We study scalar, pseudoscalar, vector, and axial-vector mesons with non-strange and strange quantum numbers in the framework of a linear sigma model with global chiral U (N f )L × U (N f )R symmetry. We perform a global fit of meson masses, decay widths, as well as decay amplitudes. The quality of the fit is, for a hadronic model that does not consider isospin-breaking effects, surprisingly good. We also investigate the question whether the scalarqq states lie below or above 1 GeV and find the scalar states above 1 GeV to be preferred asqq states. Additionally, we also describe the axial-vector resonances asqq states.

We present a two-flavour linear sigma model with global chiral symmetry and vector and axialvector mesons. We calculate ππ scattering lengths and the decay widths of scalar, vector, and axial-vector mesons. It is demonstrated that vector and axial-vector meson degrees of freedom play an important role in these low-energy processes and that a reasonable theoretical description requires globally chirally invariant terms other than the vector meson mass term. An important question for meson vacuum phenomenology is the quark content of the physical scalar f0(600) and a0(980) mesons. We investigate this question by assigning the quark-antiquark σ and a0 states of our model with these physical mesons. We show via a detailed comparison with experimental data that this scenario can describe all vacuum properties studied here except for the decay width of the σ, which turns out to be too small. We also study the alternative assignment f0(1370) and a0(1450) for the scalar mesons. In this case the decay width agrees with the experimental value, but the ππ scattering length a 0 0 is too small. This indicates the necessity to extend our model by additional scalar degrees of freedom.

Investigation at a φ-factory can shed light on several debated issues in particle physics. We discuss: i) recent theoretical development and experimental progress in kaon physics relevant for the Standard Model tests in the flavor sector, ii) the sensitivity we can reach in probing CPT and Quantum Mechanics from time evolution of entangled kaon states, iii) the interest for improving on the present measurements of non-leptonic and radiative decays of kaons and η/η′ mesons, iv) the contribution to understand the nature of light scalar mesons, and v) the opportunity to search for narrow di-lepton resonances suggested by recent models proposing a hidden dark-matter sector. We also report on the e + e − physics in the continuum with the measurements of (multi)hadronic cross sections and the study of γγ processes.

We study the three-flavor chirally and dilatation invariant extended Linear Sigma Model with (pseudo)scalar and (axial-)vector mesons as well as a scalar dilaton field whose excitations are interpreted as a glueball. The model successfully describes masses and decay widths of quarkantiquark mesons in the low-energy region up to 1.6 GeV. Here we study in detail the vacuum properties of the scalar-isoscalar J P C = 0 ++ channel and find that (i) a narrow glueball is only possible if the vacuum expectation value of the dilaton field is (at tree-level) quite large (i.e., larger than what lattice QCD and QCD sum rules suggest) and (ii) that only solutions in which f0 (1710) is predominantly a glueball are found. Moreover, the resonance f0(1370) turns out to be mainly (ūu +dd)/ √ 2 and thus corresponds to the chiral partner of the pion, while the resonance f0(1500) is mainlyss.

The strong and electromagnetic decays of the ground-state tensor mesons are studied in an effective field approach. A fit to the well-known experimental data is performed. The decay ratios of the tensor glueball are evaluated and possible candidates are discussed.

We investigate a linear sigma model with global chiral Uð2Þ R Â Uð2Þ L symmetry. The mesonic degrees of freedom are the standard scalar and pseudoscalar mesons and the vector and axial-vector mesons. The baryonic degrees of freedom are the nucleon, N, and its chiral partner, N Ã , which is usually identified with Nð1535Þ. The chiral partner is incorporated in the so-called mirror assignment, where the nucleon mass is not solely generated by the chiral condensate but also by a chirally invariant mass term, m 0 . The presence of (axial-) vector fields modifies the expressions for the axial-coupling constants of the nucleon, g N A , and its partner, g N Ã A . Using experimental data for the decays N Ã ! N and a 1 ! , as well as lattice results for g N Ã A we infer that in our model m 0 $ 500 MeV, i.e., an appreciable amount of the nucleon mass originates from sources other than the chiral condensate. We test our model by evaluating the decay N Ã ! N and the s-wave nucleon-pion scattering lengths a ðAEÞ 0 .

We compute the one-loop polarization tensor Π for the on-shell, massless mode in a thermalized SU(2) Yang-Mills theory being in its deconfining phase. Postulating that SU(2) CMB today = U(1) Y , we discuss Π's effect on the lowmomentum part of the black-body spectrum at temperatures ∼ 2 · · · 4 T CMB where T CMB ∼ 2.73 K. A table-top experiment is proposed to test the above postulate. As an application, we point out a possible connection with the stability of dilute, cold, and old innergalactic atomic hydrogen clouds. We also compute the two-loop correction to the pressure arising from the instantaneous massless mode in unitary-Coulomb gauge, which formerly was neglected, and present improved estimates for subdominant corrections.

In the framework of the U (2) R × U (2) L symmetric linear sigma model with (axial)vector mesons generalized by including a dilaton field we study the phenomenology of the scalar-isoscalar resonances below 2 GeV. It turns out that, in our favoured scenario, the resonance f 0 (1370) is predominantly aqq state and f 0 (1500) is predominantly a glueball state. Additionally we are able to calculate the value of the gluon condensate, which is in agreement with lattice QCD results.