Flavor Violation Research Articles

Riding the Seesaw: what Higgsstrahlung may reveal about massive neutrinos

We investigate if the projected high-precision measurements of the cross section of the Higgsstrahlung process e^+ e^- rightarrow Zh at a future electron–positron collider can be utilised to indirectly probe the fermionic Seesaw models. We consider the two centre-of-mass energies sqrt{s}=240 GeV and 365 GeV, and compare the collider reaches to constraints from electroweak observables, probes of lepton flavour universality and the existing and prospective bounds from searches for lepton flavour violation. For the analysis we assume the limit of an exactly conserved lepton-number symmetry. We find that while any appreciable correction to the Higgsstrahlung cross section is already strictly constrained in the Type-I Seesaw model, effects of up to {mathcal {O}}(10%) are possible within Type-III Seesaw.

Axion dark matter from lepton flavor-violating decays

We propose simple scenarios where lepton flavor-violating couplings generate the observed dark matter abundance through freeze-in of an axion-like particle with mass in the few keV range. Compared to flavor-diagonal freeze-in, this mechanism enhances dark matter stability, softens stellar cooling constraints and improves the experimental sensitivity of accelerator-based searches. These scenarios can be tested by future X-ray telescopes, and in some cases will be almost entirely probed by new searches for lepton flavor violation at high-intensity experiments such as Mu3e and MEG II.

Constraining lepton flavor violating Higgs couplings at the HL-LHC in the vector boson fusion channel

We explore the parameter space of lepton flavor violating (LFV) neutral Higgs Yukawa couplings with the muon and tau leptons that can be probed at the high-luminosity Large Hadron Collider (HL-LHC) via the vector boson fusion (VBF) Higgs production process. Our projected sensitivities for the Standard Model Higgs ($h$) LFV branching ratio $\text{Br}(h\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau})$ in the $pp\ensuremath{\rightarrow}hjj\ensuremath{\rightarrow}(h\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau})jj$ channel at the HL-LHC are contrasted with the current and future low-energy constraints from the anomalous magnetic moment and electric dipole moment of the muon, as well as with other LFV observables, such as $\ensuremath{\tau}\ensuremath{\rightarrow}3\ensuremath{\mu}$ and $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$. We also study the LFV prospects of a generic beyond the Standard Model neutral Higgs boson ($H$) with a mass in the range of ${m}_{H}\ensuremath{\in}[20,800]\text{ }\text{ }\mathrm{GeV}$ and give the projected model-independent upper limits on the VBF production cross section of $Hjj$ times the branching ratio of $H\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ at the HL-LHC. We interpret these results in the context of a two-Higgs doublet model as a case study.

Variant Nelson-Barr mechanism with minimal flavor violation

Within the general framework of using spontaneous CP violation to solve the strong CP problem, we construct a variant Nelson-Barr model in which the Standard Model (SM) quark contribution to the strong CP phase is cancelled by new heavy QCD-charged fermions. This cancellation is ensured by choosing conjugate representations for the new colored states under the same global flavor symmetry of SM quarks. Choosing the global flavor symmetry to be that of minimal flavor violation, we suppress higher-order corrections to the strong CP phase to well below current experimental constraints. More than two dozen massless Goldstone bosons emerge from spontaneous flavor symmetry breaking, which yield strong astrophysical constraints on the symmetry breaking scale. In the early universe, the Goldstone bosons can be thermally produced from their interactions with the heavy colored fermions and contribute to ∆Neff at a measurable level. As a function of reheating temperature, the predicted ∆Neff shows an interesting plateau behavior we dub the “flavor stairway”, which encodes information about the SM quark flavor structure.

HighPT: A tool for high-pT Drell-Yan tails beyond the standard model

HighPT is a Mathematica package for the analysis of high-energy data of semileptonic transitions at hadron colliders. It allows to compute high-pT tail observables for semileptonic processes, i.e. Drell-Yan cross sections, for dilepton and monolepton final states at the LHC. These observables can be calculated at tree level within the Standard Model Effective Field Theory, including the relevant operators up to dimension eight to ensure a consistent description of the cross section including terms of O(Λ−4) in the cutoff scale Λ. For New Physics models with new mediators that can be resolved at LHC energies, HighPT can also account for the full propagation effects of these new bosonic states at tree level. Using the available data from the high-pT tails in the relevant LHC run-II searches by the ATLAS and CMS collaborations, HighPT can also construct the corresponding likelihoods for all possible flavors of the leptonic final states. As an illustration, we derive and compare constraints on Wilson coefficients at different orders in the Effective Field Theory expansion, and we investigate lepton flavor violation for the S3 leptoquark model. The HighPT code is publicly available at ▪. Program summaryProgram title:HighPTCPC Library link to program files:https://doi.org/10.17632/zh5yx6j4xj.1Developer's repository link:https://github.com/HighPT/HighPTLicensing provisions: MIT licenseProgramming language: MathematicaNature of problem: Constraining New Physics scenarios with high-pT Drell-Yan data.Solution method: We present the first tool allowing to extract bounds, computed at leading order, on semileptonic transitions in the Standard Model Effective Field Theory, including operators up to dimension eight, and in explicit New Physics models.

Zee-model predictions for lepton flavor violation

The Zee model provides a simple model for one-loop Majorana neutrino masses. The new scalars can furthermore explain the long-standing deviation in the muon's magnetic moment and the recent CDF measurement of the W-boson mass. Together, these observations yield predictions for lepton flavor violating processes that are almost entirely testable in the near future. The remaining parameter space makes testable predictions for neutrino masses.

Searches for heavy neutrinos at multi-TeV muon collider: a resonant leptogenesis perspective

In this work, the Standard Model (SM) is extended with two right-handed (RH) neutrinos and two singlet neutral fermions to yield active neutrino masses via (2,2) inverse see-saw mechanism. We first validate the multi-dimensional model parameter space with neutrino oscillation data, obeying the experimental bounds coming from the lepton flavor violating (LFV) decays: mu rightarrow e gamma ,~ tau rightarrow e gamma , ~ tau rightarrow mu gamma . Besides we also search for the portion of the parameter space which yield the observed baryon asymmetry of the universe via resonant leptogenesis. Further, we pick up a few benchmark points from the aforementioned parameter space with TeV scale heavy neutrinos and perform an exhaustive collider analysis of the final states: 2l + E_T !!!!!!/;, and 1,l + 2j + not ! E_T in multi-TeV muon collider.

Commissioning the Muon Campus external beamline: first beam optics measurement

The Fermilab Mu2e experiment aims to observe charged lepton flavor violation through the direct conversion of a muon to an electron. This can be accomplished with the considerable challenge of manipulating and transporting large numbers of protons confined to a narrow phase space region without significant particle losses or deterioration of beam quality before reaching the production target. In this paper we report the first results from the commissioning the beamline upstream the Mu2e production target. We first discuss the evolution of the beam distribution in x-y plane after collecting a series of beam profiles along the line. Then, we discuss the beam evolution in phase-space by measuring the beam's Twiss parameters using two independent techniques. Finally, with the aid of numerical simulations we compare our measured data with design parameters and discuss similarities and differences observed.

Muon g−2 anomaly in a left-right model with an inverse seesaw mechanism

We investigate the possibility of explanation for the muon anomalous magnetic moment ${g}_{\ensuremath{\mu}}\ensuremath{-}2$ in a left-right model with an inverse seesaw mechanism. We emphasize that the observed deviation from the Standard Model predictions can be accommodated in a large part of the parameter space of this class of models, where loops with massive neutrinos and charged Higgs boson as well as the weak $W$ boson contribute significantly to ${g}_{\ensuremath{\mu}}\ensuremath{-}2$. Stringent constraints due to lepton flavor violation $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$, $\ensuremath{\mu}\text{\ensuremath{-}}e$ conversion and the electron anomalous magnetic moment ${g}_{e}\ensuremath{-}2$ are considered, and the results are compatible.

Nuclear-level effective theory of μ→e conversion: Formalism and applications

New mu-to-e conversion searches aim to advance limits on charged lepton flavor violation (CLFV) by four orders of magnitude. By considering P and CP selection rules and the structure of possible charge and current densities, we show that rates are governed by six nuclear responses. To generate a microscopic formulation of these responses, we construct in non-relativistic effective theory (NRET) the CLFV nucleon-level interaction, then embed it in a nucleus. We discuss previous work, noting the lack of a systematic treatment of the various small parameters. Because the momentum transfer is comparable to the inverse nuclear size, a full multipole expansion of the response functions is necessary, a daunting task with Coulomb-distorted electron partial waves. We perform such an expansion to high precision by introducing a simplifying local electron momentum, treating the full set of 16 NRET operators. Previous work has been limited to the simplest charge/spin operators, ignored Coulomb distortion (or alternatively truncated the partial wave expansion) and the nucleon velocity operator, which is responsible for three of the response functions. This generates inconsistencies in the treatment of small parameters. We obtain a "master formula" for mu-to-e conversion that properly treats all such effects and those of the muon velocity. We compute muon-to-electron conversion rates for a series of experimental targets, deriving bounds on the coefficients of the CLFV operators. We discuss the nuclear physics: two types of coherence enhance certain CLFV operators and selection rules blind elastic mu-to-e conversion to others. We discuss the matching of the NRET onto higher level EFTs, and the relation to mu-to-e conversion to other CLFV tests. Finally we describe a publicly available script that can be used to compute mu-to-e conversion rates in nuclear targets.

Nuclear-Level Effective Theory of μ→e Conversion.

The Mu2e and COMET μ→e conversion experiments are expected to significantly advance limits on new sources of charged lepton flavor violation. Almost all theoretical work in the field has focused on just two operators. However, general symmetry arguments lead to a μ→e conversion rate with six response functions, each of which, in principle, is observable by varying nuclear properties of targets. We construct a nucleon-level nonrelativistic effective theory (NRET) to clarify the microscopic origin of these response functions and to relate rate measurements in different targets. This exercise identifies three operators and their small parameters that control the NRET operator expansion. We note inconsistencies in past treatments of these parameters. The NRET is technically challenging, involving 16 operators, several distorted electron partial waves, bound muon upper and lower components, and an exclusive nuclear matrix element. We introduce a trick for treating the electron Coulomb effects accurately, which enables us to include all of these effects while producing transition densities whose one-body matrix elements can be evaluated analytically, greatly simplifying the nuclear physics. We derive bounds on operator coefficients from existing and anticipated μ→e conversion experiments. We discuss how similar NRET formulations have impacted dark matter phenomenology, noting that the tools this community has developed could be adapted for charged lepton flavor violation studies.

A comparative study of Z′ mediated charged lepton flavor violation at future lepton colliders

Charged lepton flavor violation (CLFV) represents a transition between charged leptons of different generations that violates lepton flavor conservation, which is a clear signature of possible new physics beyond the standard model. By exploiting a typical example model of extra Z′ gauge boson, we perform a detailed comparative study on CLFV searches at several future lepton colliders, including a 240 GeV electron-positron collider and a TeV scale muon collider. Based on detailed signal and background Monte-Carlo studies with fast detector simulations, we derive the potentials in searching for Z′ mediated CLFV couplings with eμ, eτ and μτ of different future colliders. The results are compared with the current and prospect limits set by either low-energy experiments or the high-energy LHC experiments. The sensitivity of the τ related CLFV coupling strength at future lepton colliders will be significantly improved in comparison to the current best constraints and the prospect constraints for the μτ channel.

Box-enhanced charged lepton flavor violation in the Grimus-Neufeld model

In the Grimus-Neufeld model (GNM) the neutrino mass generation from an extended Higgs sector leads to bounds for charged lepton flavor violating (cLFV) processes. Here we update bounds from a previous study by extending the parameter space to a nonvanishing Majorana phase of the Pontecorvo-Maki-Nakagawa-Sakata matrix and to heavier charged Higgs boson masses. Three-body cLFV decays are shown to contribute significantly in the large mass regions, as the boxes are enhanced relatively to photonic diagrams. This is in contrast to the smaller mass region studied before, in which the two-body decays tightly restrict the parameter space. The Majorana phase is shown to change limits within 1 order of magnitude. The tiny seesaw scale is assumed, which makes the cLFV decays in the GNM similar to the cLFV decays in the scotogenic model and the scoto-seesaw models.

Phenomenological and cosmological implications of a scotogenic three-loop neutrino mass model

We propose a scotogenic model for generating neutrino masses through a three-loop seesaw. It is a minimally extended inert doublet model with a spontaneously broken global symmetry U(1)′ and a preserved ℤ2 symmetry. The three-loop suppression allows the new particles to have masses at the TeV scale without fine-tuning the Yukawa couplings. The model leads to a rich phenomenology while satisfying all the current constraints imposed by neutrinoless double-beta decay, charged-lepton flavor violation, and electroweak precision observables. The relatively large Yukawa couplings lead to sizable rates for charged lepton flavor violation processes, well within future experimental reach. The model could also successfully explain the W mass anomaly and provides viable fermionic or scalar dark matter candidates.

3HDM with ∆(27) symmetry and its phenomenological consequences

We perform a comprehensive analysis of a version of the 3-Higgs doublet model whose scalar potential is invariant under a global ∆(27) discrete symmetry and where the three scalar doublets are chosen to transform as a triplet under this discrete group. For each of the known tree-level minima we study the mass spectra and use the oblique parameters STU as well as perturbative unitarity to constrain the parameter space of the model. We then discuss phenomenological consequences of some leading order flavour mixing quark Yukawa couplings by considering the flavour violation process b → sγ.We show that perturbative unitarity significantly constrains parameters of the model while, conversely, the beyond the Standard Model contributions to the b → sγ decay are automatically tamed by the symmetry.

Collider signatures of a scalar leptoquark and vectorlike lepton in light of muon anomaly

Despite the immense success of the Standard Model (SM), the hunt for physics beyond the Standard Model (BSM) has continued. Extension of the SM gauge group or the particle content of the SM remains a viable solution to many observed anomalies, such as the anomalous magnetic moment of muon, flavor violation, dark matter, etc. In this work, we consider a BSM model which includes a leptoquark, a vectorlike lepton, and a real scalar singlet. The model accounts for a dark matter candidate through a ${\mathbb{Z}}_{2}$ symmetry in addition to predicting the muon ($g\ensuremath{-}2$) in agreement with the experimental measurement. We also show that the experimental measurements of the lepton flavor violation are satisfied in a wide range of parameter space. The viable parameter space of the model is used to study the collider signatures arising from the pair production of the leptoquark in the $2\ensuremath{\mu}+2b+{\overline{)E}}_{T}$ channel at the 14 TeV LHC run.

Resolving the flavor structure in the MFV-SMEFT

We constrain the flavor structure of Wilson coefficients in the Standard Model Effective Field Theory (SMEFT) from data. In the SMEFT, new physics effects in couplings of up-type and down-type quarks are related through the Cabibbo-Kobayashi-Maskawa mixing matrix. We exploit this relation to pin down potential new sources of flavor symmetry breaking in a global analysis of high- and low-energy data from the LHC, LEP, and b factory experiments. We demonstrate the power of such an analysis by performing a combined fit of effective four-quark and two-quark couplings contributing to a large set of flavor, top-quark, electroweak, and dijet observables. All four sectors are needed to fully resolve the flavor structure of left-chiral four-quark couplings without leaving blind directions in the parameter space. Although we work in the framework of minimal flavor violation, our strategy applies as well to other flavor patterns, like U(2) flavor symmetry or leptoquark scenarios.

Complete beam optics analysis along the Muon Campus External Beamline

The Fermilab Mu2e experiment aims to measure the event ratio of the neutrinoless coherent conversion of muons to electrons in the field of an aluminum nucleus to ordinary capture as a means to probe the possibility of charged lepton flavor violation. Since the number of muons at the Mu2e production target is highly correlated to the upstream beam Twiss parameters, knowledge of these parameters is critical. Such measurement is not trivial since the accelerator line contains several bending magnets which can induce non-negligible dispersion. We discuss a methodology that in addition to the Twiss parameters can extract dispersion, slope of dispersion and momentum spread using only a set of beam profiles. We then apply this technique along the Fermilab Muon Campus and show it can play a significant role in maximizing the muon flux of the Mu2e Experiment.

Lepton-flavor-violating ALPs at the Electron-Ion Collider: a golden opportunity

Axion-like particles (ALPs) arise in a variety of theoretical contexts and can, in general, mediate flavor violating interactions and parity non-conservation. We consider lepton flavor violating ALPs with GeV scale or larger masses which may, for example, arise in composite dark sector models. We show that a future Electron-Ion Collider (EIC) can uncover or constrain such ALPs via processes of the type e AZ→ τ AZa, where AZ is a nucleus of charge Z and a is an ALP in the range mτ≤ ma ≲ 20 GeV. The production of the ALP can have a large Z2 enhancement from low Q2 electromagnetic scattering of the electron from a heavy ion. Using the gold nucleus (Z = 79) as an example, we show that the EIC can explore e − τ flavor violation, mediated by GeV-scale ALPs, well beyond current limits. Importantly, the EIC reach for this interaction is not sensitive to the lepton-flavor conserving ALP couplings, whose possible smallness can render searches using τ decays ineffective. We also discuss how the EIC electron beam polarization can provide a powerful tool for investigating parity violating ALPs.

Rare decays at LHCb

Flavour-changing neutral-current processes, such as b → sℓ + ℓ −, are forbidden at tree level in the Standard Model and hence might receive comparatively large corrections from new particles. This proceeding paper highlights recent measurements from LHCb on b → sℓ + ℓ − and purely-leptonic decays, including tests of lepton flavour universality and searches for lepton flavour violation.