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Global fit of modified quark couplings to EW gauge bosons and vector-like quarks in light of the Cabibbo angle anomaly


Crivellin, Andreas; Kirk, Matthew; Kitahara, Teppei; Mescia, Federico (2023). Global fit of modified quark couplings to EW gauge bosons and vector-like quarks in light of the Cabibbo angle anomaly. Journal of High Energy Physics, 2023(3):234.

Abstract

There are two tensions related to the Cabibbo angle of the CKM matrix. First, the determinations of V$_{us}$ from K$_{μ2}$, K$_{ℓ3}$, and τ decays disagree at the 3σ level. Second, using the average of these results in combination with β decays (including super-allowed β decays and neutron decay), a deficit in first-row CKM unitarity with a significance of again about 3σ is found. These discrepancies, known as the Cabibbo Angle anomaly, can in principle be solved by modifications of W boson couplings to quarks. However, due to SU(2)$_{L}$ invariance, Z couplings to quarks are also modified and flavour changing neutral currents can occur. In order to consistently assess the agreement of a new physics hypothesis with data, we perform a combined analysis for all dimension-six Standard Model Effective Field Theory operators that generate modified W couplings to first and second generation quarks. We then study models with vector-like quarks, which are prime candidates for a corresponding UV completion as they can affect W -quark couplings at tree level, and we perform a global fit including flavour observables (in particular loop effects in ∆F = 2 processes). We find that the best fit can be obtained for the SU(2)$_{L}$ doublet vector-like quark Q as it can generate right-handed W-u-d and W-u-s couplings as preferred by data.

Abstract

There are two tensions related to the Cabibbo angle of the CKM matrix. First, the determinations of V$_{us}$ from K$_{μ2}$, K$_{ℓ3}$, and τ decays disagree at the 3σ level. Second, using the average of these results in combination with β decays (including super-allowed β decays and neutron decay), a deficit in first-row CKM unitarity with a significance of again about 3σ is found. These discrepancies, known as the Cabibbo Angle anomaly, can in principle be solved by modifications of W boson couplings to quarks. However, due to SU(2)$_{L}$ invariance, Z couplings to quarks are also modified and flavour changing neutral currents can occur. In order to consistently assess the agreement of a new physics hypothesis with data, we perform a combined analysis for all dimension-six Standard Model Effective Field Theory operators that generate modified W couplings to first and second generation quarks. We then study models with vector-like quarks, which are prime candidates for a corresponding UV completion as they can affect W -quark couplings at tree level, and we perform a global fit including flavour observables (in particular loop effects in ∆F = 2 processes). We find that the best fit can be obtained for the SU(2)$_{L}$ doublet vector-like quark Q as it can generate right-handed W-u-d and W-u-s couplings as preferred by data.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Physics Institute
Dewey Decimal Classification:530 Physics
Scopus Subject Areas:Physical Sciences > Nuclear and High Energy Physics
Uncontrolled Keywords:Nuclear and High Energy Physics
Language:English
Date:29 March 2023
Deposited On:29 Dec 2023 10:04
Last Modified:28 Jun 2024 03:31
Publisher:Springer
ISSN:1029-8479
OA Status:Gold
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.1007/jhep03(2023)234
  • Content: Published Version
  • Language: English
  • Licence: Creative Commons: Attribution 4.0 International (CC BY 4.0)