QGP production studied at record energies

The very-high-energy densities reached in heavy-ion collisions at the LHC result in the production of an extremely hot form of matter, known as the quark-gluon plasma (QGP), consisting of freely roaming quarks and gluons. This medium undergoes a dynamic evolution before eventually transitioning to a collection of hadrons. But the details of this temporal evolution and phase transition are very challenging to calculate from first principles using quantum chromodynamics. The experimental study of the final-state hadrons produced in heavy-ion collisions therefore provides important insights into the nature of these processes. In particular, measurements of the pseudorapidity (η) distributions of charged hadrons help in understanding the initial energy density of the produced QGP and how this energy is transported throughout the event. These measurements involve different classes of collisions, sorted according to the degree of overlap between the two colliding nuclei; collisions with the largest overlap have the highest energy densities.

In 2022 the LHC entered Run 3, with higher collision energies and integrated luminosities than previous running periods. The CMS collaboration has now reported the first measurement using Run 3 heavy-ion data. Charged hadrons produced in lead–lead collisions at the record nucleon–nucleon centre-of-mass collision energy of 5.36 TeV were reconstructed by exploiting the pixel layers of the silicon tracker. At mid-rapidity and in the 5% most central collisions (which have the largest overlap between the two colliding nuclei), 2032 ± 91 charged hadrons are produced per unit of pseu­dorapidity. The data-to-theory comparisons show that models can successfully predict either the total charged-hadron multiplicity or the shape of its η distribution, but struggle to simultaneously describe both aspects.

Previous measurements have shown that the mid-rapidity yield of charged hadrons in proton-proton and heavy-ion collisions are comparable when scaled by the average number of nucleons parti­cipating in the collisions,〈N_part〉. Figure 1 shows measurements of this quantity in several collision systems as a function of collision energy. It was previously observed that central nucleus–nucleus collisions exhibit a power-law scaling, as illustrated by the blue dashed curve; the new CMS result agrees with this trend. In addition, the measurement is about two times larger than the values of proton–proton collisions at similar energies, indicating that heavy-ion collisions are more efficient at converting initial-state energy into final-state hadrons at mid-rapidity.

This measurement opens a new chapter in the CMS heavy-ion programme. At the end of 2023 the LHC delivered an integrated luminosity of around 2 nb^–1 to CMS, and more data will be collected in the coming years, enabling more precise analyses of the QGP features.