J/psi polarization in 800-GeV p-Cu interactions

Area of Science:

• High-energy particle physics
• Quantum chromodynamics
• Hadron spectroscopy

Background:

• The J/psi particle, a bound state of charm and anti-charm quarks, is a crucial probe for understanding quantum chromodynamics (QCD) and hadronization processes.
• Previous studies have indicated potential polarization of vector mesons, but experimental data, especially at high energies, remains limited.

Purpose of the Study:

• To measure the polarization of J/psi particles produced in proton-nucleus collisions.
• To investigate the dependence of J/psi polarization on kinematic variables like Feynman-x (x_F) and transverse momentum (p_T).
• To gain insights into the underlying mechanisms of charm quark production and hadronization.

Main Methods:

• Utilized an 800-GeV proton beam interacting with a copper target.
• Measured the polarization of inclusively produced J/psi mesons via their decay into dimuons.
• Analyzed polarization components as a function of Feynman-x (x_F) and transverse momentum (p_T).

Main Results:

• Observed a significant longitudinal polarization for J/psi particles at large Feynman-x (x_F > 0).
• Found that at small Feynman-x (x_F), the J/psi state is produced nearly unpolarized or with slight transverse polarization.
• No statistically significant variation in polarization was detected with respect to transverse momentum (p_T).

Conclusions:

• The observed kinematic dependence of J/psi polarization provides crucial data for refining theoretical models of heavy-quark production and hadronization.
• The results suggest different production mechanisms may dominate at different kinematic regimes (large vs. small x_F).
• Further studies are warranted to explore polarization in other systems and at different energy scales.