A team has achieved the first realization of strange-quark polarimetry in top-quark decays. Using 16.39 fb⁻¹ of public CMS data from proton-proton collisions at √s = 13 TeV, the authors measure the longitudinal polarization of Λ baryons in the strange jets from hadronic W → c̄s decays in semileptonic top-quark pair events.

The underlying idea is that the W-boson decay produces the strange quark fully polarized, so the polarization observed in the Λ directly measures how much of that polarization is transferred during fragmentation, the process by which the quark hadronizes. This transfer is a quantity previously known only in flavor-averaged form from LEP measurements, and its value for the specific strange quark had not been explored in this channel.

With the limited statistics of the sample used, the result is consistent both with zero and with the transfer inferred from the flavor-averaged LEP measurements, meaning current precision cannot yet discriminate between the two scenarios. The authors project the statistical reach that would be achieved with the full Run-2, Run-3 and future HL-LHC datasets, where the enlarged sample should enable a measurement sensitive to the polarization transfer.

The work also illustrates the potential of CMS Open Data for precision physics analyses beyond the studies for which the data were originally collected, and opens a new avenue for probing quark spin hadronization in the rich environment of the LHC.