Electroweak Constraints Research Articles

B physics constraints on a flavor symmetric scalar model to account for the [formula omitted] asymmetry and Wjj excess at CDF

Recently Nelson et al. proposed an interesting flavor symmetric model to account for the top quark forward–backward asymmetry and the dijet anomaly at CDF simultaneously with just three parameters: a coupling constant of order one, and two scalar masses of 160 GeV and 220 GeV. However these fiducial values of the parameters lead to the branching ratio of a almost pure penguin B→πK decay about one hundred times larger than the experimental results. Consider also the precision electroweak constraints, the scalar masses should be at least around 500 GeV. Actually with the coupling constant larger than one, it is impossible to explain either of the two CDF measurements consistently in this model. But one may raise the charged scalar mass to, for example, 250 GeV and reduce the coupling strength to 0.6 to meet the B physics constraints. With this parameter set, the Wjj cross section is found to be in the right range. But due to the scalar mass splitting, its correction to T-parameter is about 3σ away from the precision electroweak constraints. In addition, the top quark forward–backward asymmetry should be well below 0.1 with this small coupling constant.

Electroweak constraints on new physics

AbstractWe briefly review the limits on new interactions implied by electroweak precision data. Special attention is payed to the bounds on the Higgs boson mass. We also comment on the required cancellation among the new contributions to precisely measured electroweak observables in any Standard Model extension, if the new particles have to evade the indirect constraints on their couplings and masses but still remain at the LHC reach.

Bulk Higgs and gauge fields in a multiply warped braneworld model

We readdress the problems associated with bulk Higgs and the gauge fields in a 5-dimensional Randall-Sundrum model by extending the model to six dimensions with double warping along the two extra spatial dimensions. In this 6-dimensional model we have a freedom of two moduli scales as against one modulus in the 5-dimensional model. With a little hierarchy between these moduli we can obtain the right magnitude for $W$ and $Z$ boson masses from the Kaluza-Klein modes of massive bulk gauge fields where the spontaneous symmetry breaking is triggered by bulk Higgs . We also have determined the gauge couplings of the standard model fermions with Kaluza-Klein modes of the gauge fields. Unlike the case of 5-dimensional model with a massless bulk gauge field, here we have shown that the gauge couplings and the masses of the Kaluza-Klein gauge fields satisfy the precision electroweak constraints and also obey the Tevatron bounds.

Axigluons cannot explain the observed top quark forward-backward asymmetry

We study an SU(3)^2 axigluon model introduced by Frampton, Shu, and Wang to explain the recent Fermilab Tevatron observation of a significant positive enhancement in the top quark forward-backward asymmetry relative to standard model predictions. First, we demonstrate that data on neutral B_d-meson mixing excludes the region of model parameter space where the top asymmetry is predicted to be the largest. Keeping the gauge couplings below the critical value that would lead to fermion condensation imposes further limits at large axigluon mass, while precision electroweak constraints on the model are relatively mild. Furthermore, by considering an extension to an SU(3)^3 color group, we demonstrate that embedding the model in an extra-dimensional framework can only dilute the axigluon effect on the forward-backward asymmetry. We conclude that axigluon models are unlikely to be the source of the observed top quark asymmetry.

Constraints on D Dimensional Warped Spaces

In order to investigate the phenomenological implications of allowing gauge fields to propagate in warped spaces of more than five dimensions, we consider a toy model of a space warped by the presence of a anisotropic bulk cosmological constant. After solving the Einstein equation, three classes of solutions are found, those in which the additional (D > 5) dimensions are growing, shrinking or remaining constant. It is found that gauge fields propagating in these spaces have a significantly different Kaluza Klein (KK) mass spectrum and couplings from that of the Randall and Sundrum model. This leads to a greatly reduced lower bound on the KK scale, arising from electroweak constraints, for spaces growing towards the IR brane.

Electroweak constraints on warped geometry in five dimensions and beyond

Here we consider the tree level corrections to electroweak (EW) observables from standard model (SM) particles propagating in generic warped extra dimensions. The scale of these corrections is found to be dominated by three parameters, the Kaluza-Klein (KK) mass scale, the relative coupling of the KK gauge fields to the Higgs and the relative coupling of the KK gauge fields to fermion zero modes. It is found that 5D spaces that resolve the hierarchy problem through warping typically have large gauge-Higgs coupling. It is also found in $D>5$ where the additional dimensions are warped the relative gauge-Higgs coupling scales as a function of the warp factor. If the warp factor of the additional spaces is contracting towards the IR brane, both the relative gauge-Higgs coupling and resulting EW corrections will be large. Conversely EW constraints could be reduced by finding a space where the additional dimension's warp factor is increasing towards the IR brane. We demonstrate that the Klebanov Strassler solution belongs to the former of these possibilities.

Electroweak constraints from atomic parity violation and neutrino scattering

Precision electroweak physics can provide fertile ground for uncovering new physics beyond the standard model (SM). One area in which new physics can appear is in so-called ``oblique corrections,'' i.e., next-to-leading-order expansions of bosonic propagators corresponding to vacuum polarization. One may parametrize their effects in terms of quantities $S$ and $T$ that discriminate between conservation and nonconservation of isospin. This provides a means of comparing the relative contributions of precision electroweak experiments to constraints on new physics. Given the prevalence of strongly $T$-sensitive experiments, there is an acute need for further constraints on $S$, such as provided by atomic parity-violating experiments on heavy atoms. We evaluate constraints on $S$ arising from recently improved calculations in the Cs atom. We show that the top quark mass ${m}_{t}$ provides stringent constraints on $S$ within the context of the SM. We also consider the potential contributions of next-generation neutrino scattering experiments to improved $(S,T)$ constraints.

Once more on extra quark-lepton generations and precision measurements

Precisionmeasurements of Z-boson parameters andW-boson and t-quark masses put strong constraints on non SU(2) × U(1) singlet New Physics.We demonstrate that one extra generation passes electroweak constraints even when all new particle masses are well above their direct mass bounds.

Semileptonic decays of light quarks beyond the Standard Model

We describe non-standard contributions to semileptonic processes in a model independent way in terms of an SU ( 2 ) L × U ( 1 ) Y invariant effective lagrangian at the weak scale, from which we derive the low-energy effective lagrangian governing muon and beta decays. We find that the deviation from Cabibbo universality, Δ CKM ≡ | V u d | 2 + | V u s | 2 + | V u b | 2 − 1 , receives contributions from four effective operators. The phenomenological bound Δ CKM = ( − 1 ± 6 ) × 10 − 4 provides strong constraints on all four operators, corresponding to an effective scale Λ > 11 TeV (90% CL). Depending on the operator, this constraint is at the same level or better then the Z pole observables. Conversely, precision electroweak constraints alone would allow universality violations as large as Δ CKM = − 0.01 (90% CL). An observed Δ CKM ≠ 0 at this level could be explained in terms of a single four-fermion operator which is relatively poorly constrained by electroweak precision measurements.

Realistic composite Higgs models

We study the role of fermionic resonances in realistic composite Higgs models. We consider the low energy effective description of a model in which the Higgs arises as the pseudo-Goldstone boson of an SO(5)/SO(4) global symmetry breaking pattern. Assuming that only fermionic resonances are present below the cut-off of our effective theory, we perform a detailed analysis of the electroweak constraints on such a model. This includes the exact one-loop calculation of the T parameter and the anomalous Zbb coupling for arbitrary new fermions and couplings. Other relevant observables, like b to s gamma and Delta B=2 processes have also been examined. We find that, while minimal models are difficult to make compatible with electroweak precision tests, models with several fermionic resonances, such as the ones that appear in the spectrum of viable composite Higgs models from warped extra dimensions, are fully realistic in a large region of parameter space. These fermionic resonances could be the first observable signature of the model at the LHC.

The soft-wall standard model

We explore the possibility of modeling electroweak physics in a warped extra dimension with a soft wall. The infrared boundary is replaced with a smoothly varying dilaton field that provides a dynamical spacetime cutoff. We analyze gravity, gauge fields, and fermions in the soft-wall background and obtain a discrete spectrum of Kaluza-Klein states which can exhibit linear Regge-like behavior. Bulk Yukawa interactions give rise to nonconstant fermion mass terms, leading to fermion localization in the soft-wall background and a possible explanation of the standard model flavor structure. Furthermore we construct electroweak models with custodial symmetry, where the gauge symmetry is broken with a bulk Higgs condensate. The electroweak constraints are not as stringent as in hard-wall models, allowing Kaluza-Klein masses of order the TeV scale.

Triplet extended supersymmetric standard model

We revisit an extension of the minimal supersymmetric standard model (MSSM) by adding a hypercharge-neutral, $SU(2)$-triplet chiral superfield. Similar to the next-to-minimal supersymmetric standard model , the triplet gives an additional contribution to the quartic coupling in the Higgs potential, and the mass of the lightest $CP$-even Higgs boson can be greater than ${M}_{Z}$ at tree level. In addition to discussing the perturbativity, fine-tuning, and decoupling issues of this model, we compute the dominant 1-loop corrections to the mass of the lightest $CP$-even Higgs boson from the triplet sector. When the Higgs-Higgs-Triplet coupling in the superpotential is comparable to the top Yukawa coupling, we find that the Higgs mass can be as heavy as 140 GeV even without the traditional contributions from the top--s-top sector, and at the same time consistent with the precision electroweak constraints. At the expense of having Landau poles before the grand unified theory scale, this opens up a previously forbidden region in the MSSM parameter space where both s-tops are light. In addition to having relatively small fine-tuning (about one part in 30), this leads to a gluophilic Higgs boson whose production via gluon-gluon fusion at the CERN LHC can be twice as large as the standard model prediction.

Benchmarks for new strong interactions at the LHC

New strong interactions at the LHC may exhibit a richer structure than expected from simply rescaling QCD to the electroweak scale. In fact, a departure from rescaled QCD is required for compatibility with electroweak constraints. To navigate the space of possible scenarios, we use a simple framework, based on a 5D model with modifications of AdS geometry in the infrared. In the parameter space, we select two points with particularly interesting phenomenology. For these benchmark points, we explore the discovery of triplets of vector and axial resonances at the LHC.

Production of single heavy charged leptons at a linear collider

A sequential fourth generation of quarks and leptons is allowed by precision electroweak constraints if the mass splitting between the heavy quarks is between 50 and 80 GeV. Although heavy quarks can be easily detected at the LHC, it is very difficult to detect a sequential heavy charged lepton, $L$, due to large backgrounds. Should the $L$ mass be above 250 GeV, it cannot be pair-produced at a 500 GeV ILC. We calculate the cross section for the one-loop process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}L\ensuremath{\tau}$. Although the cross section is small, it may be detectable. We also consider contributions from the two-Higgs doublet model and the Randall-Sundrum model, in which case the cross section can be substantially higher.

Electroweak precision constraints on the Lee-Wick standard model

We perform an analysis of the electroweak precision observables in the Lee-Wick Standard Model. The most stringent restrictions come from the S and T parameters that receive important tree level and one loop contributions. In general the model predicts a large positive S and a negative T. To reproduce the electroweak data, if all the Lee-Wick masses are of the same order, the Lee-Wick scale is of order 5 TeV. We show that it is possible to find some regions in the parameter space with a fermionic state as light as 2.4-3.5 TeV, at the price of rising all the other masses to be larger than 5-8 TeV. To obtain a light Higgs with such heavy resonances a fine-tuning of order a few per cent, at least, is needed. We also propose a simple extension of the model including a fourth generation of Standard Model fermions with their Lee-Wick partners. We show that in this case it is possible to pass the electroweak constraints with Lee-Wick fermionic masses of order 0.4-1.5 TeV and Lee-Wick gauge masses of order 3 TeV.

Exceptional electroweak model

We consider a gauge extension of the electroweak sector of the standard model based on the group ${\mathrm{G}}_{2}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$. The exceptional group ${\mathrm{G}}_{2}$ is the smallest rank two group that contains SU(3) as a subgroup; the SU(3) prediction ${sin}^{2}{\ensuremath{\theta}}_{w}=1/4$ follows approximately in this model if the couplings of the additional SU(2) and U(1) factors are sufficiently large. We study the symmetry-breaking sector of the model, the bounds from precision electroweak constraints and the mass spectrum of exotic gauge bosons that may be produced at future colliders. We also discuss a SU(3) electroweak model in which a vectorlike sector is included explicitly to facilitate the decays of otherwise stable exotic states. The models considered here represent plausible extensions of the minimal SU(3) electroweak model with potentially distinctive TeV-scale phenomenology.

SUPERSYMMETRIC EFFECTS IN FLAVOUR PHYSICS

After a short introduction about the flavour problem and the MFV hypothesis, we discuss Bu → τν and other low-energy observables in the framework of the MSSM with MFV, large tan β, and heavy squarks [Formula: see text] . This scenario naturally satisfies all the electroweak precision constraints and, in the case of not too heavy slepton sector [Formula: see text], can also easily accommodate the (g − 2)μ anomaly. Within this framework, non-standard effects could possibly be detected in the near future in a few low-energy flavour-violating observables, such as [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]. The possibility to further constrain this scenario using dark-matter constraints is also briefly discussed.

Top hypercharge

We propose a top hypercharge model with gauge symmetry SU(3)C × SU(2)L × U(1)1 × U(1)2 where the first two families of the Standard Model (SM) fermions are charged under U(1)1 while the third family is charged under U(1)2. The U(1)1 × U(1)2 gauge symmetry is broken down to the U(1)Ygauge symmetry, when a SM singlet Higgs field acquires a vacuum expectation value. We consider the electroweak constraints, and compare the fit to experimental observables to that of the SM. We study the quark CKM mixing between the first two families and the third family, the neutrino masses and mixing, the flavour changing neutral current effects in meson mixing and decays, the Z' discovery potential at the Large Hadron Collider, the dark matter with a gauged Z2 symmetry, and the Higgs boson masses.

Next-to-minimal supersymmetric model solution to the fine-tuning problem, precision electroweak constraints, and the largest CERN LEP Higgs event excess

We present an extended study of how the next-to-minimal supersymmetric model easily avoids fine-tuning in electroweak symmetry breaking for a SM-like light Higgs with mass in the vicinity of 100 GeV, as beautifully consistent with precision electroweak data, while escaping LEP constraints due to the dominance of h{yields}aa decays with m{sub a} 30 GeV, and decays primarily to {gamma}{gamma} leading to an h{yields}aa{yields}4{gamma} Higgs signal.

Color octet scalar production at the CERN LHC

New physics at the weak scale that can couple to quarks typically gives rise to unacceptably large flavor changing neutral currents. An attractive way to avoid this problem is to impose the principle of minimal flavor violation. Recently it was noted that in minimal flavor violation only scalars with the same gauge quantum numbers as the standard model Higgs doublet or color octet scalars with the same weak quantum numbers as the Higgs doublet can couple to quarks. In this paper we compute the one-loop rate for production of a single color octet scalar through gluon fusion at the LHC, which can become greater than the tree level pair production rate for octet scalar masses around a TeV. We also calculate the precision electroweak constraint from Z-->[overline b]b; this constraint on color octet mass and Yukawa coupling affects the allowed range for single octet scalar production through gluon fusion.