy loss mechanisms, necessitating a new focusing mechanism in the sQGP.
• Including collisional energy loss and path-length fluctuations reduces the discrepancy between light meson and nonphotonic electron $R_{AA}$, improving qualitative agreement with data.
• The double ratio $R^{cb}(p_T)$ is predicted to be momentum-independent and ~0.2 in AdS/CFT, while approaching 1 at high $p_T$ in pQCD, providing a robust experimental test.
• Interference from away-side jets significantly reduces photon bremsstrahlung radiation, challenging previous estimates that neglected such terms.
• Theoretical precision in jet tomography is limited by large uncertainties, particularly from $\alpha_s$ dependence and unknown momentum cutoffs in pQCD and AdS/CFT approximations.
• The $R^{cb}(p_T)$ ratio is insensitive to input parameter variations, making it a highly robust observable for testing competing energy loss models at the LHC.