Showing papers on "Electroweak interaction published in 2007"

The Strongly-Interacting Light Higgs

Abstract: We develop a simple description of models where electroweak symmetry breaking is triggered by a light composite Higgs, which emerges from a strongly-interacting sector as a pseudo-Goldstone boson. Two parameters broadly characterize these models: mρ, the mass scale of the new resonances and gρ, their coupling. An effective low-energy Lagrangian approach proves to be useful for LHC and ILC phenomenology below the scale mρ. We identify two classes of operators: those that are genuinely sensitive to the new strong force and those that are sensitive to the spectrum of the resonances only. Phenomenological prospects for the LHC and the ILC include the study of high-energy longitudinal vector boson scattering, strong double-Higgs production and anomalous Higgs couplings. We finally discuss the possibility that the top quark could also be a composite object of the strong sector.

Techniques for the calculation of electroweak radiative corrections at the one-loop level and results for W-physics at LEP200

Abstract: We review the techniques necessary for the calculation of virtual electroweak and soft photonic corrections at the one-loop level. In particular we describe renormalization, calculation of one-loop integrals and evaluation of one-loop Feynman amplitudes. We summarize many explicit results of general relevance. We give the Feynman rules and the explicit form of the counter terms of the electroweak standard model, we list analytical expressions for scalar one-loop integrals and reduction of tensor integrals, we present the decomposition of the invariant matrix element for processes with two external fermions and we give the analytic form of soft photonic corrections. These techniques are applied to physical processes with external W-bosons. We present the full set of analytical formulae and the corresponding numerical results for the decay width of the W-boson and the top quark. We discuss the cross section for the production of W-bosons in e^{+}e^{-}-annihilaton including all O(alpha) radiative corrections and finite width effects. Improved Born approximations for these processes are given.

Light custodians in natural composite Higgs models

Abstract: We present a class of composite Higgs models arising from a warped extra dimension that can satisfy all the electroweak precision tests in a significant portion of their parameter space. A custodial symmetry plays a crucial role in keeping the largest corrections to the electroweak observables below their experimental limits. In these models the heaviness of the top quark is not only essential to trigger the electroweak symmetry breaking, but it also implies that the lowest top resonance and its custodial partners, the custodians, are significantly lighter than the other resonances. These custodians are the trademark of these scenarios. They are exotic colored fermions of electromagnetic charges $5/3$, $2/3$, and $\ensuremath{-}1/3$, with masses predicted roughly in the range 500--1500 GeV. We discuss their production and detection at the CERN LHC.

Singlet Higgs phenomenology and the electroweak phase transition

Abstract: We study the phenomenology of gauge singlet extensions of the Standard Model scalar sector and their implications for the electroweak phase transition. We determine the conditions on the scalar potential parameters that lead to a strong first order phase transition as needed to produce the observed baryon asymmetry of the universe. We analyze the constraints on the potential parameters derived from Higgs boson searches at LEP and electroweak precision observables. For models that satisfy these constraints and that produce a strong first order phase transition, we discuss the prospective signatures in future Higgs studies at the Large Hadron Collider and a Linear Collider. We argue that such studies will provide powerful probes of phase transition dynamics in models with an extended scalar sector.

Warped/composite phenomenology simplified

Abstract: This is the first of two papers aimed at economically capturing the collider phenomenology of warped extra dimensions with bulk Standard Model fields, where the hierarchy problem is solved non-supersymmetrically. This scenario is related via the AdS/CFT correspondence to that of partial compositeness of the Standard Model. We present a purely four-dimensional, two-sector effective field theory describing the Standard Model fields and just their first Kaluza-Klein/composite excitations. This truncation, while losing some of the explanatory power and precision of the full higher-dimensional warped theory, greatly simplifies phenomenological considerations and computations. We describe the philosophy and explicit construction of our two-sector model, and also derive formulas for residual Higgs fine tuning and electroweak and flavor precision variables to help identify the most motivated parts of the parameter space. We highlight several of the most promising channels for LHC exploration. The present paper focusses on the most minimal scenario, while the companion paper addresses the even richer phenomenology of the minimal scenario of precision gauge coupling unification.

Strong and electroweak corrections to the production of a Higgs boson+2 jets via weak interactions at the Large Hadron Collider.

Abstract: Radiative corrections of strong and electroweak interactions are presented at next-to-leading order for the production of a Higgs boson plus two hard jets via weak interactions at the CERN Large Hadron Collider. The calculation includes all weak-boson fusion and quark-antiquark annihilation diagrams as well as the corresponding interferences. The electroweak corrections, which are discussed here for the first time, reduce the cross sections by 5% and thus are of the same order of magnitude as the QCD corrections.

Higgs boson from an extended symmetry

Abstract: The variety of ideas put forward in the context of a composite picture for the Higgs boson calls for a simple and effective description of the related phenomenology. Such a description is given here by means of a minimal model and is explicitly applied to the example of a Higgs-top sector from an $SO(5)$ symmetry. We discuss the spectrum, the electroweak precision tests, B-physics, and naturalness. We show the difficulty in complying with the different constraints. The extended gauge sector relative to the standard $SU(2)\ifmmode\times\else\texttimes\fi{}U(1)$, if there is any, has little or no impact on these considerations. We also discuss the relation of the minimal model with its ``little Higgs'' or holographic extensions based on the same symmetry.

Four generations and Higgs physics

Abstract: In the light of the LHC, we revisit the implications of a fourth generation of chiral matter. We identify a specific ensemble of particle masses and mixings that are in agreement with all current experimental bounds as well as minimize the contributions to electroweak precision observables. Higgs masses between 115-315 (115-750) GeV are allowed by electroweak precision data at the 68% and 95% C.L. Within this parameter space, there are dramatic effects on Higgs phenomenology: production rates are enhanced, weak-boson-fusion channels are suppressed, angular distributions are modified, and Higgs pairs can be observed. We also identify exotic signals, such as Higgs decay to same-sign dileptons. Finally, we estimate the upper bound on the cutoff scale from vacuum stability and triviality.

The Bulk RS KK-gluon at the LHC

Abstract: We study the possibility of discovering and measuring the properties of the lightest Kaluza-Klein excitation of the gluon in a Randall-Sundrum scenario where the Standard Model matter and gauge fields propagate in the bulk. The KK-gluon decays primarily into top quarks. We discuss how to use the t final states to discover and probe the properties of the KK-gluon. Identification of highly energetic tops is crucial for this analysis. We show that conventional identification methods relying on well separated decay products will not work or heavy resonances but suggest alternative methods for top identification for energetic tops. We find, assuming good efficiency for this identification, that resonances with masses less than 5 TeV can be discovered if the algorithm to identify high pT tops can reject the QCD background by a factor of 10. We also find that for similar or lighter masses the spin can be determined and for lighter masses the chirality of the coupling to t can be measured. Since the energetic top pair final state is a generic signature for a large class of new physics as the top quark presumably couples most strongly to the electroweak symmetry breaking sector, the methods we have outlined to study the properties of the KK-gluon should also be important in other scenarios.

Electroweak constraints on warped models with custodial symmetry

Abstract: It has been recently argued that realistic models with warped extra dimensions can have Kaluza-Klein particles accessible at the CERN Large Hadron Collider if a custodial symmetry, $SU(2{)}_{V}\ifmmode\times\else\texttimes\fi{}{P}_{LR}$, is used to protect the $T$ parameter and the coupling of the left-handed bottom quark to the $Z$ gauge boson. In this article we emphasize that such a symmetry implies that the loop corrections to both the $T$ parameter and the $Z{b}_{L}{\overline{b}}_{L}$ coupling are calculable. In general, these corrections are correlated, can be sizable, and should be considered to determine the allowed parameter space region in models with warped extra dimensions and custodial symmetry, including Randall-Sundrum models with a fundamental Higgs, models of gauge-Higgs unification, and Higgsless models. As an example, we derive the constraints that arise on a representative model of gauge-Higgs unification from a global fit to the precision electroweak observables. A scan over the parameter space typically leads to a lower bound on the Kaluza-Klein excitations of the gauge bosons of about 2\char21{}3 TeV, depending on the configuration. In the fermionic sector one can have Kaluza-Klein excitations with masses of a few hundred GeV. We present the constraints on these light fermions from recent Tevatron searches, and explore interesting discovery channels at the LHC.

Folded supersymmetry and the LEP paradox

Abstract: We present a new class of models that stabilize the weak scale against radiative corrections up to scales of order 5 TeV without large corrections to precision electroweak observables. In these `folded supersymmetric' theories the one loop quadratic divergences of the Standard Model Higgs field are cancelled by opposite spin partners, but the gauge quantum numbers of these new particles are in general different from those of the conventional superpartners. This class of models is built around the correspondence that exists in the large N limit between the correlation functions of supersymmetric theories and those of their non-supersymmetric orbifold daughters. By identifying the mechanism which underlies the cancellation of one loop quadratic divergences in these theories, we are able to construct simple extensions of the Standard Model which are radiatively stable at one loop. Ultraviolet completions of these theories can be obtained by imposing suitable boundary conditions on an appropriate supersymmetric higher dimensional theory compactified down to four dimensions. We construct a specific model based on these ideas which stabilizes the weak scale up to about 20 TeV and where the states which cancel the top loop are scalars not charged under Standard Model color. Its collider signatures are distinct from conventional supersymmetric theories and include characteristic events with hard leptons and missing energy.

Precision electroweak calculation of the production of a high transverse-momentum lepton pair at hadron colliders

Abstract: We present a detailed study of the production of a high transverse-momentum lepton pair at hadron colliders, which includes the exact O(�) electroweak corrections properly matched with leading logarithmic effects due to multiple photon emission, as required by the experiments at the Fermilab Tevatron and the CERN LHC. Numerical results for the relevant observables of single Z-boson production at hadron colliders are presented. The impact of the radiative corrections is discussed in detail. The presence in the proton of a photon density is considered and the effects of the photon-induced partonic subprocesses are analyzed. The calculation has been implemented in the new version of the event generator HORACE, which is available for precision simulations of the neutral and charged current Drell-Yan processes.

Minimal walking technicolor: Setup for collider physics

Abstract: Different theoretical and phenomenological aspects of the minimal and nonminimal walking technicolor theories have recently been studied. The goal here is to make the models ready for collider phenomenology. We do this by constructing the low energy effective theory containing scalars, pseudoscalars, vector mesons, and other fields predicted by the minimal walking theory. We construct their self-interactions and interactions with standard model fields. Using the Weinberg sum rules, opportunely modified to take into account the walking behavior of the underlying gauge theory, we find interesting relations for the spin-one spectrum. We derive the electroweak parameters using the newly constructed effective theory and compare the results with the underlying gauge theory. Our analysis is sufficiently general such that the resulting model can be used to represent a generic walking technicolor theory not at odds with precision data.

Angular correlations of lepton pairs from vector boson and top quark decays in Monte Carlo simulations

Abstract: We explain how angular correlations in leptonic decays of vector bosons and top quarks can be included in Monte Carlo parton showers, in particular those matched to NLO QCD computations. We consider the production of n pairs of leptons, originating from the decays of n electroweak vector bosons or of n top quarks, in the narrow-width approximation. In the latter case, the information on the n b quarks emerging from the decays is also retained. We give results of implementing this procedure in MC@NLO.

Novel effects in electroweak breaking from a hidden sector

Abstract: The Higgs boson offers a unique window to hidden sector fields S{sub i}, singlets under the standard model gauge group, via the renormalizable interactions |H|{sup 2}S{sub i}{sup 2}. We prove that such interactions can provide new patterns for electroweak breaking, including radiative breaking by dimensional transmutation consistent with CERN LEP bounds, and trigger the strong enough first-order phase transition required by electroweak baryogenesis.

Radiative corrections to the semileptonic and hadronic Higgs-boson decays H→WW/ZZ→4 fermions

Abstract: The radiative corrections of the strong and electroweak interactions are calculated for the Higgs-boson decays H→WW/ZZ→4f with semileptonic or hadronic four-fermion final states in next-to-leading order. This calculation is improved by higher-order corrections originating from heavy-Higgs-boson effects and photonic final-state radiation off charged leptons. The W- and Z-boson resonances are treated within the complex-mass scheme, i.e. without any resonance expansion or on-shell approximation. The calculation essentially follows our previous study of purely leptonic final states. The electroweak corrections are similar for all four-fermion final states; for integrated quantities they amount to some per cent and increase with growing Higgs-boson mass MH, reaching 7–8% at MH ~ 500 GeV. For distributions, the corrections are somewhat larger and, in general, distort the shapes. Among the QCD corrections, which include corrections to interference contributions of the Born diagrams, only the corrections to the squared Born diagrams turn out to be relevant. These contributions can be attributed to the gauge-boson decays, i.e. they approximately amount to αs/π for semileptonic final states and 2αs/π for hadronic final states. The discussed corrections have been implemented in the Monte Carlo event generator PROPHECY4F.

Supersymmetry without a Light Higgs Boson

Abstract: Motivated by the absence, so far, of any direct signal of conventional low-energy supersymmetry, we explore the consequences of making the lightest Higgs boson in supersymmetry relatively heavy, up to about 300 GeV, in the most straightforward way, i.e. via the introduction of a chiral singlet S with a superpotential interaction with the Higgs doublets, {lambda}SH{sub 1}H{sub 2}. The coupling {lambda} dominates over all the other couplings and, to maintain the successful perturbative analysis of the electroweak precision tests, is only restricted to remain perturbative up to about 10 TeV. The general features of this '{lambda}SUSY' framework, which deviates significantly from the minimal supersymmetric standard model or the standard next to minimal supersymmetric standard model, are analyzed in different areas: electroweak precision tests, dark matter, naturalness bounds on superparticle masses, and LHC signals. There is a rich Higgs/Higgsino sector in the (200-700) GeV mass region, which may include LSP Higgsino dark matter. All other superpartners, apart from the top squarks, may naturally be heavier than 1-2 TeV.

Radion phenomenology in realistic warped space models

Abstract: We investigate the phenomenology of the Randall-Sundrum (RS) radion in realistic models of electroweak symmetry breaking with bulk gauge and fermion fields, since the radion may turn out to be the lightest particle in such models. We calculate the coupling of the radion in such scenarios to bulk fermion and gauge modes. Special attention needs to be devoted to the coupling to massless gauge fields (photon, gluon), since it is well known that loop effects may be important for these fields. We also present a detailed explanation of these couplings from the conformal field theory interpretation. We then use these couplings to determine the radion branching fractions and discuss some of the discovery potential of the LHC for the radion. We find that the $\ensuremath{\gamma}\ensuremath{\gamma}$ signal is enhanced over most of the range of the radion mass over the $\ensuremath{\gamma}\ensuremath{\gamma}$ signal of a standard model Higgs boson, as long as the RS scale is sufficiently low. However, the signal significance depends strongly on free parameters that characterize the magnitude of bare brane-localized kinetic terms for the massless gauge fields. In the absence of such terms, the signal can be enhanced over the traditional RS1 models (where all standard model fields are localized on the IR brane), but the signal can also be reduced compared to RS1 if the brane-localized terms are sizeable. We also show that for larger radion masses, where the $\ensuremath{\gamma}\ensuremath{\gamma}$ signal is no longer significant, one can use the usual 4 lepton signal to discover the radion.

Explaining the Electroweak Scale and Stabilizing Moduli in M Theory

Abstract: In a recent paper [1] it was shown that in M theory vacua without fluxes, all moduli are stabilized by the effective potential and a stable hierarchy is generated, consistent with standard gauge unification. This paper explains the results of [1] in more detail and generalizes them, finding an essentially unique de Sitter (dS) vacuum under reasonable conditions. One of the main phenomenological consequences is a prediction which emerges from this entire class of vacua: namely gaugino masses are significantly suppressed relative to the gravitino mass. We also present evidence that, for those vacua in which the vacuum energy is small, the gravitino mass, which sets all the superpartner masses, is automatically in the TeV - 100 TeV range.

Gauge-Higgs Unification and Radiative Electroweak Symmetry Breaking in Warped Extra Dimensions

Abstract: of large corrections to the T parameter and the coupling of the Z to the bottom quark. We demonstrate that electroweak symmetry breaking may be realized, with proper generation of the top and bottom quark masses for the same region of bulk mass parameters that lead to good agreement with precision electroweak data in the presence of a light Higgs. We compute the Higgs mass and demonstrate that for the range of parameters for which the Higgs boson has Standard Model-like properties, the Higgs mass is naturally in a range that varies between values close to the LEP experimental limit and about 160 GeV. This mass range may be probed at the Tevatron and at the LHC. We analyze the KK spectrum and briey discuss the phenomenology of the light resonances arising in our model.

The Supersymmetric Parameter Space in Light of B-physics Observables and Electroweak Precision Data

Abstract: 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 combine 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, incorporating the latest theoretical calculations of these observables within the Standard Model and supersymmetric extensions. We perform a chi^2 fit to the parameters of the constrained minimal supersymmetric extension of the Standard Model (CMSSM), in which the SUSY-breaking parameters are universal at the GUT scale, and the non-universal Higgs model (NUHM), in which this constraint is relaxed for the soft SUSY-breaking contributions to the Higgs masses. Assuming that the lightest supersymmetric particle (LSP) provides the cold dark matter density preferred by WMAP and other cosmological data, we scan over the remaining parameter space. Within the CMSSM, we confirm the preference found previously for a relatively low SUSY-breaking scale, though there is some slight tension between the EWPO and the BPO. In studies of some specific NUHM scenarios compatible with the cold dark matter constraint we investigate M_A-tan_beta planes and find preferred regions that have values of chi^2 somewhat lower than in the CMSSM.

Supersymmetric twin Higgs mechanism

Abstract: We present a supersymmetric realization of the twin Higgs mechanism, which cancels off all contributions to the Higgs mass generated above a scale f. Radiative corrections induced by the top-quark sector lead to a breaking of the twin sector electroweak symmetry at a scale f{approx}TeV. In our sector, below the scale f, these radiative corrections from the top quark are present but greatly weakened, naturally allowing a Z boson mass an order of magnitude below f, even with a top squark mass of order 1 TeV and a messenger scale near the Planck mass. A sufficient quartic interaction for our Higgs boson arises from the usual gauge contribution together with a radiative contribution from a heavy top squark. The mechanism requires the presence of an SU(2)-adjoint superfield, and can be simply unified. Naturalness in these theories is usually associated with light winos and sleptons, and is largely independent of the scale of the colored particles. The assumption of unification naturally predicts the existence of many exotic fields. The theory often has particles which may be stable on collider time scales, including an additional color octet superfield. In the limit that m{sub SUSY}>>f, the mechanism yields a UV completion of themore » nonsupersymmetric twin Higgs, with the notable improvement of a tree-level quartic for the standard model Higgs. In this framework, a successful UV completion requires the existence of new charged fields well below the scale f.« less

Twisted custodial symmetry in two-Higgs-doublet models.

Abstract: In the Standard Model for electroweak interactions, the Higgs sector is known to display a "custodial" symmetry protecting the mass relation m_{W^\pm}^2=m_{W_3}^2 from large corrections. When considering extensions of the scalar sector, this symmetry has to be introduced by hand in order to pass current electroweak precision tests in a natural way. In this Letter we implement a generalized custodial symmetry in the two-Higgs-doublet model. Assuming the invariance of the potential under CP transformations, we prove the existence of a new custodial scenario characterized by m_{H^\pm}^2=m_{H^0}^2 instead of m_{H^\pm}^2=m_{A^0}^2. Consequently the pseudoscalar A^0 may be much lighter than the charged H^\pm, giving rise to interesting phenomenology. Comment: 4 pages, RevTeX4

Neutrino mass in radiatively broken scale-invariant models

Abstract: Scale invariance may be a classical symmetry which is broken radiatively. This provides a simple way to stabilize the scale of electroweak symmetry breaking against radiative corrections. The simplest phenomenologically successful model of this type involves the addition of one real scalar field to the standard model. In this minimal model the electroweak Higgs can be interpreted as the pseudo-Goldstone boson of broken scale invariance. We study the possible origin of neutrino mass in such models, both at tree-level and radiatively.

A Confining Strong First-Order Electroweak Phase Transition

Abstract: In the Randall-Sundrum model where the radion is stabilized by a Goldberger-Wise (GW) potential there is a supercooled transition from a deconfined to a confined phase at temperatures orders of magnitude below the typical Standard Model critical temperature. When the Higgs is localized at the IR brane the electroweak phase transition is delayed and becomes a strong first-order one where the Universe expands by a few e-folds. This generates the possibility of having the out-of-equilibrium condition required by electroweak baryogenesis in the electroweak phase transition. We have studied numerically the region of the GW parameter space where the theory is consistent and the latter possibility is realized. We have found that in most of the parameter space the nucleation temperature is so low that sphalerons are totally inactive inside the bubbles. The condition for sphalerons to be inactive after reheating imposes an upper bound on the reheating temperature that is weaker for heavy Higgs bosons so that the out-of-equilibrium condition seems to favor heavy over light Higgses. The condition for sphalerons to be active outside the bubbles puts an upper bound on the number of e-folds at the phase transition, roughly consistent with the critical value required by low-scale inflation to solve the cosmological horizon problem.