J/$ψ$ polarization in pp collisions at $\sqrt{s}=7$ TeV

We have studied J/psi production in pp collisions at $\sqrt{s}=7$ TeV at the LHC through its muon pair decay. The polar and azimuthal angle distributions of the decay muons were measured, and results on the J/$\psi$ polarization parameters $\lambda_{\theta}$ and $\lambda_\phi$ were obtained. The study was performed in the kinematic region 2.5<~y<~4, 2<~$p_{\rm T}$<~8 GeV/$c$, in the helicity and Collins-Soper reference frames. In both frames, the polarization parameters are compatible with zero, within uncertainties.


Phys.Rev.Lett. 108 (2012) 082001
HEP Data
e-Print: arXiv:1111.1630 | PDF | inSPIRE

Figure 1

The dimuon invariant mass spectrum for $2< p_{\rm t}< 3$ GeV/$c$, $0< |\cos\theta_{\rm HE}|< 0.15$, together with the result of the fit. The contributions of the signal and background are also shown as dashed lines.}

Figure 2

The acceptance corrected angular distributions for the J/$\psi$ decay muons, for $2< p_{\rm t}< 3$ GeV/$c$. The simultaneous fit to the results in the CS and HE frames is also shown. The plotted errors are purely statistical. The horizontal bars represent the bin width.}

Figure 3

$\lambda_{\theta}$ and $\lambda_{\phi}$ as a function of $p_{\rm t}$ for inclusive J/$\psi$, measured in the HE (closed squares) and CS (open circles) frames. The error bars represent statistical errors, while systematic uncertainties are shown as boxes.