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

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 $2<p_{\mathrm{T}}<15$ GeV/$c$. The three polarization parameters (${\lambda }_{\theta }$, ${\lambda }_{\phi }$, ${\lambda }_{\theta \phi }$) are measured as a function of $p_{\mathrm{T}}$ 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.

20 pages, 6 captioned figures, 4 tables, authors from page 15, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/4282

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