Time-reversal asymmetries in \Lambda_b \to \Lambda(\to p \pi) \ell^+ \ell^-

Abstract

Abstract We study the decays of $\Lambda_b \to \Lambda(\to p \pi^-) \ell ^+ \ell^-$ with $\ell = (e, \mu, \tau)$. We examine the full angular distributions with polarized $\Lambda_b$, where the T-odd observables are identified. We discuss the possible effects of new physics~(NP) and find that the T-odd observables are sensitive to them as they vanish in the standard model. 
Special attentions are given to the interference of (pseudo)scalar operators with (axial)vector operators in polarized
$\Lambda_b \to \Lambda(\to p \pi^-) \tau^+ \tau^-$, which are studied in the first time.
Their effects are proportional to the lepton masses and therefore
may evade the constraint from $\Lambda_b \to \Lambda(\to p \pi^-) \mu^+ \mu^-$ at LHCb naturally. As $\Lambda_b \to \Lambda(\to p \pi^-) \tau^+ \tau^-$ is uncontaminated by the charmonia resonance, it provides a clean background to probe NP. 
In addition, we show that the experimental central value of $K_{10}$ in $\Lambda_b \to \Lambda(\to p \pi^-) \mu^+ \mu^-$ at LHCb can be explained by the NP case, which couples to the right-handed quarks and leptons.
The polarization fraction of $\Lambda_b$ at LHCb is found to be consistent with zero regardless to the NP scenarios. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.