Search for $CP$ violation and observation of $P$ violation in ${\mathrm{\Lambda }}_b^0\to p{\pi }^{-}{\pi }^{+}{\pi }^{-}$ decays

A search for $CP$ violation in the ${\mathrm{\Lambda }}_b^0\to p{\pi }^{-}{\pi }^{+}{\pi }^{-}$ decay is performed using LHCb data corresponding to an integrated luminosity of $6.6{\mathrm{fb}}^{-1}$ collected in $pp$ collisions at center-of-mass energies of 7, 8 and 13 TeV. The analysis uses both triple product asymmetries and the unbinned energy test method. The highest significances of $CP$ asymmetry are 2.9 standard deviations from triple product asymmetries and 3.0 standard deviations for the energy test method. Once the global $p$-value is considered, all results are consistent with no $CP$ violation. Parity violation is observed at a significance of 5.5 standard deviations for the triple product asymmetry method and 5.3 standard deviations for the energy test method. The reported deviations are given in regions of phase space.

Figure 1

Invariant-mass distribution for ${\mathrm{\Lambda }}_b^0\to p{\pi }^{-}{\pi }^{+}{\pi }^{-}$ candidates with the result of the fit overlaid. The solid and dotted lines describe the projections of the fit results for various components as listed in the legend.

Figure 2

Measured asymmetries for the binning scheme (top) $A_1$ and $A_2$ and (bottom) $B_1$ and $B_2$. The error bars represent the sum in quadrature of the statistical and systematic uncertainties. The ${\chi }^2$ per ndof is calculated with respect to the null hypothesis and includes statistical and systematic uncertainties.