Electroweak Precision Data Research Articles

A 17 MeV pseudoscalar and the LSND, MiniBooNE and ATOMKI anomalies

In the absence of any new physics signals at the Large Hadron Collider (LHC), anomalous results at low energy experiments have become the subject of increased attention. We focus on three such results from the LSND, MiniBooNE (MB), and ATOMKI experiments. A 17 MeV pseudoscalar mediator (a′) can account for two (8Be and 4He) out of the three cases in which excess events have been seen in pair creation transitions in ATOMKI. We incorporate this mediator in a gauge invariant extension of the Standard Model (SM) with a second Higgs doublet and three singlet (seesaw) neutrinos (Ni, i = 1, 2, 3). N1,2 participate in an interaction in MB and LSND which, with a′ as mediator, leads to the production of e+e− pairs. The Ni also lead to mass-squared differences for SM neutrinos in agreement with global oscillation data. We first show that such a model offers a natural joint solution to the MB and LSND excesses, providing excellent fits to their data. Next, using the values of the couplings to the quarks and electrons which are required to explain pair creation nuclear transition data for 8Be and 4He in ATOMKI, we show that these values still lead to fits for MB and LSND data. However, once ATOMKI is incorporated, we find that strong constraints from the decays K+ → π+a′ (a′ → e+e−) and π+ → e+νee+e− come into play. While our solution is in conformity with the bounds on the former decay, it remains in tension with 90% CL bounds on the latter. We also discuss other constraints from both collider and non-collider experiments and from electroweak precision data, stability and unitarity. We compute the contributions to the electron and muon g – 2 up to two loops for our model. We discuss tests of the model in upcoming experiments.

Non-universal probes of composite Higgs models: new bounds and prospects for FCC-ee

We study the leading loop-level phenomenology of composite Higgs models via the effective field theory of a strongly interacting light Higgs and top quark (SILH+TQ). We systematically analyze the renormalization group evolution (RGE) of tree-generated operators in the SILH+TQ scenario, finding large mixings of flavor non-universal operators into those affecting electroweak precision observables. We show that these model-independent RG contributions are more important than typical estimates for finite parts. Flavor non-universal effects are completely captured by examining three options for the top mixing: fully composite qL3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {q}_L^3 $$\\end{document}, equal compositeness, and fully composite tR. In the most phenomenologically viable case of a fully composite tR, we show that the strongest bound on the natural parameter space comes from next-to-leading log running of the 4-top operator Ott=t¯RγμtRt¯RγμtR\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\mathcal{O}}_{tt}=\\left({\\overline{t}}_R{\\gamma}_{\\mu }{t}_R\\right)\\left({\\overline{t}}_R{\\gamma}^{\\mu }{t}_R\\right) $$\\end{document} into the Peskin-Takeuchi T parameter. In general, we find that this 2-loop effect allows existing electroweak precision data to give better constraints on 4-top operators than high-energy probes from top production at the LHC. Independent of the top mixing, we find that a future tera-Z machine such as FCC-ee has the potential to probe composite Higgs models up to a scale of m∗ ≳ 25 TeV, and test the naturalness of the electroweak scale at the ≲ 10−4 level.

Echoes of Veltman criteria on the next-two-Higgs-doublet model

We investigate how the Next-Two-Higgs Doublet Model extension (N2HDM) should look if we are to address the naturalness problem using dimensional regularization. In such a model, new Higgs states are predicted, namely: three CP-even h1,2,3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$h_{1,2,3}$$\\end{document}, one CP-odd A, and a pair of charged Higgs boson H±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H^\\pm $$\\end{document}. Our calculations of the overall quadratic divergences have been performed with full consistency with the latest data from the Large Hadron Collider (LHC) concerning the observed 125 GeV Higgs boson, alongside precision electroweak data tests and lower mass limits on charged Higgs boson. It is shown that the quadratically divergent quantum corrections δi\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\delta _i$$\\end{document} (i=1,2,3) for the three CP-even Higgs bosons are controllably small, though hidden fine-tuning might still be required. This reveals a significant impact on the model parameter space, Higgs spectrum mass and notably the singlet-doublet admixture.

Search for heavy Majorana neutrinos in the τ final state at proton-electron colliders

We utilize the lepton number violation signal process p e− → τ+jjj to search for heavy Majorana neutrinos at future proton-electron colliders. The LHeC (FCC-eh) is considered to run with an electron beam energy of 60 GeV, a proton beam energy of 7 (50) TeV and an integrated luminosity of 1 (3) ab−1, and the electron beam is considered to be unpolarized. We apply detector configurations and simulate signal and related standard model background events for both hadronic τh and leptonic τℓ final states, ℓ being a muon. After preselection, multivariate analyses are performed to reject the background. The strategy to reconstruct the heavy neutrino mass is developed and distributions of reconstructed mass are presented. Discovery sensitivities on parameter |VτN|2|VeN|2/(|VτN|2 + |VeN|2) for the heavy neutrino mass between 10 and 3000 GeV are predicted. At the 2-σ significance, the best discovery sensitivity is ∼ 1.2 × 10−5 (5.0 × 10−6) at the LHeC (FCC-eh) when mN ∼ 100 GeV for the hadronic τh final state. Sensitivities for the leptonic τℓ final state are found to be similar to those for the hadronic τh final state for most of the parameter space investigated. We also derive the limits on mixing parameters from electroweak precision data (EWPD) and DELPHI experiment. Assuming |VτN|2 = |VeN|2 = |VℓN|2, sensitivity bounds from the LHeC and FCC-eh experiments are found to be stronger than those from EWPD when mN ≲ 900 GeV, and also stronger than those from DELPHI when mN ≳ 70 GeV. Constraints are also interpreted and compared in the |VτN|2 vs. |VeN|2 plane. Compared with current limits from EWPD, DELPHI, and LHC experiments, future pe experiments can probe large additional regions in the parameter space formed by |VτN|2 and |VeN|2, and thus significantly enhance the discovery potential for a large portion of the |VτN|2 vs. |VeN|2 plane.

Custodial symmetry violation in scalar extensions of the standard model* *X.W. is supported by the Fundamental Research Funds for the Central Universities of China (GK202003018). J.-H.Y. is supported by the National Science Foundation of China (12022514, 11875003, and 12047503), the National Key Research and Development Program of China (2020YFC2201501, 2021YFA0718304), the CAS Project for Young Scientists in Basic Research (YSBR-006),

The new measurement of the W boson mass from the CDF collaboration shows a significant tension with the standard model prediction, which evidences violation of custodial symmetry in the scalar sector. We study the scalar extensions of the standard model, which can be categorized into two classes, the scalar sector with custodial symmetry (Georgi-Machacek model and its generalizations) and the scalar sector without custodial symmetry, and explore how these extensions fit to electroweak precision data and the new CDF . The favored oblique parameters originate from either the large mass splitting in the multiplet via the loop contribution or the large vacuum expectation value, which breaks custodial symmetry at the tree level. In particular, we find that GeV new particles are allowed in the scalar extension scenarios.

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.

Probing the mixing parameter |VτN|2 for heavy neutrinos

Because of the difficulty in detecting final state taus, the mixing parameter $|V_{\tau N}|^2$ for heavy neutrino $N$ is not well studied at current experiments, compared with other mixing parameters $|V_{e N}|^2$ and $|V_{\mu N}|^2$. In this paper, we focus on a challenging scenario where $N$ mixes with active neutrino of tau flavour only, i.e. $ |V_{\tau N}|^2 \neq 0 $ and $|V_{e N}|^2 = |V_{\mu N}|^2 = 0$. We derive current constraints on $|V_{\tau N}|^2$ from the rare $Z$-boson decay and electroweak precision data (EWPD). To forecast the future limits, we also investigate the signal $p p \to \tau^{\pm} \tau^{\pm} j j $ via a Majorana heavy neutrino at future proton-proton colliders. To suppress the background, both taus are required to decay leptonically into muons, leading to the final state containing two same sign muons, at least two jets plus moderate missing energy. The signal and relevant background processes are simulated at the HL-LHC and SppC/FCC-hh with center-of-mass energy of 14 TeV and 100 TeV. The preselection and multivariate analyses based on machine-learning are performed to reduce background. Limits on $|V_{\tau N}|^2$ are shown for heavy neutrino mass in the range 10-1000 GeV based on measurements from the rare $Z$-boson decay and EWPD, and searches at the HL-LHC and SppC/FCC-hh with integrated luminosities of 3 and 20 ab$^{-1}$.

Testing the scalar triplet solution to CDF’s heavy W problem at the LHC

The Type II Seesaw model remains a popular and viable explanation of neutrino masses and mixing angles. By hypothesizing the existence of a scalar that is a triplet under the weak gauge interaction, the model predicts strong correlations among neutrino oscillation parameters, signals at lepton flavor experiments, and collider observables at high energies. We investigate reports that the Type II Seesaw can naturally accommodate recent measurements by the CDF collaboration, which finds the mass of the $W$ boson to be significantly larger than allowed by electroweak precision data, while simultaneously evading constraints from direct searches. Experimental scrutiny of this parameter space in the Type II Seesaw has long been evaded since it is not characterized by ``golden channels'' at colliders but instead by cascade decays, moderate mass splittings, and many soft final states. In this work, we test this parameter space against publicly released measurements made at the Large Hadron Collider. By employing a newly developed tool chain combining MadGraph5\_aMC@NLO and Contur, we find that most of the favored space for this discrepancy is already excluded by measurements of Standard Model final states. We give suggestions for further exploration at Run III of the LHC, which is now underway.

Impact of the Recent Measurements of the Top-Quark and W-Boson Masses on Electroweak Precision Fits.

We assess the impact of the very recent measurement of the top-quark mass by the CMS Collaboration on the fit of electroweak data in the standard model and beyond, with particular emphasis on the prediction for the mass of the W boson. We then compare this prediction with the average of the corresponding experimental measurements including the new measurement by the CDF Collaboration, and discuss its compatibility in the standard model, in new physics models with oblique corrections, and in the dimension-six standard model effective field theory. Finally, we present the updated global fit to electroweak precision data in these models.

The Contribution of Charged Bosons with Right-Handed Neutrinos to the Muon g − 2 Anomaly in the Twin Higgs Models

We examine the charged boson and right-handed neutrino contribution to the muon g−2 anomaly in twin Higgs models with joint constraints of Higgs global fit data, precision electroweak data, leptonic flavor-changing decay μ→eγ, and the mass requirement of heavy-gauge bosons. It comes with the conclusion that some parameters, such as the coupling of charged Higgs to the lepton yμ, the top Yukawa yt, and heavy-gauge boson coupling to the lepton Vμ are constrained roughly in the range of 0.12≲yμ≲0.4, 0.4≲yt≲0.9, and 0.47≲Vμ≲1, respectively.

Lepton flavor violation within the simplest little Higgs model

We carry out an exhaustive analysis of lepton flavor violating processes within the Simplest Little Higgs model. Its discovery could be expected from either $\mu\to e$ conversion in nuclei, $\mu\to e\gamma$ or $\mu\to3e$ decays. Then, the tau sector could help discriminating this model not only via $\tau\to\ell\gamma$ ($\ell=\mu,e$) and $\tau\to3\ell$ decays, but also by means of $\ell\to\tau$ conversion in nuclei, which is promising in this respect. Although the model violates slightly custodial symmetry, acommodating the recent CDF $M_W$ measurement is in tension with electroweak precision data.

SMEFT analysis of mW

We use the Fitmaker tool to incorporate the recent CDF measurement of mW in a global fit to electroweak, Higgs, and diboson data in the Standard Model Effective Field Theory (SMEFT) including dimension-6 operators at linear order. We find that including any one of the SMEFT operators mathcal{O} HWB, mathcal{O} HD, mathcal{O} ℓℓ or {mathcal{O}}_{Hell}^{(3)} with a non-zero coefficient could provide a better fit than the Standard Model, with the strongest pull for mathcal{O} HD and no tension with other electroweak precision data. We then analyse which tree-level single-field extensions of the Standard Model could generate such operator coefficients with the appropriate sign, and discuss the masses and couplings of these fields that best fit the CDF measurement and other data. In particular, the global fit favours either a singlet Z′ vector boson, a scalar electroweak triplet with zero hypercharge, or a vector electroweak triplet with unit hypercharge, followed by a singlet heavy neutral lepton, all with masses in the multi-TeV range for unit coupling.

Kinetic mixing, custodial symmetry, and a lower bound on the mass of a dark gauge boson

Abstract We consider the extension of the standard model by dark fields with an Abelian spontaneously broken gauge symmetry in a hidden dark matter scenario. The dimension-four gauge-invariant terms include a kinetic mixing term and a Higgs mixing term, and we show that, after spontaneous symmetry breaking, the tree-level relation $M^{2}_{W}=M^{2}_{\tilde{Z}} \cos ^{2} \tilde{\theta }_{w}$ holds and permits us to write the mixing angle induced by the kinetic mixing in the neutral massive gauge boson sector, θζ, in terms of the values of MZ, the weak mixing angle, and of the mass of the physical dark gauge boson ZD. At the loop level, a similar relation is obtained in the $\overline{MS}$ scheme. Using the result extracted from the global fit to electroweak precision data for the ratio $\rho _{0}=M^{2}_{W}/\hat{c}^{2}_{Z} M^{2}_{Z}\hat{\rho }$, we obtain the lower bound $M_{Z_{D}}\gt M_{Z}$ for the dark gauge boson mass at the $94\%$ confidence level. We argue that this lower bound holds in the general case of theories for physics beyond the standard model with an extra U(1) gauge factor subgroup, whenever the extended Higgs potential respects custodial symmetry.

Interpreting electroweak precision data including the W-mass CDF anomaly

We perform a global fit of electroweak data, finding that the anomaly in the W mass claimed by the CDF collaboration can be reproduced as a universal new-physics correction to the T parameter or |H†DμH|2 operator. Contributions at tree-level from multi-TeV new physics can fit the anomaly compatibly with collider bounds: we explore which scalar vacuum expectation values (such as a triplet with zero hypercharge), Z′ vectors (such as a Z′ coupled to the Higgs only), little-Higgs models or higher-dimensional geometries provide good global fits. On the other hand, new physics that contributes at loop-level must be around the weak scale to fit the anomaly. Thereby it generically conflicts with collider bounds, that can be bypassed assuming special kinematics like quasi-degenerate particles that decay into Dark Matter (such as an inert Higgs doublet or appropriate supersymmetric particles).

Violation of custodial symmetry from W -boson mass measurements

The new measurement of the $W$-boson mass from the CDF collaboration shows a significant tension with the Standard Model prediction. We quantify this discrepancy within a state-of-the-art analysis of electroweak precision data and scrutinize the leading deformations of the Standard Model Effective Field Theory arising at dimension six. We find evidence for a non-zero value of the $T$ parameter, i.e. for a novel source of violation of custodial symmetry, pointing to physics beyond the Standard Model at the 4.5$\sigma$ level. We contextualize the implications of our findings in light of other present anomalies in Particle Physics.

Next-to-leading order corrections to decays of the heavier CP-even Higgs boson in the two Higgs doublet model

We investigate the impact of electroweak (EW), scalar and QCD corrections to the full set of decay branching ratios of an additional CP-even Higgs boson (H) in the two Higgs doublet model with a softly broken Z2 symmetry. We employ the improved gauge independent on-shell scheme in the renormalized vertices. We particularly focus on the scenario near the alignment limit in which couplings of the discovered 125 GeV Higgs boson (h) coincide with those of the standard model while the Hhh coupling vanishes at tree level. The renormalized decay rate for H→hh can significantly be changed from the prediction at tree level due to non-decoupling loop effects of additional Higgs bosons, even in the near alignment case. We find that the radiative corrections to the branching ratio of H→hh can be a few ten percent level in the case with the masses of additional Higgs bosons being degenerate under the constraints of perturbative unitarity, vacuum stability and the EW precision data. Further sizable corrections can be obtained for the case with a mass difference among the additional Higgs bosons.

Jet charge: A new tool to probe the anomalous [formula omitted] couplings at the EIC

We propose to probe the Zbb¯ interactions at the Electron-Ion Collider (EIC) by utilizing the average jet charge weighted single-spin asymmetry AebQ, which is induced by different cross sections of a left-handed and right-handed electron beam scattering off an unpolarized proton beam in the neutral current deeply-inelastic scattering processes with one observed b-tagged jet. This novel observable at the EIC is sensitive to the axial-vector component of the Zbb¯ coupling, providing similar information as the gg→Zh cross section measurement at the high-luminosity Large Hadron Collider, and is complementary to the single-spin asymmetry measurement at the EIC which is sensitive to the vector component of the Zbb¯ coupling. We show that the apparent degeneracy of the allowed Zbb¯ coupling in its vector and axial-vector components, implied by the Large Electron-Positron Collider (LEP) and the Stanford Linear Collider (SLC) precision electroweak data, can be broken by the AebQ measurement at the EIC. With a large enough integrated luminosity collected at the EIC, the measurement of AebQ could also resolve the long-standing discrepancy between bottom quark forward-backward asymmetry AFBb at the LEP and the Standard Model prediction, with a strong dependence on the b-tagging efficiency.

Triple Higgs couplings in the 2HDM: the complete picture

The measurement of the triple Higgs coupling is one of the main tasks of the (HL-)LHC and future lepton colliders. Similarly, triple Higgs couplings involving BSM Higgs bosons are of high interest. Within the framework of Two Higgs Doublet Models (2HDM) we investigate the allowed ranges for all triple Higgs couplings involving at least one light, SM-like Higgs boson. We present newly the allowed ranges for 2HDM type III and IV and update the results within the type I and II. We take into account theoretical constraints from unitarity and stability, experimental constraints from direct BSM Higgs-boson searches, measurements of the rates of the SM-like Higgs-boson at the LHC, as well as flavor observables and electroweak precision data. For the SM-type triple Higgs coupling w.r.t. its SM value, lambda _{hhh}/lambda _{mathrm{SM}}, we find allowed intervals of sim [{-0.5}, {1.3}] in type I and sim [{0.5}, {1.0}] in the other Yukawa types. These allowed ranges have important implications for the experimental determination of this coupling at future collider experiments. We find the coupling lambda _{hhH} between sim -1.5 and sim +1.5 in the four Yukawa types. For the triple Higgs couplings involving two heavy neutral Higgs bosons, lambda _{hHH} and lambda _{hAA} we find values between sim -0.5 and sim 16, and between sim -1 and sim 32 for lambda _{hH^+H^-}. These potentially large values could lead to strongly enhanced production of two Higgs-bosons at the HL-LHC or high-energy lepton colliders.

Comprehensive study of the light charged Higgs boson in the type-I two-Higgs-doublet model

In the type-I two-Higgs-doublet model, existing theoretical and experimental constraints still permit the light charged Higgs boson with a mass below the top quark mass. We present a complete roadmap for the light charged Higgs boson at the LHC through the comprehensive phenomenology study, focusing on the normal scenario where the lighter \textit{CP}-even Higgs boson is the observed Higgs boson. In type-I, it is challenging to simultaneously accommodate the light mass of the charged Higgs boson and the constraints from theory, electroweak precision data, Higgs data, $b\to s \gamma$, and direct search bounds. Consequently, the parameter space is extremely curtailed, which predicts somewhat definite phenomenological signatures. We find that the mass of the pseudoscalar Higgs boson, $M_A$, is the most crucial factor in the phenomenology of the charged Higgs boson. If $M_A$ is light, the charged Higgs boson decays mainly into $A W^\pm$. When $M_A$ is above the $A W^\pm$ threshold, the dominant decay mode is into $ \tau^\pm \nu$. Over the whole viable parameter space, we study all the possible production and decay modes of charged Higgs bosons at the LHC, and suggest three efficient channels: (i) $pp \to H^+ H^-\to [\tau \nu] [\tau \nu]$; (ii) $pp \to HA/HH/AA \to H^\pm W^\mp H^\pm W^\mp \to [\tau \nu] [\tau \nu] W W$; (iii) $pp \to H^+ H^-\to [b \bar b W ] [b \bar b W ] $. Based on the sophisticated signal-background analyses including detector simulation, we showed that the significance of the first final state is large, that of the second one is marginal around three, but the third one suffers from huge $t\bar t$ related backgrounds.

Probing the [formula omitted] anomalous couplings via exclusive Z boson decay

We propose to utilize the exclusive Z-boson rare decays Z→ϒ(ns)+γ to constrain the Zbb¯ couplings at the HL-LHC and 100 TeV proton-proton collider. We demonstrate that the event yield of the proposed processes is sensitive to the axial-vector component of the Zbb¯ coupling and can provide complementary information to the jet-charge weighted single-spin asymmetry measurement at the EIC and the gg→Zh production rate measurement at the LHC. By applying the NRQCD factorization formalism, we calculate the partial decay width of Z→ϒ(ns)+γ to the NLO accuracy in strong interaction, which is found to agree with those obtained from the light-cone distribution amplitude approach. We show that the HL-LHC can break the degeneracy of the Zbb¯ couplings, as implied by the precision electroweak data at LEP and SLC, if the signal efficiency can be improved by a factor of 1.7 from the present ATLAS analysis at the 13 TeV LHC with an integrated luminosity of 36.1fb−1.