Measuring c-quark polarization in W+c samples at ATLAS and CMS

TL;DR

This paper proposes a method for ATLAS and CMS to measure the polarization of charm quarks via $\Lambda_c$ baryons in W+c events at the LHC, providing insights into polarization transfer during hadronization.

Contribution

It introduces a novel approach to measure charm quark polarization in W+c samples, potentially the first such measurement of longitudinal charm polarization at the LHC.

Findings

Feasibility of measuring $\Lambda_c$ polarization in W+c samples.

Potential to be the first measurement of charm quark longitudinal polarization.

Application of the technique to new physics charm samples.

Abstract

The process $pp \to W^-c$ produces polarized charm quarks. The polarization is expected to be partly retained in $\Lambda_c$ baryons when those form in the $c$-quark hadronization. We argue that it will likely be possible for ATLAS and CMS to measure the $\Lambda_c$ polarization in the $W$+$c$ samples in Run 2 of the LHC. This can become the first measurement ever of a longitudinal polarization of charm quarks. Its results will provide a unique input to the understanding of polarization transfer in fragmentation. They will also allow applying the same measurement technique to other (e.g., new physics) samples of charm quarks in which the polarization is a priori unknown. The proposed analysis is similar to the ATLAS and CMS measurements of the $W$+$c$ cross section in the 7 TeV run that used reconstructed $D$-meson decays for charm tagging.