Decay Rates of Heavy Neutrinos in the Grimus–Neufeld Model

We study phenomenological implications of an axion that arises as a pseudo Nambu-Goldstone boson due to the spontaneous breaking of anomaly-free global flavor symmetry. One interesting possibility for such anomaly-free axion to explain dark matter (DM) is when it has a mass of order keV and an intermediate scale decay constant, since it can be explored through direct search experiments, X-ray observations, various stellar cooling processes, and the misalignment mechanism naturally explains the DM abundance. As a concrete renormalizable model of such axion, we consider an extended Higgs sector with global flavor symmetry, which consists of three Higgs doublet fields and three singlet Higgs fields with U(1)B−L charges. We identify viable parameter regions that satisfy theoretical bounds on the Higgs potential and various experimental limits on this model, and evaluate the mass spectra of the axion and extra Higgs bosons. We find that even an anomaly-free axion can generally couple to photons through mixing with CP-odd Higgs, and that its strength depends on the vacuum expectation values of the Higgs doublets as well as the axion mass. As a result, the ratios of the vacuum expectation values of the Higgs doublets are tightly constrained to satisfy the X-ray constraints. We show the favored parameter region where axion DM explains the XENON1T excess. We also demonstrate that the axion-electron coupling is correlated with the extra Higgs boson masses and mixing angles for CP-even Higgs bosons. Thus, if the axion is detected in future observations, the extra Higgs boson masses and the coupling of the standard model-like Higgs boson with the weak gauge bosons are restricted. This is a good example of the synergy between searches for the axion DM and the BSM around the electroweak scale.

H-COUP Version 3: A program for one-loop corrected decays of any Higgs bosons in non-minimal Higgs models

The Dine-Seiberg-Thomas model (DSTM) is the simplest version of the new physics beyond the minimal supersymmetric standard model (MSSM), in the sense that its Higgs sector has just two dimension-five operators, which are obtained from the power series of the energy scale for the new physics in the effective action analysis. We study the possibility of spontaneous CP violation in the Higgs sector of the DSTM, which consists of two Higgs doublets. We find that the CP violation may be triggered spontaneously by a complex phase, obtained as the relative phase between the vacuum expectation values of the two Higgs doublets. At the tree level, for a reasonably established parameter region, the masses of the three neutral Higgs bosons and their corresponding coupling coefficients to a pair of Z bosons in the DSTM are calculated such that the results are inconsistent with the experimental constraint by the LEP data. Thus, the LEP2 data exclude the possibility of spontaneous CP violation in the DSTM at the tree level. On the other hand, we find that, for a wide area in the parameter region, the CP symmetry may be broken spontaneously in the Higgs sector of the DSTM at the one-loop level, more » where top quark and scalar top quark loops are taken into account. The upper bound on the radiatively corrected mass of the lightest neutral Higgs boson of the DSTM is about 87 GeV, in the spontaneous CP violation scenario. We confirm that the LEP data does not exclude this numerical result. « less

Collider probes of the MSSM Higgs sector with explicit CP violation

A search is presented for additional neutral Higgs bosons in the τ τ final state in proton-proton collisions at the LHC. The search is performed in the context of the minimal supersymmetric extension of the standard model (MSSM), using the data collected with the CMS detector in 2016 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb−1. To enhance the sensitivity to neutral MSSM Higgs bosons, the search includes production of the Higgs boson in association with b quarks. No significant deviation above the expected background is observed. Model-independent limits at 95% confidence level (CL) are set on the product of the branching fraction for the decay into τ leptons and the cross section for the production via gluon fusion or in association with b quarks. These limits range from 18 pb at 90 GeV to 3.5 fb at 3.2 TeV for gluon fusion and from 15 pb (at 90 GeV) to 2.5 fb (at 3.2 TeV) for production in association with b quarks, assuming a narrow width resonance. In the mhhod + scenario these limits translate into a 95% CL exclusion of tan β > 6 for neutral Higgs boson masses below 250 GeV, where tan β is the ratio of the vacuum expectation values of the neutral components of the two Higgs doublets. The 95% CL exclusion contour reaches 1.6 TeV for tan β = 60.

Where is the Higgs boson?

We present a systematic study of an extension of the Standard Model (SM) with two Higgs doublets and one complex singlet (2HDM+S). In order to gain analytical understanding of the parameter space, we re-parameterize the 27 parameters in the Lagrangian by quantities more closely related to physical observables: physical masses, mixing angles, trilinear and quadratic couplings, and vacuum expectation values. Embedding the 125 GeV SM-like Higgs boson observed at the LHC places stringent constraints on the parameter space. In particular, the mixing of the SM-like interaction state with the remaining states is severely constrained, requiring approximate alignment without decoupling in the region of parameter space where the additional Higgs bosons are light enough to be accessible at the LHC. In contrast to 2HDM models, large branching ratios of the heavy Higgs bosons into two lighter Higgs bosons or a light Higgs and a Z boson, so-called Higgs cascade decays, are ubiquitous in the 2HDM+S. Using currently available limits, future projections, and our own collider simulations, we show that combining different final states arising from Higgs cascades would allow to probe most of the interesting region of parameter space with Higgs boson masses up to 1 TeV at the LHC with L = 3000 fb−1 of data.

We consider the minimal U(1)' extension of the standard model (SM) with the classically conformal invariance, where an anomaly-free U(1)' gauge symmetry is introduced along with three generations of right-handed neutrinos and a U(1)' Higgs field. Since the classically conformal symmetry forbids all dimensional parameters in the model, the U(1)' gauge symmetry is broken by the Coleman-Weinberg mechanism, generating the mass terms of the U(1)' gauge boson ($Z$' boson) and the right-handed neutrinos. Through a mixing quartic coupling between the U(1)' Higgs field and the SM Higgs doublet field, the radiative U(1)' gauge symmetry breaking also triggers the breaking of the electroweak symmetry. In this model context, we first investigate the electroweak vacuum instability problem in the SM. Employing the renormalization group equations at the two-loop level and the central values for the world average masses of the top quark ($m_t=173.34$ GeV) and the Higgs boson ($m_h=125.09$ GeV), we perform parameter scans to identify the parameter region for resolving the electroweak vacuum instability problem. Next we interpret the recent ATLAS and CMS search limits at the LHC Run-2 for the sequential $Z$' boson to constrain the parameter region in our model. Combining the constraints from the electroweak vacuum stability and the LHC Run-2 results, we find a bound on the $Z$' boson mass as $m_{Z'} \gtrsim 3.5$ TeV. We also calculate self-energy corrections to the SM Higgs doublet field through the heavy states, the right-handed neutrinos and the $Z$' boson, and find the naturalness bound as $m_{Z'} \lesssim 7$ TeV, in order to reproduce the right electroweak scale for the fine-tuning level better than 10\%. The resultant mass range of $3.5$ TeV $\lesssim m_{Z'} \lesssim 7$ TeV will be explored at the LHC Run-2 in the near future.

Bounds on the number of possible Higgs particles using grand unification and exceptional Lie groups

The ATLAS experiment at the LHC has measured the Higgs boson couplings and mass, and searched for invisible Higgs boson decays, using multiple production and decay channels with up to 4.7 fb -1 of pp collision data at s = 7 TeV and 20.3 fb -1 at s = 8 TeV. In the current study, the measured production and decay rates of the observed Higgs boson in the , ZZ, W W , Z, bb, , and decay channels, along with results from the associated production of a Higgs boson with a top-quark pair, are used to probe the scaling of the couplings with mass. Limits are set on parameters in extensions of the Standard Model including a composite Higgs boson, an additional electroweak singlet, and two-Higgs-doublet models. Together with the measured mass of the scalar Higgs boson in the and ZZ decay modes, a lower limit is set on the pseudoscalar Higgs boson mass of m A > 370 GeV in the "hMSSM" simplified Minimal Supersymmetric Standard Model. Results from direct searches for heavy Higgs bosons are also interpreted in the hMSSM. Direct searches for invisible Higgs boson decays in the vector-boson fusion and associated production of a Higgs boson with W/Z (Z , W/Z jj) modes are statistically combined to set an upper limit on the Higgs boson invisible branching ratio of 0.25. The use of the measured visible decay rates in a more general coupling fit improves the upper limit to 0.23, constraining a Higgs portal model of dark matter.

We investigate the possibility of electroweak phase transition in the minimal supersymmetric standard model (MSSM) with an extra $U(1)'$. This model has two Higgs doublets and a singlet, in addition to a singlet exotic quark superfield. We find that at the one-loop level this model may accommodate the electroweak phase transitions that are strongly first-order in a reasonably large region of the parameter space. In the parameter region where the phase transitions take place, we observe that the lightest scalar Higgs boson has a smaller mass when the strength of the phase transition becomes weaker. Also, the other three heavier neutral Higgs bosons get more large masses when the strength of the phase transition becomes weaker.

In the minimal supersymmetric standard madel, it is shown that a radiative correction of the top and stop loops gives a finite, but non-negligible contribution to Higgs scalar masses if m, ' 150-250 GeV. The upper limit to the lightest-scalar mass becomes 70-190 GeV in the range of heavy top quark. 1 The mechanism of electroweak symmetry breaking is one of the most important issues in the present particle physics_ In the standard electroweak model a funda­ mental Higgs doublet is introduceq to cause the spontaneous symmetry breaking. Supersymmetry (SUSY), eliminating all quadratic divergences, may provide a better theoretical basis to describe a fundamental Higgs boson with a relatively small mass to a high energy cutoff scale, say the Planck scale for example. I) In the minimal SUSY extension of the standard electroweak model the Higgs sector consists of two chiral superfields of Higgs doublets (/JHl and (/JH2 with opposite hypercharges. They are required to give masses for all quarks and leptons and to guarantee the ~bsence of the gauge anomaly. Five physical Higgs bosons among them survive the gauge symmetry breaking, namely, there appear two neutral scalars rpa and rpb, a neutral pseudoscalar X and a pair of charged scalars x± as physical particles. It has been, furthermore, shown by many authors 2 ) that there is at least one neutral scalar boson lighter than ZO(m<p< mzo) in the minimal SUSY model. This has strongly motivated many recent analyses of Higgs boson production at LEP energies. 3 ) In this paper, however, we stress that a radiative correctiQn gives a significant contribution to the Higgs mass term if the top quark is 8uficiently heavy as mt ~150-250 GeV. Therefore, the presence of the Higgs scalar lighter than mzo is not an inevitable prediction of the minimal SUSY standard model. Let us discuss the Higgs sedor in the minimal SUSY model. With general soft-breaking terms 4 ) of SUSY the tree-level Higgs potential is given by

It is well known that generic two-Higgs-doublet models (2HDMs) suffer from potentially large Higgs-mediated flavor-changing neutral current (FCNC) problem, unless additional symmetries are imposed on the Higgs fields thereby respecting the Natural Flavor Conservation Criterion (NFC) by Glashow and Weinberg. A common way to respect the NFC is to impose Z_2 symmetry which is softly broken by a dim-2 operator. Another new way is to introduce local U(1)_H Higgs flavor symmetry that distinguishes one Higgs doublet from the other. In this paper, we consider the Higgs phenomenology in Type-I 2HDMs with the U(1)_H symmetry with the simplest U(1)_H assignments that the SM fermions are all neutral under U(1)_H, and we make detailed comparison with the ordinary Type-I 2HDM. After imposing various constraints such as vacuum stability and perturbativity as well as the electroweak precision observables and collider search bounds on charged Higgs boson, we find that the allowed Higgs signal strengths in our model are much broader than those in the ordinary Type-I 2HDM, because of newly introduced U(1)_H-charged singlet scalar and U(1)_H gauge boson. Still the ATLAS data on gg to h to gamma gamma cannot be accommodated. Our model could be distinguished from the ordinary 2HDM with the Z_2 symmetry in a certain parameter region and some channels. If the couplings of the new boson turn out to be close to those in the SM, it would be essential to search for extra U(1)_H gauge boson and/or one more neutral scalar boson to distinguish two models.

During the last decades, particle physicists have studied the tiniest building blocks of matter--the quarks and the leptons--and the forces between them in great detail. From these experiments, a theoretical framework has been built that describes the observed results with high precision. The achievement of this theory, which is referred to as the Standard Model of elementary particle physics, was the elaboration of a unified description of the strong, weak and electromagnetic forces in the framework of quantum gauge-field theories. Moreover, the Standard Model combines the weak and electromagnetic forces in a single electroweak gauge theory. The fourth force which is realized in nature, gravity, is too weak to be observable in laboratory experiments carried out in high-energy particle physics and is not part of the Standard Model. Although the Standard Model has proven highly successful in correlating a huge amount of experimental results, a key ingredient is as yet untested: the origin of electroweak symmetry breaking. Currently, the only viable ansatz that is compatible with observation is the Higgs mechanism. It predicts the existence of a scalar particle, called the Higgs boson, and the couplings to the fundamental Standard Model particles, however not its mass. An upper limit on the mass of the Higgs boson of ~ 1 TeV can be inferred from unitarity arguments. One of the key tasks of particle physics in the next years will be to verify the existence of this particle. The introduction of an elementary scalar particle in a quantum field theory is highly problematic. The Higgs boson mass is subject to large quantum corrections, which makes it difficult to understand how its mass can be less than a TeV as required by theory. In addition, the Standard Model does not provide an answer to fundamental questions like the values of free parameters of the model, the pending integration of gravity or the evolution of the coupling constants of the fundamental forces at large energy regimes. Hence there are strong reasons to believe that the Standard Model is only a low-energy approximation to a more fundamental theory. One of the best studied candidates for an extension of the Standard Model is supersymmetry, which predicts the existence of a supersymmetric partner for each fundamental particle that differs only in spin. To allow different masses for Standard Model particles and their corresponding supersymmetric partners, supersymmetry must be broken. The mechanism behind supersymmetry breaking is currently unknown, however, various hypotheses exist. Supersymmetric models do not only solve the problem of the large quantum corrections to the Higgs boson mass, but they also allow the unification of the coupling constants at a common scale. In addition, certain supersymmetric models provide a suitable candidate for cold dark matter, which represents a large fraction of mass in our universe. Searches for supersymmetric particles have been performed by the four LEP experiments (ALEPH, DELPHI, L3, OPAL) up to the kinematic limit. Since no evidence for supersymmetric particles has been found, lower limits on their masses have been derived. The search for supersymmetry is now continuing at the Tevatron collider, located at the Fermi National Accelerator Laboratory in Batavia, Illinois. Two dedicated detector systems, CDF and D0, are installed at the Tevatron to analyze proton-antiproton collisions at a center-of-mass energy of 1.96 TeV. A particular promising discovery channel for supersymmetry within the Tevatron energy range is the trilepton channel. In this channel, the lighter supersymmetric partners of the Higgs and gauge bosons, the charginos and neutralinos, decay into final states with leptons or hadrons and missing energy. Using the leptonic final states, the signal can be separated from the large Standard Model background. Supersymmetry requires an extension of the Standard Model Higgs sector, leading to more than one neutral Higgs boson. Enhanced couplings result in sizable cross sections for Higgs boson production, and the decay into a tau pair becomes an important Higgs boson discovery channel. Within the present thesis, a search for new physics predicted by constrained supersymmetric models is performed in final states consisting of an electron and a tau using data collected with the D0 detector from April 2002 to July 2004. The first analysis searches for the associated production of the lightest chargino and the second lightest neutralino in final states with an electron, a hadronically decaying tau, an additional lepton and missing transverse energy: e + τb h + ℓ + ET. The second analysis searches for neutral supersymmetric Higgs bosons in the decay mode Φ → ττ → e + τh + ET. To improve the sensitivity, the results are interpreted in combination with other channels.

H-COUP Version 2: A program for one-loop corrected Higgs boson decays in non-minimal Higgs sectors