We compute one-loop corrections to the S and T parameters in the Unhiggs scenario. In that scenario, the Standard Model Higgs is replaced by a non-local object, called the Unhiggs, whose spectral function displays a continuum above the mass gap. The Unhiggs propagator has effectively the same UV properties as the Standard Model Higgs propagator, which implies that loop corrections to the electroweak precision observables are finite and calculable. We show that the Unhiggs is consistent with electroweak precision tests when its mass gap is at the weak scale; in fact, it then mimics a light SM Higgs boson. We also argue that the Unhiggs, while being perfectly visible to electroweak precision observables, is invisible to detection at LEP.
We examine a scenario where the Higgs is part of an approximate conformal field theory, and has a scaling dimension greater than one. Such an unparticle Higgs (or Unhiggs) can still break electroweak symmetry and unitarize WW scattering, but its gauge couplings are suppressed. An Unhiggs model has a reduced sensitivity of the weak scale to the cutoff, and can thus provide a solution to the little hierarchy problem.
We explore the consequences of an electroweak symmetry breaking sector which exhibits approximately scale invariant dynamics -- i.e., nontrivial fixed point behavior, as in unparticle models. One can think of an unHiggs as a composite Higgs boson with a continuous mass distribution. We find it convenient to represent the unHiggs in terms of a Kallen-Lehmann spectral function, from which it is simple to verify the generation of gauge boson and fermion masses, and unitarization of WW scattering. We show that a spectral function with broad support, which corresponds to approximate fixed point beh…
Recent measurements by the ATLAS and CMS experiments have excluded the Standard Model Higgs boson in the high mass region, even if it is produced with a significantly smaller cross section than expected. The bounds are dominated by the non-observation of a signal in the clean gold-plated mode $h\to ZZ\to 4\ell$ and, hence, are directly related to the special role of the Higgs in electroweak symmetry breaking. A smaller cross section in comparison to the Standard Model is expected if the Higgs is realized as an unparticle in the Unhiggs scenario. With the LHC probing $σ/σ^{SM}<1$, we can theref…
We consider electroweak symmetry breaking by a certain class of non-local Higgs sectors. Extending previous studies employing the Mandelstam condition, a straight Wilson line is used to make the Higgs action gauge invariant. We show the unitarization of vector-boson scattering for a wide class of non-local actions, but find that the Wilson-line model leads to tree-level corrections to electroweak precision observables, which restrict the parameter space of the model. We also find that Unhiggs models cannot address the hierarchy problem, once the parameters are expressed in terms of low-energy …
In this paper, we present the one-loop radiative corrections to the electroweak precision observable $Δρ$ coming from the $I_W=1$ multiplet excited leptons. We have calculated the couplings of the exotic lepton triplet to the vector bosons and ordinary leptons using effective Lagrangian approach. These couplings are then used to estimate the excited lepton triplet contribution to the $Δρ$ parameter. The mass degenerate excited lepton contribution to $Δρ$ is small and can be neglected. However, if the excited leptons are non-degenerate, their contribution can be large which can result in more s…
In this proceeding, we highlight the computation of leading fermionic three-loop corrections to electroweak precision observables (EWPOs) accomplished recently. We summarize the numerical analysis and provide an outlook.
The role of the top quark in the Standard Model predictions for electroweak precision observables is reviewed and the implications of the experimental data for the indirect determination of the top mass range is discussed.
Phenomenologically relevant electroweak precision pseudo-observables related to Z-boson physics are discussed in the context of the strong experimental demands of future $e^+e^-$ colliders. The recent completion of two-loop Z-boson results is summarized and a prospect for the 3-loop Standard Model calculation of the Z-boson decay pseudo-observable is given.
One of the most precise measurements of the strong coupling constant alpha_s(MZ) is obtained in the context of global analyses of precision electroweak data. This article reviews the sensitivity of different electroweak observables to alpha_s and describes the perturbative uncertainties related to missing higher orders. The complete renormalisation scale dependence for the relevant observables is calculated at next-to-next-to-leading order and a new method is presented to determine the corresponding perturbative uncertainty for measurements of alpha_s based on these observables.
A critical assessment is given of the comparison between the new versions of the programs TOPAZ0 40i and ZFITTER 510. The relevance for precision calculations around the $\zb$ resonance is briefly discussed. << more references added >>
Precise measurements of the interactions of electroweak vector bosons at the ILC yield information about the physics of electroweak symmetry breaking. In order to combine this with the possible observation of new resonances or the effects of new strong interactions at the LHC, we need to relate resonance parameters with low-energy observables. We derive these relations for a generic setup and draw conclusions about the new-physics reach of the ILC in the electroweak sector.
I summarise the virtual effects of the new particles predicted by supersymmetric extensions of the Standard Model on the high-precision electroweak observables measured at LEP/SLC, the Tevatron and CLEO. I will then discuss in some details the two-loop SUSY-QCD corrections to the $ρ$ parameter.
The calculation of electroweak precision observables in the MSSM is reviewed. The description of the 1997 data and the results of updated global fits are discussed in comparison with the Standard Model.
The processes of the single W-production in hadron-hadron collisions are suggested for investigation of the nucleon spin. An approach is proposed for the determination of quark spin densities at low x. The lowest order electroweak radiative corrections to the observable quantities are calculated. The numerical calculations of the cross sections and the single spin asymmetries taking into consideration the electroweak corrections at RHIC energies have been made.
The predictions for electroweak observables following from Higgs-Free Model for Fundamental Interactions are derived. It is shown that these predictions are close to the Standard Model predictions and they are in a surprising agreement with the experimental data. The analysis of electroweak observables at low and high energy suggests that the Higgs mass $m_H$ is just the ultraviolet cutoff which increases if the process energy scale increases. We propose several experimental tests which can discriminate between the Standard and the Higgs-Free Models.
Warped extra dimensions provide a new playground to study electroweak brea-king and the nature of the higgs field. In this paper we reanalyze the electroweak observables in theories with one extra dimension and a completely general warp factor. We demonstrate that, regardless of the 5D background, SU(2)_R is needed in order to avoid excessive contributions to the T parameter. For higgsless theories cancelations between different contributions to the S parameter are needed.
Electroweak precision observables (EWPO) measured at the W and Z poles provide stringent limits on possible beyond the Standard Model physics scenarios. In an effective field theory (EFT) framework, the next-to-leading order QCD and electroweak results for EWPO yield indirect limits on possible 4-fermion operators that do not contribute to the observables at tree level. Here we calculate the next-to-leading corrections to EWPO induced by flavor non-universal 4-fermion interactions and find that the extracted limits on EFT coefficients have a strong dependence on the flavor structure of the 4-f…
We study vertex corrections to the leptonic electroweak observables in the general MSSM at tan(beta) < 35. In particular, we address the question of whether supersymmetry can be responsible for the observed 2 sigma deviation from the Standard Model prediction in the invisible width of the Z. We find that the presence of a light (around 100 GeV) chargino and sleptons hinted by the muon g-2 measurements makes the agreement with experiment slightly better and improves the electroweak fit.
We analyze the bounds on the Higgs pseudo-observables following from electroweak constraints, under the assumption that the Higgs particle is the massive excitation of an $SU(2)_L$ doublet. Using such bounds, detailed predictions for $h\to 4\ell$ decay rates, dilepton spectra, and lepton-universality ratios are presented.
We compute the next-to-leading order QCD and electroweak corrections to $Z$ and $W$ pole observables using the dimension-6 Standard Model effective field theory and present numerical results that can easily be included in global fitting programs. Limits on SMEFT coefficient functions are presented at leading order and at next-to-leading order under several assumptions.
The presence of an extra dimension of size R \approx TeV^{-1} introduces a tower of Kaluza-Klein gauge boson excitations that affects the standard model (SM) relations between electroweak observables. The mixing of the W and Z bosons with their excitations changes their masses and couplings to fermions. This effect depends on the Higgs field, which may live in the bulk of the extra dimension, on its boundary, or may be a combination of both types of fields. We use high-precision electroweak data to constrain 1/R. We find limits from 1 to 3 TeV from different observables, with a model independe…
The leading electroweak MSSM two-loop corrections to the electroweak precision observables are calculated. They are obtained by evaluating the two-loop O(α_t^2), O(α_t α_b), O(α_b^2) contributions to the quantity Δρin the limit of heavy scalar quarks, i.e. we consider the contributions of a Two-Higgs-Doublet model with MSSM restrictions. The full analytic result for arbitrary values of the lightest CP-even Higgs boson mass is presented. The numerical effects of the leading electroweak MSSM two-loop corrections on the precision observables M_W and sin^2(theta_eff) are analyzed. The electroweak …
We briefly review recent progress in the analysis of the Higgs sector of the Minimal R-symmetric Supersymmetric Standard Model. Importance of the interplay between W and Higgs boson masses in constraining the parameter space of the model is shown.
We compute the bounds from precision observables on alternative theories of electroweak symmetry breaking. We show that a cut-off as large as 3 TeV can be accomodated by the present data, without unnatural fine tuning.
We perform the fit of electroweak precision observables within the Standard Model with a 126 GeV Higgs boson, compare the results with the theoretical predictions and discuss the impact of recent experimental and theoretical improvements. We introduce New Physics contributions in a model-independent way and fit for the S, T and U parameters, for the $ε_{1,2,3,b}$ ones, for modified $Zb\bar{b}$ couplings and for a modified Higgs coupling to vector bosons. We point out that composite Higgs models are very strongly constrained. Finally, we compute the bounds on dimension-six operators relevant fo…
The dispersive approach to QCD is briefly overviewed and its application to the assessment of hadronic contributions to electroweak observables is discussed.
Is the weak scale natural? This ever pending question makes the search for new particle production a highly motivated primary goal of the next LHC phase. These searches may or may not be successful. While waiting for a needed higher energy collider to extend the direct exploration, the search for signs of new physics might be confined to indirect tests for quite some time. In a few fully calculable models, weakly or semi-strongly interacting, we compare the significance to measure the Higgs couplings versus the electroweak observables.
Measurements of electroweak precision observables at future electron-positron colliders, such as the CEPC, FCC-ee, and ILC, will be sensitive to physics at multi-TeV scales. To achieve this sensitivity, precise predictions for the Standard Model expectations of these observables are needed, including corrections at the three- and four-loop level. In this article, results are presented for the calculation of a subset of three-loop mixed electroweak-QCD corrections, stemming from diagrams with a gluon exchange and two closed fermion loops. The numerical impact of these corrections is illustrated…
In these proceedings we briefly review the basic concepts underlying indirect bounds on the Wilson coefficients of some Standard Model dimension-6 operators, relevant to electroweak and Higgs observables, obtained via renormalization group (RG) mixing to strongly constrained observables. With the present data we derive RG-induced bounds, stronger than the direct constraints, on some Higgs couplings and anomalous triple gauge couplings. Any deviation from these bounds would suggest a particular pattern of correlations among the Wilson coefficients, thus offering a new window on the new physics …
We perform a complete study of flavour and CP conserving electroweak observables in a slight refinement of a recently proposed five--dimensional model on R^4XS^1/Z_2, where the Higgs is the internal component of a gauge field and the Lorentz symmetry is broken in the fifth dimension. Interestingly enough, the relevant corrections to the electroweak observables turn out to be of universal type and essentially depend only on the value of the Higgs mass and on the scale of new physics, in our case the compactification scale 1/R. The model passes all constraints for 1/R > 4.7 TeV at 90% C.L., with…
Indirect information about the possible scale of supersymmetry (SUSY) breaking is provided by B physics observables (BPO) as well as electroweak precision observables (EWPO). We review the combination of the constraints imposed by recent measurements of the BPO BR(b -> s gamma), BR(B_s -> mu+ mu-), BR(B_u -> tau nu_tau) and Delta M_{B_s} with those obtained from the experimental measurements of the EWPO M_W, sin^2 theta_eff, Gamma_Z, (g-2)_mu and M_h. We perform a chi^2 fit to the parameters of the constrained minimal supersymmetric extension of the Standard Model (CMSSM), in which the SUSY-br…
High energy observables are characterized by large electroweak radiative corrections of infrared origin; double logarithms are present even for inclusive cross sections, thus violating the Bloch Nordsieck theorem. This effect, related to the initial states carrying nonabelian isospin charges, is here investigated for {\sl any} inclusive cross section in the SU(2)$\otimes$U(1) symmetric limit, that is appropriate for energies much higher than the weak scale. We develop a general formalism allowing to calculate the all order double log resummed cross sections once the hard (tree level) ones are …
Little Higgs theories, in which the Higgs particle is realized as the pseudo-Goldstone boson of an approximate global chiral symmetry have generated much interest as possible alternatives to weak scale supersymmetry. In this paper we analyze precision electroweak observables in the Minimal Moose model and find that in order to be consistent with current experimental bounds, the gauge structure of this theory needs to be modified. We then look for viable regions of parameter space in the modified theory by calculating the various contributions to the S and T parameters.
We calculate the contributions to electroweak precision observables (EWPOs) due to a fourth generation of fermions with the most general (quark-)flavour structure (but assuming Dirac neutrinos and a trivial flavour structure in the lepton sector). The new-physics contributions to the EWPOs are calculated at one-loop order using automated tools (FeynArts/FormCalc). No further approximations are made in our calculation. We discuss the size of non-oblique contributions arising from Z--quark--anti-quark vertex corrections and the dependence of the EWPOs on all CKM mixing angles involving the fourt…
Higher-order results for electroweak precision observables in the Standard Model are analyzed in view of the experimental accuracies achievable at the present and the next generation of colliders, and the indirect prediction of the Higgs-boson mass from the precision data is discussed. Within the MSSM, two-loop results for the lightest CP-even Higgs-boson mass are confronted with the exclusion limits from LEP. Possible precision tests of the MSSM at a future linear collider are furthermore investigated.
The simple independent quark models have difficulties explaining simultaneously the totality of the known hyperon magnetic moments and hyperon semi-leptonic decay rates. We show that both the Goldstone boson loop contributions and the two-quark effective exchange currents are essential in explaining these observables.
Constraining new physics effective interactions via a global fit of electroweak, Drell-Yan, Higgs, top, and flavour observables
Universidad de Granada（格拉纳达大学）；Florida State University（佛罗里达州立大学）；Universitat d’Alacant（阿利坎特大学）；University of Manchester（曼彻斯特大学）；INFN, Sezione di Roma（罗马国家核物理研究所）
In the framework of general 3-3-1 models, we perform a global $χ^2$ fit at $95\%$ C.L. to the updated Z pole observables, including new data from atomic parity violation. We found scenarios where specific realizations of 3-3-1 models are largely excluded above the current limit on the extra neutral gauge boson ($Z'$) research at LHC. We derive a relation between the $Z-Z'$ mixing angle and the parameter $\tan(β)$ of the effective Two-Higgs-Doublet Model contained into the 3-3-1 spectrum. We show that particular versions of 3-3-1 models containing particles with exotic electric charges are disf…
Large top quark mass is responsible for the enhancement of the oblique radiative corrections in SUSY models. We present the analytical formulas for these corrections to the $W$-boson mass $m_W$ and to $Z$ $l^+ l^-$ coupling constants. The comparison with the result of the Standard Model fit is made.
We calculate the two-loop QCD correction to the scalar quark contributions to the electroweak gauge-boson self-energies at zero momentum-transfer in the supersymmetric extension of the Standard Model. We then derive the $O(α_s)$ correction to the contribution of the scalar top and bottom quark loops to the rho parameter, which is the most sizable supersymmetric contribution to the electroweak mixing angle and the W-boson mass. The two-loop corrections modify the one-loop contribution by up to 30%; the gluino decouples for large masses. Contrary to the SM case where the QCD corrections are nega…
The evaluation of the two-loop MSSM-contributions to the electroweak precision observables M_W and s_w^eff at order(alpha_t^2), order(alpha_t alpha_b), order(alpha_b^2) is presented. These contributions enter via Delta(rho), and it is explained in detail how one can retain the true, non-vanishing value of the MSSM Higgs boson mass M_h in spite of using the gauge-less limit in the calculation. The numerical results can be sizeable, in particular for strong squark mixing. By comparing the results in the on-shell and DR-bar renormalization schemes, the remaining theoretical uncertainty is found t…
In the supersymmetric extension of the Standard Model we derive the two-loop QCD corrections to the scalar quark contributions to the electroweak precision observables entering via the rho parameter. A very compact expression is derived for the gluon-exchange contribution. The complete analytic result for the gluino-exchange contribution is very lengthy; we give expressions for several limiting cases that were derived from the general result. The two-loop corrections, generally of the order of 10 to 30% of the one-loop contributions, canbe very significant. Contrary to the Standard Model case,…
We derive dispersion relations for the electroweak oblique observables measured at LEP in the context of $SO(5)/SO(4)$ composite Higgs models. It is shown how these relations can be used and must be modified when modeling the spectral functions through a low-energy effective description of the strong dynamics. The dispersion relation for the parameter $ε_3$ is then used to estimate the contribution from spin-1 resonances at the 1-loop level. Finally, it is shown that the sign of the contribution to the $\hat S$ parameter from the lowest-lying spin-1 states is not necessarily positive definite,…
The current status of the theoretical predictions for the electroweak precision observables M_W, sin^2_theta_eff and m_h within the MSSM is briefly reviewed. The impact of recent electroweak two-loop results to the quantity Delta_rho is analysed and the sensitivity of the electroweak precision observables to the top-quark Yukawa coupling is investigated. Furthermore the level of precision necessary to match the experimental accuracy at the next generation of colliders is discussed.
We calculate the typical size of loop corrections to electroweak observables arising from non-standard $Z {\overline t } t$ and $W t b$ vertices. We use an effective Lagrangian formalism based on the electroweak gauge group $SU(2)_L\times U(1)_Y \rightarrow U(1)_{EM}$. Limits on the non-standard model top quark couplings from electroweak observables are presented and compared with previously obtained limits.
Asymptotic safety offers a field theory based UV completion to gravity. For low Planck scales, gravitational effects on low-energy precision observables cannot be neglected. We compute the contribution to the rho parameter from asymptotically safe gravitons and find that in contrast to effective theory, constraints on models with more than three extra dimensions are significantly weakened. The relative size of the trans-Planckian contribution increases proportional to the number of extra dimensions.
The two-loop QCD corrections to the $ρ$ parameter are derived in the Minimal Supersymmetric Standard Model. They turn out to be sizable and modify the one-loop result by up to 30%. Furthermore exact results for the gluonic corrections to $Δr$ are presented and compared with the leading contribution entering via the $ρ$ parameter.
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 $ε_{B,W}$ parameters and the aid of some other parameters to cancel the negative $T^{\prime}$, the recent CDF $W$-mass anomaly can therefore be expl…
The leading corrections to electroweak precision observables in the MSSM with non-minimal flavor violation (NMFV) are calculated and the effects on M_W and sin^2 theta_eff are analyzed. The corrections are obtained by evaluating the full one-loop contributions from the third and second generation scalar quarks, including the mixing in the scalar top and charm, as well as in the scalar bottom and strange sector. Furthermore the leading corrections to the mass of the lightest MSSM Higgs boson, m_h, is obtained. The electroweak one-loop contribution to M_W can amount up to 140 MeV and up to 70x10…
Dispersive approach to quantum chromodynamics is applied to the assessment of hadronic contributions to electroweak observables. The employed approach merges the corresponding perturbative input with intrinsically nonperturbative constraints, which originate in the respective kinematic restrictions. The evaluated hadronic contributions to the muon anomalous magnetic moment and to the shift of the electromagnetic fine structure constant at the scale of Z boson mass conform with recent assessments of these quantities.
The hadronic contributions to the muon anomalous magnetic moment and to the shift of the electromagnetic fine structure constant at the scale of Z boson mass are evaluated within dispersively improved perturbation theory (DPT). The latter merges the corresponding perturbative input with intrinsically nonperturbative constraints, which originate in the respective kinematic restrictions. The obtained results conform with recent assessments of the quantities on hand.
Indirect information about the possible scale of supersymmetry (SUSY) breaking can be obtained from the comparison of precisely measured observables (and also of exclusion limits) with accurate theory predictions incorporating SUSY loop corrections. Recent results are reviewed obtained from a combined analysis of the most sensitive electroweak precision observables (EWPO), M_W, sin^2_theta^eff, Gamma_Z, (g-2)_μand M_h, and B-physics observables (BPO), BR(b -> s γ), BR(B_s -> μ^+μ^-), BR(B_u -> τν_τ) and ΔM_{B_s}. Assuming that the lightest supersymmetric particle (LSP) provides the cold dark m…
I discuss the importance of the $O(g^4 m_t^2/\mw^2)$ corrections to the effective electroweak angle and Mw in the indirect determination of the Higgs mass. I emphasize the rôle of a very precise Mw measurement on the Mh estimate.
The leading electroweak MSSM two-loop corrections to the $ρ$-parameter are calculated. They are obtained by evaluating the two-loop self-energies of the Z and the W boson at O(Gf^2 m_t^4) in the limit of heavy scalar quarks. A very compact expression is derived, depending on the ratio of the CP-odd Higgs boson mass, M_A, and the top quark mass, m_t. Expressions for the limiting cases M_A >> m_t and M_A << m_t are also given. The decoupling of the non-SM contribution in the limit M_A -> oo is verified at the two-loop level. The numerical effect of the leading electroweak MSSM two-loop correctio…
