Heavy-ion physics is the study of the hot dense medium created shortly after the Big Bang. Physicists examine this medium in three collision systems: lead-lead, proton-lead and proton-proton collisions.
ATLAS prepared a variety of new results using data collected since 2010. These include exciting new measurements of di-jet asymmetry, jet sub-structures, muon suppression in lead-lead collisions, and measurements of the "ridge" in proton-proton collisions. ATLAS also presented the first measurement of the forward-backward multiplicity correlations in all three collision systems.
ATLAS is now looking forward to the first lead-lead collision data since 2011, with the next heavy-ion run scheduled to start on 20 November 2015.
One of the most exciting results from the LHC heavy-ion programme was the 2010 observation in lead-lead collision events of jets with asymmetric momentum (known as asymmetric di-jet events). This is direct evidence of jet quenching (the reduction of the jet energy as it interacts with the hot dense medium created by heavy-ion collisions).
ATLAS has released new measurements of the momentum imbalance in the di-jet system, using a distribution parameter known as xJ (see Figure 1). The distribution of this parameter has been directly compared to theoretical models. In proton-proton collisions, di-jets are more often symmetric (xJ distributions peak at ~1). While, in stark contrast, in central lead-lead collisions asymmetric di-jet events are the most likely configuration (with a local maximum at xJ~0.5). This result was one of the main highlights of the conference.
Analysing jet sub-structures
ATLAS also presented a new study of the sub-structure of particle jets in lead-lead collisions. As fast quarks and gluons (partons) in the jets move through the hot, dense medium created by heavy-ion collisions, there can be an energy loss. Using proton-proton collisions as a reference can help identify characteristic features of lead-lead collisions and help constrain theoretical models for this parton energy loss. The new result (see Figure 2) examines the difference between the jet momentum for proton-proton and lead-lead collisions, with respect to the number of participating nucleons.
Lead-lead collisions prompt muon suppression
ATLAS presented precision measurements of the number of single muons produced from the decays of heavy-flavour hadrons containing charm and bottom quarks. These heavy quarks are expected to have significantly less energy loss in the quark gluon plasma before they form jets (through hadronization). As a result, there should be fewer muons produced compared to those from inclusive charged hadrons.
The new results show that the muon yield is lower by factor of approximately 2.2 in most central lead-lead collisions, and this suppression has a very weak dependence over a low transverse momentum range (see Figure 4). The magnitude of suppression is in agreement with theoretical models.
New "ridge" results
Since the discovery of the "ridge" in proton-proton and proton-lead collisions in 2010, it has been a hot topic in the heavy ion community. At this year’s Quark Matter conference, two parallel sessions and one discussion session were dedicated to the topic. Just before the conference, ATLAS submitted a paper on the first measurement of the ridge using the 2.76 and 13 TeV data. The result stimulated interesting discussions and was one of the main highlights of the conference. For the first time, ATLAS showed that the ridge magnitude is controlled by event multiplicity and has very weak beam energy dependence. Find out more about this result by reading the physics briefing: "New insight into the proton-proton ridge".
Examining multiplicity correlations
ATLAS presented a comprehensive measurement of the forward-backward multiplicity correlation in proton-proton, proton-lead and lead-lead collisions. The analysis studies the number of particles produced (multiplicity) that leave the hot dense medium created in heavy-ion collisions. The correlation between the multiplicity measured in the forwards and backwards regions was measured, and can lead to uncovering new properties of the hot dense medium (see Figure 3).
The result shows an interesting comparison of the magnitude of the correlations in the short and long range from the collision. The former depends strongly on the collision system as well as the number of charged particles, while the latter depends only on the number of charged particles. Interestingly, this long-range correlation dependence on the number of charged particles follows a power-law with an index close to 0.5, suggesting that sources for particle production are similar in all the three collision systems.
These new results not only provide insight on the early particle production mechanism, but also facilitate current theoretical efforts in developing the full 3+1D models of collisions.
Find the full list of ATLAS heavy-ion results here: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HeavyIonsPublicResults