Electroweak Precision Tests Research Articles

Interplay between vector-like lepton and seesaw mechanism: oblique corrections

The non-vanishing neutrino mass strongly hints the existence of right-handed neutrinos (RHNs), singlets of the standard model (SM). However, they are highly decoupled from the SM and difficult to probe. In this work, we consider the Majorana RHNs from the type-I seesaw mechanism may well mix with the heavy neutral lepton dwelling in certain vector-like lepton (VLL), thus acquiring a sizable electroweak charge. Such a simple scenario yields many interesting consequences, and the imprint on oblique corrections, well expected from the mass splitting between components of VLL by virtue of VLL-RHN mixing, is our focus here. We analytically calculate the Peskin-Takeuchi parameters S, T and U with full details, carefully treating the Majorana loop to obtain the self consistent expressions free of divergence. Then, we constrain on the VLL-RHN system which only gives a sizable T parameter using the PDG-2021 data and CDF-II data, separately, by imposing T ≲ 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}(0.1). It is found that for the RHN and VLL below the TeV scale, with a properly large mixing, stands in the frontier of the electroweak precision test such as W-boson mass.

Neutrino masses from new Weinberg-like operators: phenomenology of TeV scalar multiplets

The unique dimension-5 effective operator, LLHH, known as the Weinberg operator, generates tiny Majorana masses for neutrinos after electroweak spontaneous symmetry breaking. If there are new scalar multiplets that take vacuum expectation values (VEVs), they should not be far from the electroweak scale. Consequently, they may generate new dimension-5 Weinberg-like operators which in turn also contribute to Majorana neutrino masses. In this study, we consider scenarios with one or two new scalars up to quintuplet SU(2) representations. We analyse the scalar potentials, studying whether the new VEVs can be induced and therefore are naturally suppressed, as well as the potential existence of pseudo-Nambu-Goldstone bosons. Additionally, we also obtain general limits on the new scalar multiplets from direct searches at colliders, loop corrections to electroweak precision tests and the W-boson mass.

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.

An unfamiliar way to generate the hierarchy of standard model fermion masses

While the properties of the observed Higgs boson agree with the Standard Model predictions, the hierarchy of fermion masses lacks an explanation within the model. In this work, we consider a fresh approach to this problem, involving a different Higgs doublet responsible for each quark mass. We construct a model with a gauged, non-anomalous U(1) family symmetry that fixes which fermion couples to which doublet with an 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}(1) Yukawa coupling. The hierarchy of masses is generated by the hierarchy of vacuum expectation values of the Higgs fields. The model generically predicts a light, weakly coupled pseudoscalar. We verify that the model satisfies constraints from flavour changing neutral currents, Higgs phenomenology and electroweak precision tests.

Heavy neutral lepton corrections to SM boson decays: lepton flavour universality violation in low-scale seesaw realisations

We study lepton flavour universality violation in SM boson decays in low-scale seesaw models of neutrino mass generation, also addressing other electroweak precision observables. We compute the electroweak next-to-leading order corrections, which turn out to be important – notably in the case of the invisible decay width of the Z boson, for which the corrections can be as large as the current experimental uncertainty. As a well-motivated illustrative study case, we choose a realisation of the Inverse Seesaw mechanism, and discuss the complementary role of lepton flavour conserving, lepton flavour violating and precision observables, both in constraining and in probing such models of neutrino mass generation. Our findings suggest that invisible Z decays are especially important, potentially at the origin of the most stringent constraints for certain regimes of the Inverse Seesaw (while complying with charge lepton flavour violation and other electroweak precision tests). We also discuss the probing power of the considered observables in view of the expected improvement in experimental precision at FCC-ee.

Closing in on new chiral leptons at the LHC

We study the phenomenological viability of chiral extensions of the Standard Model, with new chiral fermions acquiring their mass through interactions with a single Higgs. We examine constraints from electroweak precision tests, Higgs physics and direct searches at the LHC. Our analysis indicates that purely chiral scenarios are perturbatively excluded by the combination of Higgs coupling measurements and LHC direct searches. However, allowing for a partial contribution from vector-like masses opens up the parameter space and non-decoupled exotic leptons could account for the observed 2σ deviation in h → Zγ. This scenario will be further tested in the high-luminosity phase of the LHC.

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.

A new viable mass region of dark matter and dirac neutrino mass generation in a scotogenic extension of SM

In this paper, we propose a scotogenic extension of the Standard Model (SM) which can provide a scalar dark matter (DM) candidate in the new, theoretically previously unaddressed, intermediate region ([Formula: see text][Formula: see text]GeV) and also generate light Dirac neutrino masses. In this framework, the SM is extended by three gauge singlet fermions, two singlet scalar fields and one additional scalar doublet along with a continuous global symmetry [Formula: see text] and discrete symmetries [Formula: see text] and [Formula: see text]. These additional symmetries prevent the singlet fermions from obtaining Majorana mass terms along with providing the stability to the DM candidate. It is known that in the case of the scalar singlet DM model, the only region which is not yet excluded is a narrow region close to the Higgs resonance [Formula: see text] — others ruled out from different experimental and theoretical bounds. In the case of the inert doublet model, the mass region ([Formula: see text]–80[Formula: see text]GeV) and the high-mass region (heavier than 550[Formula: see text]GeV) are allowed. This motivates us to explore a parameter range in the intermediate-mass region [Formula: see text][Formula: see text]GeV, which we do in a scotogenic extension of SM with a scalar doublet and scalar singlets. The DM in our model is a mixture of singlet and doublet scalars, in the freeze-out scenario. We constrain the allowed parameter space of the model using the Planck bound on present DM relic abundance, neutrino mass and the latest bound on spin-independent DM-nucleon scattering cross-section from the XENON1T experiment. Further, we constrain the DM parameters from the indirect detection bounds arising from the global analysis of the Fermi-LAT observations of dwarf spheroidal satellite (dSphs) galaxies, Higgs invisible decay and Electroweak Precision Test (EWPT) as well. We find that our model findings may provide a viable DM candidate satisfying all the constraints on DM parameters in the new, previously unexplored mass range ([Formula: see text][Formula: see text]GeV). This new window for the DM candidate could be searched in future experiments along with an explanation of the Dirac mass of neutrinos since so far there is no strong evidence in support of the Majorana nature of neutrino mass.

Electroweak precision tests for triplet scalars

Electroweak precision observables are fundamentally important for testing the standard model (SM) or its extensions. The influences to observables from new physics within the electroweak sector can be expressed in terms of oblique parameters S, T, U. The recently reported W mass excess anomaly by CDF modifies these parameters in a significant way. By performing the global fit with the CDF new W mass measurement data, we obtain $S=0.03 \pm 0.03$, $T=0.06 \pm 0.02$ and $U=0.16 \pm 0.03$ (or $S=0.14 \pm 0.03$, $T=0.24 \pm 0.02$ with $U=0$) which is significantly away from zero as SM would predict. The CDF excess strongly indicates the need of new physics beyond SM. We carry out a global fit to study the influence of two different cases of simple extensions by a hyper-charge $Y=1$ triplet scalar $\Delta$ (corresponding to the type-II seesaw model) and a $Y=0$ real triplet scalar $\Sigma$ on electroweak precision tests to determine parameter space in these models to solve the W mass anomaly and discuss the implications. We find that these triplets can affect the oblique parameters significantly at the tree and loop levels.

331 model predictions for rare B and K decays, and ∆F = 2 processes: an update

Motivated by the improved results from the HPQCD lattice collaboration on the hadronic matrix elements entering ∆Ms,d in {B}_{s,d}^0-{overline{B}}_{s,d}^0 mixings and the increase of the experimental branching ratio for Bs→ μ+μ−, we update our 2016 analysis of various flavour observables in four 331 models, M1, M3, M13 and M16 based on the gauge group SU(3)C× SU(3)L× U(1)X. These four models, which are distinguished by the quantum numbers, are selected among 24 331 models through their consistency with the electroweak precision tests and simultaneously by the relation {C}_9^{textrm{NP}}=-{bC}_{10}^{textrm{NP}} with 2 ≤ b ≤ 5, which after new result on Bs→ μ+μ− from CMS is favoured over the popular relation {C}_9^{textrm{NP}}=-{C}_{10}^{textrm{NP}} predicted by several leptoquark models. In this context we investigate in particular the dependence of various observables on |Vcb|, varying it in the broad range [0.0386, 0.043], that encompasses both its inclusive and exclusive determinations. Imposing the experimental constraints from εK, ∆Ms, ∆Md and the mixing induced CP asymmetries {S}_{psi {K}_S} and {S}_{psi {K}_S} , we investigate for which values of |Vcb| the four models can be made compatible with these data and what is the impact on B and K branching ratios. In particular we analyse NP contributions to the Wilson coefficients C9 and C10 and the decays Bs,d→ μ+μ−, {K}^{+}to {pi}^{+}nu overline{nu} and {K}_Lto {pi}^0nu overline{nu} . This allows us to illustrate how the value of |Vcb| determined together with other parameters of these models is infected by NP contributions and compare it with the one obtained recently under the assumption of the absence of NP in εK, ∆Ms, ∆Md and {S}_{psi {K}_S} .

A scotogenic model with two inert doublets

In this work, we present a scotogenic model, where the neutrino mass is generated at one-loop diagrams. The standard model (SM) is extended by three singlet Majorana fermions and two inert scalar doublets instead of one doublet as in the minimal scotogenic model. The model scalar sector includes two CP-even, two CP-odd and two charged scalars in addition to the Higgs. The dark matter (DM) candidate could be either the light Majorana fermion (Majorana DM), or the lightest among the CP-even and the CP-odd scalars (scalar DM). We show that the model accommodates both Majorana and scalar DM within a significant viable parameter space, while considering all the relevant theoretical and experimental constraints such as perturbativity, vacuum stability, unitarity, the di-photon Higgs decay, electroweak precision tests and lepton flavor violating constraints. In addition to the collider signatures predicted by the minimal scotogenic model, our model predicts some novel signatures that can be probed through some final states such as .

Constraining the Georgi-Machacek model with a light Higgs boson

In this work, we investigate the viability of a light Higgs ($\eta$) scenario in the Georgi-Machacek (GM) model, where we consider all theoretical and experimental constraints such as the perturbativity, vacuum stability, unitarity, electroweak precision tests, the Higgs di-photon and undetermined decays and the Higgs total decay width. In addition, we consider more recent experimental bounds from the searches for doubly-charged Higgs bosons in the VBF channel $H_{5}^{++}\rightarrow W^{+}W^{+}$, Drell-Yan production of a neutral Higgs boson $pp\rightarrow H_{5}^{0}(\gamma\gamma)H_{5}^{+}$, and for the light scalars at LEP $e^{-}e^{+}\rightarrow Z\eta$, and at ATLAS and CMS in different final states such as $pp\rightarrow\eta\rightarrow2\gamma$ and $pp\rightarrow h\rightarrow\eta\eta\rightarrow4\gamma,2\mu2\tau,2\mu2b,2\tau2b$. By combining these bounds together, we found a parameter space region that is significant as the case of the SM-like Higgs to be the light CP-even eigenstate, and this part of the parameter space would be tightened by the coming analyses.

Precision μ+μ+ and μ+e− elastic scatterings

Abstract The expected measurement precisions of elastic scattering cross sections are estimated for μ+μ+ and μ+e− colliders, which have recently been proposed as future realistic possibilities (μTRISTAN). Compared with contributions from possible new physics represented by higher-dimensional operators, we find that the measurements at a TeV energy μ+μ+ collider can probe the scale of new physics up to O(100) TeV. A μ+e− collider for the Higgs boson factory can also improve the electroweak precision test. For those studies, we assume the expected integrated luminosity at μTRISTAN, Lint = 120 fb−1 (μ+μ+) and 1 ab−1 (μ+e−).

Comprehensive analysis of charged lepton flavour violation in the symmetry protected type-I seesaw

The type-I seesaw model is probably the most straightforward and best studied extension of the Standard Model that can account for the tiny active neutrino masses determined from neutrino oscillation data. In this article, we calculate the complete set of one-loop corrections to charged lepton flavour violating processes within this model. We give the results both using exact diagonalisation of the neutrino mass matrix, and at leading order in the seesaw expansion (i.e. mathcal{O} (v2/ {M}_R^2 )). Furthermore, we perform the matching onto the SU(2)L invariant Standard Model Effective Field Theory at the dimension 6 level. These results can be used as initial conditions for the renormalisation group evolution from the right-handed neutrino scale down to the scale of the physical processes, which resums large logarithms. In our numerical analysis, we study the inverse seesaw limit, i.e. the symmetry protected type-I seesaw, where the Wilson coefficient of the Weinberg operator is zero such that sizeable neutrino Yukawas are permissible and relevant effects in charged lepton flavour violating observables are possible. We correlate the different charged lepton flavour violating processes, e.g. ℓ → ℓ′γ, ℓ → 3ℓ′, μ → e conversion and Z → ℓℓ′, taking into account the constraints from electroweak precision observables and tests of lepton flavour universality.

Dark matter in a singlet-extended inert Higgs-doublet model

In this work, we consider an extension of the Standard Model with an inert Higgs doublet and a real scalar singlet, in order to address problems around the origin of dark matter (DM). In this model, the lightest among the $CP$-odd and $CP$-even neutral inert components plays the role of a DM candidate, where the model parameters are subject to many theoretical and experimental constraints. These constraints include vacuum stability, perturbativity, LEP negative searches, electroweak precision tests, Higgs diphoton, Higgs invisible and Higgs undetermined decays, DM relic density, and DM direct detection bounds. Using these constraints, we find that the allowed parameter space for these models is quite sizable and could be explored in upcoming collider and astrophysical searches.

Corrections to electroweak precision observables from mixings of an exotic vector boson in light of the CDF W -mass anomaly

We enumerate various effective couplings that contribute to the mixings between an exotic vector boson $Z^{\prime}$ and the neutral electroweak vector bosons. The miscellaneous mixing patterns can be evaluated perturbatively. The effective oblique parameters $S^{\prime}$, $T^{\prime}$, and $U^{\prime}$ are calculated to compare with the electroweak precision test results. With the contributions to the non-negligible $U^{\prime}$ parameter from the $\epsilon_{B,W}$ parameters and the aid of some other parameters to cancel the negative $T^{\prime}$, the recent CDF $W$-mass anomaly can therefore be explained.

W -boson mass, electroweak precision tests, and SMEFT

Recently the CDF collaboration at the Tevatron reported a significant discrepancy between the direct measurement of the $W$-boson mass and its Standard Model (SM) prediction based on electroweak precision tests (EWPTs). In this paper we explore the potential origin of this discrepancy from physics beyond the SM. Explicitly, we work on a set of six-dimensional operators in the SM effective field theory (SMEFT) which are relevant to the EWPTs. By fitting to the data, we demonstrate that an upward shift in $m_W$ is driven by the operator $\mathcal{O}_{T}=\frac{1}{2}(H^{\dagger}\overset{\text{$\leftrightarrow$}}{D}_{\mu}H)^2$ with a coefficient $c_T ({\rm TeV}/\Lambda)^2 \gtrsim 0.01$. This suggests that the new physics scale favored by the CDF data should be multiple TeV for tree-level effects and sub TeV for loop-level effects. One simple example is to introduce a hypercharge-free electroweak triplet scalar which can raise the $c_T$ value at tree level. We also study the potential to further test the relevant SMEFT by measuring Higgs-coupling, $m_W$ and other EWPTs at future circular $e^-e^+$ colliders.

W-mass anomaly in the simplest linear seesaw mechanism

The simplest linear seesaw mechanism can accommodate the new CDF-II W mass measurement. In addition to Standard Model particles, the model includes quasi-Dirac leptons, and a second, leptophilic, scalar doublet seeding small neutrino masses. Our proposal is consistent with electroweak precision tests, neutrino physics, rare decays and collider restrictions, requiring a new charged scalar below a few TeV, split in mass from the new degenerate scalar and pseudoscalar neutral Higgs bosons.

Gravitational-wave signatures of chiral-symmetric technicolor

A chiral-symmetric technicolor model successfully reconciles the tension between electroweak precision tests and traditional technicolor models. Focusing on its simplest realization preserving the conventional Higgs mechanism, we study its primordial gravitational wave signatures originating from first order phase transitions in the early Universe. We found that abundant phase transition patterns arise from a physically viable parameter space. Besides, we have also found gravitational wave signals possibly visible by future experiments, such as LISA, BBO and u-DECIGO. Our results stress the importance of gravitational wave detectors in exploring new physics complementary to ground colliders in the multi-messenger astronomy era.

Road map through the desert with scalars

In the context of the gauge coupling unification, we present a comprehensive analysis of the extensions of the Standard Model with vector-like fermions and scalars. We find 145 models that satisfy the unification condition, which are distinguishable by the number of new particles in the spectrum and by their transformation properties under the gauge symmetry group of the Standard Model. For all models we derive lower bounds on the exotic fermion and scalar masses, stemming from the measurement of the strong gauge coupling scale dependence, from the heavy stable charged particle searches, and from the electroweak precision tests. We also discuss the potential of testing the unification scenarios at the future 100 TeV collider and in the proton decay experiments. We show that many models can already be excluded based on the current data, while many others will be entirely probed in the coming years.