$\alpha_s$ and $\rm V_{cs}$ determination, and CKM unitarity test, from W decays at NNLO

TL;DR

This paper uses NNLO calculations of W boson widths to precisely determine the strong coupling constant $oldsymbol{ ext{alpha}_s}$, CKM matrix element $oldsymbol{V_{cs}}$, and test CKM unitarity, with implications for collider experiments.

Contribution

It provides the first NNLO theoretical predictions for W decay widths and uses them to extract fundamental parameters and test CKM unitarity with high precision.

Findings

Precise prediction of W hadronic branching ratio at NNLO.

Extraction of $ ext{alpha}_s(m_Z)$ with uncertainties.

Most accurate CKM unitarity test for lighter quarks.

Abstract

The hadronic ($\Gamma^{\rm W}_{\rm had}$) and total ($\Gamma^{\rm W}_{\rm tot}$) widths of the W boson, computed at least at next-to-next-to-leading-order (NNLO) accuracy, are combined to derive a new precise prediction for the hadronic W branching ratio ${\cal B}^{\rm W}_{\rm had} \equiv \Gamma^{\rm W}_{\rm had}/\Gamma^{\rm W}_{\rm tot}$ = $0.682 \pm 0.011_{\rm par}$, using the experimental Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, or ${\cal B}^{\rm W}_{\rm had} = 0.6742 \pm 0.0002_{\rm th} \pm 0.0001_{\rm par}$ assuming CKM unitarity, with uncertainties dominated by the input parameters of the calculations. Comparing the theoretical predictions and experimental measurements for various W decay observables, the NNLO strong coupling constant at the Z pole, $\alpha_s(m_Z) = 0.117 \pm 0.042_{\rm exp} \pm 0.004_{\rm th} \pm 0.001_{\rm par}$, as well as the charm-strange CKM element, $\rm V_{cs}$ = 0.973 $\pm$ 0.004$_{\rm exp}$ $\pm$ 0.002$_{\rm par}$, can be extracted under different assumptions. We also show that W decays provide today the most precise test of CKM unitarity for the 5 quarks lighter than $m_{\rm W}$, $\sum_{ u,c,d,s,b} |V_{ ij}|^2 = 1.999 \pm 0.008_{\rm exp} \pm 0.001_{\rm th}$. Perspectives for $\alpha_s$ and $\rm V_{cs}$ extractions from W decays measurements at the LHC and future $e^+e^-$ colliders are presented.