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

The first measurement of the polarisation of coherently photoproduced J/$\psi$ mesons in ultra-peripheral Pb$-$Pb collisions, using data at $\sqrt{s_{\rm NN}}=5.02$ 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 $\mu{\rm b}^{-1}$ $\pm$ 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 $\mathbf{\lambda_{\theta}}$, $\mathbf{\lambda_{\varphi}}$, $\mathbf{\lambda_{\theta\varphi}}$ 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.


Submitted to: PLB
e-Print: arXiv:2304.10928 | PDF | inSPIRE
Figure group

Figure 1

A fit to the raw invariant mass (a) and \pt distributions for events in the $-0.20 < \cos\theta < -0.12$ range evaluated in the helicity frame. The bottom panel (b) shows the dimuon $p_{\rm T}$ around the J/$\psi$ mass range. The displayed uncertainties are statistical.

Figure 2

Two-dimensional angular distribution of coherent J/$\psi$ candidates in the helicity frame, after being unfolded in $\varphi$ and corrected for $A \times \epsilon$ in $\cos\theta$.

Figure 3

Projections of the coherent J/$\psi$ data points in (a) $\cos\theta$ and (b) $\varphi$, for events in the $-4 < y < -2.5$ rapidity interval. The curves are obtained by setting the lambda parameters in Eq. 2 to the results of the fit to the distribution shown in Fig. 2. The uncertainties on the data points are statistical.

Figure 4

Fitted results for the three polarisation parameters with a central value displayed by the dot along with contours at 68.3%, 95.4%, and 99.7% confidence levels in red, blue, and black lines, respectively.

Figure 5

ALICE results displayed in terms of spin density matrix elements, along with statistical and systematic uncertainties. A comparison with the available lowest $Q^2$ ranges of the results from the H1 and ZEUS collaborations is also provided. H1 results  correspond to $Q^2 \sim 0.05$ GeV$^2$ consistent with photoproduction, while the shown ZEUS measurement  is for $2 < Q^2 < 5$ GeV$^2$.