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.
Eur. Phys. J. C 78 (2018) 562
HEP Data
e-Print: arXiv:1805.04374 | PDF | inSPIRE
CERN-EP-2018-098
Figure 1
($A$$\times$$\epsilon$) 2-D maps in the planes ($\cos\theta$, $p_{\rm T}$) (left) and ($\varphi$, $p_{\rm T}$) (right) in the Collins-Soper frame. The plots illustrate the symmetry with respect to $\cos\theta = 0$ (left) and with respect to $\varphi = \pi$ and $\pi/2$ (right), while the grid shows the binning used to build the $W_1(\cos\theta)$ and $W_2(\varphi)$ distributions in each $p_{\rm T}$ range. |  |
Figure 2
Acceptance corrected angular distributions of J/$\psi$ reconstructed in the di-muon decay channel $W_1(\cos\theta)$, $W_2(\varphi)$ and $W_3(\widetilde{\varphi})$ in the helicity frame for the transverse momentum interval $4 < p_{\rm T} < 5$ \GeVc, together with the results of the simultaneous fit (see text for details). Vertical bars correspond to statistical uncertainties. |  |
Figure 3
ALICE inclusive J/$\psi$ polarization parameters in pp collisions at $\sqrt{s} = 8$ TeV (black points) compared with ALICE inclusive J/$\psi$ (orange squares, shifted horizontally by $- 0.3$ \GeVc) and LHCb prompt J/$\psi$ (blue open diamonds, shifted horizontally by $+ 0.3$ \GeVc) measurements at $\sqrt{s} = 7$ TeV in the rapidity interval $2.5 < y < 4.0$. The error bars represent the total uncertainties. Left and right plots show results in the Collins-Soper and helicity frames, respectively, for $\lambda_\theta$ (top plots), $\lambda_\varphi$ (middle plots) and $\lambda_{\theta\varphi}$ (bottom plots). |  |
Figure 4
Inclusive J/$\psi$ polarization parameters in pp collisions at $\sqrt{s} = 8$ TeV (black points, error bars represent the total uncertainties) compared with model predictions: NLO CSM (blue filled bands), NRQCD (red shaded bands) and NRQCD2 (light blue hatched band). Left and right plots show the results in the Collins-Soper and helicity frames, respectively, for $\lambda_\theta$ (top plots), $\lambda_\varphi$ (middle plots) and $\lambda_{\theta\varphi}$ (bottom plots). |  |
Figure 5
Inclusive J/$\psi$ frame-invariant quantity $\widetilde{\lambda}$ in pp collisions at $\sqrt{s} = 8$ TeV in the Collins-Soper (red points, shifted horizontally by $-0.1$ \GeVc) and helicity (green squares, shifted horizontally by $+0.1$ \GeVc) frames compared with the NLO CSM (blue full band) and NRQCD (red shaded band) model predictions from . |  |
Figure 6
Average $p_{\rm T}$-integrated (in rapidity range $2.5 < y < 4.0$) inclusive J/$\psi$ polarization parameters $\langle\lambda_\theta\rangle$, $\langle\lambda_\varphi\rangle$ and $\langle\lambda_{\theta\varphi}\rangle$ in allowed 2-D regions (white areas) for $3 < p_{\rm T} < 15$ \GeVc. Full (dashed) ellipses show 1-$\sigma$ (2-$\sigma$) contours in Collins-Sopper (CS, red) and helicity (HX, green) frames. Model predictions are represented by filled contours, full filled for the CSM and shaded filled for the NRQCD model, in green for the HX frame and in red for the CS frame. |    |