Showing papers on "Electroweak interaction published in 2017"
TL;DR: In this article, an Effective Field Theory (EFT) approach based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons is proposed.
Abstract: Motivated by additional experimental hints of Lepton Flavour Universality violation in B decays, both in charged- and in neutral-current processes, we analyse the ingredients necessary to provide a combined description of these phenomena. By means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons, we show how this is possible. We demonstrate, in particular, how to solve the problems posed by electroweak precision tests and direct searches with a rather natural choice of model parameters, within the context of a U(2)q ×U(2)l flavour symmetry. We further exemplify the general EFT findings by means of simplified models with explicit mediators in the TeV range: coloured scalar or vector leptoquarks and colour-less vectors. Among these, the case of an SU(2)L-singlet vector leptoquark emerges as a particularly simple and successful framework.
375 citations
TL;DR: In this article, it was shown that the LHC provides unprecedented sensitivity to the ALP-photon and ALP-lepton couplings in the mass region above a few MeV, even if the relevant ALP couplings are loop suppressed and the a → ππππ branching ratios are significantly less than 1.
Abstract: Axion-like particles (ALPs), which are gauge-singlets under the Standard Model (SM), appear in many well-motivated extensions of the SM. Describing the interactions of ALPs with SM fields by means of an effective Lagrangian, we discuss ALP decays into SM particles at one-loop order, including for the first time a calculation of the a → πππ decay rates for ALP masses below a few GeV. We argue that, if the ALP couples to at least some SM particles with couplings of order (0.01 − 1) TeV−1, its mass must be above 1 MeV. Taking into account the possibility of a macroscopic ALP decay length, we show that large regions of so far unconstrained parameter space can be explored by searches for the exotic, on-shell Higgs and Z decays h → Za, h → aa and Z → γa in Run-2 of the LHC with an integrated luminosity of 300 fb−1. This includes the parameter space in which ALPs can explain the anomalous magnetic moment of the muon. Considering subsequent ALP decays into photons and charged leptons, we show that the LHC provides unprecedented sensitivity to the ALP-photon and ALP-lepton couplings in the mass region above a few MeV, even if the relevant ALP couplings are loop suppressed and the a → γγ and a → l+l− branching ratios are significantly less than 1. We also discuss constraints on the ALP parameter space from electroweak precision tests.
366 citations
TL;DR: The vector leptoquark representation, Uμ ¼ ð3; 1; 2=3Þ, was recently identified as an exceptional single-parameter model to address experimental hints on lepton flavor universality violation in semileptonic B-meson decays, both in neutral (b → sμμ) and charged (b→ cτν) current processes.
Abstract: The vector leptoquark representation, Uμ ¼ ð3; 1; 2=3Þ, was recently identified as an exceptional single
mediator model to address experimental hints on lepton flavor universality violation in semileptonic
B-meson decays, both in neutral (b → sμμ) and charged (b → cτν) current processes. Nonetheless, it is
well known that massive vectors crave an ultraviolet (UV) completion. We present the first full-fledged UV
complete and calculable gauge model which incorporates this scenario while remaining in agreement with
all other indirect flavor and electroweak precision measurements, as well as, direct searches at high-pT . The
model is based on a new non-Abelian gauge group spontaneously broken at the TeV scale, and a specific
flavor structure suppressing flavour violation in ΔF ¼ 2 processes while inducing sizeable semileptonic
transitions
270 citations
TL;DR: In this paper, the authors revisited their previous argument that electroweak bubble walls can "run away", that is, achieve extreme ultrarelativistic velocities γ ~ 1014.
Abstract: In extensions of the Standard Model with extra scalars, the electroweak phase transition can be very strong, and the bubble walls can be highly relativistic. We revisit our previous argument that electroweak bubble walls can "run away," that is, achieve extreme ultrarelativistic velocities γ ~ 1014. We show that, when particles cross the bubble wall, they can emit transition radiation. Wall-frame soft processes, though suppressed by a power of the coupling α, have a significance enhanced by the γ-factor of the wall, limiting wall velocities to γ ~ 1/α. Though the bubble walls can move at almost the speed of light, they carry an infinitesimal share of the plasma's energy.
231 citations
TL;DR: In this paper, a simple extension of the SM with an additional scalar singlet coupled to the Higgs boson was proposed, and the possible probes for electroweak baryogenesis in this model including collider searches, gravitational wave and direct dark matter detection signals.
Abstract: We analyse a simple extension of the SM with just an additional scalar singlet coupled to the Higgs boson. We discuss the possible probes for electroweak baryogenesis in this model including collider searches, gravitational wave and direct dark matter detection signals. We show that a large portion of the model parameter space exists where the observation of gravitational waves would allow detection while the indirect collider searches would not.
209 citations
TL;DR: The low-energy effective Lagrangian is constructed taking into account the running effects from Λ down to v through the one-loop renormalization group equations (RGEs) in the limit of exact electroweak symmetry and QED RGEs from v down to the 1 GeV scale.
Abstract: Lepton flavor universality (LFU) in B decays is revisited by considering a class of semileptonic operators defined at a scale Λ above the electroweak scale v. The importance of quantum effects, so far neglected in the literature, is emphasized. We construct the low-energy effective Lagrangian taking into account the running effects from Λ down to v through the one-loop renormalization group equations (RGEs) in the limit of exact electroweak symmetry and QED RGEs from v down to the 1 GeV scale. The most important quantum effects turn out to be the modification of the leptonic couplings of the vector boson Z and the generation of a purely leptonic effective Lagrangian. Large LFU breaking effects in Z and τ decays and visible lepton flavor violating effects in the processes τ→μll, τ→μρ, τ→μπ, and τ→μη^{(')} are induced.
209 citations
TL;DR: The newest SusHi version 1.6.0 allows to calculate the Higgs production cross section from the annihilation of heavy quarks and includes various new features which improve the gluon-fusion cross-section prediction and the associated uncertainty estimate.
206 citations
TL;DR: In this article, the correlation between the baryon-to-entropy ratio produced by electroweak baryogenesis and the gravitational wave signal from the electroweak phase transition was studied.
Abstract: We consider a real scalar singlet field which provides a strong first-order electroweak phase transition via its coupling to the Higgs boson, and gives a $CP$ violating contribution on the top quark mass via a dimension-6 operator. We study the correlation between the baryon-to-entropy ratio produced by electroweak baryogenesis, and the gravitational wave signal from the electroweak phase transition. We show that future gravitational wave experiments can test, in particular, the region of the model parameter space where the observed baryon-to-entropy ratio can be obtained even if the new physics scale, which is explicit in the dimension-6 operator, is high.
196 citations
TL;DR: In this paper, high energy measurements of Drell-Yan at the LHC can serve as electroweak precision tests, which can surpass LEP already with 8 TeV data.
183 citations
TL;DR: In this paper, the leading effective interactions between the Standard Model fields and a generic singlet CP-odd (pseudo-) Goldstone boson are studied. And the basis of leading effective operators is determined and compared with that for the linear expansion.
Abstract: We study the leading effective interactions between the Standard Model fields and a generic singlet CP-odd (pseudo-) Goldstone boson. Two possible frameworks for electroweak symmetry breaking are considered: linear and non-linear. For the latter case, the basis of leading effective operators is determined and compared with that for the linear expansion. Associated phenomenological signals at colliders are explored for both scenarios, deriving new bounds and analyzing future prospects, including LHC and High Luminosity LHC sensitivities. Mono-Z, mono-W, W-photon plus missing energy and on-shell top final states are most promising signals expected in both frameworks. In addition, non-standard Higgs decays and mono-Higgs signatures are especially prominent and expected to be dominant in non-linear realisations.
154 citations
TL;DR: In this paper, the authors studied the renormalization group evolution of new physics contributions to (semi)leptonic charged-current meson decays, focusing on operators involving a chirality flip at the quark level.
TL;DR: In this article, it was shown that the most important quantum effects are the modifications of the leptonic couplings of the W and Z vector bosons and the generation of a purely lepton effective Lagrangian.
Abstract: The growing experimental indication of Lepton Flavour Universality Violation (LFUV) both in charged- and neutral-current semileptonic B-decays, has triggered many theoretical interpretations of such non-standard phenomena. Focusing on popular scenarios where the explanation of these anomalies requires New Physics at the TeV scale, we emphasise the importance of including electroweak corrections to obtain trustable predictions for the models in question. We find that the most important quantum effects are the modifications of the leptonic couplings of the W and Z vector bosons and the generation of a purely leptonic effective Lagrangian. Although our results do not provide an inescapable no-go theorem for the explanation of the B anomalies, the tight experimental bounds on Z-pole observables and τ decays challenge an explanation of the current non-standard data. We illustrate how these effects arise, by providing a detailed discussion of the running and matching procedure which is necessary to derive the low-energy effective Lagrangian.
TL;DR: In this article, a complete analysis of the three muonic lepton-flavour violating processes μ → eγ, μ → 3e and coherent nuclear μ→ e conversion is performed in the framework of an effective theory with dimension six operators defined below the electroweak symmetry breaking scale m ≥ 0.
Abstract: In this article, a complete analysis of the three muonic lepton-flavour violating processes μ → eγ, μ → 3e and coherent nuclear μ → e conversion is performed in the framework of an effective theory with dimension six operators defined below the electroweak symmetry breaking scale m
W
. The renormalisation-group evolution of the Wilson coefficients between m
W
and the experimental scale is fully taken into account at the leading order in QCD and QED, and explicit analytic and numerical evolution matrices are given. As a result, muonic decay and conversion rates are interpreted as functions of the Wilson coefficients at any scale up to m
W
. Taking the experimental limits on these processes as input, the phenomenology of the mixing effects is investigated. It is found that a considerable set of Wilson coefficients unbounded in the simplistic tree-level approach are instead severely constrained. In addition, correlations among operators are studied both in the light of current data and future experimental prospects.
TL;DR: It is shown that, depending on the model parameters, a dramatically different scenario may happen: A first-order, six massless quark QCD phase transition occurs first, which then triggers the electroweak symmetry breaking, which potentially rich in cosmological consequences.
Abstract: If the electroweak sector of the standard model is described by classically conformal dynamics, the early Universe evolution can be substantially altered. It is already known that-contrarily to the standard model case-a first-order electroweak phase transition may occur. Here we show that, depending on the model parameters, a dramatically different scenario may happen: A first-order, six massless quark QCD phase transition occurs first, which then triggers the electroweak symmetry breaking. We derive the necessary conditions for this dynamics to occur, using the specific example of the classically conformal B-L model. In particular, relatively light weakly coupled particles are predicted, with implications for collider searches. This scenario is also potentially rich in cosmological consequences, such as renewed possibilities for electroweak baryogenesis, altered dark matter production, and gravitational wave production, as we briefly comment upon.
TL;DR: In this article, simple Two Higgs Doublet models might still provide a viable explanation for the matter-antimatter asymmetry of the Universe via electroweak baryogenesis, even after taking into account the recent order-of-magnitude improvement on the electron-EDM experimental bound by the ACME Collaboration.
Abstract: We show that simple Two Higgs Doublet models might still provide a viable explanation for the matter-antimatter asymmetry of the Universe via electroweak baryogenesis, even after taking into account the recent order-of-magnitude improvement on the electron-EDM experimental bound by the ACME Collaboration. Moreover we show that, in the region of parameter space where baryogenesis may be possible, the gravitational wave spectrum generated at the end of the electroweak phase transition is within the sensitivity reach of the future space-based interferometer LISA.
Australian Research Council1, Monash University2, University of Oslo3, University of Glasgow4, Polish Academy of Sciences5, University of Zurich6, Stockholm University7, McGill University8, University of Adelaide9, CERN10, École normale supérieure de Lyon11, University of California, Los Angeles12, University of Savoy13, Harvard University14, Spanish National Research Council15, University of Sydney16, Imperial College London17, University of Amsterdam18
TL;DR: In this article, the constrained minimal supersymmetric standard model (CMSSM) and its non-universal Higgs Mass generalisations NUHM1/NUHM2 were compared with a large collection of electroweak precision and flavour observables and direct searches for supersymmetry at LEP and runs I and II of the LHC.
Abstract: We present the most comprehensive global fits to date of three supersymmetric models motivated by grand unification: the constrained minimal supersymmetric standard model (CMSSM), and its Non-Universal Higgs Mass generalisations NUHM1 and NUHM2. We include likelihoods from a number of direct and indirect dark matter searches, a large collection of electroweak precision and flavour observables, direct searches for supersymmetry at LEP and Runs I and II of the LHC, and constraints from Higgs observables. Our analysis improves on existing results not only in terms of the number of included observables, but also in the level of detail with which we treat them, our sampling techniques for scanning the parameter space, and our treatment of nuisance parameters. We show that stau co-annihilation is now ruled out in the CMSSM at more than 95% confidence. Stop co-annihilation turns out to be one of the most promising mechanisms for achieving an appropriate relic density of dark matter in all three models, whilst avoiding all other constraints. We find high-likelihood regions of parameter space featuring light stops and charginos, making them potentially detectable in the near future at the LHC. We also show that tonne-scale direct detection will play a largely complementary role, probing large parts of the remaining viable parameter space, including essentially all models with multi-TeV neutralinos.
TL;DR: In this article, the authors investigated the possibility of a strong first order electroweak phase transition in the CP-conserving 2-Higgs-Doublet Model (2HDM) type I and type II where either of the CP even Higgs bosons is identified with the SM-like Higgs Boson.
Abstract: The discovery of the Higgs boson by the LHC experiments ATLAS and CMS has marked a milestone for particle physics. Yet, there are still many open questions that cannot be answered within the Standard Model (SM). For example, the generation of the observed matter-antimatter asymmetry in the universe through baryogenesis can only be explained qualitatively in the SM. A simple extension of the SM compatible with the current theoretical and experimental constraints is given by the 2-Higgs-Doublet Model (2HDM) where a second Higgs doublet is added to the Higgs sector. We investigate the possibility of a strong first order electroweak phase transition in the CP-conserving 2HDM type I and type II where either of the CP-even Higgs bosons is identified with the SM-like Higgs boson. The renormalisation that we apply on the loop-corrected Higgs potential allows us to efficiently scan the 2HDM parameter space and simultaneously take into account all relevant theoretical and up-to-date experimental constraints. The 2HDM parameter regions found to be compatible with the applied constraints and a strong electroweak phase transition are analysed systematically. Our results show that there is a strong interplay between the requirement of a strong phase transition and collider phenomenology with testable implications for searches at the LHC.
TL;DR: In this paper, the authors review the challenges that have been overcome in developing EFT methods for LHC studies, and discuss the developing SMEFT and HEFT approaches that are consistent versions of such EFTs, systematically improvable with higher order corrections, and comment on the pseudo-observable approach.
Abstract: Projecting measurements of the interactions of the known Standard Model (SM) states into an effective field theory (EFT) framework is an important goal of the LHC physics program. The interpretation of measurements of the properties of the Higgs-like boson in an EFT allows one to consistently study the properties of this state, while the SM is allowed to eventually break down at higher energies. In this review, basic concepts relevant to the construction of such EFTs are reviewed pedagogically. Electroweak precision data is discussed as a historical example of some importance to illustrate critical consistency issues in interpreting experimental data in EFTs. A future precision Higgs phenomenology program can benefit from the projection of raw experimental results into consistent field theories such as the SM, the SM supplemented with higher dimensional operators (the SMEFT) or an Electroweak chiral Lagrangian with a dominantly $J^P = 0^+$ scalar (the HEFT). We discuss the developing SMEFT and HEFT approaches, that are consistent versions of such EFTs, systematically improvable with higher order corrections, and comment on the pseudo-observable approach. We review the challenges that have been overcome in developing EFT methods for LHC studies, and the challenges that remain.
TL;DR: In this article, experimental and cosmological constraints on the extension of the Standard Model by three right-handed neutrinos with masses between those of the pion and W boson were studied.
TL;DR: In this article, the ingredients necessary to provide a combined description of B decays were analyzed by means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons.
Abstract: Motivated by additional experimental hints of Lepton Flavour Universality violation in B decays, both in charged- and in neutral-current processes, we analyse the ingredients necessary to provide a combined description of these phenomena. By means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons, we show how this is possible. We demonstrate, in particular, how to solve the problems posed by electroweak precision tests and direct searches with a rather natural choice of model parameters, within the context of a $U(2)_q \times U(2)_\ell$ flavour symmetry. We further exemplify the general EFT findings by means of simplified models with explicit mediators in the TeV range: coloured scalar or vector leptoquarks and colour-less vectors. Among these, the case of an $SU(2)_L$-singlet vector leptoquark emerges as a particularly simple and successful framework.
TL;DR: In this article, a new class of renormalisable simplified models for dark matter searches at the LHC were proposed based on two Higgs doublet models with an additional pseudoscalar mediator.
Abstract: We study a new class of renormalisable simplified models for dark matter searches at the LHC that are based on two Higgs doublet models with an additional pseudoscalar mediator. In contrast to the spin-0 simplified models employed in analyses of Run I data these models are self-consistent, unitary and bounds from Higgs physics typically pose no constraints. Predictions for various missing transverse energy (E
T,miss) searches are discussed and the reach of the 13 TeV LHC is explored. It is found that the proposed models provide a rich spectrum of complementary observables that lead to non-trivial constraints. We emphasise in this context the sensitivity of the $$ t\overline{t}+{E_T}_{,\mathrm{miss}} $$
, mono-Z and mono-Higgs channels, which yield stronger limits than mono-jet searches in large parts of the parameter space. Constraints from spin-0 resonance searches, electroweak precision measurements and flavour observables are also derived and shown to provide further important handles to constraint and to test the considered dark matter models.
TL;DR: In this paper, the authors evaluate the bubble nucleation temperature throughout the parameter space of this model where a first-order transition is expected, and find that bubbles in fact do not nucleate at any finite temperature, eliminating these models as viable electroweak baryogenesis (EWBG) scenarios.
Abstract: An addition to the Standard Model of a real, gauge-singlet scalar field, coupled via a Higgs portal interaction, can reopen the possibility of a strongly first-order electroweak phase transition (EWPT) and successful electroweak baryogenesis (EWBG). If a discrete symmetry that forbids doublet-singlet mixing is present, this model is notoriously difficult to test at the Large Hadron Collider. As a result, it emerged as a useful benchmark for evaluating the capabilities of proposed future colliders to conclusively test EWPT and EWBG. In this paper, we evaluate the bubble nucleation temperature throughout the parameter space of this model where a first-order transition is expected. We find that in large parts of this parameter space, bubbles in fact do not nucleate at any finite temperature, eliminating these models as viable EWBG scenarios. This constraint eliminates most of the region where a ``two-step'' phase transition is naively predicted, while the ``one-step'' transition region is largely unaffected. In addition, expanding bubble walls must not reach relativistic speeds during the transition for baryon asymmetry to be generated. We show that this condition further reduces the parameter space with viable EWBG.
TL;DR: In this article, the neutrino masses and flavor mixings are incorporated in the type-I seesaw extension of the SM with heavy Majorana neutrinos being singlet under the SM gauge group.
University of Southampton1, Kanazawa University2, Abdelmalek Essaâdi University3, Aix-Marseille University4, Spanish National Research Council5, Benemérita Universidad Autónoma de Puebla6, University of Helsinki7, University of Mainz8, National Institute of Chemical Physics and Biophysics9, University of Toyama10, Birzeit University11, University of Warsaw12, École normale supérieure de Lyon13, CERN14, University of Bergen15, Paul Scherrer Institute16, University of Lisbon17, Instituto Superior de Engenharia de Lisboa18, University of Adelaide19, Stockholm University20
TL;DR: In this article, the authors summarize the current situation and discuss possible search strategies for charged scalars, in non-supersymmetric extensions of the Standard Model at the LHC.
Abstract: The goal of this report is to summarize the current situation and discuss possible search strategies for charged scalars, in non-supersymmetric extensions of the Standard Model at the LHC. Such scalars appear in Multi-Higgs-Doublet models, in particular in the popular Two-Higgs-Doublet model, allowing for charged and additional neutral Higgs bosons. These models have the attractive property that electroweak precision observables are automatically in agreement with the Standard Model at the tree level. For the most popular version of this framework, Model II, a discovery of a charged Higgs boson remains challenging, since the parameter space is becoming very constrained, and the QCD background is very high. We also briefly comment on models with dark matter which constrain the corresponding charged scalars that occur in these models. The stakes of a possible discovery of an extended scalar sector are very high, and these searches should be pursued in all conceivable channels, at the LHC and at future colliders.
TL;DR: In this paper, the authors analyze the collider signatures of the real singlet extension of the Standard Model in regions consistent with a strong first-order electroweak phase transition and a singlet-like scalar heavier than the standard model-like Higgs.
Abstract: We analyze the collider signatures of the real singlet extension of the Standard Model in regions consistent with a strong first-order electroweak phase transition and a singlet-like scalar heavier than the Standard Model-like Higgs. A definitive correlation exists between the strength of the phase transition and the trilinear coupling of the Higgs to two singlet-like scalars, and hence between the phase transition and non-resonant scalar pair production involving the singlet at colliders. We study the prospects for observing these processes at the LHC and a future 100 TeV pp collider, focusing particularly on double singlet production. We also discuss correlations between the strength of the electroweak phase transition and other observables at hadron and future lepton colliders. Searches for non-resonant singlet-like scalar pair production at 100 TeV would provide a sensitive probe of the electroweak phase transition in this model, complementing resonant di-Higgs searches and precision measurements. Our study illustrates a strategy for systematically exploring the phenomenologically viable parameter space of this model, which we hope will be useful for future work.
TL;DR: In this article, a scenario that the universe undergoes a two-step phase transition with the first step happened to the dark matter sector and the second step being the transition between the dark mass and the electroweak vacuums, where the barrier between the two vacuacs, that is necessary for a strongly first-order electroweak phase transition (EWPT) as required by the baryogenesis mechanism, arises at the tree-level.
Abstract: We study in this work a scenario that the universe undergoes a two step phase transition with the first step happened to the dark matter sector and the second step being the transition between the dark matter and the electroweak vacuums, where the barrier between the two vacuums, that is necessary for a strongly first order electroweak phase transition (EWPT) as required by the electroweak baryogenesis mechanism, arises at the tree-level. We illustrate this idea by working with the standard model (SM) augmented by a scalar singlet dark matter and an extra scalar singlet which mixes with the SM Higgs boson. We study the conditions for such pattern of phase transition to occur and especially for the strongly first order EWPT to take place, as well as its compatibility with the basic requirements of a successful dark matter, such as observed relic density and constraints of direct detections. We further explore the discovery possibility of this pattern EWPT by searching for the gravitational waves generated during this process in spaced based interferometer, by showing a representative benchmark point of the parameter space that the generated gravitational waves fall within the sensitivity of eLISA, DECIGO and BBO.
TL;DR: In this article, a set of renormalizable, SU(2)×U(1) invariant extensions of the Standard Model, each comprising an inert ℤ 2-odd scalar field and one or more vector-like pairs of colorless fermions that communicate to the muons through Yukawa-type interactions, are defined.
Abstract: We investigate simplified models of new physics that can accommodate the measured value of the anomalous magnetic moment of the muon and the relic density of dark matter. We define a set of renormalizable, SU(2)×U(1) invariant extensions of the Standard Model, each comprising an inert ℤ
2-odd scalar field and one or more vector-like pairs of colorless fermions that communicate to the muons through Yukawa-type interactions. The new sectors are classified according to their transformation properties under the Standard Model gauge group and all models are systematically confronted with a variety of experimental constraints: LEP mass bounds, direct LHC searches, electroweak precision observables, and direct searches for dark matter. We show that scenarios featuring only one type of new fermions become very predictive once the relic density and collider constraints are taken into account, as in this case (g − 2)
μ
is not enhanced by chirality flip. Conversely, for models where an additional source of chiral-symmetry violation is generated via fermion mixing, the constraints are much looser and new precision experiments with highly suppressed systematic uncertainties may be required to test the parameter space.
TL;DR: In this article, the authors discuss how to perform consistent extractions of anomalous triple gauge couplings (aTGC) from electroweak boson pair production at the LHC in the Standard Model Effective Field Theory (SMEFT), and provide a method to set (conservative) aTGC bounds in this situation, and recast the present searches accordingly.
Abstract: We discuss how to perform consistent extractions of anomalous triple gauge couplings (aTGC) from electroweak boson pair production at the LHC in the Standard Model Effective Field Theory (SMEFT). After recasting recent ATLAS and CMS searches in pp → W Z(W W ) → l′νl+l−(νl) channels, we find that: (a) working consistently at order Λ−2 in the SMEFT expansion the existing aTGC bounds from Higgs and LEP-2 data are not improved, (b) the strong limits quoted by the experimental collaborations are due to the partial Λ−4 corrections (dimension-6 squared contributions). Using helicity selection rule arguments we are able to explain the suppression in some of the interference terms, and discuss conditions on New Physics (NP) models that can benefit from such LHC analyses. Furthermore, standard analyses assume implicitly a quite large NP scale, an assumption that can be relaxed by imposing cuts on the underlying scale of the process (
$$ \sqrt{\widehat{s}} $$
). In practice, we find almost no correlation between $$ \sqrt{\widehat{s}} $$
and the experimentally accessible quantities, which complicates the SMEFT interpretation. Nevertheless, we provide a method to set (conservative) aTGC bounds in this situation, and recast the present searches accordingly. Finally, we introduce a simple NP model for aTGC to compare the bounds obtained directly in the model with those from the SMEFT analysis.
TL;DR: In this paper, the spectrum of gravitational waves originated from strongly first order electroweak phase transition in the extended Higgs model with a real singlet scalar field, and the strength at the peak frequency can be detected at future space-based gravitational interferometers such as eLISA, DECIGO and BBO.
TL;DR: In this article, the phase transition dynamics in a thermal environment and the related gravitational wave phenomenology within the framework of scalar conformal extensions of the Standard Model were investigated, and it was shown that minimal extensions involving only one additional scalar field struggle to reproduce the correct phase-transition dynamics once thermal corrections are accounted for.
Abstract: Thermal corrections in classically conformal models typically induce a strong first-order electroweak phase transition, thereby resulting in a stochastic gravitational background that could be detectable at gravitational wave observatories. After reviewing the basics of classically conformal scenarios, in this paper we investigate the phase transition dynamics in a thermal environment and the related gravitational wave phenomenology within the framework of scalar conformal extensions of the Standard Model. We find that minimal extensions involving only one additional scalar field struggle to reproduce the correct phase transition dynamics once thermal corrections are accounted for. Next-to-minimal models, instead, yield the desired electroweak symmetry breaking and typically result in a very strong gravitational wave signal.