Evidence for Nonlinear Gluon Effects in QCD and Their Mass Number Dependence at STAR

Area of Science:

• High Energy Physics
• Nuclear Physics
• Quantum Chromodynamics

Background:

• Understanding particle production in high-energy collisions is crucial for probing the fundamental properties of matter.
• Previous studies have investigated particle correlations to understand the complex dynamics within nuclear matter.

Purpose of the Study:

• To measure back-to-back azimuthal correlations of di-pion0 pairs at forward pseudorapidities.
• To investigate the effects of nuclear matter on particle correlations in proton-nucleus (p+Al, p+Au) collisions compared to proton-proton (p+p) collisions.
• To explore the role of nonlinear gluon dynamics and saturation phenomena in these interactions.

Main Methods:

• Utilizing data from the STAR Collaboration at a center-of-mass energy of 200 GeV.
• Analyzing di-pion0 production at forward pseudorapidities (2.6 < eta < 4.0).
• Comparing correlated yields in p+p, p+Al, and p+Au collisions as a function of transverse momentum.

Main Results:

• A significant suppression of back-to-back pi0 pair yields was observed in p+Al and p+Au collisions relative to p+p collisions.
• The suppression exhibits a dependence on the transverse momentum of the di-pion0 pairs.
• A larger suppression was found in p+Au compared to p+Al collisions, indicating a dependence on the mass number (A).
• The suppression scales linearly with A^(1/3) with a slope of -0.09 ± 0.01.

Conclusions:

• The observed suppression suggests the onset of nonlinear gluon dynamics at high parton densities within the colliding nuclei.
• The mass number dependence of the suppression provides evidence for the saturation of gluon densities, characterized by the saturation scale Qs^2.
• These findings offer insights into the structure of the proton and the behavior of nuclear matter under extreme conditions.