Observation of a ${\mathrm{\Lambda }}_b^0-{\bar{\mathrm{\Lambda }}}_b^0$ production asymmetry in proton-proton collisions at $\sqrt{s}=7\text{ and }8\text{TeV}$

This article presents differential measurements of the asymmetry between ${\mathrm{\Lambda }}_b^0$ and ${\bar{\mathrm{\Lambda }}}_b^0$ baryon production rates in proton-proton collisions at centre-of-mass energies of $\sqrt{s}=7$ and $8\text{TeV}$ collected with the LHCb experiment, corresponding to an integrated luminosity of $3{\text{fb}}^{-1}$. The ${\mathrm{\Lambda }}_b^0$ baryons are reconstructed through the inclusive semileptonic decay ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\mu }^{-}{\bar{\nu }}_{\mu }X$. The production asymmetry is measured both in intervals of rapidity in the range $2.15<y<4.10$ and transverse momentum in $2<p_T<27\text{GeV}\mathrm{/}c$. The results are found to be incompatible with symmetric production with a significance of 5.8 standard deviations for both $\sqrt{s}=7$ and $8\text{TeV}$ data, assuming no $C\text{\hspace{0.17em}⁣}P$ violation in the decay. There is evidence for a trend as a function of rapidity with a significance of 4 standard deviations. Comparisons to predictions from hadronisation models in PYTHIA and heavy-quark recombination are provided. This result constitutes the first observation of a particle-antiparticle asymmetry in $b$-hadron production at LHC energies.