Recent Heavy Ion Results with the ATLAS Detector at the LHC

TL;DR

This paper reports on recent heavy ion collision results from the ATLAS detector at the LHC, including measurements of particle multiplicities, flow, and hard probes, revealing similarities to RHIC results and new insights into jet suppression and particle production.

Contribution

It provides new measurements of heavy ion collision phenomena at the LHC, extending previous RHIC results and exploring jet suppression and flow harmonics with high precision.

Findings

Charged particle multiplicity doubles compared to RHIC energy.

Jets are suppressed by a factor of two in central collisions.

W boson yields scale with binary collisions, consistent with expectations.

Abstract

Results are presented from the ATLAS collaboration from the 2010 LHC heavy ion run, during which nearly 10 inverse microbarns of luminosity were delivered. Soft physics results include charged particle multiplicities and collective flow. The charged particle multiplicity, which tracks initial state entropy production, increases by a factor of two relative to the top RHIC energy, with a centrality dependence very similar to that already measured at RHIC. Measurements of elliptic flow out to large transverse momentum also show similar results to what was measured at RHIC, but no significant pseudorapidity dependence. Extensions of these measurements to higher harmonics have also been made, and can be used to explain structures in the two-particle correlation functions that had long been attributed to jet-medium interactions. New hard probe measurements include single muons, jets and high $p_T$ hadrons. Single muons at high momentum are used to extract the yield of $W^{\pm}$ bosons and are found to be consistent within statistical uncertainties with binary collision scaling. Conversely, jets are found to be suppressed in central events by a factor of two relative to peripheral events, with no significant dependence on the jet energy. Fragmentation functions are also found to be the same in central and peripheral events. Finally, charged hadrons have been measured out to 30 GeV, and their centrality dependence relative to peripheral events is similar to that found for jets.