Measurement of the photon polarization in ${\Lambda }_b\to \Lambda \gamma$ decays

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 $6f{b}^{-1}$ collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of $13TeV$ is used. The photon polarization is measured to be ${\alpha }_{\gamma }=0.82_{-0.26-0.13}^{+0.17+0.04}$, 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.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2021-030.html (LHCb public pages)

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