The Quark Matter conference returned to its birthplace, Darmstadt, in its 24th edition. Results from proton-Lead collisions at the LHC continued to generate interest, with contributions from MIT and CMS at the forefront. The most intriguing results from the previous Quark Matter conference concerned energy loss in the hot dense medium formed in heavy ion collisions. This conference continued in that vein, and MIT presented exciting new results exploring the origin of this energy loss in proton-Lead and Lead-Lead collisions.
In PbPb collisions, Doga Gulhan presented an extensive study of parton energy loss traversing the quark gluon plasma in dijet events as part of a parallel talk. In dijet events of high momentum imbalance, the missing momentum is recovered by summing over tracks scattered through large angles in the medium. By projecting the momentum of tracks onto an axis symmetric with respect to the dijet system, we are able to study the angular distribution of particles in both the leading and subleading jet hemispheres. The accompanying slide of particle angular distribution is taken directly from Doga?s presentation.
Yet another way to investigate this energy loss is to use heavy-flavored particles. By looking at the spectra of B-mesons, we can try to understand whether or not the energy lost by a particle in the medium depends on the flavor of the quarks which constitute it. Gian Michele?s poster, discussing B-meson reconstruction and spectra in proton-Lead collisions was one of 5 posters selected (from nearly 400 entries) for a five minute flash talk on the final day of the conference.
While dijet and heavy-flavor systems can offer insight into the state of the probe after interaction with the quark gluon plasma, it is difficult to disentangle what the state of the probe was before traversing the medium. One solution is to use a much rarer event topology using photons. These photon-jet events allow an unbiased characterization of the energy both before and after interaction with the medium. Alex Barbieri presented updated results of our previous photon-jet paper confirming the earlier result (significant quenching in PbPb) with higher statistics pp reference data. In addition, no modification of the probe was observed in the new pPb data.
Using the new pPb data collected in 2013, a very sensitive measurement of the change of parton distribution functions (PDFs) due to nuclear effects was conducted using the distribution of the average eta of dijet systems as the ruler. Alex presented this result as part of his parallel talk, and there was significant interest from the theoretical community for this result, as it is one of the only measurements sensitive enough to differentiate between multiple competing models of the nuclear PDFs.
Another highlight of the conference was the ATLAS confirmation of a baffling result first presented by CMS at the Hard Probes 2013 conference which shows that there is an excess of high transverse momentum charged particles in proton-Lead collisions as compared to proton-proton collisions. We hope to explore in further detail these, and other interesting phenomena in heavy ion collisions in the next few months!