Measurement of the inclusive J/$\psi$ polarization at forward rapidity in pp collisions at $\sqrt{s} = 8$ TeV

TL;DR

This paper measures the polarization of J/ψ particles produced in proton-proton collisions at 8 TeV, finding results consistent with no polarization, challenging existing theoretical predictions.

Contribution

First measurement of J/ψ polarization parameters at 8 TeV in the specified rapidity and transverse momentum range, providing new data to test quantum chromodynamics models.

Findings

J/ψ polarization parameters are compatible with zero within uncertainties.

Results contradict all existing theoretical predictions.

Comparison with 7 TeV data shows consistency in polarization measurements.

Abstract

We report on the measurement of the inclusive J/$\psi$ polarization parameters in pp collisions at a center of mass energy $\sqrt{s} = 8$ TeV with the ALICE detector at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 1.23 pb$^{-1}$. J/$\psi$ resonances are reconstructed in their di-muon decay channel in the rapidity interval $2.5 < y < 4.0$ and over the transverse-momentum interval $2 < p_{\rm T} < 15$ GeV/$c$. The three polarization parameters ($\lambda_\theta$, $\lambda_\varphi$, $\lambda_{\theta\varphi}$) are measured as a function of $p_{\rm T}$ both in the helicity and Collins-Soper reference frames. The measured J/$\psi$ polarization parameters are found to be compatible with zero within uncertainties, contrary to expectations from all available predictions. The results are compared with the measurement in pp collisions at $\sqrt{s} = 7$ TeV.