Effective gauge theory and the effect of heavy quarks in Higgs boson decays

A general framework is given for evaluating the contributions of as yet undiscovered heavy quarks to the gluonic decay rate of the Weinberg-Salam type Higgs boson. Since the Yukawa coupling of the Higgs boson to a quark pair is proportional to the quark mass, loop graphs involving heavy quarks have a non-vanishing effect on the gluonic decay width of the Higgs boson. This effect of heavy quarks with massesM _j(j=t,...) much greater than the Higgs boson massm _H is calculated in an effective gauge theory. The effects of two different kinds of large logarithms, lnM _j ² /μ m _h ² /μ ² are separated and summed up by the renormalization group method. It is found that the higher order QCD corrections are large and that the gluonic contribution to the hadronic decay width is significant if there are more than three generations. The Higgs decay width can therefore be used to probe the number of generations of heavy quarks.     

The hadronic tau decay signature of a heavy charged Higgs boson at LHC

The hadronic tau decay channel offers by far the best signature for heavy charged Higgs boson search at the LHC in the large tanβ region. By exploiting the distinct polarization of the tau and its large transverse mass, along with the accompanying missing–p_T, one can probe for a charged Higgs boson up to a mass of about 600 GeV in an essentially background-free environment. The transverse mass distribution of the tau jet also provides a fairly unambiguous estimate of the charged Higgs boson mass.     

Search for invisibly decaying Higgs bosons with large decay width using the OPAL detector at LEP

This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e⁺e^-→HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV corresponding to a total integrated luminosity of 629 pb^-1. In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state. For moderate Higgs boson decay widths, these range from about 0.07 pb (M_H=60 GeV) to 0.57 pb (M_H=114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario experimentally accessible at LEP 2.     

Cosmic-ray induced vacuum decay in the standard model

In the standard model, if the Higgs boson mass, m_H, and the top quark mass, m_t, satisfy the relationship m_t≳95GeV + 0.60 m_H, then the vacuum is unstable. However, if the top quark mass is less than 190 GeV, then the lifetime is greater than the age of the universe. There is thus a large region of parameter space in which the vacuum is unstable, but sufficiently long-lived. We examine the possibility that high energy cosmic ray collisions could induce the decay of the vacuum, and show that this region of parameter space can be excluded.     

Search for a SM Higgs boson in dilepton plus missing transverse energy final state with the D??detector at {\sqrt s}{=}{1.96}?TeV

A search is presented for the Standard Model (SM) Higgs boson optimized in the decay channel H??W ^?+? W ^??, where both W bosons decay leptonically. The final state considered contains dileptons and missing transverse energy from the neutrinos. A multivariate analysis is used to suppress the background. No significant excess above the SM background has been observed and limits set on the Higgs boson production cross-section ? the branching ratio for m _H?= 115?C200?GeV are computed. Results using 8.1?fb^?1 of data are presented.     

Associated production of heavy Higgs boson with top quarks

Associated production of a heavy Higgs boson (m_H > 100 GeV) with top quarks at Juratron energies is studied. It is natural to differentiate between the “light” (2m_t < m_H < 2m_W) and “heavy” (m_H > 2m_W) Higgs search. It is assumed that the mass value of the top quarks is in the interval m_t ≈ 30–80 GeV. m_W is the W-boson mass. If m_H < 2m_W a dangerous background is given by the QCD production of four top quarks. We have calculated the cross sections for both the Higgs production and the background reaction. The disappointing result found is that the background is overwhelmingly large. However the Higgs search in this mass region is not hopeless. The associated production of the Higgs boson with a W-boson may have a clear experimental signature, its background given by the reaction $\text{p}\text{+}\text{p}\text{→}\text{W}\text{+}\text{t}\text{+}\text{t}$ might be suppressed. The difficulty with this mechanism is that the rate is rather low. If m_H > 2m_W the background is different and its contribution is expected to be small. The associated production of a Higgs boson with a pair of top quarks might be a useful method in the Higgs search in this case.     

Production on heavy Higgs bosons ate + e −-colliders

The production of Higgs bosons in the mass range of 200 GeV/c²<M _H <1TeV/c ² ate ⁺ e ^? super colliders is studied. We consider the possibilities to detect Higgs bosons via their decays into vector boson pairs. Single particle distributions of vektor bosons arising from this decay are compared with background reactions. We find, that the signal of Higgs bosons with masses up to 60- GeV/c² should be visible over the background already at \(\sqrt s = 1\) TeV, provided that the vektor bosons can be identified via their hadronic decay modes.     

Improving the heavy-Higgs four-charged-lepton signature

Assuming the Higgs scalar boson H⁰ of the standard model has mass m_H>2M_Z, we show that the popular four-lepton decay signature H⁰→ZZ→ℓ₁⁺ℓ₁⁻ℓ₂⁺ℓ₂⁻ at the SSC can be significantly enhanced relative to the continuum ZZ background by requiring high transverse momentum for at least one Z. The calculated signal is sensitive to the top quark mass.     

A numerical estimate of the upper limit for the Higgs boson mass

TheSU (2) Higgs model with a scalar doublet field is studied by Monte Carlo calculations on 12⁴ and 16⁴ lattices. The gauge coupling is chosen to be similar in magnitude to the physical value in the standard model. The numerical results at large scalar self-coupling imply an upper limitm _H /m _W ?9 for the ratio of the Higgs boson mass to the W-mass.     

On the decays of heavy Higgs bosons

The decay of a neutral heavy Higgs bosonH to a weak vector bosonV+lepton pair is discussed and found to be relevant. Its width is strongly growing with increasingm _H and fromm _H≈200 GeV it exceeds ≈10 MeV.     

On the lattice approach to the maximum Higgs boson mass

If the triviality upper bound on the Higgs boson mass m_H occurs for strong self-coupling, inferring properties of the Higgs from the euclidean propagator is in principle theoretically difficult whether in coordinate or momentum space. In that case, common methods of identifying m_H in lattice field theory simulations may produce a value for which is at best distantly related to the true upper limit. We discuss some shortcomings and ambiguities of recent results suggesting that the maximum occurs for weak coupling and emphasize potential complications due to finite-size and non-Lorentz-invariant effects of the lattice. The situation is illustrated by reference to the behavior in an analytically soluble approximation based on a 1/N expansion.     

Four weak gauge boson production at photon linear collider and heavy Higgs signal

We study the signals and backgrounds for a heavy Higgs boson in the processes γγ → WWWW, γ → WWZZ at the proton linear collider. The results are based on the complete tree-level SM calculation for these reactions. We show that the invariant mass spectrum of central WW, ZZ pairs is sensitive to the signal from Higgs boson with a mass up to 1 TeV linear collider for integrated luminosity of 300 fb⁻¹. At 1.5 TeV PLC Higgs boson with a mass up to 700 GeV can be studied. The nonresonant longitudinal gauge boson scattering (m_H = ∞) can be detected in photon-photon collisions at e⁺e⁻ center-of-mass energy of 3 TeV.     

Simplified parametric scenarios of the Minimal Supersymmetric Standard Model after the discovery of the Higgs boson

Constraints on the parameter space of theMinimal Supersymmetric StandardModel (MSSM) that are imposed by the experimentally observed mass of the Higgs boson (m_H = 125 GeV) upon taking into account radiative corrections within an effective theory for the Higgs sector in the decoupling limit are examined. It is also shown that simplified approximations for radiative corrections in theMSSM Higgs sector could reduce, to a rather high degree of precision, the dimensionality of the multidimensionalMSSM parameter space to two.     

Low-energy impact of a heavy Higgs boson sector

The effective theory which characterizes the low-energy sensitivity of the minimal Weinberg-Salam model to a heavy Higgs boson sector is shown to be the gauged SU(2)_L × U(1) non-linear θ model. This theory is the limit of the Weinberg-Salam model as the Higgs boson mass, M_H, is removed (M_H → ∞). Using the symmetry properties of the non-linear theory, along with a power-counting analysis, we are able to classify low-energy observables according to their sensitivity to the regulator (M_H). At one loop, the greatest sensitivity is a logarithmic dependence on the Higgs boson mass. The M_H dependent corrections to some specific, experimentally accessible observables are calculated, and other possible applications of this technique are discussed.     

Search for a standard model Higgs boson in CMS via vector boson fusion in the H→WW→lνlν channel

We present the potential for discovering the standard model Higgs boson produced via the vector-boson fusion mechanism. We considered the decay of Higgs bosons to the W⁺W^- final state, with both W-bosons subsequently decaying leptonically. The main background is tt̄ produced in association with one or more jets. This study is based on a full simulation of the CMS detector. The result is that a signal of 5σ significance can be obtained with an integrated luminosity of 12–72 fb^-1 for Higgs boson masses in the range 130<m_H< 200 GeV. In addition, the major background can be measured directly to 7% from the data with an integrated luminosity of 30 fb^-1. We also suggest a method to determine the Higgs mass using template transverse mass distributions. PACS 14.80.Bn     

Bounds on a top quark with charged Higgs decay

If the top quark decays to a charged Higgs boson t→bH⁺, semileptonic decay signatures are suppressed and the mass bounds set by the UA1 top search are evaded. However the charged Higgs decay mode H⁺→τ⁺ν_τ gives a missing-ϱ_T signature; depending on the decay branching fraction and using the UA1 limits on new physics contributions to missing-ϱ_T, we set bounds on the top mass in this scenario. We also evaluate the feasibility of seeking visible-τ-jet signals for top at a pp̄ collider.     

Nonlocal Yukawa interaction and fermion mass-scale nonlocality

A nonlocal Yukawa interaction between the Higgs boson and the fundamental fermions is introduced. A simple form of this interaction allows us to calculate a particular mass-scale nonlocality for all fundamental fermions. A prediction is given for the mass of the Higgs boson (m _H ≈ 200 GeV).     

The standard model process e^ + e^ - \to \nu \bar \nu b\bar b and its Higgs signal at LEP II

We present and study the results for the standard model process $e^ + e^ - \to \nu \bar \nu b\bar b$ at c.m. energies 150 ≤ √s(GeV) ≤ 240 and for Higgs boson masses 60 GeV ≤ m _H ≤ 110 GeV, obtained from all tree-level diagrams and including the most important radiative corrections. The matrix elements have been calculated by the ‘spinor bracket’ method without neglecting masses, which is presented in detail. The √s-dependence and the interference properties of the Higgs boson contributions and of various coherent background contributions to the total cross section are examined and compared. The important differential distributions for the Higgs boson and the background components are studied, providing information useful for choosing cuts in Higgs searches. We also examine the effect of a minimal set of cuts and evaluate the importance of the WW fusion for detecting a higher mass Higgs boson at LEP II.     

Z′ bosons, the NuTeV anomaly, and the Higgs boson mass

Fits to the precision electroweak data that include the NuTeV measurement are considered in family universal, anomaly free U(1) extensions of the Standard Model. In data sets from which the hadronic asymmetries are excluded, some of the Z′ models can double the predicted value of the Higgs boson mass, from ～60 to ～120 GeV, removing the tension with the LEP II lower bound, while also modestly improving the χ ² confidence level. The effect of the Z′ models on both m _H and the χ ² confidence level is increased when the NuTeV measurement is included in the fit. Both the original NuTeV data and a revised estimate by the PDG are considered.     

Higgs boson bremsstrahlung in top quark decay

In top quark decay a neutral Higgs boson may be emitted in bremsstrahlung from the top quark or from the W-boson. We evaluate the branching fraction for this hitherto overlooked t→H⁰bW⁺ decay mode, where the W⁺ can be virtual or real. It has the standard model values of 0.02%, 0.2%, 0.8% for m_t=50, 100, 200 GeVand m_H=10 GeV.