The Fragility of High-pT Hadron Spectra as a Hard Probe

TL;DR

The study couples LO perturbative QCD with medium-induced radiative parton energy loss via quenching weights to examine high-$p_T$ hadron suppression in heavy-ion collisions. It finds that the nuclear modification factor $R_{AA}(p_T)$ is largely independent of $p_T$ at RHIC and LHC, due to the slope of the partonic spectrum and energy-loss dynamics, and that for time-averaged transport coefficients $\hat{q} \gtrsim 5$ GeV$^2$/fm the sensitivity of $R_{AA}$ to the produced energy density diminishes because the outer corona dominates particle production. The inferred interaction strength between hard partons and the medium exceeds perturbative estimates by a factor of about 4–5, reminiscent of the opacity problem seen in elliptic flow, and this motivates jet-based measurements to probe the medium more directly. Extrapolations to the LHC predict substantial, nearly $p_T$-independent suppression ($R_{AA} \sim 0.2 \pm 0.1$) up to very high $p_T$, highlighting the fragility of leading-hadron measurements as penetrating probes and the need to resolve jet substructure to access interior medium properties.

Abstract

We study the suppression of high-pT hadron spectra in nuclear collisions, supplementing the perturbative QCD factorized formalism with radiative parton energy loss. We find that the nuclear modification factor which quantifies the degree of suppression, is almost pT-independent both for RHIC (in agreement with data) and for the LHC. This is a consequence of the shape of the partonic pT-spectrum in elementary collisions which implies that for the same value of the nuclear modification factor at higher pT, an increasingly smaller fraction of parton energy loss is needed. When the values of the time-averaged transport coefficient exceed 5 GeV^2/fm, the nuclear modification factor gradually loses its sensitivity to the corresponding produced energy density. This is due to particle production in the outer corona of the medium, which remains almost unsuppressed even for extreme densities. Thus, even for the highest experimentally accessible transverse momentum at the LHC and in contrast to jets, the measurement of leading partons via leading hadrons is not a penetrating probe of the dense matter, but a rather fragile probe which fragments for the opacities reached below the skin of the medium. Relating the transport coefficient to the energy density produced in the collision region, we find and discuss a phenomenon reminiscent of the opacity problem of elliptic flow: namely, the interaction of the hard parent parton with the medium appears to be much stronger than that expected for perturbative interactions of the hard parton with an ideal quark gluon plasma.

Figures (10)

• Figure 1: Continuous part of the quenching weights for light quarks (upper row) and gluons (lower row) computed in the multiple soft scattering approximation with $\alpha_S=0.5$.
• Figure 2: Discrete part of the quenching weights for light quarks and gluons computed in the multiple soft scattering approximation with $\alpha_S=0.5$.
• Figure 3: (Colour online) The nuclear modification factor $R_{AA}$ for charged hadrons $h\equiv (h^++ h^-)/2$ in the $0-5\%$ most central Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. Different lines are for a maximal lifetime of the medium $\tau\sim L_{\rm cut}$=5 fm and different values of the transport coefficient $\hat{q}$. The shaded region between the curves calculated with reweighted (solid curves) and non-reweighted (dotted-dashed curves) quenching weights is indicative of uncertainties related to finite energy corrections. For comparison, the most central $R_{AA}$ data from the RHIC collaborations are shown (with statistical errors only): $0-5\%$ STAR data for $\vert\eta\vert <0.5$Adams:2003kv, $0-10\%$ PHENIX data for $\vert\eta\vert<0.35$Adler:2003au, $0-10\%$ BRAHMS data for $\eta=0$Arsene:2003yk and $0-6\%$ PHOBOS data for $0.2<\eta<1.4$Back:2003qr.
• Figure 4: (Colour online) $R_{AA}$ for charged hadrons $h\equiv (h^++ h^-)/2$ in the $0-5\%$ most central Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV as a function of $\hat{q}$ for $p_T$=10 GeV.
• Figure 5: (Colour online) The nuclear modification factor for charged hadrons $h\equiv (h^++ h^-)/2$ in $0-5\%$ most central Pb+Pb collisions at the LHC, calculated for $L_{\rm cut}$=10 fm and various values of the transport coefficient. The largest value $\hat{q}_{\rm LHC}=68$${\rm GeV}^2/{\rm fm}$ lies in the range favoured by current multiplicity estimates. See the text for more details.