Measurement of the photon polarization in $\Lambda_b \to \Lambda \gamma$ decays

TL;DR

This paper reports the first measurement of photon polarization in $ ext{Lambda}_b o ext{Lambda} ext{gamma}$ decays, using LHCb data, confirming Standard Model predictions and exploring CP symmetry.

Contribution

It provides the first measurement of photon polarization in b-baryon decays and investigates CP violation effects in this context.

Findings

Photon polarization measured as $ ext{alpha}_ ext{gamma} = 0.82^{+0.17+0.04}_{-0.26-0.13}$

Result consistent with Standard Model predictions

No evidence of CP violation found in the observable.

Abstract

The photon polarization in $b \to s \gamma$ transitions is measured for the first time in radiative b-baryon decays exploiting the unique spin structure of $\Lambda_b \to \Lambda \gamma$ decays. A data sample corresponding to an integrated luminosity of $6\;fb^{-1}$ collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of $13\;TeV$ is used. The photon polarization is measured to be $\alpha_{\gamma}= 0.82^{\,+\,0.17\,+\,0.04}_{\,-\,0.26\,-\,0.13}$, where the first uncertainty is statistical and the second systematic. This result is in agreement with the Standard Model prediction and previous measurements in b-meson decays. Charge-parity breaking effects are studied for the first time in this observable and found to be consistent with $CP$ symmetry.