Search

Abstract The Froggatt-Nielsen (FN) mechanism, a prominent framework for explaining the observed flavor hierarchies, generically predicts the existence of an axion-like particle (ALP). This work examines a class of FN models based on ℤ N discrete symmetries. We chart the allowed parameter space from a set of theoretical considerations and construct explicit renormalizable completions with minimal field content necessary to generate consistent textures. We then conduct comprehensive phenomenological analyses of two particularly elegant ℤ4 and ℤ8 models, highlighting the interplay between the effects of the ALP and the associated UV fields. We find that the FN scale can be as low as a few TeV.

Abstract In our recent attempt to explain flavor hierarchies [1], a gauged SU(2) flavor symmetry acting on left-handed fermions provides a ground to introduce three independent rank-one contributions to the Yukawa matrices: a renormalizable one for the third family, a mass-suppressed one for the second family, and an additional loop-suppressed factor for the first family. Here, we demonstrate how minimal quark-lepton unification à la Pati-Salam, relating down-quarks to charged leptons, can significantly improve this mechanism. We construct and thoroughly analyze a renormalizable model, performing a comprehensive one-loop matching calculation that reveals how all flavor hierarchies emerge from a single ratio of two scales. The first signatures may appear in the upcoming charged lepton flavor violation experiments.

Abstract We propose an economical model to address the mass hierarchies of quarks and charged leptons. The light generations of the left-handed fermions form doublets under an $$ \textrm{SU}(2) $$ SU ( 2 ) flavor symmetry, which is gauged. The generational hierarchies emerge from three independent rank-one contributions to the Yukawa matrices: one is a renormalizable contribution, the second is suppressed by a mass ratio, and the last by an additional loop factor. The model is renormalizable, features only a handful of new fields, and is remarkably simple compared to typical completions of gauged flavor symmetries or Froggatt–Nielsen. The model has a rich phenomenology, and we highlight promising signatures, especially in the context of K and B meson physics. This includes an interpretation of the latest $$B^+ \rightarrow K^+ \nu \bar{\nu }$$ B + → K + ν ν ¯ measurement from Belle II.

Abstract Exclusive semileptonic b hadron decays (b → uℓν) serve as a sandbox for probing strong and electroweak interactions and for extracting the CKM element V ub . Instead, this work investigates their underexplored potential to reveal new short-distance physics. Utilizing SMEFT as a conduit to chart territory beyond the SM, we demonstrate that substantive new physics contributions in b → uℓν are necessarily linked to correlated effects in rare neutral-current b decays, neutral B meson mixing or high-mass Drell-Yan tails. We find that measurements of the latter processes strongly restrict the allowed deviations in the former. A complete set of tree-level mediators, originating from a perturbative ultraviolet model and matching at dimension 6, is thoroughly explored to support this assertion. As a showcase application, we examine the feasibility of a new physics interpretation of the recent tension in exclusive |V ub | extraction from B → Vℓν where V = (ρ, ω).

Abstract A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work.

Abstract Short-distance new physics at (or slightly) above the TeV scale should not excessively violate the approximate flavor symmetries of the SM in order to comply with stringent constraints from flavor-changing neutral currents. In this respect, flavor symmetries provide an effective organizing principle for the vast parameter space of the SMEFT. In this work, we classify all possible irreducible representations under U(3)5 flavor symmetry of new heavy spin-0, 1/2, and 1 fields which integrate out to dimension-6 operators at the tree level. For a general perturbative UV model, the resulting flavor-symmetric interactions are very restrictive and, in most cases, predict a single Hermitian operator with a definite sign. These leading directions in the SMEFT space deserve particular attention. We derive an extensive set of present experimental constraints by utilizing the existing global SMEFT fits, which incorporate data from top quark, Higgs boson, and electroweak measurements, along with constraints on dilepton and 4-lepton contact interactions. The derived set of bounds comprehensively summarises the present knowledge from indirect searches of flavor-blind new physics mediators.

Abstract Rare b hadron decays are considered excellent probes of new semileptonic four-fermion interactions of microscopic origin. However, the same interactions also correct the high-mass Drell-Yan tails. In this work, we revisit the first statement in the context of this complementarity and chart the space of short-distance new physics that could show up in rare b decays. We analyze the latest b → qℓ + ℓ − measurements, where q = d or s and ℓ = e or μ, including the most recent LHCb R K ∗ $$ {R}_{K^{\left(\ast \right)}} $$ update, together with the latest charged and neutral current high-mass Drell-Yan data, pp → ℓν and pp → ℓ + ℓ − . We implement a sophisticated interpretation pipeline within the flavio framework, allowing us to investigate the multidimensional SMEFT parameter space thoroughly and efficiently. To showcase the new functionalities of flavio, we construct several explicit models featuring either a Z′ or a leptoquark, which can explain the tension in b → sμ + μ − angular distributions and branching fractions while predicting lepton flavor universality (LFU) ratios to be SM-like, R K ∗ ≈ R K ∗ SM $$ {R}_{K^{\left(\ast \right)}}\approx {R}_{K^{\left(\ast \right)}}^{\textrm{SM}} $$ , as indicated by the recent data. Those models are then confronted against the global likelihood, including the high-mass Drell-Yan, either finding tensions or compatibility.

Abstract Recent progress in calculating lepton density functions inside the proton and simulating lepton showers laid the foundations for precision studies of resonant leptoquark production at hadron colliders. Direct quark-lepton fusion into a leptoquark is a novel production channel at the LHC that has the potential to probe a unique parameter space for large masses and couplings. In this work, we build the first Monte Carlo event generator for a full-fledged simulation of this process at NLO for production, followed by a subsequent decay using the POWHEG method and matching to the parton showers utilizing HERWIG. The code can handle all scalar leptoquark models with renormalisable quark-lepton interactions. We then comprehensively study the differential distributions, including higher-order effects, and assess the corresponding theoretical uncertainties. We also quantify the impact of the improved predictions on the projected (HL-)LHC sensitivities and initiate the first exploration of the potential at the FCC-hh. Our work paves the way toward performing LHC searches using this channel.

Abstract We study the flavor structure of the lepton and baryon number-conserving dimension-6 operators in the Standard Model effective field theory (SMEFT). Building on the work of [1], we define several well-motivated flavor symmetries and symmetry-breaking patterns that serve as competing hypotheses about the ultraviolet (UV) dynamics beyond the SM, not far above the TeV scale. In particular, we consider four different structures in the quark sector and seven in the charged lepton sector. The set of flavor-breaking spurions is (almost) always taken to be the minimal one needed to reproduce the observed charged fermion masses and mixings. For each case, we explicitly construct and count the operators to the first few orders in the spurion expansion, providing ready-for-use setups for phenomenological studies and global fits. We provide a Mathematica package SMEFTflavor ( https://github.com/aethomsen/SMEFTflavor ) to facilitate similar analyses for flavor symmetries not covered in this work.

Abstract Rare flavour-changing neutral-current transitions b → sμ + μ − probe higher energy scales than what is directly accessible at the LHC. Therefore, the presence of new physics in such transitions, as suggested by the present-day LHCb anomalies, would have a major impact on the motivation and planning of future high-energy colliders. The two most prominent options currently debated are a proton-proton collider at 100 TeV (FCC-hh) and a multi-TeV muon collider (MuC). In this work, we compare the discovery prospects at these colliders on benchmark new physics models indirectly detectable in b → sμ + μ − decays but beyond the reach of the high-p T searches at the HL-LHC. We consider a comprehensive set of scenarios: semileptonic contact interactions, Z′ from a gauged U 1 B 3 − L μ $$ \textrm{U}{(1)}_{B_3-{L}_{\mu }} $$ and U 1 L μ − L τ $$ \textrm{U}{(1)}_{L_{\mu }-{L}_{\tau }} $$ , the scalar leptoquark S 3, and the vector leptoquark U 1. We find that a 3 TeV MuC has a sensitivity reach comparable to the one of the FCC-hh. However, for a heavy enough mediator, the new physics effects at a 3 TeV MuC are only observed indirectly via deviations in the highest energy bin, while the FCC-hh has a greater potential for the discovery of a resonance. Finally, to completely cover the parameter space suggested by the bsμμ anomalies, among the proposed future colliders, only a MuC of 10 TeV (or higher) can meet the challenge.

Abstract We investigate an economical explanation for the (g − 2) μ anomaly with a neutral vector boson from a spontaneously broken U(1) X gauge symmetry. The Standard Model fermion content is minimally extended by 3 right-handed neutrinos. Using a battery of complementary constraints, we perform a thorough investigation of the renormalizable, quark flavor-universal, vector-like U(1) X models, allowing for arbitrary kinetic mixing. Out of 419 models with integer charges not greater than ten, only 7 models are viable solutions, describing a narrow region in model space. These are either L μ − L τ or models with a ratio of electron to baryon number close to −2. The key complementary constraints are from the searches for nonstandard neutrino interactions. Furthermore, we comment on the severe challenges to chiral U(1) X solutions and show the severe constraints on a particularly promising such candidate.

Abstract We develop an economical theoretical framework for combined explanations of the flavor physics anomalies involving muons: (g − 2) μ , R K ∗ $$ {R}_{K^{\left(\ast \right)}} $$ , and b → sμ + μ − angular distributions and branching ratios, that was first initiated by some of us in ref. [1]. The Standard Model (SM) is supplemented with a lepton-flavored U(1) X gauge group. The U(1) X gauge boson with the mass of O $$ \mathcal{O} $$ (0.1) GeV resolves the (g − 2) μ tension. A TeV-scale leptoquark, charged under the U(1) X , carries a muon number and mediates B-decays without prompting charged lepton flavor violation or inducing proton decay. We explore the theory space of the chiral, anomaly-free U(1) X gauge extensions featuring the above scenario, and identify many suitable charge assignments for the SM+3ν R fermion content with the integer charges in the range X F i $$ {X}_{F_i} $$ ∈ [−10, 10]. We then carry out a comprehensive phenomenological study of the muonic force in representative benchmark models. Interestingly, we found models which can resolve the tension without conflicting the complementary constraints, and all of the viable parameter space will be tested in future muonic resonance searches. Finally, the catalog of the anomaly-free lepton-non-universal charge assignments motivated us to explore different directions in model building. We present a model in which the muon mass and the (g − 2) μ are generated radiatively from a common short-distance dynamics after the U(1) X breaking. We also show how to charge a vector leptoquark under U(1) μ−τ in a complete gauge model.

We explore a novel mechanism to restrict TeV-scale leptoquark interactions and render the proton exactly stable to all orders in the effective field theory expansion. A scalar condensate breaks a lepton-flavoured U(1)X gauge symmetry in the ultraviolet and generates neutrino masses, leaving a discrete Z9 or Z18 gauge symmetry in the infrared, forbidding ΔB=1 processes. This provides an elegant framework to address the flavour anomalies and can be adapted to many other new-physics models. The U(1)X can emerge from a gauge–flavour unified SU(12)×SU(2)×SU(2) theory at even higher energies.

Abstract In this work, we reinterpret ATLAS and CMS dijet resonance searches to set robust constraints on all hypothetical tree-level scalar and vector mediators with masses up to 5 TeV, assuming a diquark or a quark-antiquark coupling with an arbitrary flavor composition. To illustrate the application of these general results, we quantify the permissible size of new physics in B ¯ q → D q ∗ + π K $$ {\overline{B}}_q\to {D}_q^{\left(\ast \right)+}\left\{\pi, K\right\} $$ consistent with the absence of signal in dijet resonance searches. Along the way, we perform a full SMEFT analysis of the aforementioned non-leptonic B meson decays at leading-order in α s . Our findings uncover a pressing tension between the new physics explanations of recently reported anomalies in these decays and the dijet resonant searches. The high-p T constraints are crucial to drain the parameter space consistent with the low-p T flavor physics data.

Abstract The high-energy tails of charged- and neutral-current Drell-Yan processes provide important constraints on the light quark and anti-quark parton distribution functions (PDFs) in the large-x region. At the same time, short-distance new physics effects such as those encoded by the Standard Model Effective Field Theory (SMEFT) would induce smooth distortions to the same high-energy Drell-Yan tails. In this work, we assess for the first time the interplay between PDFs and EFT effects for high-mass Drell-Yan processes at the LHC and quantify the impact that the consistent joint determination of PDFs and Wilson coefficients has on the bounds derived for the latter. We consider two well-motivated new physics scenarios: 1) electroweak oblique corrections ( W ̂ , Y ̂ $$ \hat{W},\hat{Y} $$ ) and 2) four-fermion interactions potentially related to the LHCb anomalies in R(K (*)). We account for available Drell-Yan data, both from unfolded cross sections and from searches, and carry out dedicated projections for the High-Luminosity LHC. Our main finding is that, while the interplay between PDFs and EFT effects remains moderate for the current dataset, it will become a significant challenge for EFT analyses at the HL-LHC.

Abstract When a TeV-scale leptoquark has a sizeable Yukawa coupling, its dominant production mechanism at hadron colliders is the partonic-level lepton-quark fusion. Even though the parton distribution functions for leptons inside the proton are minuscule, they get compensated by the resonant enhancement. We present the first computation of higher order radiative corrections to the resonant leptoquark production cross section at the Large Hadron Collider (LHC). Next-to-leading (NLO) QCD and QED corrections are similar in size but come with the opposite sign. We compute NLO K-factors for a wide range of scalar leptoquark masses, as well as, all possible combinations of quark and lepton flavors and leptoquark charges. Theoretical uncertainties due to the renormalisation and factorisation scale variations and the limited knowledge of parton distribution functions are quantified. We finally discuss how to disentangle the flavor structure of leptoquark interactions by exploiting the interplay between different production channels.

Abstract We present a systematic survey of possible short-distance new-physics effects in (semi)leptonic charged- and neutral-current charmed meson decays. Using the Standard Model Effective Field Theory (SMEFT) to analyze the most relevant experimental data at low and high energies, we demonstrate a striking complementarity between charm decays and high invariant mass lepton tails at the LHC. Interestingly enough, high-p T Drell-Yan data offer competitive constraints on most new physics scenarios. Furthermore, the full set of correlated constraints from K, π and τ decays imposed by SU(2) L gauge invariance is considered. The bounds from D (s) decays, high-p T lepton tails and SU(2) L relations chart the space of the SMEFT affecting semi(leptonic) charm flavor transitions.

The Standard Model (SM) is augmented with a U(1)B−3Lμ gauge symmetry spontaneously broken above the TeV scale when an SM-singlet scalar condenses. Scalar leptoquarks S1(3)=(3‾,1(3),13) charged under U(1)B−3Lμ mediate the intriguing effects observed in muon (g−2), RK(⁎), and b→sμ+μ− angular distributions, while generically evading all other phenomenological constraints. The fermionic sector is minimally extended with three right-handed neutrinos, and a successful type-I seesaw mechanism is realized. Charged lepton flavor violation is effectively suppressed, and proton decay—a common prediction of leptoquarks—is postponed to the dimension-6 effective Lagrangian. Unavoidable radiative corrections in the Higgs mass and muon Yukawa favor leptoquark masses interesting for collider searches. The parameters of the model are radiatively stable and can be evolved by the renormalization group to the Planck scale without inconsistencies. Alternative lepton-flavored gauge extensions of the SM, under which leptoquarks become muoquarks, are proposed for comparison.

Abstract We study, from theoretical and phenomenological angles, the Higgs boson oblique parameter Ĥ, as the hallmark of off-shell Higgs physics. Ĥ is defined as the Wilson coefficient of the sole dimension-6 operator that modifies the Higgs boson propagator, within a Universal EFT. Theoretically, we describe self-consistency conditions on Wilson coefficients, derived from the Källén-Lehmann representation. Phenomenologically, we demonstrate that the process gg → h ∗ → V V is insensitive to propagator corrections from Ĥ, and instead advertise four-top production as an effective high-energy probe of off-shell Higgs behaviour, crucial to break flat directions in the EFT.

Abstract 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-p T . 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.

Abstract The “golden” channel, in which the newly-discovered Higgs boson decays to four leptons by means of intermediate vector bosons, is important for determining the properties of the Higgs boson and for searching for subtle new physics effects. Different approaches exist for parametrizing the relevant Higgs couplings in this channel; here we relate the use of pseudo-observables to methods based on specifying the most general amplitude or Lagrangian terms for the HVV interactions. We also provide projections for sensitivity in this channel in several novel scenarios, illustrating the use of pseudo-observables, and analyze the role of kinematic distributions and (ratios of) rates in such H → 4ℓ studies.

Abstract We provide an ultraviolet (UV) complete model for the R(D (∗)) anomalies, in which the additional contribution to semi-tauonic b → c transitions arises from decay to a right-handed sterile neutrino via exchange of a TeV-scale SU(2) L singlet W ′. The model is based on an extension of the Standard Model (SM) hypercharge group, U(1) Y , to the SU(2) V × U(1)′ gauge group, containing several pairs of heavy vector-like fermions. We present a comprehensive phenomenological survey of the model, ranging from the low-energy flavor physics, direct searches at the LHC, to neutrino physics and cosmology. We show that, while the W ′ and Z ′-induced constraints are important, it is possible to find parameter space naturally consistent with all the available data. The sterile neutrino sector also offers rich phenomenology, including possibilities for measurable dark radiation, gamma ray signals, and displaced decays at colliders.

We construct a model of quark–lepton unification at the TeV scale based on an SU(4) gauge symmetry, while still having acceptable neutrino masses and enough suppression in flavor changing neutral currents. An approximate U(2) flavor symmetry is an artifact of family-dependent gauge charges leading to a natural realization of the CKM mixing matrix. The model predicts sizeable violation of PMNS unitarity as well as a gauge vector leptoquark U1μ=(3,1,2/3) which can be produced at the LHC – both effects within the reach of future measurements. In addition, recently reported experimental anomalies in semi-leptonic B-meson decays, both in charged b→cτν and neutral b→sμμ currents, can be accommodated.

Abstract 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-p T 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.

Abstract We present the HiggsPO UFO model for Monte Carlo event generation of electroweak VH and VBF Higgs production processes at NLO in QCD in the formalism of Higgs pseudo-observables (PO). We illustrate the use of this tool by studying the QCD corrections, matched to a parton shower, for several benchmark points in the Higgs PO parameter space. We find that, while being sizable and thus important to be considered in realistic experimental analyses, the QCD higher-order corrections largely factorize. As an additional finding, based on the NLO results, we advocate to consider 2D distributions of the two-jet azimuthal-angle difference and the leading jet $$p_T$$ p T for new physics searches in VBF Higgs production. The HiggsPO UFO model is publicly available.

Abstract Motivated by additional experimental hints of Lepton Flavour Universality violation in B decays, both in charged- and in neutral-current processes, we analyse the ingredients necessary to provide a combined description of these phenomena. By means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons, we show how this is possible. We demonstrate, in particular, how to solve the problems posed by electroweak precision tests and direct searches with a rather natural choice of model parameters, within the context of a U(2) q ×U(2)ℓ flavour symmetry. We further exemplify the general EFT findings by means of simplified models with explicit mediators in the TeV range: coloured scalar or vector leptoquarks and colour-less vectors. Among these, the case of an SU(2) L -singlet vector leptoquark emerges as a particularly simple and successful framework.

Abstract We investigate the impact of flavor-conserving, non-universal quark-lepton contact interactions on the dilepton invariant mass distribution in $$p~p \rightarrow \ell ^+ \ell ^-$$ p p → ℓ + ℓ - processes at the LHC. After recasting the recent ATLAS search performed at 13 TeV with 36.1 fb $$^{-1}$$ - 1 of data, we derive the best up-to-date limits on the full set of 36 chirality-conserving four-fermion operators contributing to the processes and estimate the sensitivity achievable at the HL-LHC. We discuss how these high- $$p_T$$ p T measurements can provide complementary information to the low- $$p_T$$ p T rare meson decays. In particular, we find that the recent hints on lepton-flavor universality violation in $$b \rightarrow s \mu ^+ \mu ^-$$ b → s μ + μ - transitions are already in mild tension with the dimuon spectrum at high- $$p_T$$ p T if the flavor structure follows minimal flavor violation. Even if the mass scale of new physics is well beyond the kinematical reach for on-shell production, the signal in the high- $$p_T$$ p T dilepton tail might still be observed, a fact that has been often overlooked in the present literature. In scenarios where new physics couples predominantly to third generation quarks, instead, the HL-LHC phase is necessary in order to provide valuable information.

The basic features of quark and lepton mass matrices can be successfully explained by natural minima of a generic potential with dynamical Yukawa fields invariant under the [SU(3)]5×O(3) flavor symmetry. If this symmetry is gauged, in order to avoid potentially dangerous Goldstone bosons, and small perturbations are added to exactly fit the observed pattern of fermion masses, the spectrum of massive flavor gauge bosons can naturally explain the hints for new physics in b→sℓ+ℓ− transitions, including RK. In particular, the desired pattern of the Standard Model Yukawa couplings is compatible with a gauged U(1)q in the quark sector, and U(1)μ−τ in the lepton sector spontaneously broken around the TeV scale. In order to explain the aforementioned experimental hints, the corresponding neutral gauge bosons are required to mix, yielding to potentially observable signals in dimuon resonance searches at the LHC.

We confront the indications of lepton flavor universality (LFU) violation observed in semi-tauonic B meson decays with new physics (NP) searches using high pT tau leptons at the LHC. Using effective field theory arguments we correlate possible non-standard contributions to semi-tauonic charged currents with the τ+τ− signature at high energy hadron colliders. Several representative standard model extensions put forward to explain the anomaly are examined in detail: (i) weak triplet of color-neutral vector resonances, (ii) second Higgs doublet and (iii) scalar or (iv) vector leptoquark. We find that, in general, τ+τ− searches pose a serious challenge to NP explanations of the LFU anomaly. Recasting existing 8 TeV and 13 TeV LHC analyses, stringent limits are set on all considered simplified models. Future projections of the τ+τ− constraints as well as caveats in interpreting them within more elaborate models are also discussed.