Colorful Mirror Solution to the Strong CP Problem.

We propose theories of a complete mirror world with parity (P) solving the strong CP problem. P exchanges the entire standard model with its mirror copy. We derive bounds on the two new mass scales that arise: v^{'} where parity and mirror electroweak symmetry are spontaneously broken, and v_{3} where the color groups break to the diagonal strong interactions. The strong CP problem is solved even if v_{3}≪v^{'}, when heavy colored states at the scale v_{3} may be accessible at LHC and future colliders. Furthermore, we argue that the breaking of P introduces negligible contributions to θ[over ¯]_{QCD}, starting at three-loop order. The symmetry breaking at v_{3} can be made dynamical, without introducing an additional hierarchy problem.

Fermi Constant from Muon Decay Versus Electroweak Fits and Cabibbo-Kobayashi-Maskawa Unitarity.

The Fermi constant G_{F} is extremely well measured through the muon lifetime, defining one of the key fundamental parameters in the standard model (SM). Therefore, to search for physics beyond the SM (BSM) via G_{F}, the constraining power is determined by the precision of the second-best independent determination of G_{F}. The best alternative extractions of G_{F} proceed either via the global electroweak (EW) fit or from superallowed ß decays in combination with the Cabibbo angle measured in kaon, τ, or D decays. Both variants display some tension with G_{F} from muon decay, albeit in opposite directions, reflecting the known tensions within the EW fit and hints for the apparent violation of Cabibbo-Kobayashi-Maskawa unitarity, respectively. We investigate how BSM physics could bring the three determinations of G_{F} into agreement using SM effective field theory and comment on future perspectives.

Combined Explanation of the Z→bb[over ¯] Forward-Backward Asymmetry, the Cabibbo Angle Anomaly, and τ→µνν and b→sℓ

In this Letter, we propose a simple model that can provide a combined explanation of the Z→bb[over ¯] forward-backward asymmetry, the Cabibbo angle anomaly (CAA), τ→µνν and b→sℓ^{+}ℓ^{-} data. This model is obtained by extending the standard model (SM) by two heavy vectorlike quarks (an SU(2)_{L} doublet (singlet) with hypercharge -5/6 (-1/3), two new scalars (a neutral and a singly charged one), and a gauged L_{µ}-L_{τ} symmetry. The mixing of the new quarks with the SM ones, after electroweak symmetry breaking, does not only explain Z→bb[over ¯] data, but also generates a lepton flavor universal contribution to b→sℓ^{+}ℓ^{-} transitions. Together with the lepton flavor universality violating effect, generated by loop-induced Z^{'} penguins involving the charged scalar and the heavy quarks, it gives an excellent fit to data (6.1σ better than the SM). Furthermore, the charged scalar (neutral vector) gives a necessarily constructive tree-level (loop) effect in µâ†’eνν (τ→µνν), which can naturally account for the CAA (Br[τ→µνν]/Br[τ→eνν] and Br[τ→µνν]/Br[µâ†’eνν]).

Hadronic Vacuum Polarization: (g-2)_{µ} versus Global Electroweak Fits.

Hadronic vacuum polarization (HVP) is not only a critical part of the standard model (SM) prediction for the anomalous magnetic moment of the muon (g-2)_{µ}, but also a crucial ingredient for global fits to electroweak (EW) precision observables due to its contribution to the running of the fine-structure constant encoded in Δα_{had}^{(5)}. We find that with modern EW precision data, including the measurement of the Higgs mass, the global fit alone provides a competitive, independent determination of Δα_{had}^{(5)}|_{EW}=270.2(3.0)×10^{-4}. This value actually lies below the range derived from e^{+}e^{-}→hadrons cross section data, and thus goes into the opposite direction as would be required if a change in HVP were to bring the SM prediction for (g-2)_{µ} into agreement with the Brookhaven measurement. Depending on the energy where the bulk of the changes in the cross section occurs, reconciling experiment and SM predictions for (g-2)_{µ} by adjusting HVP would thus not necessarily weaken the case for physics beyond the SM (BSM), but to some extent shift it from (g-2)_{µ} to the EW fit. We briefly explore some options of BSM scenarios that could conceivably explain the ensuing tension.

Global Fit to Modified Neutrino Couplings and the Cabibbo-Angle Anomaly.

Recently, discrepancies of up to 4σ between the different determinations of the Cabibbo angle were observed. In this context, we point out that this "Cabibbo-angle anomaly" can be explained by lepton flavor universality violating new physics in the neutrino sector. However, modified neutrino couplings to standard model gauge bosons also affect many other observables sensitive to lepton flavor universality violation, which have to be taken into account in order to assess the viability of this explanation. Therefore, we perform a model-independent global analysis in a Bayesian approach and find that the tension in the Cabibbo angle is significantly reduced, while the agreement with other data is also mostly improved. In fact, nonzero modifications of electron and muon neutrino couplings are preferred at more than 99.99% C.L. (corresponding to more than 4σ). Still, since constructive effects in the muon sector are necessary, simple models with right-handed neutrinos (whose global fit we update as a by-product) cannot fully explain data, pointing towards more sophisticated new physics models.