by Antonella del Rosso. Published: 20 August 2013

When a single proton (p) is smashed against a lead ion (Pb), unexpected events may occur: in the most violent p-Pb collisions, correlations of particles exhibit similar features as in lead-lead collisions where quark-gluon plasma is formed. This and other amazing results were presented by the ALICE experiment at the SQM2013 conference held in Birmingham from 21 to 27 July.

“Jet quenching” is one of the most powerful signatures of quark-gluon plasma (QGP) formed in high-energy lead-lead collisions. QGP is expected to exist only in specific conditions involving extremely hot temperatures and a very high particle concentration. These conditions are not expected to apply in the case of less “dense” particle collisions such as proton-lead collisions. “When we observe the results of these collisions in ALICE, we do not see a strong particle-jet suppression; however, when studying the most violent p-Pb collisions we observe signatures in particle production characteristic of a hydrodynamic nature,” explains Mateusz Ploskon from the ALICE collaboration. “Indeed, some of the properties of the correlations of particles produced in proton-lead collisions resemble those associated with the formation of QGP in lead-lead collisions.”

More data is needed to resolve the conundrum but in the meantime the physics community is excited as the phenomena observed in proton-lead collisions could have strong implications for our understanding of the QCD – the theory that describes the interactions of strongly interacting subatomic particles. “The proton-lead data already provide an extremely useful baseline for the collisions of heavy ions; however, we need more time and more data to understand the intriguing observations from proton-lead collisions – it remains to be seen whether we learn something new about hadronic and nuclear collisions at high energies, and whether these observations have any unexpected implications for our understanding of QGP based on lead-lead collisions,” says Mateusz.

Event display from the proton-lead run, in January 2013. This event was generated by the High Level Trigger (HLT) of the ALICE experiment.

At the “Strangeness in Quark Matter” conference, the ALICE collaboration also presented results on the behaviour of heavy quarks and quarkonia (bound states of charm or beauty quark-antiquark pairs) in proton-lead and lead-lead collisions. “The data we have show that the charm quark travelling through QGP loses significantly more energy (jet quenching) compared to the much heavier beauty quark. These findings, combined with the clear indication that the relatively heavy charm quark flows together with the light quarks and gluons of the QGP, present an important challenge to existing theoretical models. We are exploring the hot and dense matter as we never did before, but we still need more accurate information. Many of the puzzles we are facing today will be clarified in the new LHC runs beyond 2015,” concludes Mateusz.

The story originally appeared in the issue 33-35 of the CERN Bulletin and can be found here