Electroweak Precision Data Research Articles

Following the trail of new physics via the vector boson fusion Higgs boson signal at the Large Hadron Collider

We investigate the modification of the Higgs signals from vector boson fusion at the LHC arising from higher-dimensional effective operators involving quarks, electroweak gauge bosons and the 125-GeV scalar discovered in 2012. Taking a few of the admissible dimension-6 operators as illustration, we work within the framework of the Standard Model Effective Field Theory (SMEFT) and identify kinematic variables that can reflect the presence of such effective operators. The useful variables turn out to be the geometric mean of the transverse momenta of the two forward jets produced in VBF and the rapidity difference between the two forward jets. We identify the shift in event population caused by the effective operators in the same, spanned by the above kinematic variables. Minimum values of the Wilson coefficients of the chosen dimension-6 operators are identified, for which they can be probed at the $3\sigma$ level in the high luminosity run of the LHC at 14 TeV. Projected exclusion limits on some of the couplings, obtained from our analysis, can significantly improve the limits on such couplings derived from electroweak precision data.

Electroweak Higgs effective field theory after LHC run 2

We analyze the electroweak interactions in the framework of the Higgs effective field theory using the available Higgs and electroweak diboson production results from LHC run 2 as well as the electroweak precision data. Assuming universality of the weak current, our study considers 25 possible anomalous couplings. To unveil the nature of the Higgs boson, i.e., isosinglet versus part of $SU(2{)}_{L}$ doublet, we explore the correlation effects between observables that are predicted to exist in the linear realization of the electroweak gauge symmetry but not in its nonlinear counterpart. This improves previous studies aimed at investigating the Higgs nature and the origin of the electroweak symmetry breaking.

Neutrino masses and magnetic moments of electron and muon in the Zee Model

We explore parameter space in the Zee Model to resolve the long-standing tension of the electron and muon anomalous magnetic moment (AMM). The model comprises a second Higgs doublet and a charged singlet at electroweak scale and generates Majorana neutrino masses at one-loop level; the neutral partner of the SU(2)L doublet contributes to the AMM of electron and muon via one loop and two-loop corrections. We propose two minimal flavor structures that can explain these anomalies while fitting the neutrino oscillation data. We find that the neutral Higgs resides in the mass range of roughly 10–300 GeV or 1–30 GeV, depending on the flavor structures. The model is consistent with constraints from colliders, electroweak precision data, and lepton flavor violation. To be comprehensive, we examine the constraints from the electric dipole moment (EDM) and find a region of parameter space that gives a sizable contribution to muon EDM while simultaneously giving corrections to muon AMM. In addition to the light scalar, the two charged scalars with masses as low as 100 GeV can induce nonstandard neutrino interactions εee as large as 8%, potentially hinting at new physics. We also investigate the projected capability of future lepton colliders to probe the currently allowed parameter space consistent with both electron and muon AMMs via direct searches in the ℓ+ℓ− → ℓ+ℓ−(H → ℓ+ℓ−) channel.

Electroweak legacy of the LHC run II

We present a comprehensive study of the electroweak interactions using the available Higgs and electroweak diboson production results from LHC runs 1 and 2 as well as the electroweak precision data in terms of the dimension-six operators. Under the assumption that no new tree-level sources of flavor violation or violation of the universality of the weak current are introduced, the analysis involves 21 operators. We assess the impact of the data on kinematic distributions for the Higgs production at the LHC by comparing the results obtained by including the simplified template cross-section data with those in which only total Higgs signal strengths are considered. We also compare the results obtained when including the dimension-six anomalous contributions to order $1/{\mathrm{\ensuremath{\Lambda}}}^{2}$ and to order $1/{\mathrm{\ensuremath{\Lambda}}}^{4}$. As an illustration of the LHC potential to indirectly learn about specific forms of new physics, we adapt the analysis to constrain the parameter space for a few simple extensions of the standard model which generate a subset of the dimension-six operators at tree level.

Seesaw lepton masses and muon $$g-2$$ from heavy vector-like leptons

We propose a model for the vector-like lepton to explain the small muon mass by a seesaw mechanism, based on lepton-specific two Higgs doublet models with a local $U(1)'$ symmetry. There is no bare muon mass for a nonzero $U(1)'$ charge of the leptophilic Higgs doublet, so the physical muon mass is generated due to the mixing between the vector-like lepton and the muon after the leptophilic Higgs doublet and the dark Higgs get VEVs. In this scenario, the non-decoupling effects of the vector-like lepton give rise to leading contributions to the muon $g-2$, thanks to the light $Z'$ and the light dark Higgs boson. We discuss various constraints on the model from lepton flavor violation, electroweak precision and Higgs data, as well as collider searches.

A Sub-GeV Low Mass Hidden Dark Sector of SU(2)H\xd7 U(1)X

We present a detailed study of the non-abelian vector dark matter candidate Wt with a MeV–GeV low mass range, accompanied by a dark photon A′ and a dark Z′ of similar masses, in the context of a gauged two-Higgs-doublet model with the hidden gauge group that has the same structure as the Standard Model electroweak gauge group. The stability of dark matter is protected by an accidental discrete Z2 symmetry (h-parity) which was usually imposed ad hoc by hand. We examine the model by taking into account various experimental constraints including dark photon searches at NA48, NA64, E141, ν-CAL, BaBar and LHCb experiments, electroweak precision data from LEP, relic density from Planck satellite, direct (indirect) detection of dark matter from CRESST-III, DarkSide-50, XENON1T (Fermi-LAT), and collider physics from the LHC. The theoretical requirements of bounded from below of the scalar potential and tree level perturbative unitarity of the scalar sector are also imposed. The viable parameter space of the model consistent with all the constraints is exhibited. While a dark Z′ can be the dominant contribution in the relic density due to resonant annihilation of dark matter, a dark photon is crucial to dark matter direct detection. We also demonstrate that the parameter space can be further probed by various sub-GeV direct dark matter experimental searches at CDEX, NEWS-G and SuperCDMS in the near future.

Imprint of SUSY in radiative B -meson decays

We study supersymmetric (SUSY) effects on $C_7(\mu_b)$ and $C'_7(\mu_b)$ which are the Wilson coefficients (WCs) for $b \to s \gamma$ at b-quark mass scale $\mu_b$ and are closely related to radiative $B$-meson decays. The SUSY-loop contributions to $C_7(\mu_b)$ and $C'_7(\mu_b)$ are calculated at leading order (LO) in the Minimal Supersymmetric Standard Model (MSSM) with general quark-flavour violation (QFV). For the first time we perform a systematic MSSM parameter scan for the WCs $C_7(\mu_b)$ and $C'_7(\mu_b)$ respecting all the relevant constraints, i.e. the theoretical constraints from vacuum stability conditions and the experimental constraints, such as those from $K$- and $B$-meson data and electroweak precision data, as well as recent limits on SUSY particle masses and the 125 GeV Higgs boson data from LHC experiments. From the parameter scan we find the following: (1) The MSSM contribution to Re($C_7(\mu_b)$) can be as large as $\sim \pm 0.05$, which could correspond to about 3$\sigma$ significance of New Physics (NP) signal in the future LHCb and Belle II experiments. (2) The MSSM contribution to Re($C'_7(\mu_b)$) can be as large as $\sim -0.08$, which could correspond to about 4$\sigma$ significance of NP signal in the future LHCb and Belle II experiments. (3) These large MSSM contributions to the WCs are mainly due to (i) large scharm-stop mixing and large scharm/stop involved trilinear couplings, (ii) large sstrange-sbottom mixing and large sstrange-sbottom involved trilinear couplings and (iii) large bottom Yukawa coupling $Y_b$ for large $\tan\beta$ and large top Yukawa coupling $Y_t$. In case such large NP contributions to the WCs are really observed in the future experiments at Belle II and LHCb Upgrade, this could be the imprint of QFV SUSY (the MSSM with general QFV).

The anomalous [formula omitted] couplings at the HERA and EIC

To resolve the long-standing discrepancy between the precision measurement of bottom quark forward-backward asymmetry at LEP/SLC and the Standard Model prediction, we propose a novel method to probe the Zbb¯ coupling by measuring the single-spin asymmetry Aeb of the polarized lepton cross section in neutral current DIS processes with a b-tagged jet at HERA and EIC. Depending on the tagging efficiency of the final state b-jet, the measurement of Aeb at HERA can already partially break the degeneracy found in the anomalous Zbb¯ coupling, as implied by the LEP and SLC precision electroweak data. In the first year run of the EIC, the measurement of Aeb can already break the degeneracy, due to its much larger luminosity and higher electron beam polarization. With enough integrated luminosity collected at the EIC, it is possible to either verify or exclude the LEP data and resolve the AFBb puzzle. We also discuss the complementary roles between the proposed Aeb measurement at EIC and the measurement of gg→Zh cross section at the HL-LHC in constraining the anomalous Zbb¯ coupling.

EWPD in the SMEFT to dimension eight

We calculate the mathcal{O}left({leftlangle {H}^{dagger }Hrightrangle}^2/{Lambda}^4right) corrections to LEP electroweak precision data using the geometric formulation of the Standard Model Effective Field Theory (SMEFT). We report our results in simple-to-use interpolation tables that allow the interpretation of this data set to dimension eight for the first time. We demonstrate the impact of these previously unknown terms in the case of a general analysis in the SMEFT, and also in the cases of two distinct models matched to dimension eight. Neglecting such dimension-eight corrections to LEP observables introduces a theoretical error in SMEFT studies. We report some preliminary studies defining such a theory error, explicitly demonstrating the effect of previously unknown dimension-eight SMEFT corrections on LEP observables.

Complementary searches of low mass non-Abelian vector dark matter, dark photon, and dark Z′

We present a novel study of the non-abelian vector dark matter candidate $W^\prime$ with a MeV$-$GeV low mass range, accompanied by a dark photon $A^\prime$ and a dark $Z^\prime$ of similar masses in the context of a simplified gauged two-Higgs-doublet model. The model is scrutinized by taking into account various experimental constraints including dark photon searches, electroweak precision data, relic density of dark matter together with its direct and indirect searches, mono-jet and Higgs collider physics from LHC. The viable parameter space of the model consistent with all experimental and theoretical constraints is exhibited. While a dark $Z^\prime$ can be the dominant contribution in the relic density due to resonant annihilation of dark matter, a dark photon is crucial to dark matter direct detection. We demonstrate that the parameter space can be further probed in the near future by sub-GeV dark matter experiments like CDEX, NEWS-G and SuperCDMS.

Uncovering the high-scale Higgs singlet model

The scalar singlet model extends the Standard Model with the addition of a new gauge singlet scalar. We reexamine the limits on the new scalar from oblique parameter fits and from a global fit to precision electroweak observables and present analytic expressions for our results. For the case when the new scalar is much heavier than the weak scale, we map the model onto the dimension-six Standard Model effective field theory and review the allowed parameter space from unitarity considerations and from the requirement that the electroweak minimum be stable. A global fit to precision electroweak data, along with LHC observables, is used to constrain the parameters of the high scale singlet model, and we determine the numerical effects of performing the matching at both tree level and one loop.

CP violation at ATLAS in effective field theory

CP violation beyond the Standard Model (SM) is a crucial missing piece for explaining the observed matter-antimatter symmetry in the Universe. Recently, the ATLAS experiment at the Large Hadron Collider has performed an analysis of electroweak $Zjj$ production, thereby excluding the SM locally at 95\% confidence level in the measurement of CP-sensitive observables. We take the excess' interpretation in terms of anomalous gauge-Higgs interactions at face value and discuss further steps that are required to scrutinize its origin. In particular, we discuss the relevance of multi-boson production using adapted angular observables to show how they can be used to directly tension the reported $Zjj$ excess in a more comprehensive analysis. To connect the excess to a concrete UV scenario for which the underlying assumptions of the $Zjj$ analysis are valid, we identify vector-like leptons as a candidate theory consistent with the observed CP-odd Wilson coefficient hierarchy observed by ATLAS. We perform a complete one-loop matching calculation to motivate further model-specific and correlated new physics searches. In parallel, we provide estimates of the sensitivity reach of the LHC's high luminosity phase for this particular scenario of CP-violation in light of electroweak precision and Run-2 Higgs data. These provide strong constraints on the model's CP-even low-energy phenomenology, but also inform the size of the CP-odd SM deformation indirectly via our model hypothesis.

Breaking up the Proton: An Affair with Dark Forces.

Deep inelastic scattering of e^{±} off protons is sensitive to contributions from "dark photon" exchange. Using HERA data fit to HERA's parton distribution functions (PDFs), we obtain the model-independent bound ε≲0.02 on the kinetic mixing between hypercharge and the dark photon for dark photon masses ≲10 GeV. This slightly improves on the bound obtained from electroweak precision observables. For higher masses, the limit weakens monotonically; ε≲1 for a dark photon mass of 5TeV. Utilizing PDF sum rules, we demonstrate that the effects of the dark photon cannot be (trivially) absorbed into refit PDFs and, in fact, lead to non-Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (Bjorken x_{B}-independent) scaling violations that could provide a smoking gun in data. The proposed e^{±}p collider operating at sqrt[s]=1.3 TeV (Large Hadron Electron Collider) is anticipated to accumulate 10^{3} times the luminosity of HERA, providing substantial improvements in probing the effects of a dark photon: sensitivity to ε well below that probed by electroweak precision data is possible throughout virtually the entire dark photon mass range, as well as being able to probe to much higher dark photon masses, up to 100TeV.

Impact of fermionic operators on the Higgs boson width measurement

The off-shell Higgs production in pp -> ZZ at LHC provides at present the most direct measurement of the Higgs width in the absence of beyond the standard model contributions. Here, we analyze the impact of anomalous Z couplings to fermions on the Higgs width determination. We show that, despite these couplings being strongly constrained by the available electroweak precision data, they can substantially affect the Higgs width determination at the LHC Runs 2 and 3. Conversely, in larger integrated luminosities runs, such as those foreseen at the high luminosity LHC and high energy LHC setups, the effect of such anomalous interactions in the Higgs width measurement is minimal.

B anomalies under the lens of electroweak precision

The measurements carried out at LEP and SLC projected us into the precision era of electroweak physics. This has also been relevant in the theoretical interpretation of LHCb and Belle measurements of rare B semileptonic decays, paving the road for new physics with the inference of lepton universality violation in {R}_{K^{left(ast right)}} ratios. The simplest explanation of these flavour anomalies — sizeable one-loop contributions respecting Minimal Flavour Violation — is currently disfavoured by electroweak precision data. In this work, we discuss how to completely relieve the present tension between electroweak constraints and one-loop minimal flavour violating solutions to {R}_{K^{left(ast right)}} . We determine the correlations in the Standard Model Effective Field Theory that highlight the existence of such a possibility. Then, we consider minimal extensions of the Standard Model where our effective-field-theory picture can be realized. We discuss how these solutions to b → sℓℓ anomalies, respecting electroweak precision and without any new source of flavour violation, may point to the existence of a Z′ boson at around the TeV scale, within the discovery potential of LHC, or to leptoquark scenarios.

What can a heavy boson do to the muon anomaly and to a new Higgs boson mass? * *J.P.R is supported by the project PTDC/FIS-PAR/31000/2017. The work of A.P.M. has been performed in the framework of COST Action CA16201 “Unraveling new physics at the LHC through the precision frontier” (PARTICLEFACE). A.P.M. is supported by the Center for Research and Development

The minimal extension of the Standard Model (B-L-SM) offers an explanation for neutrino mass generation via a seesaw mechanism; it also offers two new physics states, namely an extra Higgs boson and a new gauge boson. The emergence of a second Higgs particle as well as a new gauge boson, both linked to the breaking of a local symmetry, makes the B-L-SM rather constrained by direct searches in Large Hadron Collider (LHC) experiments. We investigate the phenomenological status of the B-L-SM by confronting the new physics predictions with the LHC and electroweak precision data. Taking into account the current bounds from direct LHC searches, we demonstrate that the prediction for the muon anomaly in the B-L-SM yields at most a contribution of approximately , which represents a tension of standard deviations, with the current uncertainty, by means of a boson if its mass is in the range of to , within the reach of future LHC runs. This means that the B-L-SM, with heavy yet allowed boson mass range, in practice, does not resolve the tension between the observed anomaly in the muon and the theoretical prediction in the Standard Model. Such a heavy boson also implies that the minimal value for the new Higgs mass is of the order of 400 GeV.

Sensitivity reach on heavy neutral leptons and τ -neutrino mixing |UτN|2 at the HL-LHC

The model of heavy neutral leptons (HNLs) is one of the well-motivated models beyond the standard model (BSM) from both theoretical and phenomenological point of views. It is an indispensable ingredient to explain the puzzle of tiny neutrino masses and the origin of the matter-antimatter asymmetry in our Universe, based on the models in which the simplest Type-I seesaw mechanism can be embedded. The HNL with a mass up to the electroweak scale is an attractive scenario which can be readily tested in present or near-future experiments including the LHC. In this work, we study the decay rates of HNLs and find the sensitive parameter space of the mixing angles between the active neutrinos and HNLs. Since the mixing between $ \nu_{\tau} $ and HNL is not well established in literature compared with those of $\nu_e$ and $\nu_{\mu}$ for the HNL of mass in the electroweak scale, we focus on the channel $ pp\rightarrow W^{\pm(\ast)} + X\rightarrow \tau^{\pm} N + X$ to search for HNLs at the LHC 14 TeV. The targeted signature consists of three prompt charged leptons, which include at least two tau leptons. After the signal-background analysis, we further set sensitivity bounds on the mixing $ |U_{\tau N}|^2 $ with $ M_N $ at High-Luminosity LHC (HL-LHC). We predict the testable bounds from HL-LHC can be stronger than the previous LEP constraints and Electroweak Precision Data (EWPD), especially for $ M_N \lesssim $ 50 GeV can reach down to $ |U_{\tau N}|^2 \approx 2\times 10^{-6} $.

Dimension-8 operators in the Standard Model Effective Field Theory

We present a complete basis of dimension-8 operators in the Standard Model Effective Field Theory. Attention is paid to operators that vanish in the absence of flavor structure. The 44,807 operators are encoded in 1,031 Lagrangian terms. We also briefly discuss a few aspects of phenomenology involving dimension-8 operators, including light-by-light scattering and electroweak precision data.

Exploring sizable triple Higgs couplings in the 2HDM

An important task at future colliders is the measurement of the triple Higgs coupling. Depending on its size relative to the Standard Model (SM) value, certain collider options result in a higher experimental accuracy. Within the framework of Two Higgs Doublet Models (2HDM) types I and II we investigate the allowed ranges for all triple Higgs couplings involving at least one light, SM-like Higgs boson. We take into account theoretical constraints (unitarity, stability), experimental constraints from direct Higgs-boson searches, measurements of the SM-like Higgs-boson properties, flavor observables and electroweak precision data. We find that the SM-type triple Higgs coupling w.r.t. its SM value, lambda _{hhh}/lambda _{mathrm {SM}}, can range between sim -0.5 and sim 1.5. Depending on which value is realized, the HL-LHC can compete with, or is clearly inferior to the ILC. We find the coupling lambda _{hhH} between sim -1.5 and sim 1.5. Triple Higgs couplings involving two heavy Higgs bosons, lambda _{hHH}, lambda _{hAA} and lambda _{hH^+H^-} can reach values up to {{mathcal {O}}}(10), roughly independent of the 2HDM type. This can lead to potentially strongly enhanced production of two Higgs-bosons at the HL-LHC or high-energy e^+e^- colliders.

Hadronic Vacuum Polarization: (g-2)_{μ} versus Global Electroweak Fits.

Hadronic vacuum polarization (HVP) is not only a critical part of the standard model (SM) prediction for the anomalous magnetic moment of the muon (g-2)_{μ}, but also a crucial ingredient for global fits to electroweak (EW) precision observables due to its contribution to the running of the fine-structure constant encoded in Δα_{had}^{(5)}. We find that with modern EW precision data, including the measurement of the Higgs mass, the global fit alone provides a competitive, independent determination of Δα_{had}^{(5)}|_{EW}=270.2(3.0)×10^{-4}. This value actually lies below the range derived from e^{+}e^{-}→hadrons cross section data, and thus goes into the opposite direction as would be required if a change in HVP were to bring the SM prediction for (g-2)_{μ} into agreement with the Brookhaven measurement. Depending on the energy where the bulk of the changes in the cross section occurs, reconciling experiment and SM predictions for (g-2)_{μ} by adjusting HVP would thus not necessarily weaken the case for physics beyond the SM (BSM), but to some extent shift it from (g-2)_{μ} to the EW fit. We briefly explore some options of BSM scenarios that could conceivably explain the ensuing tension.