Constraints on couplings of a fast decaying axion

Constraints are imposed on the couplings of an axion-like pseudoscalar particle φ that decays into e⁺e⁻ and γγ. The electron g−2 sets a severe bound not only on the φe⁺e⁻ coupling but also on the φγγ coupling. The most reliable bound on the φ coupling to the light quarks comes from the decay ∑⁺ → pe⁺e⁻ which was measured in a hydrogen bubble-chamber experiment with good sensitivity and resolution covering the entire range of e⁺e⁻ mass practically down to 2m_e. These upper bounds, together with a constraint from the decay K⁺→π⁺γγ, reinforce arguments against the particle-physics interpretation of the anomalous positrons in heavy-ion collisions, since the couplings are by orders of magnitude smaller than needed to produce e⁺e⁻ pairs at the observed level by known mechanisms.     

Visions: The coming revolutions in particle physics

We study the branching ratio, CP-violating asymmetry, forward-backward asymmetry and the CP-violating asymmetry in the forward-backward asymmetry for the exclusive decay B → Kτ⁺τ⁻ in the two Higgs doublet model with tree level flavor changing neutral currents (model III). We analyse the dependencies of these quantities on the neutral Higgs boson contributions and the CP parameter sinθ in the model III. We observe that to determine the neutral Higgs boson effects, the measurements of the forward-backward asymmetry and the CP-violating asymmetry in the forward-backward asymmetry for the decay B → Kτ⁺τ⁻ are promising.     

Probing ZWW couplings through Z0→Wff′

We investigated the possibility of probing the ZWW couplings through the rare decay mode Z⁰→Wff′ of Z⁰. Neglecting masses of the fermions, we computed the combined polarization density matrix of Z and W as a function of the most general ZWW couplings. Integrations over phase space are carried out and the diagonal matrix elements are expressed as quadratic forms of the seven independent couplings. We found that several charge symmetrix polarization channels are particularly sensitive to the CP-violating coupling f₆, which is proportional to the weak magnetic quadrupole moment.     

Searching for the standard model Higgs particle in Bs decays

Purely leptonic B_s decay, B_s→τ⁺τ⁻ and B_s→μ⁺μ⁻ are shown to be very sensitive to a standard model (SM) Higgs particle, for a t quark of mass ≳ 100 GeV, and for a Higgs mass of up to about 7 GeV, the latter being exchanged in the s-channel. B_d→ℓ⁺ℓ⁻ decays are also discussed. Theoretical cleanliness (i.e. absence of long-distance effects) of these modes also suggests that they should be very useful as precision tests of the SM and indicators of new physics.     

Limits on a lepton-number violating process from e+e− collisions

If a new hypothetical particle induces a lepton-number violating process, it could also affect the reactions e⁺e⁻→μ⁺μ⁻ and e⁺e⁻→τ⁺τ⁻ through the t-channel exchange of the particle. A fit of the cross sections and forward-backward asymmetries measured by experiments at PEP and PETRA yields the 90% confidence level limits on the coupling of the partile: G_eμ < 0.119G_F and G_eτ < 0.085G_F, where G_F is the Fermi coupling constant. These limits exclude certain regions of couplings allowed by other lepton-number violating searches.     

Dephasing in the semiclassical limit is system-dependent

Dephasing in open quantum chaotic systems has been investigated in the limit of large system sizes to the Fermi wavelength ratio, L/λ_F 〉 1. The weak localization correction g ^wl to the conductance for a quantum dot coupled to (i) an external closed dot and (ii) a dephasing voltage probe is calculated in the semiclassical approximation. In addition to the universal algebraic suppression g ^wl ∝ (1 + τ_D/τ_?)^?1 with the dwell time τ_D through the cavity and the dephasing rate τ _? ^?1 , we find an exponential suppression of weak localization by a factor of ∝ exp[? $\tilde \tau $ /τ_?], where $\tilde \tau $ is the system-dependent parameter. In the dephasing probe model, $\tilde \tau $ coincides with the Ehrenfest time, $\tilde \tau $ ∝ ln[L/λ_F], for both perfectly and partially transparent dot-lead couplings. In contrast, when dephasing occurs due to the coupling to an external dot, $\tilde \tau $ ∝ ln[L/ξ] depends on the correlation length ξ of the coupling potential instead of λ_F.     

A CP violating mixing matrix compatible with neutral flavour conservation and spontaneous CP violation

A specific ansatz for the Yukawa couplings of a four-generation SU(2)_L×U(1) model with two Higgs doublets is discussed which leads to neutral flavour conservation, spontaneous CP violation and to a genuinely complex mixing matrix. W exchange conserves CP in the limit m_t′ = m_t only. The decay rate for t→b is reduced by factor two compared to the standard model wit three generations. The phenomenological implications for K⁰−K̄⁰ and B⁰−B̄⁰ are investigated.     

Search for the decay τ− → vτηπ−

We have searched for the decay τ⁻ → v_τηπ⁻ using data accumulated by the ARGUS detector at the e⁺e⁻ storage ring DORIS II at DESY. No η signal was found in the π⁺π⁻π⁰ subsystems of the decay τ⁻ →v_τπ⁻π⁻π⁺ π⁰. We obtain an upper limit for the branching ratio of the decay τ⁻ → v_τηπ⁻ of 1.3% at the 95% confidence level.     

Two-loop results from improved one loop computations

A nonperturbative method is proposed for the approximative solution of the exact evolution equation which describes the scale dependence of the effective average action. It consists of a combination of exact evolution equations for independent couplings with renormalization group improved one loop expressions of secondary couplings. Our method is illustrated by an example: We compute the β-function of the quartic coupling λ of anO(N) symmetric scalar field theory to order λ³ as well as the anomalous dimension to order λ² using only one loop expressions and find agreement with the two loop perturbation theory. We also treat the case of very strong coupling and confirm the existence of a “triviality bound”.     

The constraints of non-standardWWγ couplings from FCNC processes

It was reported that the contributions of the non-standardWWγ couplings called λ_γ and \(\bar \lambda _\gamma \) to the anomalous magnetic moments and the electric dipole moments of fermions depend on regularization schemes of the loop integral. We find that this dependence does not exist in the contributions of the λ_γ and \(\bar \lambda _\gamma \) couplings to the flavor changing neutral current (FCNC) processes of quarks such asb→sγ. By utilizing the experimental upper bound of the branching ratio of the processB→K ^*γ, we obtain the allowed regions of λ_γ and \(\bar \lambda _\gamma \) that do not depend on regularization schemes of the loop integral. The cut-off independent constraints of the other non-standardWWγ couplings called Δκ_λ and \(\tilde \kappa _\gamma \) are also obtained.     

Measurement of e+e−→τ+τ− at high energy and properties of the τ lepton

The e⁺e^?→τ⁺τ^? process has been measured using the CELLO detector at a mean total centre of mass energy of 34.2 GeV using essentially all the decay channels of the τ lepton. The measured cross section yields R_τ=1.03±0.05 (stat)±0.07 (syst). Topological branching fraction are given for τ → 1, 3 or 5 charged tracks. The angular distribution shows a clear 1 + cos²θ dependance with a forward-backward asymmetry of -0.103 ± 0.052 corresponding to an axial-vector coupling a_τ of the τ to the weak neutral current given by a_τ=?1.12 ± 0.57.     

Radiative Kaon decays and CP violation in chiral perturbation theory

Chiral perturbation theory is a very useful framework for testing the standard model in processes where long-distance effects are expected to play an essential rôle. We analyze the rare K decays K⁰→γℓ⁺ℓ⁻, K⁺→π⁺γ3 and K_L→π⁰ℓ⁺ℓ⁻ in the effective chiral formulation of the standard model. These processes, like the decays K⁰→γγ, K⁺→π⁺ℓ⁺ℓ⁻, K_S→π⁰ℓ⁺ℓ⁻ and K⁰→π⁰γγ discussed in previous work, have the property that the corresponding amplitudes vanish to lowest order in chiral perturbation theory. Precise predictions for decay rates and spectra are made in terms of a few coupling constants not restricted by softly broken chiral symmetry alone. Special consideration is given to various possible tests of CP noninvariance in these decays, in particular to effects due to intrinsic CP violation in the amplitudes. We find that chiral perturbation theory correlates different CP-violating observables such as the charge asymmetries in K^±→π^±γγ and K^±→π^±ℓ⁺ℓ⁻, the one-photon exchange contribution to K_L→π⁰e⁺e⁻ and the transverse polarization in K_L →π⁰μ⁺μ⁻. Detailed numerical results are shown.     

Uplifted supersymmetric Higgs region

We show that the parameter space of the Minimal Supersymmetric Standard Model includes a region where the down-type fermion masses are generated by the loop-induced couplings to the up-type Higgs doublet. In this region the down-type Higgs doublet does not acquire a vacuum expectation value at tree level, and has sizable couplings in the superpotential to the tau leptons and bottom quarks. Besides a light standard-like Higgs boson, the Higgs spectrum includes the nearly degenerate states of a heavy spin-0 doublet which can be produced through their couplings to the b quark and decay predominantly into τ ⁺ τ ^? or τν.     

Gaussian effective potential for the U(1) Higgs model

In order to investigate the Higgs mechanism nonperturbatively, we compute the Gaussian effective potential of the U(1) Higgs model (“scalar electrodynamics”). We show that the same simple result is obtained in three different formalisms. A general covariant gauge is used, with Landau gauge proving to be optimal. The renormalization generalizes the “autonomous” renormalization for λ?⁴ theory and requires a particular relationship between the bare gauge coupling e _B and the bare scalar self-coupling λ _B. When both couplings are small, then λ is proportional to e⁴ and the scalar/vector mass-squared ratio is of order e², as in the classic 1-loop analysis of Coleman and Weinberg. However, as λ increases, e reaches a maximum value and then decreases, and in this “nonperturbative” regime the Higgs scalar can be much heavier than the vector boson. We compare our results to the autonomously renormalized 1-loop effective potential, finding close agreement in the physical predictions. The main phenomenological implication is a Higgs mass of about 2 TeV.     

Theoretical upper bound on F → φπ decay rate by current algebras

A tight bound is derived on the F → φπ decay rate from current algebras and a few safe assumptions which are well justified theoretically and experimentally. In terms of the branching ratio and the F meson lifetime, the bound is written as B(F → φπ) ? (2.1 × 10¹¹) s^?1 × τ_F. No soft-pion limit is taken to derive the bound.     

Nuclear electric dipole moment of He

A permanent electric dipole moment (EDM) of a physical system requires time-reversal (T) and parity (P) violation. Experimental programs are currently pushing the limits on EDMs in atoms, nuclei, and the neutron to regimes of fundamental theoretical interest. Here we calculate the magnitude of the P-, T-violating EDM of ³He and the expected sensitivity of such a measurement to the underlying P-, T-violating interactions. Assuming that the coupling constants are of comparable magnitude for π-, ρ-, and ω-exchanges, we find that the pion-exchange contribution dominates. Our results suggest that a measurement of the ³He EDM is complementary to the planned neutron and deuteron experiments, and could provide a powerful constraint for the theoretical models of the pion–nucleon P-, T-violating interaction.     

An improved upper limit on the ντ-mass from the decay π−→π−π−π−π+π+ντ

Using the ARGUS detector at the e⁺e⁻ storage ring DORIS II, we have observed the decay τ⁻→π⁻π⁻π⁻π⁺π⁺ν_τ in tau-pair events produced at center-or-mass energies between 9.4 and 10.6 GeV. From the 5π invariant mass distribution we derive an upper limit of m(ν_τ)<35 MeV/c² at the 95% confidence level. The branching ratio for this decay channel is found to be (0.064±0.023±0.01)%.     

The twistM λ,T=0 giant modes in spherical nuclei

The theory of twisting nuclear vibrations developed by Holzwarth and Eckart to elucidate the nature of the 2^? giant mode is extended to high-multipole, spin-independentM λ,T=0 resonances. Within the framework of the distorted Fermi-surface model the parameters of inertia and stiffness against multipolarity for the nuclear incompressible Fermi-drop are derived. The energies and probabilities of the twistingM λ,T=0 modes calculated in fact without adjustable constants readE(M λ)=?ω _F \(\left[ {\frac{{(2\lambda + 3)(\lambda - 1)}}{5}} \right]^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}} \) andB(M λ) = γ_λ Z² A ^(2λ-4)/3 μ² fm^(2λ-2)respectively. The comparison with data for the 2^? resonance and predictions for higher multipoles are presented.     

On the Spectral Theory of Operator Pencils in a Hilbert Space

Abstract

Consider the operator pencil L _λ = A ? λB ? λ ² C, where A, B, and C are linear, in general unbounded and nonsymmetric, operators densely defined in a Hilbert space H. Sufficient conditions for the existence of the eigenvalues of L _λ are investigated in the case when A, B and C are K-positive and K-symmetric operators in H, and a method to bracket the eigenvalues of L _λ is developed by using a variational characterization of the problem (i) L _λ u = 0. The method generates a sequence of lower and upper bounds converging to the eigenvalues of L _λ and can be considered an extension of the Temple-Lehman method to quadratic eigenvalue problems (i).