Low-energy Observables Research Articles

Neutrinoless double beta decay from scalar leptoquarks: interplay with neutrino mass and flavor physics

We perform a comprehensive analysis of neutrinoless double beta (0νββ) decay and its interplay with low-energy flavor observables in a radiative neutrino mass model with scalar leptoquarks S1(3¯, 1, 1/3) and R~2321/6. We carve out the parameter region consistent with constraints from neutrino mass and mixing, collider searches, as well as measurements of several flavor observables, such as muon and electron anomalous magnetic moments, charged lepton flavor violation and rare (semi)leptonic kaon and B-meson decays, including the recent anomalies in RD∗ and B→Kνν¯ observables. We perform a global analysis to all the existing constraints and show the (anti)correlations between all relevant Yukawa couplings satisfying these restrictions. We find that the most stringent constraint on the parameter space comes from μ → e conversion in nuclei and K+→π+νν¯ decay. We also point out a tension between the muon and electron (g – 2) anomalies in this context. Taking benchmark values from the combined allowed regions, we study the implications for 0νββ decay including both the canonical light neutrino and the leptoquark contributions. We find that for normal ordering of neutrino masses, the leptoquark contribution removes the cancellation region that occurs for the canonical case. The effective mass in presence of leptoquarks can lie in the desert region between the standard normal and inverted ordering cases, and this can be probed in future ton-scale experiments like LEGEND-1000 and nEXO.

Positivity bounds on massive vectors

In this paper, we explore positivity bounds for the effective field theory (EFT) of a single weakly coupled massive vector field. The presence of both mass and spin makes the crossing properties of the amplitudes vastly complicated — we address this by parametrizing the amplitudes as products of a polarization matrix and a vector of appropriately chosen functions with simpler crossing properties. The resulting framework involves sum rules and null constraints that allows us to constrain any combination of low-energy observables, such as EFT amplitudes. By varying the value of the vector mass over the cutoff scale, some of our bounds asymptote to the bounds obtained in the context of photons and massless scalars. This work paves the way for future applications to e.g. non-abelian massive vectors, glueballs and theories with spin larger than one.

Dimension-8 SMEFT contact-terms for vector-pair production via on-shell Higgsing

We derive the dimension-8 standard-model effective theory (SMEFT) contact terms relevant for vector-pair production at the LHC and lepton colliders. We first list the relevant dimension-8 massless SMEFT amplitudes, and then obtain the low-energy amplitudes using on-shell Higgsing. In all cases, the contributions we calculate are the leading-order contributions to 4-point contact-terms; the dimension-6 SMEFT merely corrects the three-point couplings entering the amplitudes. Since they are given in terms of physical quantities, namely momenta and polarizations, the results allow for a direct mapping of EFT effects to low-energy observables. The vector amplitudes are sensitive to both anomalous vector couplings and Higgs self-couplings. The left-handed fermion amplitudes feature SU(2) violating effects first generated at dimension-8. We also compare our results to HEFT predictions. Interestingly, the dimension-8 SMEFT populates almost all the novel structures generated by the dimension-8 HEFT.

CPT-Odd effects on the electromagnetic properties of charged leptons in the Standard Model Extension

Abstract The impact of the CPT-Odd electroweak gauge sector of the Standard Model Extension on the magnetic dipole moment of charged leptons is studied. This gauge sector is characterized by the (k1)µ and (k2)µ Lorentz coefficients, which have positive mass dimension because they are associated with a UY (1)-invariant and with an SUL(2)-invariant dimension-three operators, respectively. They belong to the category of relevant interactions, which have strong effects on low-energy observables. We present a comprehensive study on the impact of this sector on the magnetic dipole moment of charged leptons, up to second order in these Lorentz coefficients, both at the tree and one-loop levels. We find that the first-order contributions in the Lorentz coefficients at the tree and one-loop levels depend on energy, while second-order ones at the tree-level do not. As for second-order one-loop effects, there are both energy-dependent and energy-independent contributions, but we have focused only on those of the latter type. We find that the tree-level contributions are suppressed with respect to the one-loop ones by at least a factor of (m2l /m2Z). We find that the contribution to the dipole associated with the electron is by far the dominant one, as it can be up to fourteen orders of magnitude greater than that of the muon and up to sixteen orders greater than that of the tau. The Lorentz coefficient (kAF )µ of the Carroll-Field-Jackiw’s QED is given by a linear combination of the (k1)µ and (k2)µ vectors. Assuming thatcollinear, we obtain an upper bound of|k12|,|k22| ≫ |k2AF| and taking (kAF)µ = 0, which implies that (|||−(3.49 × 10-4)(k2)0me + k2 | k1)µ and (k2)µ are.

Extraordinary Nature of the Nucleon Scalar Charge and Its Densities as a Signal of Nontrivial Vacuum Structure of QCD

It is widely known that the nucleon scalar charge is proportional to the pion–nucleon sigma term as one of the important low-energy observables of QCD. Especially interesting to us is the physics of the nucleon scalar charge densities. This comes from the fact that the corresponding operator has the same quantum number as the physical vacuum. It indicates unusual behavior of the nucleon scalar density as a function of the distance r from the nucleon center. Namely, it would not be reduced down to zero at the spatial infinity but rather approach some nonzero constant corresponding to the vacuum quark condensate. Naturally, this unique nature of the nucleon scalar density in the position space also affects the corresponding density in the momentum space, i.e., the corresponding parton distribution function (PDF) as a function of the Bjorken variable x. This PDF is known as the chiral-odd twist-3 PDF e(x). We argue that e(x) is likely to have a delta-function-type singularity at x=0 and that the appearance of this singularity can be interpreted as a signal of the nontrivial vacuum structure of QCD.

Phonons reveal coupled cholesterol-lipid dynamics in ternary membranes

Experimental studies of collective dynamics in lipid bilayers have been challenging due to the energy resolution required to observe these low-energy phonon-like modes. However, inelastic x-ray scattering (IXS) measurements—a technique for probing vibrations in soft and biological materials—are now possible with sub-meV resolution, permitting direct observation of low-energy, phonon-like modes in lipid membranes. Here, IXS measurements with sub-meV energy resolution reveal a low-energy optic-like phonon mode at roughly 3 meV in the liquid-ordered (Lo) and liquid-disordered phases of a ternary lipid mixture. This mode is only observed experimentally at momentum transfers greater than 5 nm−1 in the Lo system. A similar gapped mode is also observed in all-atom molecular dynamics (MD) simulations of the same mixture, indicating that the simulations accurately represent the fast, collective dynamics in the Lo phase. Its optical nature and the Q range of the gap together suggest that the observed mode is due to the coupled motion of cholesterol-lipid pairs, separated by several hydrocarbon chains within the membrane plane. Analysis of the simulations provides molecular insight into the origin of the mode in transient, nanoscale substructures of hexagonally packed hydrocarbon chains. This nanoscale hexagonal packing was previously reported based on MD simulations and, later, by NMR measurements. Here, however, the integration of IXS and MD simulations identifies a new signature of the Lo substructure in the collective lipid dynamics, thanks to the recent confluence of IXS sensitivity and MD simulation capabilities.

The stringy S-matrix bootstrap: maximal spin and superpolynomial softness

We explore the space of meromorphic amplitudes with extra constraints coming from the shape of the leading Regge trajectory. This information comes in two guises: it bounds the maximal spin of exchanged particles of a given mass; it leads to sum rules obeyed by the discontinuity of the amplitude, which express the softness of scattering at high energies. We assume that the leading Regge trajectory is linear, and we derive bounds on the low-energy Wilson coefficients using the dual and primal approaches. For the graviton-graviton scattering in four dimensions, the maximal spin constraint leads to slightly more stringent bounds than those that follow from general constraints of analyticity, crossing, and unitarity. The exponential softness at high energies is manifest in our primal approach and is not used in our implementation of the dual approach. Nevertheless, we observe the agreement between the bounds obtained from both. We conclude that high-energy superpolynomial softness does not leave an obvious imprint on the low-energy observables. We exhibit a unitary three-parameter deformation of the Veneziano amplitude for the open string case. It has a novel, exponentially soft behavior at high energies and fixed angles. We generalize the previous analysis of this regime and present a stringy version of the lower bound on high-energy, fixed-angle scattering by Cerulus and Martin.

Lepton flavor violation in exclusive b→dℓiℓj and b→sℓiℓj decay modes

We discuss the exclusive lepton flavor violating (LFV) decays modes based on b→dℓiℓj and b→sℓiℓj by considering the ground state mesons and baryons. After spelling out the expressions for such decay rates in a low energy effective theory which includes generic contributions arising from physics beyond the Standard Model, we show that the experimental bounds on meson decays can be used to bound the corresponding modes involving baryons. We find, for example, B(Λb→Λμτ)≲4×10−5. We also consider two specific models and constrain the relevant LFV couplings by using the low energy observables. In the first model, we assume the Higgs mediated LFV and find the resulting decay rates to be too small to be experimentally detectable. We also emphasize that the regions favored by the bounds B(h→μτ)Atlas and B(h→eτ)Atlas are not compatible with B(μ→eγ)MEG to 1σ. In the second model, we assume LFV mediated by a heavy Z′ boson and find that the corresponding b-hadron branching fractions can be O(10−6), thus possibly within experimental reach at LHCb and Belle II. Published by the American Physical Society 2024

Anomalous dimensions via on-shell methods: Operator mixing and leading mass effects

We elaborate on the application of on shell and unitarity-based methods for evaluating renormalization group coefficients, and generalize this framework to account for the mixing of operators with different dimensions and leading mass effects. We derive a master formula for anomalous dimensions stemming from the general structure of operator mixings, up to two-loop order, and show how the Higgs low-energy theorem can be exploited to include leading mass effects. A few applications on the renormalization properties of popular effective field theories showcase the strength of the proposed approach, which drastically reduces the complexity of standard loop calculations. Our results provide a powerful tool to interpret experimental measurements of low-energy observables, such as flavor violating processes or electric and magnetic dipole moments, as induced by new physics emerging above the electroweak scale. Published by the American Physical Society 2024

Supernova limits on QCD axionlike particles

In this paper, we explore the phenomenology of massive axionlike particles (ALPs) coupled to quarks and gluons, dubbed “QCD ALPs,” with an emphasis on the associated low-energy observables. ALPs coupled to gluons and quarks not only induce nuclear interactions at scales below the QCD scale, relevant for ALP production in supernovae (SNe), but naturally also couple to photons similarly to the QCD axion. We discuss the link between the high-energy formulation of ALP theories and their effective couplings with nucleons and photons. The induced photon coupling allows ALPs with masses ma≳1 MeV to efficiently decay into photons, and astrophysical observables severely constrain the ALP parameter space. We show that a combination of arguments related to SN events rule out ALP-nucleon couplings down to gaN≳10−11–10−10 for ma≳1 MeV—a region of the parameter space that was hitherto unconstrained. Published by the American Physical Society 2024

Weinberg, effective field theories, and time-reversal violation

I offer some glimpses of Steven Weinberg as a mentor, taking as example his influential work on time-reversal violation. I also review some of the implications of this work to low-energy observables using nuclear effective field theories.

Probing lepton number violation: a comprehensive survey of dimension-7 SMEFT

Observation of lepton number violation would represent a groundbreaking discovery with profound consequences for fundamental physics and as such, it has motivated an extensive experimental program searching for neutrinoless double beta decay. However, the violation of lepton number can be also tested by a variety of other observables. We focus on the possibilities of probing this fundamental symmetry within the framework of the Standard Model Effective Field Theory (SMEFT) beyond the minimal dimension-5. Specifically, we study the bounds on ∆L = 2 dimension-7 effective operators beyond the electron flavor imposed by all relevant low-energy observables and confront them with derived high-energy collider limits. We also discuss how the synergy of the analyzed multi-frontier observables can play a crucial role in distinguishing among different dimension-7 SMEFT operators.

Neutrino model with broken [formula omitted] -[formula omitted] symmetry and unflavored leptogenesis with dihedral flavor symmetry

We propose a new neutrino flavor model based on a D4×U(1) flavor symmetry providing predictions for neutrino masses and mixing along with a successful generation of the observed Baryon Asymmetry of the Universe (BAU). After the spontaneous breaking of the flavor symmetry, the type I seesaw mechanism leads to a light neutrino mass matrix with broken μ−τ symmetry. By performing a numerical analysis, we find that the model favors a normal mass hierarchy with the lightest neutrino mass lies in the range m1∈[2.516,21.351] meV. The phenomenological implications of the neutrino sector are explored in detail and the results are discussed. Moreover, the generation of BAU is addressed via the leptogenesis mechanism from the decay of three right-handed neutrinos Ni. Through analytical and numerical analysis of the baryon asymmetry parameter YB, a successful unflavored leptogenesis takes place within the allowed parameter space obtained from neutrino phenomenology. We also examine interesting correlations between YB and low energy observables and provide a comprehensive discussion of the results.

Dark showers from Z-dark Z′ mixing

We discuss dark shower signals at the LHC from a dark QCD sector, containing GeV-scale dark pions. The portal with the Standard Model is given by the mixing of the Z boson with a dark Z′ coupled to the dark quarks. Both mass and kinetic mixings are included, but the mass mixing is the essential ingredient, as it is the one mediating visible decays of the long-lived dark pions. We focus especially on the possibility that the dark Z′ is lighter than the Z. Indirect constraints are dominated by electroweak precision tests, which we thoroughly discuss, showing that both Z-pole and low-energy observables are important. We then recast CMS and LHCb searches for displaced dimuon resonances to dark shower signals initiated by the production of on-shell Z or Z′, where the visible signature is left by a dark pion decaying to μ+μ−. We demonstrate how dark shower topologies have already tested new parameter space in Run 2, reaching better sensitivity on a light dark Z′ compared to the flavor-changing decays of B mesons, which can produce a single dark pion at a time, and the electroweak precision tests.

Pauli–Villars Regularization of Kaluza–Klein Casimir Energy with Lorentz Symmetry

Abstract The Pauli–Villars regularization is appropriate to discuss the UV sensitivity of low-energy observables because it mimics how the contributions of new particles at high energies cancel large quantum corrections from the light particles in the effective field theory. We discuss the UV sensitivity of the Casimir energy density and pressure in an extra-dimensional model in this regularization scheme, and clarify the condition on the regulator fields to preserve the Lorentz symmetry of the vacuum state. Some of the conditions are automatically satisfied in spontaneously broken supersymmetric models, but supersymmetry is not enough to ensure the Lorentz symmetry. We show that the necessary regulators can be introduced as bulk fields. We also evaluate the Casimir energy density with such regulators, and its deviation from the result obtained in the analytic regularization.

The chiral Lagrangian of CP-violating axion-like particles

We discuss the construction of the most general CP-violating chiral Lagrangian for an axion-like particle (ALP). Starting with an effective Lagrangian containing light quarks and gluons, we provide its matching onto a chiral effective Lagrangian at O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(p2) described in terms of mesons and baryons, identifying the correspondence between the Jarlskog invariants of the two theories. After deriving the ALP interactions with mesons and baryons, we analyse a few relevant phenomenological implications such as the permanent electric dipole moments of nucleons and the CP-violating ALP and kaon decays. This work provides the necessary tools for further phenomenological analyses connecting low-energy observables with the couplings of the underlying ultraviolet complete theory.

B meson anomalies within the triplet vector boson model to the light of recent measurements from LHCb

The triplet vector boson (TVB) is a simplified new physics model involving massive vector bosons transforming as a weak triplet vector. Such a model has been proposed as a combined explanation of the anomalous b→sμ+μ− and b→cτν¯τ data (the so-called B meson anomalies). In this work, we carry out a revamped view of the TVB model by incorporating the most recent 2022 and 2023 LHCb measurements on the lepton-flavor universality ratios R(D(*))=BR(B→D(*)τν¯τ)/BR(B→D(*)ℓ′ν¯ℓ′), R(Λc)=BR(Λb→Λcτν¯τ)/BR(Λb→Λcμν¯μ), and RK(*)=BR(B→K(*)μ+μ−)/BR(B→K(*)e+e−). We perform a global fit to explore the allowed parameter space by the new data and all relevant low-energy flavor observables including the recent experimental progress from Belle, Belle II, and LHCb. Our results are confronted with the recent high-mass dilepton searches at the LHC. We find that for a heavy TVB mass of 1 TeV a common explanation of the B meson anomalies is possible for all data with the recent LHCb measurements on R(D(*)), consistent with LHC constraints. However, this framework is in strong tension with LHC bounds when one considers all data along with the world average values reported by the Heavy Flavor Averaging Group (, Belle, and LHCb) on R(D(*)). Future measurements will be required in order to clarify such a situation. Published by the American Physical Society 2024

Correlation function for the Tbb state: Determination of the binding, scattering lengths, effective ranges, and molecular probabilities

We perform a study of the B*+B0,B*0B+ correlation functions using an extension of the local hidden gauge approach which provides the interaction from the exchange of light vector mesons and gives rise to a bound state of these components in I=0 with a binding energy of about 21 MeV. After that, we face the inverse problem of determining the low energy observables, scattering length and effective range for each channel, the possible existence of a bound state, and, if found, the couplings of such a state to each B*+B0,B*0B+ component as well as the molecular probabilities of each of the channels. We use the bootstrap method to determine these magnitudes and find that, with errors in the correlation function typical of present experiments, we can determine all these magnitudes with acceptable precision. In addition, the size of the source function of the experiment from where the correlation functions are measured can be also determined with a high precision. Published by the American Physical Society 2024

Indirect constraints on top quark operators from a global SMEFT analysis

We perform a model-independent analysis of top-philic New Physics scenarios, under the assumption that only effective operators involving top quarks are generated at tree level. Within the SMEFT framework, we derive indirect constraints on Wilson Coefficients by combining a large set of low-energy observables: B-meson and kaon decays, meson mixing observables, precision electroweak and Higgs measurements, anomalous magnetic moments, lepton flavour violating processes, lepton flavour universality tests, and measurements of the Cabibbo angle. We consider the renormalization group evolution of the operators and use the one-loop matching of the SMEFT onto the LEFT. The global analysis is then used to perform one-parameter, two-parameter, and global fits, as well as applications to explicit ultraviolet models. We find that the inclusion of measurements from different physics sectors reveals a strong interplay and complementarity among the observables. The resulting constraints are also compared to direct bounds provided by top quark productions at the LHC.

Exploring inelasticity in the S-matrix Bootstrap

The modern S-Matrix Bootstrap provides non-perturbative bounds on low-energy aspects of scattering amplitudes, leveraging the constraints of unitarity, analyticity and crossing. Typically, the solutions saturating such bounds also saturate the unitarity constraint as much as possible, meaning that they are almost exclusively elastic. This is expected to be unphysical in d>2 because of Aks' theorem. We explore this issue by adding inelasticity as an additional input, both using a primal approach in general dimensions which extends the usual ansatz, and establishing a dual formulation in the 2d case. We then measure the effects on the low-energy observables where we observe stronger bounds than in the standard setup.