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.     

An SU(2) model for weak interactions

It is shown that it is possible to obtain neutral current weak interaction in agreement with experiments in a pure SU(2) model. The main experimental consequences are the relation mW₃? √2 mW± and the prediction that mW± ? 53 GeV and mW₃ ? 75GeV.     

Role of heavy fermions and heavy Higgs scalars in the electroweak correction to the MW − MZ relation

The role of heavy fermions and heavy Higgs-scalars in the M_W ? M_Z relation resulting from the one-loop-corrected μ decay width Γ⁽¹⁾ and its experimental data Γ^exp is studied in the framework of the standard electroweak theory. Exact and approximate formulae are both given for these heavy particle effects. The quadratic dependence of Γ⁽¹⁾ on large fermion mass m_heavy gives a positive contribution to the calculation of M_W from Γ⁽¹⁾ = Γ^exp for a given M_Z, and cancels the light fermion contributions of the form ～ αln(m_light/M_W) at the value of m_heavy ～ 200 GeV. On the other hand, the Higgs mass dependence of the calculation is, at best, logarithmic, and does not produce visible effects. Applications for deriving constraints for the top-quark mass (or heavier fermion mass) are discussed, and a concrete example is given of the relation between experimental uncertainties in measurements of M_W,Z and the corresponding allowed region for m_t.     

Enhancement of CP-violating phenomena at tree level due to heavy quark propagator singularities

We discuss new interesting CP-violating phenomena in scattering processes resulting from heavy quarks with masses larger than the W-mass (m _q >M _W ). Such a situation is connected with a singularity in the heavy quark propagator which, properly regularized, gives rise to measurable CP-odd effects. The observability of these CP-odd signals in the most representative Kobayashi-Maskawa (KM) models (like fourth generation model, left-right symmetric model and two Higgs doublet model) is investigated. We find that the necessary conditions for measurable CP-asymmetries imply the enlargement of the Standard Model (SM) in the generations of quarks. In addition a significant mixing between the fourth and the other generations is required.     

Heavy neutrino mixing and single production at linear collider

We study the single production of heavy neutrinos via the processes e⁻e⁺ → νN and e⁻γ → W⁻N at future linear colliders. As a base of our considerations we take a wide class of models, both with vanishing and non-vanishing left-handed Majorana neutrino mass matrix m_L. We perform a model independent analyses of the existing experimental data and find connections between the characteristic of heavy neutrinos (masses, mixings, CP eigenvalues) and the m_L parameters. We show that with the present experimental constraints heavy neutrino masses almost up to the collision energy can be tested in the future experiments.     

On the Meissner effect in gauge theories

The phase structure of spontaneously broken scalar electrodynamics in an external electromagnetic field is analyzed. With no external field, the spectrum comprises a scalar boson of mass m_H and a vector boson of mass m_W. If m_H ≤ m_W, it is shown that in the tree approximation, as the external field is increased, a first order phase transition to a restored symmetry phase occurs, and the critical field strength is calculated. Below the critical point the external field is completely screened, this being the analogue of the Meissner effect in superconductivity. If m_H > m_W, a third phase, characterized by vortex solutions of the field equations, occurs. Quantum effects, such as pair production in an electric field, are considered at the one (and two) loop level in the massless theory (the Coleman-Weinberg model). The leading correction to the critical magnetic field strength is calculated, and it is shown that for an external electric field the phase transition does not exist.     

Physical null zones and radiation representation

General conditions for reconstructing physical null radiation zones in single photon tree amplitudes are given. The systematic analysis has been carried out using invariant quantities. For arbitrary values of masses and charges these zones are always smaller than in the massless and equal charges case. As an application the radiative W boson decay into heavy quarks is studied. This process turns out be a rather sensitive test of the current quark masses m_q(M_W²), as well as of the $\text{q}$qW, q$\text{q}$γ and W⁺W^?γ vertices. This is to the presence of a null line in the photon phase space with a location which strongly depends on m_q. A recently proposed radiation representation for single photon tree amplitudes is analyzed. Explicit examples are given for a number of cases including fermion and vector boson lines.     

Effects of unknown fermion generations on the mw, mZ interdependence

The effect of unknown fermion generations on the m_W, m_Z interdependence is analyzed. It is shown that an additional fermion generation can increase the quantum correction Δr and, therefore, the predicted mass difference m_Z - m_W for given m_Z, but such positive contributions are bounded above by small quantities. In particular, as previously pointed out by Veltman, massive degenerate fermions do not fully decouple but lead to small positive corrections. On the other hand, it is known that significant negative contributions to Δr and m_Z - m_W can arise from exotic values for the mass of the top quark and the isodoublet splittings in higher generations. A method of obtaining information about such masses when m_W and m_Z become precisely known is discussed. The analytic methods of obtaining the general features of the corrections, for essentially arbitrary fermion masses, exploit the convexity properties of elementary functions.     

Generalization of the radiative corrections to β and μ decays in the SU(2)L × U(1) gauge model

Previous results for the radiative corrections to the ratio of decay probabilities of β and μ decays are generalized to the case of arbitrary symmetry breaking in the framework of the SU(2)_L × U(1) gauge model. In this more general case, the results remain very simple: the corrections are given by the corresponding expressions of the local V-A theory with m_W substituted for the cutoff Λ plus a positive definite contribution which depends on θ_W and R = m_W²/m_Z².Thus, the corrections in the general case are still constrained to have large values as required by Cabibbo universality and the present experimental values on β and μ decays. For reasonable values of m_Z and m_W the results in the general case are numerically close to those previously obtained in the simplest version of the theory. To illustrate this point the corrections are compared for two different choices of the parameters of the theory. In an appendix we discuss, rather briefly, the theoretical arguments that allow us to generalize pprevious discussions the case of fractionally charged quarks.     

Radiatively induced spontaneous symmetry breaking and phase transitions in curved spacetime

Local gauge symmetries which are spontaneously broken in flat spacetime are shown to be restored for large spacetime curvatures. The case of symmetry breaking due to radiative quantum corrections in gauge theories with elementary scalar fields is considered explicitly. In spacetimes with a positive Ricci curvature scalar R and a cosmological event horizon, the critical curvature R_C is of O(m_H²) or O(m_W²), depending on whether the theory is formulated with conformal or minimal scalar fields. In Ricci flat spacetimes with a conventional event horizon the symmetry is expected to be restored when the temperature of the Hawking thermal radiation is of O(m_W). This phenomenon is described in detail, using functional integral methods and dimensional renormalization, for massless scalar electro-dynamics in de-Sitter spacetime. For conformal scalars, the symmetry restoring phase transition is first order, the critical curvature being R_C = 0.910 m_H². For minimal scalars, an anomalous, curvature dependent mass counterterm is required. The phase transition in this case is second order, and occurs at R_C = 83.57 m_W². Symmetry restoration at finite temperature in flat spacetime is considered in an appendix. The critical temperature at which a first-order phase transition occurs in the Weinberg-Salam model is found to be T_C = 0.329 m_W.     

Limits to possible scalar-boson mass and coupling from e+e− collisions

Measurements of e⁺e^? → e⁺e^? at 2.8 GeV are reported and interpreted in terms of limits for the mass and coupling of a possible scalar boson of the type introduced in recent renormalizable models of weak interactions. In particular, in the Georgi-Glashow scheme of leptons we find that the scalar boson mass must be larger than 10 GeV for an m_W = 10 GeV (m_W mass of the W-boson) and of 6.5 GeV for m_W = 15 GeV. Alternatively its coupling is extremely weak.     

Universal behavior in the scattering of heavy, weakly interacting dark matter on nuclear targets

Particles that are heavy compared to the electroweak scale (M?mW), and that are charged under electroweak SU(2) gauge interactions display universal properties such as a characteristic fine structure in the mass spectrum induced by electroweak symmetry breaking, and an approximately universal cross section for scattering on nuclear targets. The heavy particle effective theory framework is developed to compute these properties. As illustration, the spin independent cross section for low-velocity scattering on a nucleon is evaluated in the limit M?mW, including complete leading-order matching onto quark and gluon operators, renormalization analysis, and systematic treatment of perturbative and hadronic-input uncertainties.     

Associated production of intermediate-mass Higgs particle with a large-pT jet at SSC energy

We calculate the higher-order Higgs particle production process gg→gH for a large top-quark mass (2m_t > m_H). We compute the resulting associated cross section for intermediate-mass Higgs particle (m_W<m_H<2m_W) at SSC, followed by its dominant decay mode into a bottom-quark pair. At large p_T the cross section becomes comparable to that of the QCD background while remaining sufficiently large for detection at SSC.     

Pseudoscalars and vector mesons in a unitary and self-consistently broken SU(6) W scheme

We estimate hadronic self-energy effects to “bare” pseudoscalar (P) and vector (V) meson states due to theP→PV→P, P→VV→P, V→ PP→V, V→PV→V andV→VV→V loops. We simulate higher order diagrams by consistently requiring external and internal particles to have the same mass. We find good agreement with all the experimental masses (exceptm _π), widths and mixing angles. The “bare”P andV states are heavy (≈1.26 GeV) and degenerate up to a smallm _s?m_u quark mass difference term. The “bare” coupling constants for thePPV, PVV andVVV vertices obey exact OZI rule and almost exact SU(6) _W symmetry. We use a common cut-off ofk _cm?0.7 GeV/c corresponding to a harmonic oscillator radius of ?0.7 fm for all SU(6) _W related thresholds except for the pion.     

Upper bound estimate for the Higgs mass from the lattice regularized Weinberg-Salam model

There are indications that, as a consequence of the trivality of the scalar φ⁴ theory, the Weinberg-Salam model is massive free field theory. However, considering theWS model as an effective field theory, an upper bound for the Higgs mass can exist. We have studied this phenomenon by simulating the SU(2) gauge Higgs model on a lattice. The results indicate that R_max = m_H/m_W|_max = 9.3 ± 1. However, serious finite size effects make it unfeasible to explore the region m_H ⩽ Λ_cutoff with Monte Carlo simulation.     

A two-particle exchange interaction model

The magnetisation reversal of two interacting particles was investigated within a simple model describing exchange coupling of magnetically uniaxial single-domain particles. Depending on the interaction strength W, the reversal may be cooperative or non-cooperative. A non-collinear reversal mode is obtained even for two particles with parallel easy axes. The model yields different phenomena as observed in spring magnets such as recoil hysteresis in the second quadrant of the field-magnetisation-plane, caused by exchange bias, as well as the mentioned reversal-rotation mode. The Wohlfarth’s remanence analysis performed on aggregations of such pairs of interacting particles shows that the deviation δM(H_m) usually being considered as a hallmark of magnetic interaction vanishes for all maximum applied fields H_m not only at W=0, but also for sufficiently large values of W. Furthermore, this so-called δM-plot depends on whether the sample is ac-field or thermally demagnetised.     

Large mW, mZ behaviour of the O(α) corrections to semileptonic processes mediated by W

Using the current algebra formulation of radiative corrections and working in the framework of the SU(2)_L × U(1) × SU(3)_c theory, we derive a theorem that governs the large m_W, m_Z behaviour of the O(α) corrections to general semileptonic processes mediated by W. The leading asymptotic dependence is logarithmic with a universal coefficient not affected by the strong interactions. As a byproduct, we obtain the leading asymptotic effect induced perturbatively by the strong interactions, which is of O(ln ln (m_W/Λ)).     

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.     

Radiative corrections to νμ + N → μ− + X and their effect on the determination of ϱ2 and sin2θW

We study the O(α) corrections to σ_T(ν_μ + N → μ^? + X). The coefficient of the leading O(αln m_Z) contribution is given by a general theorem as in the case of β decay, while the energy scale accompanying m_Z in the argument of the logarithm as well as mass singularities and non-logarithmic terms are obtained by a detailed quark-parton model calculation. The effect of muon mass singularities, when there is an experimental vut in the low range of the y distribution, is also investigated. Combining these new results with our previous analysis of σ_T(ν_μ + N → ν_μ + X), we calculate theSU(2)_L × U(1) parameter ?_(νμ;h)² and evaluate the effect of the O(α) corrections on the determination of sin²θ_W^exp from deep inelastic ν_μ scattering. Applying these corrections to existing data, we obtain the weigthed average sin²θ_W^exp = 0.216 ± 0.010 ± 0.004. This value when used in conjuction with our previous analysis of the W^± and Z⁰ masses provides the predictions m_W = 83.0_?2.8^+3.0 GeV and m_Z = 93.8_?2.4^+2.5 GeV. For the weak interaction angle defined by modified minimal subtraction we find sin²θ_W(m_W) = 0.215 ± 0.010 ± 0.004, which is in very good agreement with the SU(5) prediction we have recently given.     

Cabibbo angle,CP violation and quark masses

By adding suitable discrete flavor symmetries to SU(2)_L ? SU(2)_R ? U(1) left-right symmetric gauge models of weak and electromagnetic interactions, we are able to express all the mixing angles between the quark flavors (u, d, s,c) in terms of the quark masses. This enables us to compute the Cabibbo angle and the CP violating phases using plausible values for the quark masses. The CP violating K ar 2π decay amplitude η_+? (and η₀₀ in the model is then given purely in terms of the parity violating parameter of the model (m_WL⁺/m_WR⁺)².