Earlier this month, almost 700 physicists from all over the world met in Houston, Texas, to attend the 30th edition of the Quark Matter conference, the largest conference in the field of heavy-ion physics. At this meeting, the ALICE collaboration presented its first results based on data collected with the upgraded detector in 2022, the first year of Run 3 of the LHC. Before the start of Run 3, ALICE underwent a major upgrade of its experimental apparatus to allow the recording of 50-100 times more Pb-Pb collisions and up to 500 times more proton-proton collisions than in previous runs. In addition, upgrades of the tracking detectors improved the pointing resolution by a factor 3-6. All in all, many new high-precision results will become available in the coming years.
One of the new results presented at the Quark Matter conference was the measurement of the production of two different states of charmonia in proton-proton collisions. Charmonia are particles that consist of a charm and an anti-charm quark, with a total mass of about 3 GeV, more than 3 times that of the proton. Charmonia have a characteristic decay signature, producing an electron-positron pair or a positive and a negative muon.
There are a variety of charmonium states, with different binding energies, from the tightly bound J/ψ (binding energy of approximately 650 MeV) to the weakly bound – and two times larger – ψ(2S) (binding energy of 50 MeV). In heavy-ion collisions, these states melt in the quark–gluon plasma (QGP) and a reduced number of them is observed in the final state, a phenomenon known as charm suppression. Physicists can determine the temperature of the plasma by measuring how the different states are suppressed. Such measurements have played an important role in the field over the years, starting from early measurements at the SPS in the 1990s.
The key to measuring charmonium suppression is knowing the production rates. These rates can be determined by measuring the production of quarkonia in proton-proton collisions, where there is no suppression. This provides the reference for the measurements performed in Pb-Pb collisions.
The upgraded ALICE detector has a broad kinematic coverage that allows it to study J/ψ and ψ(2S) down to zero transverse momentum in two different and complementary regions. In the central region, charmonium is reconstructed from its decay into an e+e- pair in the central barrel detectors, while in the forward region it is detected in its decay channel µ+µ-, in the muon spectrometer.
The proton-proton statistics collected in LHC Runs 1 and 2 allowed ALICE to study the ψ(2S) yields in the forward region, but not in the central region. The data from 2022 represents an increase of the total number of collisions by a factor of 300, making it possible to measure the production rate of the ψ(2S) in the central region for the first time. The results, based on 500 billion minimum-bias proton-proton collisions, show that both the excited and the ground charmonium states can be accessed over the whole ALICE kinematic region and this will constrain the models of quarkonium production and open the way for more detailed measurements in the upcoming heavy-ion run.