cern.ch

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

[to restricted-access page]

Abstract

An angular analysis of the decay $J/\psi \rightarrow \mu^{+} \mu^{-}$ is performed to measure the polarization of prompt $J/\psi$ mesons produced in $pp$ collisions at $\sqrt{s}=7\; \mathrm{TeV}$. The dataset corresponds to an integrated luminosity of 0.37 fb$^{-1}$ collected with the LHCb detector. The measurement is presented as a function of transverse momentum, $p_\mathrm{T}$, and rapidity, $y$, of the $J/\psi$ meson, in the kinematic region $2 < p_\mathrm{T} < 15\; \mathrm{GeV}/c$ and $2.0<y<4.5$.

Figures and captions

(${\it Left}$) Invariant mass distribution of muon pairs passing the selection criteria. In the plot, $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ candidates are required to have 5 $<$ $p_{\rm T}$ $<$ 7 $ {\mathrm{\,GeV\!/}c}$ and $3.0<y<3.5$. The solid (dashed) vertical lines indicate the signal (sideband) regions. (${\it Right}$) Pseudo decay-time significance ($S_{\tau}$) distribution for background subtracted $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ candidates in the same kinematic bin. The solid vertical lines indicate the $S_\tau$ selection region. The right tail of the distribution is due to $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ production through the decay of $b$ hadrons.

Fig1a.pdf [14 KiB]
HiDef png [247 KiB]
Thumbnail [82 KiB]
*.C file
Fig1a.pdf
Fig1b.pdf [9 KiB]
HiDef png [116 KiB]
Thumbnail [50 KiB] *.C file
Fig1b.pdf

Global efficiency (area normalized to unity) as a function of (top) $\cos \theta$ and (bottom) $\phi$ for (left) 3 $<$ $p_{\rm T}$ $<$ 4 $ {\mathrm{\,GeV\!/}c}$ and for (right) 7 $<$ $p_{\rm T}$ $<$ 10 $ {\mathrm{\,GeV\!/}c}$ of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ mesons in the helicity frame. The efficiency is determined from simulation.

Fig2a.pdf [8 KiB]
HiDef png [227 KiB]
Thumbnail [73 KiB]
*.C file
Fig2a.pdf
Fig2b.pdf [8 KiB]
HiDef png [228 KiB]
Thumbnail [74 KiB] *.C file
Fig2b.pdf
Fig2c.pdf [8 KiB]
HiDef png [227 KiB]
Thumbnail [72 KiB] *.C file
Fig2c.pdf
Fig2d.pdf [8 KiB]
HiDef png [217 KiB]
Thumbnail [66 KiB] *.C file
Fig2d.pdf

(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 (open circles) and simulated sample (open squares) after the weighting based on the $B^{+}$ and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ kinematics and (right) their ratio.

Fig3a.pdf [5 KiB]
HiDef png [165 KiB]
Thumbnail [61 KiB]
*.C file
Fig3a.pdf
Fig3b.pdf [5 KiB]
HiDef png [145 KiB]
Thumbnail [56 KiB] *.C file
Fig3b.pdf

Measurements of $\lambda_{\theta}$ in bins of $p_{\rm T}$ for five rapidity bins in (left) the helicity frame and (right) the Collins-Soper frame. The error bars represent the statistical and systematic uncertainties added in quadrature. The data points are shifted slightly horizontally for different rapidities to improve visibility.

Fig4a.pdf [17 KiB]
HiDef png [209 KiB]
Thumbnail [64 KiB]
*.C file
Fig4a.pdf
Fig4b.pdf [17 KiB]
HiDef png [207 KiB]
Thumbnail [63 KiB] *.C file
Fig4b.pdf

Comparison of LHCb and ALICE results for $\lambda_{\theta}$ in different $p_{\rm T}$ bins integrating over the rapidity range $2.5 < y < 4.0$ in (left) the helicity frame and (right) the Collins-Soper frame. Error bars represent the statistical and systematic uncertainties added in quadrature.

Fig5a.pdf [5 KiB]
HiDef png [117 KiB]
Thumbnail [51 KiB]
*.C file
Fig5a.pdf
Fig5b.pdf [5 KiB]
HiDef png [116 KiB]
Thumbnail [50 KiB] *.C file
Fig5b.pdf

Comparison of LHCb prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ polarization measurements of $\lambda_{\theta}$ with direct NLO color singlet (magenta diagonal lines [39]) and three different NLO NRQCD (blue diagonal lines (1) [39], red vertical lines (2) [40] and green hatched (3) [41]) predictions as a function of the $p_{\rm T}$ of the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ meson in the rapidity range $2.5<y<4.0$ in the helicity frame.

Fig6.pdf [7 KiB]
HiDef png [294 KiB]
Thumbnail [77 KiB]
*.C file
Fig6.pdf

Differential cross-section of prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ production as a function of $p_{\rm T}$ and in bins of $y$. The vertical error bars show the quadratic sum of the statistical and systematic uncertainties.

Fig7.pdf [14 KiB]
HiDef png [162 KiB]
Thumbnail [64 KiB]
*.C file
Fig7.pdf

Animated gif made out of all figures.

PAPER-2013-008.gif
Thumbnail
thumbnail_PAPER-2013-008.gif

Tables and captions

Main contributions to the absolute systematic uncertainty on the parameter $\lambda_\theta$ in the helicity and Collins-Soper frames. While the systematic uncertainties are evaluated separately for all $p_{\rm T}$ and $y$ bins, we give here only the average, the minimum and the maximum values of all bins.

Table_1.pdf [25 KiB]
HiDef png [77 KiB]
Thumbnail [11 KiB]
tex code
Table_1.pdf

Measured $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ polarization parameters in bins of $p_{\rm T}$ and $y$ in the helicity frame. The first uncertainty is statistical (from the fit and the background subtraction) while the second is the systematic uncertainty.

Table_2.pdf [42 KiB]
HiDef png [178 KiB]
Thumbnail [22 KiB]
tex code
Table_2.pdf

Measured $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ polarization parameters in bins of $p_{\rm T}$ and $y$ in Collins-Soper frame. The first uncertainty is statistical (from the fit and the background subtraction) while the second is the systematic uncertainty.

Table_3.pdf [35 KiB]
HiDef png [148 KiB]
Thumbnail [23 KiB]
tex code
Table_3.pdf

Double-differential cross-section $d ^{2} \sigma/ d p_{\rm T} \, dy$ in nb/( $ {\mathrm{\,GeV\!/}c}$ ) for prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ production in bins of $p_{\rm T}$ and $y$, with statistical, systematic and polarization uncertainties.

Table_3.pdf [35 KiB]
HiDef png [148 KiB]
Thumbnail [23 KiB]
tex code
Table_3.pdf

Supplementary Material [file]

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2013-008. The files are: LHCB-PAPER-2013-008-additional.pdf: In this file we present extra results in figures and also explicitly in tables, short discussions are made. *.pdf *.eps *.png *.C : The figures used in the file LHCB-PAPER-2013-008-additional.pdf mycommands_italic.tex: predefined symbols for tex

Lambda[..].pdf [17 KiB]
HiDef png [207 KiB]
Thumbnail [63 KiB]
*C file
LambdaInvCS.pdf
Lambda[..].pdf [17 KiB]
HiDef png [212 KiB]
Thumbnail [65 KiB]
*C file
LambdaInvHX.pdf
LHCB-P[..].pdf [165 KiB]
HiDef png [468 KiB]
Thumbnail [86 KiB]
*C file
LHCB-PAPER-2013-008-additional.pdf

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