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.
Highlights
Fraction carried by the Λc is currently unknown
Regardless of whether any new physics is discovered at the LHC, the polarization measurements will advance our understanding of fragmentation
While we have focused on Λ+c → pK−π+, other reconstructible decay modes of the Λ+c can potentially be used for the polarization measurement as well
Summary
When a polarized c quark produces a Λc, a large fraction of its polarization is expected to be preserved in the Λc polarization. The likely dominant polarization loss effect is due to events in which the c quark first hadronizes to a Σc or Σ∗c ( known as Σc(2455) and Σc(2520), respectively) and a Λc is produced via Σ(c∗) → Λcπ [12].2. This is different from the case of a direct hadronization into a Λc, where the light diquark has spin 0, and the polarization of the Λc is the polarization of the original c quark:. Taking into account that one-loop QCD corrections to the hard process reduce the c-quark polarization by about 3% [46], one gets the estimate P(Λc) ≈ −0.66, where the negative sign. Polarized FFs are instrumental in computing the renormalization group evolution of the polarization retention with the scale of the hard process [53] (as exemplified in [21]), which has been neglected in the above discussion
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