First polarisation measurement of coherently photoproduced J/$\psi$ in ultra-peripheral Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

Abstract

The first measurement of the polarisation of coherently photoproduced J$/\psi$ mesons in ultra-peripheral Pb-Pb collisions, using data at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV, is presented. The J/$\psi$ meson is measured via its dimuon decay channel in the forward rapidity interval $-4.0<y<-2.5$ using the ALICE detector at the CERN LHC. An event sample corresponding to an integrated luminosity of 750 $\mu {\text{b}}^{-1}$ $\pm$ 5% (syst) is analysed. Hadronic activity is highly suppressed since the interaction is mediated by a photon. The polar and azimuthal angle distributions of the decay muons are measured, and the polarisation parameters ${\lambda }_{\theta }$, ${\lambda }_{\phi }$, ${\lambda }_{\theta \phi }$ are extracted. The analysis is carried out in the helicity frame. The results are found to be consistent with a transversely polarised J/$\psi$. These values are compared with previous measurements by the H1 and ZEUS experiments. The polarisation parameters of coherent J/$\psi$ photoproduction in Pb-Pb collisions are found to be consistent with the $s$-channel helicity conservation hypothesis.

19 pages, 5 captioned figures, 2 tables, authors from page 14, figures at http://alice-publications.web.cern.ch/node/9316

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Cited by in corpus (1)

• Physics with high-luminosity proton-nucleus collisions at the LHC