Measuring polarizations of bottom, charm, strange, up and down quarks in top decays

Abstract

Standard Model $t\bar t$ samples in ATLAS and CMS provide an opportunity to conduct measurements of the polarization of quarks produced in top-quark decays. The bottom quarks from the $t \to W^+ b$ decays, as well as the charm, strange, up and down quarks from the subsequent $W^+ \to c\bar s, u\bar d$ decays, are polarized. When a polarized quark hadronizes to a baryon, its polarization is partly preserved, and is reflected in the angular distributions of the baryon decay products. For the strange and bottom quarks, this effect has already been seen in hadronic $Z$ decays at LEP. In Run 2 of the LHC, $t\bar t$ samples will have comparable statistics and several advantages from the point of view of flavor tagging. We propose strategies for such measurements in ATLAS and CMS. We estimate that precision of order $10\%$ is attainable for the polarizations of the strange, charm and bottom quarks. In future higher-statistics runs, polarizations of up and down quarks might be measurable as well.