Standard Higgs Boson Research Articles

Pragmatic Approach to the Little Hierarchy Problem: The Case for Dark Matter and Neutrino Physics

We show that the addition of real scalars (gauge singlets) to the standard model can both ameliorate the little hierarchy problem and provide realistic dark matter candidates. To this end, the coupling of the new scalars to the standard Higgs boson must be relatively strong and their mass should be in the 1-3 TeV range, while the lowest cutoff of the (unspecified) UV completion must be > or ~ 5 TeV, depending on the Higgs boson mass and the number of singlets present. The existence of the singlets also leads to realistic, and surprisingly reach, neutrino physics. The resulting light neutrino mass spectrum and mixing angles are consistent with the constraints from the neutrino oscillations.

Higgs signal forh→aaat hadron colliders

We assess the prospect of observing a neutral Higgs boson at hadron colliders in its decay to two spin-zero states, a, for a Higgs mass of 90-130 GeV, when produced in association with a W or Z boson. Such a decay is allowed in extensions of the MSSM with CP-violating interactions and in the NMSSM, and can dominate Higgs boson final states, thereby evading the LEP constraints on standard Higgs boson production. The light spin-zero state decays primarily via a to bb and tau+tau-, so this signal channel retains features distinct from the main backgrounds. Our study shows that at the Tevatron, there may be potential to observe a few events in the bb tau+tau- or bbbb channels with relatively small background, although this observation would be statistically limited. At the LHC, the background problem is more severe, but with cross sections and integrated luminosities orders of magnitude larger than at the Tevatron, the observation of a Higgs boson in this decay mode would be possible. The channel h to aa to bbbb would provide a large statistical significance, with a signal-to-background ratio on the order of 1:2. In these searches, the main challenge would be to retain the adequate tagging efficiency of b's and tau's in the low p_T region.

Precision calculations in the Minimal Supersymmetric Standard Model

Recent progress in improving the MSSM predictions for observables with standard particles, Higgs bosons, and SUSY particles is reviewed.

Studying Standard Model-Like Higgs Bosons in Two-Photon Collisions at a Linear Collider

We examine the potential for detecting and studying a SM-like Higgs boson in two-photon collisions at a future linear collider. Our study incorporates realistic two-photon spectra based on the most probable available laser technology.

Scale of fermion mass generation

Unitarity of longitudinal weak vector boson scattering implies an upper bound on the scale of electroweak symmetry breaking, $\Lambda_{EWSB}\equiv \sqrt{8\pi}v\approx$ 1 TeV. Appelquist and Chanowitz have derived an analogous upper bound on the scale of fermion mass generation, proportional to $v^2/m_f$, by considering the scattering of same-helicity fermions into pairs of longitudinal weak vector bosons in a theory without a standard Higgs boson. We show that there is no upper bound, beyond that on the scale of electroweak symmetry breaking, in such a theory. This result is obtained by considering the same process, but with a large number of longitudinal weak vector bosons in the final state. We further argue that there is no scale of (Dirac) fermion mass generation in the standard model. In contrast, there is an upper bound on the scale of Majorana-neutrino mass generation, given by $\Lambda_{Maj}\equiv 4\pi v^2/m_\nu$. In general, the upper bound on the scale of fermion mass generation depends on the dimensionality of the interaction responsible for generating the fermion mass. We explore the scale of fermion mass generation in a variety of excursions from the standard model: models with fermions in nonstandard representations, a theory with higher-dimension interactions, a two-Higgs-doublet model, and models without a Higgs boson.

Physics potential of a Tevatron tripler

We explore the capabilities for new physics discovery in proton-antiproton collisions at 5.4 TeV center-of-mass energy with luminosity 10 33 cm −2 s −1 at a Tripler upgrade of the Tevatron collider. The prospects are robust for the usual Higgs boson and supersymmetry benchmarks. With an integrated luminosity of 40 fb −1, discoveries at 5 σ could be made for a standard Higgs boson of mass ≲680 GeV (600 GeV for 10 fb −1), a lighter chargino of mass ≲380 GeV, and an extra Z boson of mass ≲2.6 TeV; four-fermion contact interactions from new physics with scale ≲74 TeV could be excluded at the 95% confidence level.

Effects of extra dimensions on unitarity and the Higgs boson mass

We study the unitarity constraint on two body Higgs boson elastic scattering in the presence of extra dimensions. The contributions from the exchange of spin-2 and spin-0 Kaluza-Klein states can have a large effect on the partial wave amplitude. The unitarity condition restricts the maximal allowed value for the ratio $r=\sqrt{s}{/M}_{S}$ of the center of mass energy $\sqrt{s}$ to the gravity scale ${M}_{S}$ to be less than one. Although the constraint on the standard Higgs boson mass for r of order one is considerably relaxed, for small r the constraint is similar to that in the standard model. The resulting bound on the Higgs boson mass is not dramatically altered if perturbative calculations are required to be valid up to the maximal allowed value for r.

INFLUENCE OF SU(2) ⊗ U(1) SINGLET SCALARS ON HIGGS BOSON SIGNAL AT LHC

We investigate a model of SU(2) ⊗ U(1) singlet scalars coupled to Standard Model. We point out that for the case of the maximal mixing between singlet scalars and the standard Higgs boson, the Higgs boson production cross-section is smaller than the standard Higgs boson production by a factor of 1/(N+1), where N is the number of singlet scalars. For the case of large mixing, Higgs boson could be unobservable at LHC. However, there is also the possibility for the observation of scalar singlets at LHC. As a semi-realistic example, we discuss supersymmetric SU(2) ⊗ U(1) electroweak model with an additional singlet chiral superfield.

The Higgs-photon-Z boson coupling revisited

We analyze the coupling of CP-even and CP-odd Higgs bosons to a photon and a Z boson in extensions of the Standard Model. In particular, we study in detail the effect of charged Higgs bosons in two-Higgs doublet models, and the contribution of SUSY particle loops in the minimal supersymmetric extension of the Standard Model. The Higgs-$\gamma Z$ coupling can be measured in the decay $Z \to \gamma$+Higgs at $e^+e^-$ colliders running on the Z resonance, or in the reverse process Higgs $\to Z \gamma$ with the Higgs boson produced at LHC. We show that a measurement of this coupling with a precision at the percent level, which could be the case at future $e^+e^-$ colliders, would allow to distinguish between the lightest SUSY and standard Higgs bosons in large areas of the parameter space.

The Higgs-photon-Z boson coupling revisited

We analyze the coupling of CP-even and CP-odd Higgs bosons to a photon and a Z boson in extensions, of the Standard Model. In particular, we study in detail the effect of charged Higgs bosons in two-Higgs doublet models;. and the contribution of SUSY particle loops in the minimal supersymmetric extension of the Standard Model: The Higgs-γZ coupling can be measured in the decayZ → γ+Higgs ate + e − colliders running on theZ resonance, or in the reverse process Higgs →Zγ with the Higgs boson produced at LHC. We show that a measurement of this coupling with a precision at the percent level, which could be the case at futuree + e − colliders, would allow to distinguish between the lightest SUSY and standard Higgs bosons in large areas of the parameter space.

Standard Higgs boson searches at LEP 2

The process e +e − → bb −− with − = v or − = charged lepton or − = u, d the range of LEP 2 energies. In the near future LEP 2 will represent a unique opportunity for a direct search of a Higgs boson of mass > 65 GeV. The whole emphasis has been put on a selfconsistent study of the standard Higgs boson properties. Indeed there is the actual possibility of testing a calculation which uses the full matrix elements for e + e −- annihilation into four fermions with the experimental data, beyond the usual approximation of computing production cross section times branching ratios of Higgs into decay products. Of course, given the fact that the number of collected events at LEP 2 will be limited and because of the low statistics one could proceed with a different strategy: for discovery physics elaborated tools are unnecessary. However, the precise theoretical calculation is at our disposal already now, thanks to the combined effort of several groups, and it is extremely interesting and appealing to apply the corresponding machinery to an analysis - as complete as possible - of standard Higgs boson physics. Calculation of the total cross sections and of different kinds of differential distributions at the partonic level are made available to access the widest information for choosing cuts, for discussing the physics of a standard Higgs boson with a mass around 90 GeV where the Higgs and the Z signal become degenerate and for a comprehensive analysis of the various background components.

Second-order QCD corrections to scalar and pseudoscalar Higgs decays into massive bottom quarks

Quark mass effects in O ( α s 2) QCD corrections to the decay rates of intermediate Higgs bosons are studied. The total hadronic rate and the partial decay rate into bottom quarks are analyzed for the Standard (scalar) Higgs boson as well as for pseudoscalar Higgs bosons. The calculations of three different contributions are presented. First, the flavour singlet diagrams containing two closed fermion loops are calculated for a nonvanishing bottom mass in the heavy top limit. Their leading contribution, which is of the same order as the quasi-massless nonsinglet corrections, and the subleading terms are found. Large logarithms arise due to the separation of the pure gluon final state from the bottom final states. Second, quadratic bottom mass corrections originating from nonsinglet diagrams are presented. Third, nonsinglet corrections induced by virtual heavy top quarks are calculated in leading and subleading orders. It is demonstrated that, in order α 2 2, the first contribution numerically dominates over the second and the third ones.

PARTICLE SEARCHES WITH NEURAL NETS

The efficiency and robustness of neural feed-forward nets in particle searches is studied using the search for the Standard Higgs Boson at LEP-200 as an example. Methods to select the most efficient variables, to define standard cuts, and to recognize significant differences between the training-data sample and a test-data sample are presented. The efficiencies of the neural nets are significantly better than those of standard methods.

Energy spectra and energy correlations in the decayH?ZZ??+???+??

It is shown that in the sequential decay\(H \to ZZ \to (f_1 \bar f_1 ) + (f_2 \bar f_2 )\), the energy distribution of the final state particles provides a simple and powerful test of theHZZ vertex. For a standard Higgs boson, the energy spectrum of any final fermion, in the rest frame ofH, is predicted to be dΓ/dx∼1+β4−2(x−1)2, with\(\beta = \sqrt {1 - 4m_Z^2 /m_H^2 } \) and 1−β≤x=4E/m H ≤1+β. By contrast, the spectrum for a pseudoscalar Higgs is dΓ/dx∼β2+(x−1)2. There are characteristic energy correlations betweenf 1 andf 2 and betweenf 1 andf 2. These considerations are applied to the “gold-plated” reactionH→ZZ→μ+μ−μ+μ−, including possible effects ofCP-violation in theHZZ coupling. Our formalism also yields the energy spectra and correlations of leptons in the decay\(H \to W^ + W^ - \to l^ + v_l l^ - \bar v_l \).

Validation of input data for trained neural-nets

The validation of input-data samples is a very important issue for neural-net applications if high precision is required. The aim is to make sure that a neural net properly trained on one data sample gives reliable results for another. Using the search for the standard Higgs boson at LEP-200 as an example, the effect of systematic changes in the input data is studied in detail for feed-forward nets. Several methods for recognizing significant differences between the training-data sample and a test-data sample are presented.

Higgs boson production via the Bjorken process e+e--->H0 micro+ micro- at high energy e+e- colliders.

Production of the standard Higgs boson via the Bjorken process, [ital e][sup +][ital e][sup [minus]][r arrow][ital H][sup 0][mu][sup +][mu][sup [minus]], at the center-of-mass energy [radical][ital s] [approx]200 GeV (i.e., for the CERN Large Electron-Positron Collider LEP II) is investigated in detail for a Higgs boson mass ranging from 60 GeV to about 120 GeV. We focus on physical quantities which have not been emphasized so far, including the differential cross sections in the various [mu][sup +] (or [mu][sup [minus]]) scattering angles (with the remaining angles integrated out completely) and the anticipated charge asymmetry. Numerical results on these quantities, in addition to the total production cross section, are described.

Associated production of Higgs and weak bosons, with H-->bb-bar, at hadron colliders.

We consider the search for the Higgs boson at a high-luminosity Fermilab Tevatron, an upgraded Tevatron of energy 3.5 TeV, and the CERN Large Hadron Collider, via $WH/ZH$ production followed by H -> bb~ and leptonic decay of the weak vector bosons. We show that each of these colliders can potentially observe the standard Higgs boson in the intermediate-mass range, 80 GeV <m_H < 120 GeV. This mode complements the search for and the study of the intermediate-mass Higgs boson via H -> \gamma\gamma at the LHC. In addition, it can potentially be used to observe the lightest Higgs scalar of the minimal supersymmetric model in a region of parameter space not accessible to CERN LEP II or the LHC (using h -> \gamma\gamma,ZZ^*).

Higgs boson with continuously distributed mass

We consider local quantum field theories with continuously distributed masses and in particular the Weinberg-Salam model with continuously distributed Higgs boson mass. The phenomenology of the Higgs boson with continuously distributed mass differs in a drastic way from the standard Higgs boson phenomenology, namely, there are invisible decay modes for the Higgs boson.

Higgs search and neural-net analysis

Two aspects of neural-net analysis are addressed: the application of neural nets to physics analysis and the analysis of neural nets. Feed-forward nets with error back-propagation are applied to the search for the standard Higgs boson at LEP 200. New methods to select the most efficient variables in such a classification task and to analyse the nets are presented. The sensitivity of the nets for systematic effects is studied extensively. The efficiencies of the neural nets are found to be significantly better than those of standard methods.

Heavy-Higgs-boson resonance.

We study the use of the standard Higgs boson, with ${\mathit{m}}_{\mathit{H}}$&gt;700 GeV, as a model for a resonance in longitudinal-vector-boson scattering. We focus on the constraint placed upon the modulus of the amplitude by unitarity. We show that it is better to use the energy-dependent width \ensuremath{\Gamma}(s)=${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$\ifmmode\times\else\texttimes\fi{}(s/${\mathit{m}}_{\mathit{H}}^{2}$${)}^{2}$ in the Higgs-boson propagator, rather than the usual approximation \ensuremath{\Gamma}(s)=${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$. The s-channel approximation, the effective-W approximation, and the full electroweak qq\ensuremath{\rightarrow}qqVV process are discussed.