Hints of lepton flavor universality violations.

Mounting evidence shows where the standard model may be incomplete.

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]).

Correlating h→µ

Recently, both ATLAS and CMS measured the decay h→µ^{+}µ^{-}, finding a signal strength with respect to the standard model expectation of 1.2±0.6 and 1.19_{-0.39-0.16}^{+0.41+0.17}, respectively. This provides, for the first time, evidence that the standard model Higgs couples to second generation fermions. This measurement is particularly interesting in the context of the intriguing hints for lepton flavor universality violation, accumulated within recent years, as new physics explanations could also be tested in the h→µ^{+}µ^{-} decay mode. Leptoquarks are prime candidates to account for the flavor anomalies. In particular, they can provide the necessary chiral enhancement (by a factor m_{t}/m_{µ}) to address a_{µ} with tera-electron-volt scale new physics. In this Letter we point out that such explanations of a_{µ} also lead to enhanced effects in h→µ^{+}µ^{-} and we examine the correlations between h→µ^{+}µ^{-} and a_{µ} within leptoquark models. We find that the effect in the branching ratio of h→µ^{+}µ^{-} ranges from several percent up to a factor of 3, if one aims at accounting for a_{µ} at the 2σ level. Hence, the new ATLAS and CMS Collaboration measurements already provide important constraints on the parameter space, rule out specific a_{µ} explanations, and will be very important to test the flavor anomalies in the future.

ß Decays as Sensitive Probes of Lepton Flavor Universality.

Nuclear ß decays as well as the decay of the neutron are well-established low-energy probes of physics beyond the standard model (SM). In particular, with the axial-vector coupling of the nucleon g_{A} determined from lattice QCD, the comparison between experiment and SM prediction is commonly used to derive constraints on right-handed currents. Further, in addition to the CKM element V_{us} from kaon decays, V_{ud} from ß decays is a critical input for the test of CKM unitarity. Here, we point out that the available information on ß decays can be reinterpreted as a stringent test of lepton flavor universality (LFU). In fact, we find that the ratio of V_{us} from kaon decays over V_{us} from ß decays (assuming CKM unitarity) is extremely sensitive to LFU violation (LFUV) in W-µ-ν couplings thanks to a CKM enhancement by (V_{ud}/V_{us})^{2}∼20. From this perspective, recent hints for the violation of CKM unitarity can be viewed as further evidence for LFUV, fitting into the existing picture exhibited by semileptonic B decays and the anomalous magnetic moments of muon and electron. Finally, we comment on the future sensitivity that can be reached with this LFU violating observable and discuss complementary probes of LFU that may reach a similar level of precision, such as Γ(π→µν)/Γ(π→eν) at the PEN and PiENu experiments or even direct measurements of W→µν at an FCC-ee.

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.

CP Violation in Higgs-Gauge Interactions: From Tabletop Experiments to the LHC.

We investigate the interplay between the high- and low-energy phenomenology of CP-violating interactions of the Higgs boson with gauge bosons. For this purpose, we use an effective field theory approach and consider all dimension-six operators arising in so-called universal theories. We compute their loop-induced contributions to electric dipole moments and the CP asymmetry in B→X_{s}γ and compare the resulting current and prospective constraints to the projected sensitivity of the LHC. Low-energy measurements are shown to generally have a far stronger constraining power, which results in highly correlated allowed regions in coupling space-a distinctive pattern that could be probed at the high-luminosity LHC.

Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark.

In recent years experiments revealed intriguing hints for new physics (NP) in B decays involving b→cτν and b→sℓ^{+}ℓ^{-} transitions at the 4σ and 5σ level, respectively. In addition, there are slight disagreements in b→uτν and b→dµ^{+}µ^{-} observables. While not significant on their own, they point in the same direction. Furthermore, V_{us} extracted from τ decays shows a slight tension (≈2.5σ) with its value determined from Cabibbo-Kobayashi-Maskawa unitarity, and an analysis of BELLE data found an excess in B_{d}→τ^{+}τ^{-}. Concerning NP explanations, the vector leptoquark SU(2) singlet is of special interest since it is the only single particle extension of the standard model which can (in principle) address all the anomalies described above. For this purpose, large couplings to τ leptons are necessary and loop effects, which we calculate herein, become important. Including them in our phenomenological analysis, we find that neither the tension in V_{us} nor the excess in B_{d}→τ^{+}τ^{-} can be fully explained without violating bounds from K→πνν[over ¯]. However, one can account for b→cτν and b→uτν data finding intriguing correlations with B_{q}→τ^{+}τ^{-} and K→πνν[over ¯]. Furthermore, the explanation of b→cτν predicts a positive shift in C_{7} and a negative one in C_{9}, being nicely in agreement with the global fit to b→sℓ^{+}ℓ^{-} data. Finally, we point out that one can fully account for b→cτν and b→sℓ^{+}ℓ^{-} without violating bounds from τ→ϕµ, ϒ→τµ, or b→sτµ processes.

B Meson Anomalies in a Pati-Salam Model within the Randall-Sundrum Background.

Lepton number as a fourth color is the intriguing theoretical idea of the famous Pati-Salam (PS) model. While in conventional PS models, the symmetry breaking scale and the mass of the resulting vector leptoquark are stringently constrained by K_{L}→µe and K→πµe, the scale can be lowered to a few TeV by adding vectorlike fermions. Furthermore, in this case, the intriguing hints for lepton flavor universality violation in b→sµ^{+}µ^{-} and b→cτν processes can be addressed. Such a setup is naturally achieved by implementing the PS gauge group in the five-dimensional Randall-Sundrum background. The PS symmetry is broken by boundary conditions on the fifth dimension, and the resulting massive vector leptoquark automatically has the same mass scale as the vectorlike fermions and all other resonances. We consider the phenomenology of this model in the context of the hints for lepton flavor universality violation in semileptonic B decays. Assuming flavor alignment in the down sector, we find that in b→sℓ^{+}ℓ^{-} transitions, the observed deviations from the standard model predictions [including R(K) and R(K^{*})] can be explained with natural values for the free parameters of the model. Even though we find sizable effects in R(D), R(D^{*}), and R(J/Ψ), one cannot account for the current central values in the constrained setup of our minimal model due to the stringent constraints from D-D[over ¯] mixing and τ→3µ.

Searching for New Physics with b→sτ

In recent years, intriguing hints for the violation of lepton flavor universality (LFU) have been accumulated in semileptonic B decays, both in the charged-current transitions b→cℓ^{-}ν[over ¯]_{ℓ} (i.e., R_{D}, R_{D^{*}}, and R_{J/ψ}) and the neutral-current transitions b→sℓ^{+}ℓ^{-} (i.e., R_{K} and R_{K^{*}}). Hints for LFU violation in R_{D^{(*)}} and R_{J/ψ} point at large deviations from the standard model (SM) in processes involving tau leptons. Moreover, LHCb has reported deviations from the SM expectations in b→sµ^{+}µ^{-} processes as well as in the ratios R_{K} and R_{K^{*}}, which together point at new physics (NP) affecting muons with a high significance. These hints for NP suggest the possibility of huge LFU-violating effects in b→sτ^{+}τ^{-} transitions. In this Letter, we predict the branching ratios of B→Kτ^{+}τ^{-}, B→K^{*}τ^{+}τ^{-}, and B_{s}→ϕτ^{+}τ^{-}, taking into account NP effects in the Wilson coefficients C_{9(^{'})}^{ττ} and C_{10(^{'})}^{ττ}. Assuming a common NP explanation of R_{D}, R_{D^{(*)}}, and R_{J/ψ}, we show that a very large enhancement of b→sτ^{+}τ^{-} processes, of around 3 orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of B_{s}→τ^{+}τ^{-}, B_{s}→ϕτ^{+}τ^{-}, and B→K^{(*)}τ^{+}τ^{-} under these assumptions are in the observable range for LHCb and Belle II.

No-Go Theorem for Nonstandard Explanations of the τ→K_{S}πν_{τ} CP Asymmetry.

The CP asymmetry in τ→K_{S}πν_{τ}, as measured by the BABAR collaboration, differs from the standard model prediction by 2.8σ. Most nonstandard interactions do not allow for the required strong phase needed to produce a nonvanishing CP asymmetry, leaving only new tensor interactions as a possible mechanism. We demonstrate that, contrary to previous assumptions in the literature, the crucial interference between vector and tensor phases is suppressed by at least 2 orders of magnitude due to Watson's final-state-interaction theorem. Furthermore, we find that the strength of the relevant CP-violating tensor interaction is strongly constrained by bounds from the neutron electric dipole moment and D-D[over ¯] mixing. These observations together imply that it is extremely difficult to explain the current τ→K_{S}πν_{τ} measurement in terms of physics beyond the standard model originating in the ultraviolet.

Perturbed Lepton-Specific Two-Higgs-Doublet Model Facing Experimental Hints for Physics beyond the Standard Model.

BABAR, Belle, and LHCb Collaborations report evidence for new physics in B→Dτν and B→D^{*}τν of approximately 3.8σ. There is also the long lasting discrepancy of about 3σ in the anomalous magnetic moment of the muon, and the branching ratio for τ→µνν is 1.8σ (2.4σ) above the standard model expectation using the HFAG (PDG) values. Furthermore, CMS Collaboration finds hints for a nonzero decay rate of h→µτ. Interestingly, all these observations can be explained by introducing new scalars. In this Letter we consider these processes within a lepton-specific two-Higgs doublet model (i.e., of type X) with additional nonstandard Yukawa couplings. It is found that one can accommodate τ→µνν with modified Higgs-τ couplings. The anomalous magnetic moment of the muon can be explained if the additional neutral CP-even Higgs boson H is light (below 100 GeV). Also R(D) and R(D^{*}) can be easily explained by additional t-c-Higgs couplings. Combining these t-c couplings with a light H the decay rate for t→Hc can be in a testable range for the LHC. Effects in h→µτ are also possible, but in this case a simultaneous explanation of the anomalous magnetic moment of the muon is difficult due to the unavoidable τ→µÎ³ decay.

Effective Field Theory Approach to b→sℓℓ

The LHCb Collaboration reported anomalies in B→K^{*}µ^{+}µ^{-}, B_{s}→ϕµ^{+}µ^{-}, and R(K)=B→Kµ^{+}µ^{-}/B→Ke^{+}e^{-}. Furthermore, BABAR, BELLE, and LHCb Collaborations found hints for the violation of lepton-flavor universality violation in R(D^{(*)})=B→D^{(*)}τν/B→D^{(*)}ℓν. In this Letter we reexamine these decays and their correlations to B→K^{(*)}νν[over ¯] using gauge invariant dim-6 operators. For the numerical analysis we focus on scenarios in which new physics couples, in the interaction eigenbasis, to third generation quarks and lepton only. We conclude that such a setup can explain the b→sµ^{+}µ^{-} data simultaneously with R(D^{(*)}) for small mixing angles in the lepton sector (of the order of π/16) and very small mixing angles in the quark sector (smaller than V_{cb}). In these regions of parameter space, B→K^{(*)}τµ and B_{s}→τµ can be order 10^{-6}. Possible UV completions are briefly discussed.

Explaining h→µ(±)τ(∓), B→K*µ(+)µ(-), and B→Kµ(+)µ(-)/B→Ke(+)e(-) in a Two-Higgs-Doublet Model with Gauged L(µ)-L(τ).

The LHC has observed, so far, 3 deviations from the Standard Model (SM) predictions in flavor observables: LHCb reported anomalies in B→K*µ(+)µ(-) and R(K)=B→Kµ(+)µ(-)/B→Ke(+)e(-), while CMS found an excess in h→µτ. We show, for the first time, how these deviations from the SM can be explained within a single well-motivated model: a two-Higgs-doublet model with gauged L(µ)-L(τ) symmetry. We find that, despite the constraints from τ→µµµ and B(s)-B¯(s) mixing, one can explain h→µτ, B→K*µ(+)µ(-) and R(K) simultaneously, obtaining interesting correlations among the observables.

Can the differences in the determinations of V(ub) and V(cb) be explained by new physics?

The precise determination of the Cabibbo-Kobayashi-Maskawa elements V(ub) and V(ub) is crucial for any new physics analysis in the flavor sector. Their values can be determined from several tree-level decays: V(ub) can be extracted from B→Dℓν and B→D(*)ℓν while V(ub) can be obtained from B→πℓν, B→ρℓν, and B→τν. In addition, for both V(ub) and V(ub) an inclusive determination is available. There is a long lasting discrepancy between the inclusive and exclusive determinations which recently even increased for V(ub) above the 3σ level. In this Letter we study the possible effect of new physics on the inclusive and exclusive determination of V(ub) and V(ub) in a model independent way. We find that there is only one operator corresponding to a modified W coupling which can achieve this. However, respecting SU(2) gauge invariance at the high scale this would lead to very large violations of the Z to bb̅ coupling not compatible with experiment. Therefore, we conclude that a new physics explanation of the difference between the inclusive and exclusive determination of V(ub) and V(ub) is currently ruled out. Therefore, the discrepancies must be due to underestimated uncertainties in the theoretical and/or the experimental analysis.

New constraints on dark matter effective theories from standard model loops.

We consider an effective field theory for a gauge singlet Dirac dark matter particle interacting with the standard model fields via effective operators suppressed by the scale Λ â‰³ 1 TeV. We perform a systematic analysis of the leading loop contributions to spin-independent Dirac dark matter-nucleon scattering using renormalization group evolution between Λ and the low-energy scale probed by direct detection experiments. We find that electroweak interactions induce operator mixings such that operators that are naively velocity suppressed and spin dependent can actually contribute to spin-independent scattering. This allows us to put novel constraints on Wilson coefficients that were so far poorly bounded by direct detection. Constraints from current searches are already significantly stronger than LHC bounds, and will improve in the near future. Interestingly, the loop contribution we find is isospin violating even if the underlying theory is isospin conserving.