Measuring The Collective Flow With Jets

TL;DR

It is argued that data on the medium-induced broadening of jetlike particle correlations in Au + Au collisions at the Relativistic Heavy-Ion Collider may provide evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow.

Abstract

In nucleus--nucleus collisions, high-pT partons interact with a dense medium, which possesses strong collective flow components. Here, we demonstrate that the resulting medium-induced gluon radiation does not depend solely on the energy density of the medium, but also on the collective flow. Both components cannot be disentangled on the basis of leading hadron spectra, but the measurement of particle production associated to high-pT trigger particles, jet-like correlations and jets, allows for their independent characterization. In particular, we show that flow effects lead to a characteristic breaking of the rotational symmetry of the average jet energy and jet multiplicity distribution in the $η\times φ$-plane. We argue that data on the medium-induced broadening of jet-like particle correlations in Au+Au collisions at RHIC provide a first evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow.

Figures (3)

• Figure 1: Upper part: sketch of the distortion of the jet energy distribution in the presence of a medium with or without collective flow. Lower part: calculated distortion of the jet energy distribution (\ref{['eq5']}) in the $\eta \times \phi$-plane for a 100 GeV jet. The right hand-side is for an average medium-induced radiated energy of 23 GeV and equal contributions from density and flow effects, $\mu = q_0$. Scales of the contour plot are visible from Fig. \ref{['fig2']}.
• Figure 2: Jet energy distribution for a sample of jets for which the medium was moving with equal probability in the positive and negative beam direction.
• Figure 3: The width in azimuth and rapidity of the near-side distribution of charged hadrons associated to high-$p_T$ trigger particles of transverse momentum $4\, {\rm GeV} < p_T < 6\, {\rm GeV}$ in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. Black points are preliminary data from the STAR collaboration FuqiangWang. The band represents our calculation for longitudinal flow fields in the range $2< q_0/\mu < 4$, see text for further details.