Heavy baryons as polarimeters at colliders

TL;DR

This paper proposes using heavy baryons as polarimeters at colliders to measure quark polarization, providing insights into new physics and nonperturbative QCD parameters through specific decay channels.

Contribution

It extends polarization retention predictions to transverse polarization and suggests experimental methods for measuring b and c quark polarizations at the LHC.

Findings

Feasible measurement of b-quark polarization via Lambda_b decays.

Feasible measurement of c-quark polarization via Lambda_c+ decays.

Calibration method using ttbar samples with 10% precision.

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, Lambda_b and Lambda_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 Lambda_b decays, and the c-quark polarization using Lambda_c+ -> p K- pi+ decays. For calibrating both measurements we suggest to use ttbar samples in which these polarizations can be measured with precision of order 10% using 100/fb 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.