What have we learned about the quark–gluon plasma with the ATLAS detector at the LHC?

Abstract

Recent results from lead–lead and proton–lead collisions, measured by the ATLAS experiment at the LHC, are presented. In lead–lead collisions, electroweak bosons are found to be produced proportionally to the number of binary nucleon–nucleon collisions, and to have rapidity distributions compatible with perturbative QCD calculations, suggesting no need for large nuclear PDF effects. Conversely, the large suppression of inclusive jets, the elliptic flow of hadrons at high p T and the direct measurements of jet v 2 support the need for a path-length-dependent energy loss in the hot, dense medium. Proton–lead measurements provide new insights into particle production in small, longitudinally asymmetric systems, but require further insights into the fluctuating nature of proton–proton collisions. The modification factors for charged hadrons show a non-trivial dependence on centrality and rapidity, with a “Cronin” peak appearing only in the most central events, and in the lead-going direction. Finally the measurements of inclusive jets in proton–lead show a striking scaling in the R CP suppression variable that is only a function of the jet momentum, while the suppression factor relative to PYTHIA jet cross sections shows an enhanced yield in peripheral events, and a suppressed yield in central events.