The $\Lambda ^0_ b \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \Lambda $ production crosssection is measured with the LHCb detector in inelastic $pp$ collisions at a centreofmass energy of $\sqrt{s} = 7\, \mathrm{\,TeV} .$ Using 36.4 $\,pb^{1}$ of data recorded in 2010, the crosssection for the region with rapidity $2.2 <y< 4.5$ and transverse momentum $ p_{\rm T} < 13.0$ $ {\mathrm{\,GeV\!/}c}$ is found to be $\sigma(pp\rightarrow \Lambda ^0_ b X)\cal B ( \Lambda ^0_ b \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \Lambda )=4.19\pm 0.61\mathrm{(stat)} \pm 0.37\mathrm{(syst)} \,\rm \,nb $ for the baryon and $\sigma(pp\rightarrow \overline{\Lambda} ^0_ b X)\cal B ( \overline{\Lambda} ^0_ b \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \overline{\Lambda} )=2.63\pm0.48\mathrm{(stat)} \pm 0.27\mathrm{(syst)} \,\rm \,nb $ for the antibaryon decay.
$\bar{\Lambda ^0_ b } \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \overline{\Lambda} $ mass fit. Top: $\overline{\Lambda}$ is reconstructed using long tracks. Bottom: using the downstream tracks. The plots on the left and on the right used data recorded with different magnet polarities. 
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The measured $\sigma(pp\rightarrow \Lambda ^0_ b X)\cal B ( \Lambda ^0_ b \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \Lambda )$ in $\rm \,nb$ for the eight samples. The black vertical bars represent the statistical error, the red limits represent the systematic uncertainty. The green horizontal band represents the average for the two species $\Lambda ^0_ b $ and $\overline{\Lambda} ^0_ b $ . The red horizontal line represent the predictions from the LHCb simulated sample. (1)&(2) stand for the measurement using $\Lambda ^0_ b $ reconstructed with $\Lambda$ daughters as long tracks using data recorded with different polarity of the magnetic field, while (3)&(4) stand for the case in which the $\Lambda$ daughters are reconstructed as downstream tracks. 
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Animated gif made out of all figures. 
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Requirements used to select first the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ $\rightarrow$ $\mu ^+\mu ^$ then the $\Lambda \rightarrow p \pi $ and finally the $\Lambda ^0_ b \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \Lambda $ candidates. $M$ and $m$ are used to indicate the measured invariant masses and the nominal masses respectively. As the $\Lambda$ can be reconstructed from a pair of long tracks (LL) or downstream tracks (DD) different values are indicated where applied, $\tau$ is the decay time of the particle and IP$_{\chi^2}$ the difference in the $\chi^2$ of the primary vertex measured with and without the respective track. The $ K ^0_{\rm\scriptscriptstyle S}$ background is eliminated from the $\Lambda$ sample by applying a requirement on $M_{\pi \pi }m_{ K ^0_{\rm\scriptscriptstyle S} }$. 
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Systematic uncertainties for the crosssection measurements in percent. Where the uncertainty is different for the eight subsamples in which the candidates are divided, the smallest and the largest estimated values are are shown. The total systematic uncertainty is obtained assuming total correlation among the correlated variables. 
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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.