Measurement of $ψ(2S)$ polarisation in $pp$ collisions at $\sqrt{s}=7$ TeV

TL;DR

This study measures the prompt psi(2S) polarisation in pp collisions at sqrt(s)=7 TeV using 1.0 fb^-1 of LHCb data, via an angular analysis of psi(2S) -> mu+ mu- decays. Unbinned maximum-likelihood fits to the angular distributions are performed in the helicity and Collins-Soper frames, with detector acceptance obtained from unpolarised MC and background subtracted using sWeights guided by mass fits. Across five pT and five y bins, the results show no significant polarisation, with lambda_inv negative and the remaining parameters consistent with zero, disfavoring the color-singlet model and challenging NRQCD predictions at high pT. The findings provide constraints on quarkonium production mechanisms and underscore remaining tensions between theory and data in polarization at collider energies.

Abstract

The polarisation of prompt $ψ(2S)$ mesons is measured by performing an angular analysis of $ψ(2S)\rightarrow μ^{+} μ^{-}$ decays using proton-proton collision data, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, collected by the LHCb detector at a centre-of-mass energy of 7 TeV. The polarisation is measured in bins of transverse momentum $p_\mathrm{T}$ and rapidity $y$ in the kinematic region $3.5<p_\mathrm{T}<15$ GeV$/c$ and $2.0<y<4.5$, and is compared to theoretical models. No significant polarisation is observed.

Figures (5)

• Figure 1: Invariant mass distribution of $\uppsi\xspace{(2S)}$ candidates in the kinematic region $5<\hbox{$p_{\rm T}$}\xspace<7{\mathrm{\,Ge V\!/}c}\xspace$ and $2.5<y<3.0$. The solid blue line is the total fit function, the dot-dashed green line represents the linear background function and the red dashed line is the combination of the two CB functions.
• Figure 2: Detection efficiency in arbitrary units as a function of $\cos \theta$ and $\phi$ in the helicity frame for $\uppsi\xspace{(2S)}$ mesons in the range $5<\hbox{$p_{\rm T}$}\xspace<7{\mathrm{\,Ge V\!/}c}\xspace$ and $2.5<y<3.0$.
• Figure 3: (Left) Distributions of $\cos\theta$ in the helicity frame for ${\mathrm{J}\xspace / \uppsi\xspace }$ mesons from $\mathrm{B}\xspace\xspace^+\xspace\xspace \rightarrow {\mathrm{J}\xspace / \uppsi\xspace }\xspace K^+$ decays in data (filled triangles) and in the simulated sample (open circles) and (right) their ratio after the weighting based on the $\mathrm{B}\xspace\xspace^+\xspace\xspace$ and ${\mathrm{J}\xspace / \uppsi\xspace }$ kinematic properties.
• Figure 4: Polarisation parameters for prompt $\uppsi\xspace{(2S)}$ mesons as a function of $p_{\rm T}$, in five rapidity intervals, (top left) $\lambda_\theta\xspace$ and (bottom) $\lambda_{\mathrm{inv}}\xspace$, measured in the helicity frame, and (top right) $\lambda_\theta\xspace$ in the Collins-Soper frame. The uncertainties on data points are the sum in quadrature of statistical and systematic uncertainties. The horizontal bars represent the width of the $p_{\rm T}$ bins for the $\uppsi\xspace{(2S)}$ meson. The data points for each rapidity interval are displaced horizontally to improve visibility.
• Figure 5: Polarisation parameter $\lambda_\theta$ of the prompt $\uppsi\xspace{(2S)}$ meson in the helicity frame as a function of $p_{\rm T}$, in the rapidity range $2.5<y<4$. The predictions of NLO CSM Butenschoen:2012px and three NLO NRQCD models (1) Butenschoen:2012px, (2) Gong:2012ug and (3) Chao:2012iv are also shown. Uncertainties on data are the sum in quadrature of the statistical and systematic uncertainties. The horizontal bars represent the width of $p_{\rm T}$ bins for the $\uppsi\xspace{(2S)}$ meson.