ψ(2S) Suppression in Pb-Pb Collisions at the LHC.

Area of Science:

• High Energy Physics
• Nuclear Physics
• Quantum Chromodynamics

Background:

• The quark-gluon plasma (QGP) is a state of matter created in high-energy nuclear collisions.
• Charmonium states, like J/ψ and ψ(2S), are sensitive probes of the QGP properties.
• Understanding charmonium suppression in heavy-ion collisions provides insights into QGP formation and evolution.

Purpose of the Study:

• To measure the production of the ψ(2S) charmonium state in lead-lead (Pb-Pb) collisions at the Large Hadron Collider (LHC).
• To investigate the suppression of ψ(2S) production relative to J/ψ production as a function of collision centrality and transverse momentum.
• To compare these measurements with proton-proton (pp) collisions and theoretical models to understand charmonium behavior in the QGP.

Main Methods:

• Utilized the ALICE detector at the LHC to record Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon pair of 5.02 TeV.
• Measured the ψ(2S) charmonium state via its dimuon decay channel.
• Analyzed data to determine inclusive production cross sections and ratios, including the nuclear modification factor (R_AA).

Main Results:

• Observed a significant ψ(2S) signal down to zero transverse momentum at forward rapidity (2.5 < y < 4).
• Found that ψ(2S) production is suppressed by a factor of approximately 2 relative to J/ψ in central Pb-Pb collisions.
• The ψ(2S) nuclear modification factor (R_AA) shows suppression by up to a factor of 3 in Pb-Pb collisions compared to pp collisions.

Conclusions:

• The strong suppression of ψ(2S) in Pb-Pb collisions provides evidence for its interaction with the quark-gluon plasma.
• These findings contribute to understanding the mechanisms of charmonium dissociation and regeneration in the QGP.
• The results offer crucial data for refining theoretical models of heavy-ion collisions and QGP properties.