Review of results using heavy ion collisions at CMS

Abstract

Ultrarelativistic heavy ion collisions at the laboratory provide a unique chance to study quantum chromodynamics (QCD) under extreme temperature (${\approx}150\,\mathrm{MeV}$) and density (${\approx}1\,\mathrm{GeV}/\mathrm{fm}^3$) conditions. Over the past decade, experimental results from LHC have shown further evidence for the formation of the quark-gluon plasma (QGP), a phase that is thought to permeate the early Universe and is formed in the high-density neutron-star cores. Various QCD predictions that model the behavior of the low-$x$ gluon nuclear density, a poorly explored region, are also tested. Since the photon flux per ion scales as the square of the emitting electric charge $Z^2$, cross sections of so far elusive photon-induced processes are extremely enhanced as compared to nucleon-nucleon collisions. Here, we review recent progress on CMS measurements of particle production with large transverse momentum or mass, photon-initiated processes, jet-induced medium response, and heavy quark production. These high-precision data, along with novel approaches, offer stringent constraints on initial state, QGP formation and transport parameters, and even parametrizations beyond the standard model.