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Search for $C P$ violation and observation of $P$ violation in $\Lambda_b^0 \to p \pi^- \pi^+ \pi^-$ decays

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Abstract

A search for $C P$ violation in the $\Lambda_b^0 \to p \pi^- \pi^+ \pi^-$ decay is performed using LHCb data corresponding to an integrated luminosity of 6.6$ fb^{-1}$ collected in $pp$ collisions at centre-of-mass energies of 7, 8 and 13$ Te V$. The analysis uses both triple product asymmetries and the unbinned energy test method. The highest significances of $C P$ 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 $C P$ 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.

Figures and captions

Invariant-mass distribution for $\Lambda ^0_ b \rightarrow 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.

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Measured asymmetries for the binning scheme (left) $A_1$ and $A_2$ and (right) $B_1$ 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.

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Animated gif made out of all figures.

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Tables and captions

The $p$-values from the energy test for different distance scales and test configurations.

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Definition of binning scheme $A$. This binning scheme is based on the helicity angles of the decay topology $\Lambda ^0_ b \rightarrow \left( N^{*+} \rightarrow \left(\Delta ^{++} \rightarrow p \pi ^+ \right)\pi ^- \right)\pi ^- $ where $\varphi$ is the azimuthal angle of the proton in the $\Delta^{++}$ rest frame and $\theta_{\Delta ^{++}}$ ($\theta_ p $) is the polar angle of the $\Delta ^{++}$ ($ p $) in the $N^{*+}$ ($\Delta ^{++}$) rest frame.

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Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [1 MiB]
supplementary.pdf

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2019-028. The files are: Supplementary.pdf : "Search for CP violation and observation of P violation in Lb --> p pi^- pi^+ pi^- decays" Supplementary.tex

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Created on 06 March 2021.