B-meson Anomalies Research Articles

Third-family quark-lepton Unification and electroweak precision tests

We analyze the compatibility of the hypothesis of third-family quark-lepton unification at the TeV scale with electroweak precision data, lepton flavor universality tests, and high-pT constraints. We work within the framework of the UV complete flavor non-universal 4321 gauge model, which is matched at one loop to the Standard Model Effective Field Theory. For consistency, all electroweak precision observables are also computed at one loop within the effective field theory. At tree level, the most sizeable corrections are to W → τντ and Z → ντντ due to integrating out a pseudo-Dirac singlet fermion required by the model for neutrino mass generation. At loop level, the new colored states of the model generate large flavor-universal contributions to the electroweak precision observables via leading- and next-to-leading log running effects, yielding a significant improvement in the electroweak fit (including an increase in the W-boson mass). These effects cannot be decoupled if the model addresses the charged-current B-meson anomalies. Overall, we find good compatibility between the data sets, while simultaneously satisfying all low- and high-energy constraints.

Confronting the vector leptoquark hypothesis with new low- and high-energy data

In light of new data we present an updated phenomenological analysis of the simplified U_1-leptoquark model addressing charged-current B-meson anomalies. The analysis shows a good compatibility of low-energy data (dominated by the lepton flavor universality ratios R_D and R_{D^*}) with the high-energy constraints posed by pprightarrow tau {bar{tau }} Drell-Yan data. We also show that present data are well compatible with a framework where the leptoquark couples with similar strength to both left- and right-handed third-generation fermions, a scenario that is well-motivated from a model building perspective. We find that the high-energy implications of this setup will be probed at the 95% confidence level in the high-luminosity phase of the LHC.

Flavor non-universal vector leptoquark imprints in [formula omitted] and ΔF = 2 transitions

We analyze K→πνν¯ rates in a model with a TeV-scale leptoquark addressing B-meson anomalies, based on the flavor non-universal 4321 gauge group featuring third-generation quark-lepton unification. We show that, together with the tight bounds imposed by ΔF=2 amplitudes, the present measurement of B(K+→π+νν¯) already provides a non-trivial constraint on the model parameter space. In the minimal version of the model, the deviations from the Standard Model in B(K+→π+νν¯) are predicted to be in close correlation with non-standard effects in the Lepton Flavor Universality ratios RD and RD⁎. With the help of future data, these correlations can provide a decisive test of the model.

A bridge to new physics: proposing new — and reviving old — explanations of aμ

The 4.2 σ tension in the combined measurement of the anomalous magnetic moment of the muon, aμ, and the Standard Model prediction strongly suggests the existence of beyond the Standard Model physics. Following the Standard Model Effective Field Theory approach, we study a particular topology, the bridge diagram, which gives a chirally enhanced contribution to aμ. We classify all possible 2- and 3-field SM extensions that can generate this contribution and present the full aμ result for them. Within our approach, we find that several 2-field fermion-scalar extensions which had been previously discarded in the literature — when only the Yukawa-suppressed contribution was considered — can actually be viable models to explain the observed anomaly. Furthermore, the 3-field extensions which generate the bridge diagram represent a new class of models to account for aμ. We explore a particular 3-field extension which, beyond explaining aμ, can also account for the neutral B-meson anomalies and the Cabibbo angle anomaly. We present the full one-loop matching for this model and a one-loop phenomenological study.

Flavor anomalies and quark-lepton unification

We show that one can explain the neutral and charged anomalies in B-meson decays in the minimal theory for quark-lepton unification.The implications for flavor violating processes are discussed in detail. Strikingly, experimental observations suggest that the unification of quarks and leptons could be realized below the ${\cal O}(10^2)$ TeV scale.

Flavor hierarchies, flavor anomalies, and Higgs mass from a warped extra dimension

The recent B-meson anomalies are coherently explained at the TeV scale by 4321 gauge models with hierarchical couplings reminiscent of the Standard Model Yukawas. We show that such models arise as the low-energy limit of a complete theory of flavor, based on a warped fifth dimension where each Standard Model family is quasi-localized on a different brane. The Higgs is identified as a pseudo-Nambu-Goldstone boson emerging from the same dynamics responsible for 4321 symmetry breaking. This novel construction unifies quarks and leptons in a flavor non-universal manner, provides a natural description of flavor hierarchies, and addresses the electroweak hierarchy problem.

From B-meson anomalies to Kaon physics with scalar leptoquarks

In this work we study possible connections between B-meson anomalies and Kaon physics observables in the context of combined solutions with the singlet and triplet scalar leptoquarks S_1 and S_3. By assuming a flavor structure for the leptoquark couplings dictated by a minimally broken U(2)^5 flavor symmetry we can make a sharp connection between these two classes of observables. We find that the bound on {mathcal {B}}(K^+ rightarrow pi ^+ nu nu ) from NA62 puts already some tension in the model, while the present limits on {mathcal {B}}(K_L rightarrow mu ^+ mu ^-) and mu rightarrow e conversion in nuclei can be saturated. Relaxing instead the flavor assumption we study what values for {mathcal {B}}(K^+ rightarrow pi ^+ nu nu ), as well as for {mathcal {B}}(K_L rightarrow pi ^0 nu nu ) and {mathcal {B}}(K_{L,S} rightarrow mu ^+ mu ^-), are viable compatibly with all other phenomenological constraints.

SU(5) × U(1)′ Models with a Vector-like Fermion Family

Motivated by experimental measurements indicating deviations from the Standard Model predictions, we discuss F-theory-inspired models, which, in addition to the three chiral generations, contain a vector-like complete fermion family. The analysis takes place in the context of SU(5)×U(1)′ GUT embedded in an E8 covering group, which is associated with the (highest) geometric singularity of the elliptic fibration. In this context, the U(1)′ is a linear combination of four abelian factors subjected to the appropriate anomaly cancellation conditions. Furthermore, we require universal U(1)′ charges for the three chiral families and different ones for the corresponding fields of the vector-like representations. Under the aforementioned assumptions, we find 192 models that can be classified into five distinct categories with respect to their specific GUT properties. We exhibit representative examples for each such class and construct the superpotential couplings and the fermion mass matrices. We explore the implications of the vector-like states in low-energy phenomenology, including the predictions regarding the B-meson anomalies. The rôle of R-parity violating terms appearing in some particular models of the above construction is also discussed.

On the LHC signatures of SU(5)times U(1)' F-theory motivated models

We study low energy implications of F-theory GUT models based on SU(5) extended by a U(1)' symmetry which couples non-universally to the three families of quarks and leptons. This gauge group arises naturally from the maximal exceptional gauge symmetry of an elliptically fibred internal space, at a single point of enhancement, E_8supset SU(5)times SU(5)'supset SU(5)times U(1)^4. Rank-one fermion mass textures and a tree-level top quark coupling are guaranteed by imposing a Z_2 monodromy group which identifies two abelian factors of the above breaking sequence. The U(1)' factor of the gauge symmetry is an anomaly free linear combination of the three remaining abelian symmetries left over by Z_2. Several classes of models are obtained, distinguished with respect to the U(1)' charges of the representations, and possible extra zero modes coming in vector-like representations. The predictions of these models are investigated and are compared with the LHC results and other related experiments. Particular cases interpreting the B-meson anomalies observed in LHCb and BaBar experiments are also discussed.

Third-family quark-lepton unification with a fundamental composite Higgs

We present a model for third-family quark-lepton unification at the TeV scale featuring a composite Higgs sector. The model is based on a variant of the Pati-Salam model, the so-called 4321 model, consisting of the gauge group SU(4)×SU(3)′×SU(2)L×U(1)X. The spontaneous symmetry breaking to the SM gauge group is triggered dynamically by a QCD-like confining sector. The same strong dynamics also produces the Higgs as a pseudo Nambu-Goldstone boson, connecting the energy scales of both sectors. The model predicts a massive U1 vector leptoquark coupled dominantly to the third generation, recently put forward as a possible solution to the B-meson anomalies.

Rank-one flavor violation and B-meson anomalies

We assume that the quark-flavor coefficients matrix of the semileptonic operators addressing the neutral-current B-meson anomalies has rank-one, i.e. it can be described by a single vector in quark-flavor space. By correlating the observed anomalies to other flavor and high-pT observables, we constrain its possible directions and we show that a large region of the parameter space of this framework will be explored by flavor data from the NA62, KOTO, LHCb and Belle II experiments.

Pati-Salam explanations of the B-meson anomalies

We provide a combined explanation of the increasingly tantalizing B-meson anomalies, both in {R}_{K^{left(ast right)}} and {R}_{D^{left(ast right)}} , in the Pati-Salam model with minimal matter content. This well-known model, based on the gauge group SU(4)LC × SU(2)L × SU(2)R, naturally contains a variety of scalar leptoquarks with related and restricted couplings. In particular we show that the seesaw-motivated scalar leptoquark within the representation ( overline{mathbf{10}} ,3,1) and its right-handed parity partner ( overline{mathbf{10}} ,1,3) can solve both anomalies while making testable predictions for related observables such as B → Kνν and B → Kμτ. The solution of the {R}_{K^{left(ast right)}} anomaly alone can be related to a type-II seesaw neutrino mass structure. Explaining also {R}_{D^{left(ast right)}} requires the existence of a light right-handed neutrino, which constrains the UV structure of the model.

Maximal flavour violation: a Cabibbo mechanism for leptoquarks

We propose a mechanism that allows for sizeable flavour violation in quark-lepton currents, while suppressing flavour changing neutral currents in quark-quark and lepton-lepton sectors. The mechanism is applied to the recently proposed “4321” renormalizable model, which can accommodate the current experimental anomalies in B-meson decays, both in charged and neutral currents, while remaining consistent with all other indirect flavour and electroweak precision measurements and direct searches at high-pT. To support this claim, we present an exhaustive phenomenological survey of this fully calculable UV complete model and highlight the rich complementarity between indirect and direct searches.

Lepto-quark portal dark matter

We consider the extension of the Standard Model with scalar leptoquarks as a portal to dark matter (DM), motivated by the recent anomalies in semi-leptonic B-meson decays. Taking singlet and triplet scalar leptoquarks as the best scenarios for explaining B-meson anomalies, we discuss the phenomenological constraints from rare meson decays, muon (g − 2)μ, and leptoquark searches at the Large Hadron Collider (LHC). Introducing leptoquark couplings to scalar dark matter, we find that the DM annihilations into a pair of leptoquarks open a wide parameter space, being compatible with XENON1T bound, and show that there is an interesting interplay between LHC leptoquark searches and distinct signatures from cascade annihilations of dark matter.

Leptoquark toolbox for precision collider studies

We implement scalar and vector leptoquark (LQ) models in the universal FeynRules output (UFO) format assuming the Standard Model fermion content and conservation of baryon and lepton numbers. Scalar LQ implementations include next-to-leading order (NLO) QCD corrections. We report the NLO QCD inclusive cross sections in proton-proton collisions at 13 TeV, 14 TeV, and 27 TeV for all on-shell LQ production processes. These comprise (i) LQ pair production (pp → ΦΦ) and (ii) single LQ + lepton production (pp → Φℓ) for all initial quark flavours (u, d, s, c, and b). Vector LQ implementation includes adjustable non-minimal QCD coupling. We discuss several aspects of LQ searches at a hadron collider, emphasising the implications of SU(2) gauge invariance, electroweak and flavour constraints, on the possible signatures. Finally, we outline the high-pT search strategy for LQs recently proposed in the literature to resolve experimental anomalies in B-meson decays. In this context, we stress the importance of complementarity of the three LQ related processes, namely, pp → ΦΦ, pp → Φℓ, and pp → ℓℓ.

B-meson anomalies and Higgs physics in flavored $$U(1)'$$ U ( 1 ) ′ model

We consider a simple extension of the Standard Model with flavor-dependent U(1)', that has been proposed to explain some of B-meson anomalies recently reported at LHCb. The U(1)' charge is chosen as a linear combination of anomaly-free B_3-L_3 and L_mu -L_tau . In this model, the flavor structure in the SM is restricted due to flavor-dependent U(1)' charges, in particular, quark mixings are induced by a small vacuum expectation value of the extra Higgs doublet. As a result, it is natural to get sizable flavor-violating Yukawa couplings of heavy Higgs bosons involving the bottom quark. In this article, we focus on the phenomenology of the Higgs sector of the model including extra Higgs doublet and singlet scalars. We impose various bounds on the extended Higgs sector from Higgs and electroweak precision data, B-meson mixings and decays as well as unitarity and stability bounds, then discuss the productions and decays of heavy Higgs bosons at the LHC.

Footprints of leptoquarks: from R_{K^{(*)}} to K rightarrow pi nu bar{nu }

Rare K rightarrow pi nu bar{nu } decays, being dominated by short distance contributions within the standard model (SM), open a window for new physics (NP) searches at low energies. The K rightarrow pi nu bar{nu } branching ratios are expected to be measured with sim 10% accuracies by NA62/CERN and KOTO/JPARC. The theoretical uncertainties of branching ratios within the SM are well under control. In the B sector, it is tentative to explain the B-meson anomalies R_{D^{(*)}} and/or R_{K^{(*)}} by effects of physics beyond the SM. Although NP seems to be present in the third fermion generation it might also manifest in the flavor changing neutral current transition s rightarrow d . Together with the anticipated good experimental sensitivities and accurate theoretical predictions for Krightarrow pi nu bar{nu }, this motivates studies of correlated effects of NP in rare Krightarrow pi nu bar{nu } and B rightarrow K^{(*)} mu ^+ mu ^- decays. Here we consider the loop induced effects in K rightarrow pi nu bar{nu } in two leptoquark models designed to address lepton-flavor universality violation in the R_{K^{(*)}} anomalies.

Flavor non-universal gauge interactions and anomalies in B-meson decays**Supported by the Grant-in-Aid for Innovative Areas (16H06490)

Motivated by flavor non-universality and anomalies in semi-leptonic B-meson decays, we present a general and systematic discussion about how to construct anomaly-free U(1)′ gauge theories based on an extended standard model with only three right-handed neutrinos. If all standard model fermions are vector-like under this new gauge symmetry, the most general family non-universal charge assignments, (a,b,c) for three-generation quarks and (d,e,f) for leptons, need satisfy just one condition to be anomaly-free, 3(a+b+c) = − (d+e+f). Any assignment can be linear combinations of five independent anomaly-free solutions. We also illustrate how such models can generally lead to flavor-changing interactions and easily resolve the anomalies in B-meson decays. Probes with mixing, decay into τ±, dilepton and dijet searches at colliders are also discussed.

Flavor anomalies from warped space

We study the recently found anomalies in B-meson decays within a scenario with a warped extra dimension where the Standard Model (SM) fermions are propagating in the bulk. The anomalies are then interpreted as the result of the exchange of heavy vector resonances with electroweak (EW) quantum numbers. The model naturally leads to lepton-flavor universality (LFU) violation when different flavor fermions are differently localized along the extra dimension, signaling a different degree of compositeness in the dual holographic theory.

Charged-lepton mixing and lepton flavor violation

We present a model for calculating charged-lepton mixing matrices. These matrices are an essential ingredient for predicting lepton flavor-violating rates in the lepton number nonuniversal models recently proposed to explain anomalies in B-meson decays. The model is based on work on “constrained flavor breaking” by Appelquist, Bai and Piai relating the charged-lepton mass matrix, Mℓ, to those for the up- and down-type quarks, Mu,d. We use our recent model of lepton nonuniversality to illustrate the magnitudes of flavor-violating B-decay rates that might be expected. Decays with μτ final states generally have the highest rates by far.