Narrow Results

We present the NLO QCD corrections for light Higgs pair production via vector boson fusion at the LHC within the CP conserving type II two-Higgs doublet model in the form of a fully flexible parton-level Monte Carlo program. Scale dependences on integrated cross sections and distributions are reduced with QCD K-factors of order unity.

The zero-width approximation (ZWA) restricts the intermediate unstable particle state to the mass shell and, when combined with the decorrelation approximation, fully factorizes the production and decay of unstable particles. The ZWA uncertainty is expected to be of , where M and Γ are the mass and width of the unstable particle. We review the ZWA and demonstrate that errors can be much larger than expected if a significant modification of the Breit–Wigner lineshape occurs. A thorough examination of the recently discovered candidate Standard Model Higgs boson is in progress. For M_H≈125 GeV, one has Γ_H/M_H < 10^-4, which suggests an excellent accuracy of the ZWA. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalization at the 1% precision level. For gg→H→VV, V = W, Z, corrections occur due to an enhanced Higgs signal in the region M_VV > 2M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to suppress this region in search channels where the Higgs mass cannot be reconstructed. We note that H→VV decay modes in non-gluon-fusion channels are similarly affected.

We give a brief overview of beyond the Standard Model (BSM) theories with an extended scalar sector and their phenomenological status in the light of recent experimental results. We discuss the relevant theoretical and experimental constraints, and show their impact on the allowed parameter space of two specific models: the real scalar singlet extension of the Standard Model (SM) and the Inert Doublet Model. We emphasize the importance of the LHC measurements, both the direct searches for additional scalar bosons, as well as the precise measurements of properties of the Higgs boson of mass 125 GeV. We show the complementarity of these measurements to electroweak and dark matter observables.

In string theory, a scalar field often appears as a moduli of a geometrical configuration of D-branes in higher-dimensional space. In the low energy effective theory on D-branes, the distance between D-branes is translated into the energy scale of the gauge symmetry breaking. In this paper, we study a phenomenological consequence of a possibility that the Higgs field is such a moduli field and the D-brane configuration is stabilized by a stationary motion, in particular, revolution of D-branes on which we live. Then, due to the Coriolis force, Higgs mode is mixed with the angular fluctuation of branes and the Lorentz symmetry is violated in the dispersion relation of the Higgs boson. The Higgs boson mass measurements at LHC experiments give an upper bound $\sim$${𝒪}(0.1)$ GeV for the angular frequency of the revolution of D-branes.

Electroweak precision data have been extensively used to constrain models containing physics beyond that of the Standard Model (SM). When the model contains Higgs scalars in representations other than singlets or doublets, and hence ρ≠1 at tree-level, a correct renormalization scheme requires more inputs than the three commonly used for the SM case. In such cases, the one-loop electroweak results cannot be split into a SM contribution plus a piece which vanishes as the scale of new physics becomes much larger than M_W. We illustrate our results by presenting the dependence of M_W on the top-quark mass in a model with a Higgs triplet and in the SU(2)_L × SU(2)_R left–right symmetric model. In these models, the allowed range for the lightest neutral Higgs mass can be as large as a few TeV.

The next-to-minimal supersymmetric extension of the Standard Model (NMSSM) is one of the most favored supersymmetric models. After an introduction to the model, the Higgs sector and the neutralino sector are discussed in detail. Theoretical, experimental, and cosmological constraints are studied. Eventually, the Higgs potential is investigated in the approach of bilinear functions. Emphasis is placed on aspects which are different from the minimal supersymmetric extension.

Unitarity constraints for Yukawa couplings are considered in the Two-Higgs-Doublet Model type III, by using a general expansion in partial waves for fermionic scattering processes. Constraints over general Flavor Changing Neutral Currents are found from that systematic, wherein such bounds compete with those coming from Lagrangian perturbativity requirement but are weaker than those imposed from phenomenological processes and precision tests. Nevertheless, for bounds based on unitarity, the number of assumptions is the lowest among phenomenological and theoretical limits. Indeed, these new theoretical constraints are independent of scalar masses or mixing angles for this extended Higgs sector, making them less model dependent.

Current experiments do not support, as ATLAS and CMS Collaborations at the Large Hadron Collider reported, that the Higgs-like resonance discovered in July 2012 is a pure CP-odd state. We examine a general hZZ vertex which contains CP-even and CP-odd couplings, by studying the process with l₁, l₂ = e or μ, to explore the CP mixed property of the Higgs-like particle. One momentum asymmetry and two angular asymmetries have been analyzed in order to reveal the difference from different CP-couplings. Our study shows that these asymmetries could be interesting observables in the future precise experiments.

The discovery of the Higgs boson has been one the most important results in the whole story of particle physics. A review of the aspects of the long path, starting from the speculative introduction of the Brout–Englert–Higgs mechanism, continuing with the long search to detect the Higgs particle, and arriving to the discovery is given. The first measurements of the properties are described, their implications are discussed and the future prospects are given.

We consider scale invariant models where the classical scale invariance is broken perturbatively by radiative corrections at the electroweak scale. These models potentially offer an elegant and simple solution to the hierarchy problem. If we further require the cosmological constant to be small then such models are also highly predictive. Indeed, the minimal such model, comprizing a Higgs doublet and a real singlet, has the same number of parameters as the standard model. Although this minimal model is disfavored by recent LHC data, we show that two specific extensions incorporating neutrino masses and dark matter are fully realistic. That is, consistent with all experiments and observations. These models predict a light pseudo-Goldstone boson, h, with mass around 10 GeV or less. A fermionic-bosonic mass relation is also predicted. The specific models considered, as well as more generic scale invariant models, can be probed at the LHC.

In scenarios with sterile (right-handed) neutrinos with an approximate “lepton-number-like” symmetry, the heavy neutrinos (the mass eigenstates) can have masses around the electroweak scale and couple to the Higgs boson with, in principle, unsuppressed Yukawa couplings, while the smallness of the light neutrinos’ masses is guaranteed by the approximate symmetry. The on-shell production of the heavy neutrinos at lepton colliders, together with their subsequent decays into a light neutrino and a Higgs boson, constitutes a resonant contribution to the Higgs production mechanism. This resonant mono-Higgs production mechanism can contribute significantly to the mono-Higgs observables at future lepton colliders. A dedicated search for the heavy neutrinos in this channel exhibits sensitivities for the electron neutrino Yukawa coupling as small as $\sim 5\times 10^{-3}$. Furthermore, the sensitivity is enhanced for higher center-of-mass energies, when identical integrated luminosities are considered.

This report will review the Higgs boson properties: the mass, the total width and the couplings to fermions and bosons. The measurements have been performed with the data collected in 2011 and 2012 at the LHC accelerator at CERN by the ATLAS and CMS experiments. Theoretical frameworks to search for new physics are also introduced and discussed.

The Compact Linear Collider (CLIC) is a mature option for a future electron-positron collider operating at center-of-mass energies of up to 3 TeV. CLIC is foreseen in a staged approach with three center-of-mass energies, currently assumed to be 380, 1.5 and 3 TeV. This contribution discusses the physics potential of CLIC in the field of Higgs physics, based on the benchmark analyses using full detector simulation. The initial stage of operation allows study of Higgs production in Higgsstrahlung and WW-fusion, resulting in precise measurements of the production cross sections and the total Higgs-boson decay width. Operation at high energies will provide high-statistics samples of Higgs bosons enabling tight constraints on Higgs couplings and measurement of the Higgs self-coupling.

In this paper, we present general formulae for the calculation of LFV Higgs decays $H_r\to l_a{l}_b$ at one-loop, with $H_r$ being part of the Higgs spectrum of a generic multi-scalar extension of the Standard Model (SM) with neutrino masses. We develop a method based on a classification of the particles appearing in the loop diagrams (scalars, fermions and vectors), and by identifying the corresponding couplings, we are able to present compact expressions for the form factors involved in the amplitudes. Our results are applicable to models where Flavor Changing Neutral Currents (FCNC) are forbidden at the tree level, but change of flavor is induced by charged currents. Then, as applications of our formalism, we evaluate the branching ratio for the mode $h\to l_a{l}_b$, for two specific models: the See-Saw Type I-$\nu$SM and the Scotogenic model (here $h=H_1$ corresponds to the SM-like Higgs boson); we find that the largest branching ratio for SM-like Higgs $h$ boson within the $\nu$SM is of the order ${ℬ}{ℛ}(h\to \mu \tau )\simeq 10^{-16}$, while for the Scotogenic model we find ${ℬ}{ℛ}(h\to l_a{l}_b)\lesssim 10^{-9}$, which satisfy the latest experimental LHC results.

The nonconservation of the lepton number has been explored at the LHC through the lepton-flavor violating (LFV) Higgs decays $h\to {\ell }_a{\ell }_b$, with ${\ell }_{a,b}=e,\mu ,\tau$$(a\ne b)$. Current limits on these decays are a source of valuable information on the structure of the Yukawa and Higgs sectors. The LFV Higgs couplings can arise within the general two-Higgs doublet model (2HDM). The predicted rates for these decay modes depend on the specific Yukawa structure being considered, ranging from a vanishing branching ratio at tree-level for some versions (2HDM-I, II, X, Y), up to large and detectable ratios within the general 2HDM-III. An attractive scenario is given by the texturized version of the model (2HDM-Tx), with the Yukawa matrices having some texture zeros such as the minimal version with the so-called Cheng–Sher ansatz. We study the constraints on the parameter space of the 2HDM provided by experimental and theoretical restrictions, and use them to study the detection of LFV Higgs modes at LHC. We find several encouraging scenarios to the search for the decay $h\to \tau \mu$ that could be achieved in the high-luminosity LHC. On the other hand, LFV Higgs couplings can also be induced at one-loop level in the 2HDM with neutrino masses, with the loops being mediated by neutrino interactions; we find that the resulting branching ratios are of order $10^{-7}$ at best, which is out of the reach of current and future phases of the LHC.

The observed value of the Higgs mass indicates an instability of the Higgs scalar at large energy scales, and hence also at large field values. In the context of early universe cosmology, this is often considered to lead to problems. Here, we point out that we can use the instability of the Higgs field to generate an ekpyrotic phase of contraction. In the context of string theory, it is possible that at very high energy densities, extra states become massless leading to an $S$-brane which causes the transition between a contracting phase in the past and the current expanding phase. Thus, the Higgs field may be useable to generate a nonsingular bouncing cosmology in which the anisotropy problem of usual bouncing scenarios is mitigated.

The new particle recently discovered at the Large Hadron Collider has properties compatible with those expected for the Standard Model (SM) Higgs boson. However, this does not exclude the possibility that the discovered state is of non-standard origin, as part of an elementary Higgs sector in an extended model, or not at all a fundamental Higgs scalar. We review briefly the motivations for Higgs boson scenarios beyond the SM, discuss the phenomenology of several examples, and summarize the prospects and methods for studying interesting models with non-standard Higgs sectors using current and future data.

In scenarios with sterile (right-handed) neutrinos with an approximate “lepton-numberlike” symmetry, the heavy neutrinos (the mass eigenstates) can have masses around the electroweak scale and couple to the Higgs boson with, in principle, unsuppressed Yukawa couplings, while the smallness of the light neutrinos’ masses is guaranteed by the approximate symmetry. The on-shell production of the heavy neutrinos at lepton colliders, together with their subsequent decays into a light neutrino and a Higgs boson, constitutes a resonant contribution to the Higgs production mechanism. This resonant mono-Higgs production mechanism can contribute significantly to the mono-Higgs observables at future lepton colliders. A dedicated search for the heavy neutrinos in this channel exhibits sensitivities for the electron neutrino Yukawa coupling as small as ∼ 5 × 10⁻³. Furthermore, the sensitivity is enhanced for higher center-of-mass energies, when identical integrated luminosities are considered.

We pursued the question of the influence of a strong gravitational field on the structure of the Higgs effective potential in the gauge-less top-Higgs sector of the Standard Model with an additional scalar singlet. To this end, we calculated the one-loop corrected effective potential in an arbitrary curved spacetime. We have found that the gravity induced terms in the effective potential may influence its behavior in both small and large field regions. This result indicated the necessity of a more careful investigation of the effect of high curvature in the problems concerning the stability of the Higgs effective potential in the full Standard Model.

In general, effective low-energy Lagrangian models of composite electroweak symmetry breaking (EWSB) have soliton solutions that may be identified with technibaryons. The masses of such states may be related to the coefficients of fourth-order terms in the effective Lagrangian, as in QCD. We show how the current theoretical and phenomenological constraints on the corresponding fourth-order coefficients in the electroweak theory could be used to estimate qualitative lower and upper bounds on the lightest electroweak baryon mass. We point out that in strongly-interacting models of electroweak symmetry breaking, non-Skyrmion-like solitons are also possible. Future data from the LHC as well as dark matter experiments will constrain this possibility. With the currently available data the upper bound on the mass of the electroweak soliton is between 18 and 59 TeV. With improved information that LHC is likely to provide on quartic terms in the electroweak Lagrangian, these bounds might be reduced to 4.6 to 8.1 TeV.