Nonmonotonic Energy Dependence of Net-Proton Number Fluctuations

Area of Science:

• Nuclear Physics
• High-Energy Physics
• Quantum Chromodynamics

Background:

• The search for the quantum chromodynamics (QCD) critical point is a major goal in heavy-ion collision research.
• Nonmonotonic variations in net-baryon number distribution moments are theoretical signatures of the QCD critical point.
• These moments relate to system correlation length and susceptibilities.

Purpose of the Study:

• To search for experimental evidence of the QCD critical point.
• To investigate the behavior of net-baryon number distribution moments as a function of collision energy.
• To identify potential signatures of phase transitions in nuclear matter.

Main Methods:

• Analysis of net-proton number distribution moments (kurtosis times variance) in gold-on-gold (Au+Au) collisions.
• Measurements conducted at the Relativistic Heavy Ion Collider (RHIC) using the Solenoidal Tracker.
• Comparison of data from central and noncentral collisions, and with theoretical models.

Main Results:

• Observed the first evidence of a nonmonotonic variation in the kurtosis times variance of the net-proton number distribution with collision energy (sqrt[s_{NN}]).
• This nonmonotonic behavior was significant (3.1 sigma) in central Au+Au collisions.
• Monotonic variations were observed in noncentral Au+Au collisions and in models lacking a critical point.

Conclusions:

• The observed nonmonotonic variation provides compelling evidence supporting the existence of the QCD critical point.
• This finding marks a significant step in mapping the phase diagram of nuclear matter.
• Further studies are needed to confirm and characterize this critical point signature.