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

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Abstract

The polarisation of prompt $\psi(2S)$ mesons is measured by performing an angular analysis of $\psi(2S)\rightarrow \mu^{+} \mu^{-}$ 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 and captions

Invariant mass distribution of $\psi {(2S)}$ candidates in the kinematic region $5< p_{\rm T} <7 {\mathrm{\,GeV\!/}c} $ 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.

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Detection efficiency in arbitrary units as a function of $\cos \theta$ and $\phi$ in the helicity frame for $\psi {(2S)}$ mesons in the range $5< p_{\rm T} <7 {\mathrm{\,GeV\!/}c} $ and $2.5<y<3.0$.

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(Left) Distributions of $\cos\theta$ in the helicity frame for $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ mesons from $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^+$ decays in data (filled triangles) and in the simulated sample (open circles) and (right) their ratio after the weighting based on the $ B ^+ $ and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ kinematic properties.

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Polarisation parameters for prompt $\psi {(2S)}$ mesons as a function of $p_{\rm T}$ , in five rapidity intervals, (top left) $\lambda_\theta $ and (bottom) $\lambda_{\mathrm{inv}} $, measured in the helicity frame, and (top right) $\lambda_\theta $ 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 $\psi {(2S)}$ meson. The data points for each rapidity interval are displaced horizontally to improve visibility.

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Polarisation parameter $\lambda_\theta$ of the prompt $\psi {(2S)}$ 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 [39] and three NLO NRQCD models (1) [39], (2) [40] and (3) [41] 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 $\psi {(2S)}$ meson.

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Animated gif made out of all figures.

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Tables and captions

Sources of systematic uncertainties on the polarisation parameter $\lambda_\theta$ in the helicity and Collins-Soper frames. For each type of uncertainty, the average and the range over all $\psi {(2S)}$ kinematic bins are shown.

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Measured prompt $\psi {(2S)}$ polarisation parameters $\lambda_\theta$ , $\lambda_{\theta\phi}$ , $\lambda_{\phi}$ and $\lambda_{\mathrm{inv}}$ in bins of $p_{\rm T}$ and $y$ in the helicity frame. The first uncertainty is statistical and the is second systematic.

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Measured prompt $\psi {(2S)}$ polarisation parameters $\lambda_\theta$ , $\lambda_{\theta\phi}$ , $\lambda_{\phi}$ and $\lambda_{\mathrm{inv}}$ in bins of $p_{\rm T}$ and $y$ in the Collins-Soper frame. The first uncertainty is statistical and the second is systematic.

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.