Showing papers on "Electroweak interaction published in 2015"
TL;DR: A new class of solutions to the electroweak hierarchy problem is presented that does not require either weak-scale dynamics or anthropics to be presented.
Abstract: A new class of solutions to the electroweak hierarchy problem is presented that does not require either weak-scale dynamics or anthropics. Dynamical evolution during the early Universe drives the Higgs boson mass to a value much smaller than the cutoff. The simplest model has the particle content of the standard model plus a QCD axion and an inflation sector. The highest cutoff achieved in any technically natural model is 10^{8} GeV.
674 citations
TL;DR: In this paper, a review of the atomic nucleus from the ground up is presented, including the structure of light nuclei, electroweak response of nuclei relevant in electron and neutrino scattering, and the properties of dense nucleonic matter.
Abstract: Quantum Monte Carlo techniques aim at providing a description of complex quantum systems such as nuclei and nucleonic matter from first principles, i.e., realistic nuclear interactions and currents. The methods are similar to those used for many-electron systems in quantum chemistry and condensed matter physics, but are extended to include spin-isospin, tensor, spin-orbit, and three-body interactions. This review shows how to build the atomic nucleus from the ground up. Examples include the structure of light nuclei, electroweak response of nuclei relevant in electron and neutrino scattering, and the properties of dense nucleonic matter.
602 citations
TL;DR: In this paper, the current status of theoretical and experimental constraints on the real Higgs singlet extension of the standard model is discussed and the impact of perturbative unitarity, electroweak precision data with a special focus on higher-order contributions to the Higgs boson mass, perturbativity of the couplings as well as vacuum stability.
Abstract: We discuss the current status of theoretical and experimental constraints on the real Higgs singlet extension of the standard model. For the second neutral (non-standard) Higgs boson we consider the full mass range from $$1~\mathrm{GeV}$$
to $$1~\mathrm{TeV}$$
accessible at past and current collider experiments. We separately discuss three scenarios, namely, the case where the second Higgs boson is lighter than, approximately equal to, or heavier than the discovered Higgs state at around $$125~\mathrm{GeV}$$
. We investigate the impact of constraints from perturbative unitarity, electroweak precision data with a special focus on higher-order contributions to the $$W$$
boson mass, perturbativity of the couplings as well as vacuum stability. The latter two are tested up to a scale of $$\sim $$
$$4 \times 10^{10}\,\mathrm{GeV}$$
using renormalization group equations. Direct collider constraints from Higgs signal rate measurements at the LHC and $$95\,\%$$
confidence level exclusion limits from Higgs searches at LEP, Tevatron, and LHC are included via the public codes HiggsSignals and HiggsBounds, respectively. We identify the strongest constraints in the different regions of parameter space. We comment on the collider phenomenology of the remaining viable parameter space and the prospects for a future discovery or exclusion at the LHC.
285 citations
TL;DR: In this paper, the authors show that the 14 TeV LHC and a 100 TeV proton-proton collider provide powerful probes of both exotic Higgs decay channels.
Abstract: High-energy colliders offer a unique sensitivity to dark photons, the mediators of a broken dark U(1) gauge theory that kinetically mixes with the Standard Model (SM) hypercharge. Dark photons can be detected in the exotic decay of the 125 GeV Higgs boson, h→ZZ
D
→4l, and in Drell-Yan events, pp→Z
D
→ ll. If the dark U(1) is broken by a hidden-sector Higgs mechanism, then mixing between the dark and SM Higgs bosons also allows the exotic decay h → Z
D
Z
D
→ 4l. We show that the 14 TeV LHC and a 100 TeV proton-proton collider provide powerful probes of both exotic Higgs decay channels. In the case of kinetic mixing alone, direct Drell-Yan production offers the best sensitivity to Z
D
, and can probe ϵ ≳ 9 × 10−4 (4 × 10−4) at the HL-LHC (100 TeV pp collider). The exotic Higgs decay h → ZZ
D
offers slightly weaker sensitivity, but both measurements are necessary to distinguish the kinetically mixed dark photon from other scenarios. If Higgs mixing is also present, then the decay h → Z
D
Z
D
can allow sensitivity to the Z
D
for ϵ ≳ 10−9 − 10−6 (10−10 − 10−7) for the mass range $$ 2{m}_{\mu }<{m_Z}_{{}_D}<{m}_h/2 $$
by searching for displaced dark photon decays. We also compare the Z
D
sensitivity at pp colliders to the indirect, but model-independent, sensitivity of global fits to electroweak precision observables. We perform a global electroweak fit of the dark photon model, substantially updating previous work in the literature. Electroweak precision measurements at LEP, Tevatron, and the LHC exclude ϵ as low as 3 × 10−2. Sensitivity can be improved by up to a factor of ∼ 2 with HL-LHC data, and an additional factor of ∼ 4 with ILC/GigaZ data.
273 citations
TL;DR: An upper bound on the pole mass of the top quark is derived by requiring that the standard model be stable all the way up to the Planck mass scale and conservatively estimate the theoretical uncertainty.
Abstract: We perform a manifestly gauge-independent analysis of the vacuum stability in the standard model including two-loop matching, three-loop renormalization group evolution, and pure QCD corrections through four loops. All these ingredients are exact, except that light-fermion masses are neglected. We in turn apply the criterion of nullifying the Higgs self-coupling and its beta function in the modified minimal-subtraction scheme and a recently proposed consistent method for determining the true minimum of the effective Higgs potential that also avoids gauge dependence. Exploiting our knowledge of the Higgs-boson mass, we derive an upper bound on the pole mass of the top quark by requiring that the standard model be stable all the way up to the Planck mass scale and conservatively estimate the theoretical uncertainty. This bound is compatible with the Monte Carlo mass quoted by the Particle Data Group at the 1.3σ level.
268 citations
TL;DR: In this paper, the phenomenology of gauge-singlet extensions of the Standard Model scalar sector and their implications for the electroweak phase transition were discussed. And the authors showed that there exists considerable potential for observable deviations from purely Standard Model Higgs properties at these prospective future colliders.
Abstract: We update the phenomenology of gauge-singlet extensions of the Standard Model scalar sector and their implications for the electroweak phase transition. Considering the introduction of one real scalar singlet to the scalar potential, we analyze present constraints on the potential parameters from Higgs coupling measurements at the Large Hadron Collider (LHC) and electroweak precision observables for the kinematic regime in which no new scalar decay modes arise. We then show how future precision measurements of Higgs boson signal strengths and the Higgs self-coupling could probe the scalar potential parameter space associated with a strong first-order electroweak phase transition. We illustrate using benchmark precision for several future collider options, including the high-luminosity LHC, the International Linear Collider, Triple-Large Electron-Positron collider, the China Electron-Positron Collider, and a 100 TeV proton-proton collider, such as the Very High Energy LHC or the Super Proton-Proton Collider. For the regions of parameter space leading to a strong first-order electroweak phase transition, we find that there exists considerable potential for observable deviations from purely Standard Model Higgs properties at these prospective future colliders.
261 citations
Vardan Khachatryan1, Albert M. Sirunyan1, Armen Tumasyan1, Wolfgang Adam +2353 more•Institutions (181)
TL;DR: In this paper, a search for a heavy Higgs boson in the H to WW and H to ZZ decay channels is reported, based upon proton-proton collision data samples corresponding to an integrated luminosity of up to 5.1 inverse femtobarns at sqrt(s)=7 TeV and up to 19.7 inverse femto-bars at square root of 8 TeV, recorded by the CMS experiment at the CERN LHC.
Abstract: A search for a heavy Higgs boson in the H to WW and H to ZZ decay channels is reported. The search is based upon proton-proton collision data samples corresponding to an integrated luminosity of up to 5.1 inverse femtobarns at sqrt(s)=7 TeV and up to 19.7 inverse femtobarns at sqrt(s)=8 TeV, recorded by the CMS experiment at the CERN LHC. Several final states of the H to WW and H to ZZ decays are analyzed. The combined upper limit at the 95% confidence level on the product of the cross section and branching fraction exclude a Higgs boson with standard model-like couplings and decays in the range 145 < m[H] < 1000 GeV. We also interpret the results in the context of an electroweak singlet extension of the standard model.
237 citations
TL;DR: In this article, the authors compute the contribution of order αβερατε βετε γεταταβετταγαγε βαγγεγε αβδαγβαγ γγεβεγγααβαβγεαγδεγβγγγβεβααα βεγαβγγγε βεβγαστα βασε βγεσα βγ
Abstract: We compute the contribution of order α
2
α
2 to the cross section of a top-antitop pair in association with at least one heavy Standard Model boson — Z, W ±, and Higgs — by including all effects of QCD, QED, and weak origin and by working in the automated MadGraph5_aMC@NLO framework. This next-to-leading order contribution is then combined with that of order α
3
α, and with the two dominant lowest-order ones, α
2
α and α
S
α
2, to obtain phenomenological results relevant to a 8, 13, and 100 TeV pp collider.
237 citations
TL;DR: In this article, the authors define a set of effective couplings of W and Z bosons to fermions and to itself, which capture the effects of new physics corrections, and obtain numerical constraints on the coefficients of dimension-6 operator in a form that can be easily adapted to any particular basis of operators, or any particular model with new heavy particles.
Abstract: We discuss electroweak precision constraints on dimension-6 operators in the effective theory beyond the standard model. We identify the combinations of these operators that are constrained by the pole observables (the W and Z masses and on-shell decays) and by the W boson pair production. To this end, we define a set of effective couplings of W and Z bosons to fermions and to itself, which capture the effects of new physics corrections. This formalism clarifies which operators are constrained by which observable, independently of the adopted basis of operators. We obtain numerical constraints on the coefficients of dimension-6 operator in a form that can be easily adapted to any particular basis of operators, or any particular model with new heavy particles.
230 citations
TL;DR: In this article, it was shown that the successful Higgs inflation scenario can also take place if the SM vacuum is not absolutely stable, based on two effects that were overlooked previously: effective renormalization of the SM couplings at the energy scale M-P/xi and symmetry restoration after inflation due to high temperature effects.
Abstract: The measurements of the Higgs mass and top Yukawa coupling indicate that we live in a very special universe, at the edge of the absolute stability of the electroweak vacuum. If fully stable, the Standard Model (SM) can be extended all the way up to the inflationary scale and the Higgs field, nonminimally coupled to gravity with strength xi, can be responsible for inflation. We show that the successful Higgs inflation scenario can also take place if the SM vacuum is not absolutely stable. This conclusion is based on two effects that were overlooked previously. The first one is associated with the effective renormalization of the SM couplings at the energy scale M-P/xi, where M-P is the Planck scale. The second one is a symmetry restoration after inflation due to high temperature effects that leads to the (temporary) disappearance of the vacuum at Planck values of the Higgs field.
228 citations
TL;DR: In this paper, direct and indirect constraints on the second mass eigenstate and corresponding mixing angle were analyzed for the 125 GeV Higgs-like scalar at the LHC and the existence of the additional scalar can be beneficial as it can stabilise the otherwise metastable electroweak vacuum.
Abstract: In the Higgs portal framework, the Higgs field generally mixes with the Standard Model (SM) singlet leading to the existence of two states, one of which is identified with the 125 GeV scalar observed at the LHC. In this work, we analyse direct and indirect constraints on the second mass eigenstate and the corresponding mixing angle. The existence of the additional scalar can be beneficial as it can stabilise the otherwise-metastable electroweak vacuum. We find parameter regions where all of the bounds, including the stability constraints, are satisfied. We also study prospects for observing the decay of the heavier state into a pair of the 125 GeV Higgs-like scalars.
TL;DR: FlexibleSUSY, a Mathematica and C++ package, which generates a fast, precise C++ spectrum generator for any SUSY model specified by the user, making it easy to adapt and extend with new features.
TL;DR: In this paper, the authors studied the enhancement of the di-Higgs production cross section resulting from the resonant decay of a heavy Higgs boson at hadron colliders in a model with a Higgs singlet.
Abstract: We study the enhancement of the di-Higgs production cross section resulting from the resonant decay of a heavy Higgs boson at hadron colliders in a model with a Higgs singlet. This enhancement of the double Higgs production rate is crucial in understanding the structure of the scalar potential and we determine the maximum allowed enhancement such that the electroweak minimum is a global minimum. The di-Higgs production enhancement can be as large as a factor of $\ensuremath{\sim}18(13)$ for the mass of the heavy Higgs around 270(420) GeV relative to the Standard Model rate at 14 TeV for parameters corresponding to a global electroweak minimum.
TL;DR: In this article, the current status of theoretical and experimental constraints on the real Higgs singlet extension of the Standard Model is discussed, with a special focus on higher order contributions to the W boson mass, perturbativity of the couplings as well as vacuum stability.
Abstract: We discuss the current status of theoretical and experimental constraints on the real Higgs singlet extension of the Standard Model. For the second neutral (non-standard) Higgs boson we consider the full mass range from 1 GeV to 1 TeV accessible at past and current collider experiments. We separately discuss three scenarios, namely, the case where the second Higgs boson is lighter than, approximately equal to, or heavier than the discovered Higgs state at around 125 GeV. We investigate the impact of constraints from perturbative unitarity, electroweak precision data with a special focus on higher order contributions to the W boson mass, perturbativity of the couplings as well as vacuum stability. The latter two are tested up to a scale of 4 x 10^10 GeV using renormalization group equations. Direct collider constraints from Higgs signal rate measurements at the LHC and 95% C.L. exclusion limits from Higgs searches at LEP, Tevatron and LHC are included via the public codes HiggsSignals and HiggsBounds, respectively. We identify the strongest constraints in the different regions of parameter space. We comment on the collider phenomenology of the remaining viable parameter space and the prospects for a future discovery or exclusion at the LHC.
TL;DR: In this paper, a summary of the constraints from the ATLAS experiment on R-parity-conserving supersymmetry is presented, based on analysis of up to 20.3 fb(-1) of proton-proton collision data at centre-of-mass energies of root s = 7 and 8TeV at the Large Hadron Collider.
Abstract: A summary of the constraints from the ATLAS experiment on R-parity-conserving supersymmetry is presented. Results from 22 separate ATLAS searches are considered, each based on analysis of up to 20.3 fb(-1) of proton-proton collision data at centre-of-mass energies of root s = 7 and 8TeV at the Large Hadron Collider. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, in which the lightest supersymmetric particle is a neutralino, taking into account constraints from previous precision electroweak and flavour measurements as well as from dark matter related measurements. The results are presented in terms of constraints on supersymmetric particle masses and are compared to limits from simplified models. The impact of ATLAS searches on parameters such as the dark matter relic density, the couplings of the observed Higgs boson, and the degree of electroweak fine-tuning is also shown. Spectra for surviving supersymmetry model points with low fine-tunings are presented.
TL;DR: In this article, the friction on the bubble wall is computed using a kinetic theory approach and including hydrodynamic effects, and the results are used in calculating the baryon asymmetry in various singlet-driven scenarios, as well as other features related to cosmological phase transitions in the early Universe, such as the resulting spectrum of gravitational radiation.
Abstract: The standard picture of electroweak baryogenesis requires slowly expanding bubbles. This can be difficult to achieve if the vacuum expectation value (VEV) of a gauge singlet scalar field changes appreciably during the electroweak phase transition. It is important to determine the bubble wall velocity in this case, since the predicted baryon asymmetry can depend sensitively on its value. Here, this calculation is discussed and illustrated in the real singlet extension of the Standard Model. The friction on the bubble wall is computed using a kinetic theory approach and including hydrodynamic effects. Wall velocities are found to be rather large (v
w
≳ 0.2) but compatible with electroweak baryogenesis in some portions of the parameter space. If the phase transition is strong enough, however, a subsonic solution may not exist, precluding non-local electroweak baryogenesis altogether. The results presented here can be used in calculating the baryon asymmetry in various singlet-driven scenarios, as well as other features related to cosmological phase transitions in the early Universe, such as the resulting spectrum of gravitational radiation.
TL;DR: In this paper, a search for the b (b) over bar decay of the Standard Model Higgs boson is performed with the ATLAS experiment using the full dataset recorded at the LHC in Run 1.
Abstract: A search for the b (b) over bar decay of the Standard Model Higgs boson is performed with the ATLAS experiment using the full dataset recorded at the LHC in Run 1. The integrated luminosities used are 4.7 and 20.3 fb(-1) from pp collisions at root s = 7 and 8 TeV, respectively. The processes considered are associated (WIZ)H production, where W -> e nu/mu nu, Z -> ee/mu mu, and Z -> nu nu. The observed (expected) deviation from the backgroundonly hypothesis corresponds to a significance of 1.4 (2.6) standard deviations and the ratio of the measured signal yield to the Standard Model expectation is found to be mu = 0.52 +/- 0.32 (stat.) +/- 0.24 (syst.) for a Higgs boson mass of 125.36 GeV. The analysis procedure is validated by a measurement of the yield of (W/Z)Z production with Z -> b (b) over bar in the same final states as for the Higgs boson search, from which the ratio of the observed signal yield to the Standard Model expectation is found to be 0.74 +/- 0.09 (stat.) +/- 0.14 (syst.).
TL;DR: The composite Higgs scenario, in which the Higgs emerges as a composite pseudo-Nambu-Goldstone boson, is extensively reviewed in this article, with great emphasis on the conceptual and technical foundations of the construction.
Abstract: The composite Higgs scenario, in which the Higgs emerges as a composite pseudo-Nambu-Goldstone boson, is extensively reviewed in these Notes. The material is presented in a pedagogical fashion, with great emphasis on the conceptual and technical foundations of the construction. A comprehensive summary of the flavor, collider and electroweak precision phenomenology is also presented.
TL;DR: In this article, the authors consider interpretations of the recent reports by the CMS and ATLAS collaborations of a possible $X(sim 750~{\rm GeV})$ state decaying into $gg$ and $gamma \gamma$ final states, and show that the required Yukawa-like couplings between the $X$ particle and the heavy vector-like fermions are small enough to be perturbative so long as the particle has dominant decay modes into
Abstract: We consider interpretations of the recent $\sim 3 \sigma$ reports by the CMS and ATLAS collaborations of a possible $X(\sim 750~{\rm GeV})$ state decaying into $\gamma \gamma$ final states. We focus on the possibilities that this is a scalar or pseudoscalar electroweak isoscalar state produced by gluon-gluon fusion mediated by loops of heavy fermions. We consider several models for these fermions, including a single vector-like charge $2/3$ T quark, a doublet of vector-like quarks $(T, B)$, and a vector-like generation of quarks, with or without leptons that also contribute to the $X \to \gamma \gamma$ decay amplitude. We also consider the possibility that $X(750)$ is a dark matter mediator, with a neutral vector-like dark matter particle. These scenarios are compatible with the present and prospective direct limits on vector-like fermions from LHC Runs 1 and 2, as well as indirect constraints from electroweak precision measurements, and we show that the required Yukawa-like couplings between the $X$ particle and the heavy vector-like fermions are small enough to be perturbative so long as the $X$ particle has dominant decay modes into $gg$ and $\gamma \gamma$. The decays $X \to Z Z, Z \gamma$ and $W^+ W^-$ are interesting prospective signatures that may help distinguish between different vector-like fermion scenarios.
TL;DR: In this article, the authors address the recent anomalies in semi-leptonic B-meson decays using a model of fermion masses based on the U(2) flavor symmetry.
Abstract: We address the recent anomalies in semi-leptonic B-meson decays using a model of fermion masses based on the U(2) flavor symmetry. The new contributions to b → sll transitions arise due to a tree-level exchange of a Z′ vector boson gauging a U(1) subgroup of the flavor symmetry. They are controlled by a single parameter and are approximately aligned to the Standard Model prediction, with constructive interference in the e-channel and destructive interference in the μ-channel. The current experimental data on semi-leptonic B-meson decays can be very well reproduced without violating existing constraints from flavor violation in the quark and lepton sectors. Our model will be tested by new measurements of b → sll transitions and also by future electroweak precision tests, direct Z′ searches, and μ-e conversion in nuclei.
TL;DR: In this paper, the expected sensitivity of future lepton collider experiments for probing neutrino masses around the electroweak scale with up to order one mixings with the light neutrinos is discussed.
Abstract: Extending the Standard Model (SM) with sterile (“right-handed”) neutrinos is one of the best motivated ways to account for the observed neutrino masses. We discuss the expected sensitivity of future lepton collider experiments for probing such extensions. An interesting testable scenario is given by “symmetry protected seesaw models”, which theoretically allow for sterile neutrino masses around the electroweak scale with up to order one mixings with the light (SM) neutrinos. In addition to indirect tests, e.g. via electroweak precision observables, sterile neutrinos with masses around the electroweak scale can also be probed by direct searches, e.g. via sterile neutrino decays at the Z pole, deviations from the SM cross section for four lepton final states at and beyond the WW threshold and via Higgs boson decays. We study the present bounds on sterile neutrino properties from LEP and LHC as well as the expected sensitivities of possible future lepton colliders such as ILC, CEPC and FCC-ee (TLEP).
TL;DR: In this article, the authors consider the constraints imposed on the coefficients of dimension-6 operators by electroweak precision tests (EWPTs) and derive model-independent limits on the effective Standard Model after LHC Run 1.
Abstract: We treat the Standard Model as the low-energy limit of an effective field theory that incorporates higher-dimensional operators to capture the effects of decoupled new physics. We consider the constraints imposed on the coefficients of dimension-6 operators by electroweak precision tests (EWPTs), applying a framework for the effects of dimension- 6 operators on electroweak precision tests that is more general than the standard S, T formalism, and use measurements of Higgs couplings and the kinematics of associated Higgs production at the Tevatron and LHC, as well as triple-gauge couplings at the LHC. We highlight the complementarity between EWPTs, Tevatron and LHC measurements in obtaining model-independent limits on the effective Standard Model after LHC Run 1. We illustrate the combined constraints with the example of the two-Higgs doublet model.
TL;DR: In this article, spectra of the first order phase transition at the electroweak symmetry breaking were evaluated in a set of extended scalar sectors with additional N isospin-singlet fields as a concrete example of renormalizable theories and it was shown that the produced gravitational waves can be significant, so that they are detectable at future gravitational wave interferometers such as DECIGO and BBO.
Abstract: We discuss spectra of gravitational waves which are originated by the strongly first order phase transition at the electroweak symmetry breaking, which is required for a successful scenario of electroweak baryogenesis. Such spectra are numerically evaluated without high temperature expansion in a set of extended scalar sectors with additional N isospin-singlet fields as a concrete example of renormalizable theories. We find that the produced gravitational waves can be significant, so that they are detectable at future gravitational wave interferometers such as DECIGO and BBO. Furthermore, since the spectra strongly depend on N and the mass of the singlet fields, our results indicate that future detailed observation of gravitational waves can be in general a useful probe of extended scalar sectors with the first order phase transition.
TL;DR: In this paper, the authors present a fully automated implementation of next-to-leading order electroweak (NLO EW) corrections in the OpenLoops matrix-element generator combined with the Sherpa and Munich Monte Carlo frameworks.
Abstract: We present a fully automated implementation of next-to-leading order electroweak (NLO EW) corrections in the OpenLoops matrix-element generator combined with the Sherpa and Munich Monte Carlo frameworks. The process-independent character of the implemented algorithms opens the door to NLO QCD + EW simulations for a vast range of Standard Model processes, up to high particle multiplicity, at current and future colliders. As a first application, we present NLO QCD + EW predictions for the production of positively charged on-shell W bosons in association with up to three jets at the Large Hadron Collider. At the TeV energy scale, due to the presence of large Sudakov logarithms, EW corrections reach the 20-40% level and play an important role for searches of physics beyond the Standard Model. The dependence of NLO EW effects on the jet multiplicity is investigated in detail, and we find that W + multijet final states feature genuinely different EW effects as compared to the case of W + 1 jet.
TL;DR: In this paper, the authors derive model-independent constraints arising from the Z and W boson observables on dimension six operators in the effective theory beyond the Standard Model, and discuss the generic flavor structure for these operators as well as several flavor patterns motivated by simple new physics scenarios.
Abstract: We derive model-independent constraints arising from the Z and W boson observables on dimension six operators in the effective theory beyond the Standard Model. In particular, we discuss the generic flavor structure for these operators as well as several flavor patterns motivated by simple new physics scenarios.
TL;DR: In this paper, the authors show that simplified models used to describe the interactions of dark matter with standard model particles do not in general respect gauge invariance and that perturbative unitarity may be violated in large regions of the parameter space.
Abstract: We show that simplified models used to describe the interactions of dark matter with Standard Model particles do not in general respect gauge invariance and that perturbative unitarity may be violated in large regions of the parameter space. The modifications necessary to cure these inconsistencies may imply a much richer phenomenology and lead to stringent constraints on the model. We illustrate these observations by considering the simplified model of a fermionic dark matter particle and a vector mediator. Imposing gauge invariance then leads to strong constraints from dilepton resonance searches and electroweak precision tests. Furthermore, the new states required to restore perturbative unitarity can mix with Standard Model states and mediate interactions between the dark and the visible sector, leading to new experimental signatures such as invisible Higgs decays. The resulting constraints are typically stronger than the 'classic' constraints on DM simplified models such as monojet searches and make it difficult to avoid thermal overproduction of dark matter.
TL;DR: In this article, the expected sensitivity of future lepton collider experiments for probing neutrino masses around the electroweak scale with up to order one mixings with the light neutrinos is discussed.
Abstract: Extending the Standard Model (SM) with sterile ('right-handed') neutrinos is one of the best motivated ways to account for the observed neutrino masses. We discuss the expected sensitivity of future lepton collider experiments for probing such extensions. An interesting testable scenario is given by 'symmetry protected seesaw models', which theoretically allow for sterile neutrino masses around the electroweak scale with up to order one mixings with the light (SM) neutrinos. In addition to indirect tests, e.g. via electroweak precision observables, sterile neutrinos with masses around the electroweak scale can also be probed by direct searches, e.g. via sterile neutrino decays at the Z pole, deviations from the SM cross section for four lepton final states at and beyond the WW threshold and via Higgs boson decays. We study the present bounds on sterile neutrino properties from LEP and LHC as well as the expected sensitivities of possible future lepton colliders such as ILC, CEPC and FCC-ee (TLEP).
Vardan Khachatryan, Albert M. Sirunyan, Armen Tumasyan, Wolfgang Adam1 +2120 more•Institutions (146)
TL;DR: Bounds on the structure of quartic vector-boson interactions are given in the framework of dimension-eight effective field theory operators, as well as limits on the production of doubly charged Higgs bosons.
Abstract: A study of vector boson scattering in pp collisions at a center-of-mass energy of 8 TeV is presented. The data sample corresponds to an integrated luminosity of 19.4 inverse femtobarns collected with the CMS detector. Candidate events are selected with exactly two leptons of the same charge, two jets with large rapidity separation and dijet mass, and moderate missing transverse energy. The signal region is expected to be dominated by electroweak same-sign W-boson pair production. The observation agrees with the standard model prediction. The observed significance is 2.0 standard deviations, where a significance of 3.1 standard deviations is expected based on the standard model. Cross section measurements for W+/- W+/- and WZ processes in the fiducial region are reported. Bounds on the structure of quartic vector-boson interactions are given in the framework of dimension-eight effective field theory operators, as well as limits on the production of doubly-charged Higgs bosons.
TL;DR: In this paper, it was shown that only the "lepton-specific" (or "type X") 2HDM can explain the present muon (g-2) anomaly in the parameter region of large $\tan\beta$, a light CP-odd Higgs boson, and heavier CP-even and charged Higgs Bosons which are almost degenerate.
Abstract: Updating various theoretical and experimental constraints on the four different types of two-Higgs-doublet models (2HDMs), we find that only the "lepton-specific" (or "type X") 2HDM can explain the present muon (g-2) anomaly in the parameter region of large $\tan\beta$, a light CP-odd Higgs boson, and heavier CP-even and charged Higgs bosons which are almost degenerate. The severe constraints on the models come mainly from the consideration of vacuum stability and perturbativity, the electroweak precision data, $B$ physics observables like $b\to s \gamma$ as well as the 125 GeV Higgs boson properties measured at the LHC.
Posted Content•
TL;DR: In this paper, the authors compute the contribution of order α-S^2\alpha^2$ to the cross section of a top-antitop pair in association with at least one heavy Standard Model boson (Z, W^\pm, and Higgs) by including all effects of QCD, QED, and weak origin.
Abstract: We compute the contribution of order $\alpha_S^2\alpha^2$ to the cross section of a top-antitop pair in association with at least one heavy Standard Model boson -- $Z$, $W^\pm$, and Higgs -- by including all effects of QCD, QED, and weak origin and by working in the automated MadGraph5_aMC@NLO framework. This next-to-leading order contribution is then combined with that of order $\alpha_S^3\alpha$, and with the two dominant lowest-order ones, $\alpha_S^2\alpha$ and $\alpha_S\alpha^2$, to obtain phenomenological results relevant to a 8, 13, and 100~TeV $pp$ collider.