Measurement of CKM matrix elements in single top quark $t$-channel production in proton-proton collisions at $\sqrt{s} = $ 13 TeV

The CMS collaboration
Sirunyan, Albert M , Tumasyan, Armen , Adam, Wolfgang , Ambrogi, Federico , Bergauer, Thomas , Dragicevic, Marko , Erö, Janos , Escalante Del Valle, Alberto , Flechl, Martin , Fruehwirth, Rudolf
Phys.Lett.B 808 (2020) 135609, 2020.

Abstract (data abstract)
The first direct, model-independent measurement is presented of the modulus of the Cabibbo--Kobayashi--Maskawa (CKM) matrix elements $|V_{tb}|$, $|V_{td}|$, and $|V_{ts}|$, in final states enriched in single top quark $t$-channel events. The analysis uses proton-proton collision data from the LHC, collected during 2016 by the CMS experiment, at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $35.9 fb^{-1}$. Processes directly sensitive to these matrix elements are considered at both the production and decay vertices of the top quark. In the standard model hypothesis of CKM unitarity, a lower limit of $|V_{tb}| > 0.970$ is measured at the $95\%$ confidence level. Several theories beyond the standard model are considered, and by releasing all constraints among the involved parameters, the values $|V_{tb}| = 0.988 \pm 0.024 $, and $|V_{td}|^2 +|V_{ts}|^2 = 0.06 \pm 0.06$, where the uncertainties include both statistical and systematic components, are measured.

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