Heavy baryons as polarimeters at colliders

Abstract: In new-physics processes that produce b or c jets, a measurement of the initial b or c-quark polarization could provide crucial information about the structure of the new physics. In the heavy-quark limit, the b and c-quark polarizations are preserved in the lightest baryons they hadronize into, Λ_b and Λ_c, respectively. We revisit the prediction for the polarization retention after the hadronization process and extend it to the case of transverse polarization. We show how ATLAS and CMS can measure the b-quark polarization using semileptonic Λ_bdecays, and the c-quark polarization using Λ_c⁺ → pK⁻π⁺ decays. For calibrating both measurements we suggest to use tt¯samples in which these polarizations can be measured with precision of order 10% using 100 fb⁻¹ of data in Run 2 of the LHC. Measurements of the transverse polarization in QCD events at ATLAS, CMS and LHCb are motivated as well. The proposed measurements give access to nonperturbative QCD parameters relevant to the dynamics of the hadronization process.