Studies of $\Lambda^0_{\mathrm{b}} \rightarrow \mathrm{J}/\psi \Lambda$ production in $pp$ collisions at $\sqrt{s}$ = 7 TeV

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The $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ production cross-section is measured with the LHCb detector in inelastic $pp$ collisions at a centre-of-mass energy of $\sqrt{s} = 7 \mathrm{ Te V} .$ Using 36.4 $ pb^{-1}$ of data recorded in 2010, the cross-section for the region with rapidity $2.2 <y< 4.5$ and transverse momentum $ p_{\rm T} < 13.0$ $ {\mathrm{ Ge V /}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.

Figures and captions

$\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.


Tables and captions

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 cross-section measurements in percent. Where the uncertainty is different for the eight sub-samples 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 05 March 2021.