cern.ch

Measurement of the $\Lambda^0_b\rightarrow J/\psi\Lambda$ angular distribution and the $\Lambda^0_b$ polarisation in $pp$ collisions

[to restricted-access page]

Abstract

This paper presents an analysis of the $\Lambda^0_b\rightarrow J/\psi\Lambda$ angular distribution and the transverse production polarisation of $\Lambda^0_b$ baryons in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The measurements are performed using data corresponding to an integrated luminosity of 4.9 fb$^{-1}$, collected with the LHCb experiment. The polarisation is determined in a fiducial region of $\Lambda^0_b$ transverse momentum and pseudorapidity of $1 < p_{T} < 20$ GeV$/c$ and $2 < \eta < 5$, respectively. The data are consistent with $\Lambda^0_b$ baryons being produced unpolarised in this region. The parity-violating asymmetry parameter of the $\Lambda\rightarrow p\pi^-$ decay is also determined from the data and its value is found to be consistent with a recent measurement by the BES III collaboration.

Figures and captions

Definition of the five decay angles, $\theta$, $\theta_b$, $\phi_b$, $\theta_{l}$ and $\phi_l$ used to describe the kinematics of the $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ decay. The angles are described in the text.

Fig1.pdf [90 KiB]
HiDef png [151 KiB]
Thumbnail [110 KiB]
*.C file
Fig1.pdf

Mass distribution of selected $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ candidates in (top-left) the 2011, (top-right) the 2012, (bottom-left) the 2015 and (bottom-right) the 2016 data sets. The long and downstream categories have been combined. The results of fits to the distributions are overlaid.

Fig2a.pdf [55 KiB]
HiDef png [199 KiB]
Thumbnail [176 KiB]
*.C file
Fig2a.pdf
Fig2b.pdf [55 KiB]
HiDef png [170 KiB]
Thumbnail [149 KiB]
*.C file
Fig2b.pdf
Fig2c.pdf [56 KiB]
HiDef png [178 KiB]
Thumbnail [153 KiB]
*.C file
Fig2c.pdf
Fig2d.pdf [54 KiB]
HiDef png [167 KiB]
Thumbnail [143 KiB]
*.C file
Fig2d.pdf

Angular distributions of $\cos\theta_b$, $\cos\theta_l$, $\cos\theta$, $\phi_b$ and $\phi_l$ for the background-subtracted candidates. The long and downstream categories for the different data-taking years have been combined. The result of the moment analysis, folded with the angular efficiency, is overlaid. The contribution from the long and downstream categories are indicated by the green and red lines, respectively.

Fig3a.pdf [18 KiB]
HiDef png [212 KiB]
Thumbnail [195 KiB]
*.C file
Fig3a.pdf
Fig3b.pdf [18 KiB]
HiDef png [216 KiB]
Thumbnail [198 KiB]
*.C file
Fig3b.pdf
Fig3c.pdf [18 KiB]
HiDef png [211 KiB]
Thumbnail [191 KiB]
*.C file
Fig3c.pdf
Fig3d.pdf [18 KiB]
HiDef png [181 KiB]
Thumbnail [169 KiB]
*.C file
Fig3d.pdf
Fig3e.pdf [18 KiB]
HiDef png [181 KiB]
Thumbnail [169 KiB]
*.C file
Fig3e.pdf
Fig3f.pdf [18 KiB]
HiDef png [181 KiB]
Thumbnail [166 KiB]
*.C file
Fig3f.pdf
Fig3g.pdf [18 KiB]
HiDef png [180 KiB]
Thumbnail [164 KiB]
*.C file
Fig3g.pdf
Fig3h.pdf [18 KiB]
HiDef png [183 KiB]
Thumbnail [163 KiB]
*.C file
Fig3h.pdf
Fig3i.pdf [18 KiB]
HiDef png [178 KiB]
Thumbnail [155 KiB]
*.C file
Fig3i.pdf
Fig3j.pdf [18 KiB]
HiDef png [175 KiB]
Thumbnail [161 KiB]
*.C file
Fig3j.pdf
Fig3k.pdf [18 KiB]
HiDef png [183 KiB]
Thumbnail [170 KiB]
*.C file
Fig3k.pdf
Fig3l.pdf [18 KiB]
HiDef png [179 KiB]
Thumbnail [159 KiB]
*.C file
Fig3l.pdf
Fig3m.pdf [18 KiB]
HiDef png [197 KiB]
Thumbnail [181 KiB]
*.C file
Fig3m.pdf
Fig3n.pdf [18 KiB]
HiDef png [187 KiB]
Thumbnail [166 KiB]
*.C file
Fig3n.pdf
Fig3o.pdf [18 KiB]
HiDef png [198 KiB]
Thumbnail [175 KiB]
*.C file
Fig3o.pdf

Posterior probability distributions of $|a_{\pm}|$, ${\rm arg}(a_{\pm})$, $|b_{-}|$, ${\rm arg}(b_{-})$ and the transverse production polarisation of the $\Lambda ^0_ b $ baryons, $P_{b}$, at centre-of-mass energies of 7, 8 and 13 $\text{ Te V}$ assuming uniform priors. The shaded regions indicate the 68% and 95% credibility intervals.

Fig4a.pdf [34 KiB]
HiDef png [133 KiB]
Thumbnail [120 KiB]
*.C file
Fig4a.pdf
Fig4b.pdf [32 KiB]
HiDef png [124 KiB]
Thumbnail [115 KiB]
*.C file
Fig4b.pdf
Fig4c.pdf [34 KiB]
HiDef png [132 KiB]
Thumbnail [119 KiB]
*.C file
Fig4c.pdf
Fig4d.pdf [36 KiB]
HiDef png [146 KiB]
Thumbnail [130 KiB]
*.C file
Fig4d.pdf
Fig4e.pdf [35 KiB]
HiDef png [130 KiB]
Thumbnail [112 KiB]
*.C file
Fig4e.pdf
Fig4f.pdf [36 KiB]
HiDef png [142 KiB]
Thumbnail [123 KiB]
*.C file
Fig4f.pdf
Fig4g.pdf [32 KiB]
HiDef png [125 KiB]
Thumbnail [114 KiB]
*.C file
Fig4g.pdf
Fig4h.pdf [32 KiB]
HiDef png [133 KiB]
Thumbnail [122 KiB]
*.C file
Fig4h.pdf
Fig4i.pdf [32 KiB]
HiDef png [123 KiB]
Thumbnail [113 KiB]
*.C file
Fig4i.pdf

Measured transverse production polarisation of the $\Lambda ^0_ b $ baryons, $P_{b}$, as a function of the centre-of-mass energy, $\sqrt{s}$, of the data set. The points indicate the most probable value and the shaded regions the 68% and 95% credibility level intervals.

Fig5.pdf [13 KiB]
HiDef png [77 KiB]
Thumbnail [72 KiB]
*.C file
Fig5.pdf

Posterior probability distribution of the parity-violating asymmetry parameter, $\alpha_b$. The shaded regions indicate the 68% and 95% credibility intervals.

Fig6.pdf [34 KiB]
HiDef png [121 KiB]
Thumbnail [104 KiB]
*.C file
Fig6.pdf

Posterior probability distribution for $\alpha_{\Lambda }$, assuming a uniform prior, with all external constraints removed. The shaded regions indicate the 68% and 95% credibility intervals.

Fig7.pdf [33 KiB]
HiDef png [119 KiB]
Thumbnail [102 KiB]
*.C file
Fig7.pdf

Statistical correlation between the moments determined at $\sqrt{s}$ of 7 $\text{ Te V}$ .

Fig8.pdf [16 KiB]
HiDef png [212 KiB]
Thumbnail [188 KiB]
*.C file
Fig8.pdf

Statistical correlation between the moments determined at $\sqrt{s}$ of 8 $\text{ Te V}$ .

Fig9.pdf [16 KiB]
HiDef png [212 KiB]
Thumbnail [188 KiB]
*.C file
Fig9.pdf

Statistical correlation between the moments determined at $\sqrt{s}$ of 13 $\text{ Te V}$ .

Fig10.pdf [16 KiB]
HiDef png [210 KiB]
Thumbnail [188 KiB]
*.C file
Fig10.pdf

Animated gif made out of all figures.

PAPER-2020-005.gif
Thumbnail
thumbnail_PAPER-2020-005.gif

Tables and captions

Angular functions parameterising the $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ angular distribution. The numbering scheme is the same as in Ref. [37].

Table_1.pdf [72 KiB]
HiDef png [180 KiB]
Thumbnail [82 KiB]
tex code
Table_1.pdf

Signal yields in the long and downstream categories of the 2011, 2012, 2015 and 2016 data sets.

Table_2.pdf [38 KiB]
HiDef png [31 KiB]
Thumbnail [14 KiB]
tex code
Table_2.pdf

Values of the 20 moments, $M_{i}$, measured in the data collected at 7, 8 and 13 $\text{ Te V}$ centre-of-mass energies. The long and downstream categories have been combined. The first and second uncertainties are statistical and systematic, respectively.

Table_3.pdf [52 KiB]
HiDef png [243 KiB]
Thumbnail [114 KiB]
tex code
Table_3.pdf

Systematic uncertainties on the angular moments. The largest value amongst the moments is given for each source. The total systematic uncertainty varies from 0.002 to 0.010, depending on the moment considered. The sources are described in the text.

Table_4.pdf [42 KiB]
HiDef png [101 KiB]
Thumbnail [45 KiB]
tex code
Table_4.pdf

Estimates for the magnitude and phase of the decay amplitudes and the transverse production polarisation of the $\Lambda ^0_ b $ baryons, extracted using the Bayesian analysis. The most probable value (MPV) and the shortest 68% interval containing the most probable value are given.

Table_5.pdf [69 KiB]
HiDef png [101 KiB]
Thumbnail [49 KiB]
tex code
Table_5.pdf

Intervals at 95% credibility level on the amplitudes, the polarisation and $\alpha_{b}$ from the Bayesian analysis. The interval on $\alpha_{\Lambda }$, with the external constraint removed, is also provided.

Table_6.pdf [69 KiB]
HiDef png [139 KiB]
Thumbnail [65 KiB]
tex code
Table_6.pdf

Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [357 KiB]
supplementary.pdf

This ZIP file contains supplementary material for the publication LHCb-PAPER-2020-005. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in various formats paperData.json : Numerical values for moments and covariance matrices

Fig1a-supp.pdf [16 KiB]
HiDef png [115 KiB]
Thumbnail [117 KiB]
*C file
Fig1a-supp.pdf
Fig1b-supp.pdf [15 KiB]
HiDef png [115 KiB]
Thumbnail [116 KiB]
*C file
Fig1b-supp.pdf
Fig1c-supp.pdf [16 KiB]
HiDef png [113 KiB]
Thumbnail [114 KiB]
*C file
Fig1c-supp.pdf
Fig1d-supp.pdf [15 KiB]
HiDef png [111 KiB]
Thumbnail [113 KiB]
*C file
Fig1d-supp.pdf
Fig1e-supp.pdf [15 KiB]
HiDef png [107 KiB]
Thumbnail [102 KiB]
*C file
Fig1e-supp.pdf
Fig1f-supp.pdf [15 KiB]
HiDef png [110 KiB]
Thumbnail [107 KiB]
*C file
Fig1f-supp.pdf
Fig1g-supp.pdf [16 KiB]
HiDef png [110 KiB]
Thumbnail [110 KiB]
*C file
Fig1g-supp.pdf
Fig1h-supp.pdf [15 KiB]
HiDef png [107 KiB]
Thumbnail [107 KiB]
*C file
Fig1h-supp.pdf
Fig1i-supp.pdf [15 KiB]
HiDef png [104 KiB]
Thumbnail [99 KiB]
*C file
Fig1i-supp.pdf
Fig1j-supp.pdf [15 KiB]
HiDef png [103 KiB]
Thumbnail [96 KiB]
*C file
Fig1j-supp.pdf
Fig2a-supp.pdf [24 KiB]
HiDef png [132 KiB]
Thumbnail [118 KiB]
*C file
Fig2a-supp.pdf
Fig2b-supp.pdf [16 KiB]
HiDef png [115 KiB]
Thumbnail [105 KiB]
*C file
Fig2b-supp.pdf
Fig2c-supp.pdf [17 KiB]
HiDef png [109 KiB]
Thumbnail [102 KiB]
*C file
Fig2c-supp.pdf
Fig2d-supp.pdf [16 KiB]
HiDef png [112 KiB]
Thumbnail [101 KiB]
*C file
Fig2d-supp.pdf
Fig2e-supp.pdf [16 KiB]
HiDef png [116 KiB]
Thumbnail [104 KiB]
*C file
Fig2e-supp.pdf
Fig2f-supp.pdf [17 KiB]
HiDef png [119 KiB]
Thumbnail [109 KiB]
*C file
Fig2f-supp.pdf
Fig3a-supp.pdf [17 KiB]
HiDef png [95 KiB]
Thumbnail [54 KiB]
*C file
Fig3a-supp.pdf
Fig3b-supp.pdf [16 KiB]
HiDef png [96 KiB]
Thumbnail [53 KiB]
*C file
Fig3b-supp.pdf

Created on 19 February 2021.