A team from IRFU, a member of the ALICE collaboration at CERN, has coordinated the analysis of J/ψ particles produced during lead-lead collisions at the LHC (CERN). Their results provide a solid indication of the existence of a quark–gluon plasma.
The quark–gluon plasma (QGP) is an exceptional, very dense and very hot state of matter where the most elementary constituents (quarks and gluons) are free. Analogous to the state of the universe a few microseconds after the Big Bang, it could be created by collisions of high-energy lead nuclei at the LHC (CERN).
To prove the existence of this plasma and study its properties, the physicists' approach consists in tracking down rare particles in their detectors, composed of a charm quark and its antiquark (c-cbar pairs): the J/ψ meson. They use J/ψ as a probe for the QGP. Indeed, the QGP can dissociate c-cbar pairs, thus suppressing part of the production of J/ψbar. Moreover, when the density of c quarks and cbar increases in the QGP, these particles recombine more easily and the J/ψ regenerate.
These two antagonistic effects were observed at the LHC in 2012, with limited accuracy for the regeneration of J/ψbar. Starting in 2015, the energy of collisions of lead nuclei increased from 2.76 TeV to 5.02 TeV. The physicists were then able to observe both effects with greater accuracy and confirm their interpretation.
 ALICE Collaboration, J/ψ suppression at forward rapidity in Pb-Pb collisions at √sNN = 5.02 TeV, Phys. Lett. B766 (2017) 212-224.
Contact: Javier Castillo Castellanos