CERN Accelerating science

Wealth of new results in Quark Matter 2017

by Panos Charitos

Last month, the LHC experiment collaborations presented their latest results at the Quark Matter 2017 conference on how matter behaved in the very early moments of the universe.

The programme includes a plethora of presentations on a variety of topics, spanning QCD at high temperature, QGP in small systems, initial state physics, collective dynamics, correlations and fluctuations, electroweak probes, jets, quarkonia and others. Four parallel sessions ran on the first two days, and plenaries on the last two days. A session was dedicated to posters, and a few of them were awarded a prize and the authors invited to give a flash talk.  A public lecture by Rocky Kolb entitled “From Quarks to Cosmos” was also included.

"Quark matter demonstrates the wealth of physics results on a topic which is inherently very complex, heavy ion physics,” said Eckhard Elsen, CERN Director for Research and Computing. "With the LHC performing so well and in so many different beam constellations, we have the experimental tools at hand to shed light on the state of matter that dominated in the early beginning of our universe.”

Several new results from the higher energy and luminosity samples of pp, p-Pb and Pb-Pb collisions of the LHC Run2 were presented. Production and flow coefficients of light flavour and strange hadrons measured in Pb-Pb collisions at 5 TeV were discussed, as well as strange meson and baryon yields as a function of multiplicity in pp at 13 TeV.

Individual heavy ion collisions create only a small droplet of quark-gluon plasma; the multitude of tracks left by the particle collisions allows scientists to look at how particles behave in such a medium. As an example, among the new results presented, the ALICE collaboration has shown that heavy quarks directly “feel” the shape and size of the quark-gluon-plasma droplet created within the region of the collision. This means that even the heaviest quarks move with the plasma, which is primarily formed of light quarks and gluons. ALICE also presented new results on the distribution of particle species – e.g. pions and kaons, among a zoo of other particles – in collisions of lead nuclei that help to measure the pressure and density in the quark-gluon plasma.

Particles are also used as direct probes to measure characteristics of the plasma. ALICEATLAS and CMS all presented new results in this area at a new lead collision energy per nucleon pair of 5 TeV. These have been compared to previous measurements at the collision energy of 2.76 TeV. Significant progress has been made on jet quenching with many new results reported at the conference.

All of the LHC experiments now collect large samples of collisions of lead nuclei and lead nuclei with protons. The ATLAS and CMS collaborations presented key features of collective particle behaviour in high-multiplicity collisions, which are key to better understanding the microscopic mechanisms at play in the quark-gluon plasma, as well as new methods for measuring the collective effects in small systems. The LHCb collaboration also presented its first public result from fixed-target collisions with argon – a completely new programme at the LHC also allowing for very high energy density where, again, particles containing heavy-quarks exquisitely reconstructed in their detector, play an important role.