[en] The scalar mesons have caused much debate amongst hadronic physicists for many years. Even today the number of scalars is hotly contested, and there is almost no agreement on the composition of any of the experimentally observed states, except perhaps for the K₀* (1430). This thesis attempts to shed light on both of these problems via the application of analyticity to two different quantities. Recently a number of authors have proposed the existence of a light, strange, scalar meson known as the κ. We perform a direct search of the best available πK scattering data to determine whether or not this resonance exists. This is done by constructing contour integrals from these data and determining the number of poles present inside the contour. We do not need to model either the internal dynamics of the state nor the form of the background scattering. The number of poles found tells us the number of resonances present and their positions allow us to estimate the resonance parameters. We find that there is only one resonance in scalar πK scattering below 1800 MeV and this is identified with the established K₀*(1430). We find no evidence for the κ. Secondly, applying Cauchy's Theorem to the vacuum polarisation function leads to a relation between experimental and theoretical integrals known as a Finite Energy Sum Rule (FESR). FESRs are used to explore the scalar, isoscalar non-strange current and allow us to determine which of the experimentally observed scalar, isoscalar mesons is most likely to be the uu-bar + dd-bar state. We find that the lightest scalar, isoscalar uu-bar + dd-bar state is not the f₀(980) as suggested by some authors, but is rather the light, broad object known as the f₀(400 - 1200). We are also able to estimate the average light quark mass and find m_q(1 GeV²) = 4.7 ± 0.9 MeV which is consistent with the recent estimates of this quantity from unquenched lattice QCD. (author)

[en] Interference between the amplitudes of the reactions e⁺e^-→ρ→γπ⁺π^- and e⁺e^-→φ→γ(f₀+σ)→γπ⁺π^- is considered with allowance for the phase of an elastic background and for the phase of the mixing of the f₀ and σ mesons. It is shown that, in the reaction e⁺e^-→ρ→γπ⁺π^-, the Fermi-Watson theorem for final-state interaction is inapplicable in the case of soft photons (ω<100 MeV) in the φ-meson region. Interference patterns are obtained in the differential cross sections with respect to photon energies and in the total cross section as a function of the beam energy

[en] The production and decay of vector mesons (ρ, ω) in pA reactions at COSY energies is studied with particular emphasis on their in-medium spectral functions. It is explored within transport calculations, if hadronic in-medium decays like π⁺π^- or π⁰γ might provide complementary information to their dilepton (e⁺e^-) decays. Whereas the π⁺π^- signal from the ρ-meson is found to be strongly distorted by pion rescattering, the ω- meson Dalitz decay to π⁰γ appears promising even for more heavy nuclei. The perspectives of scalar meson ( f₀, a₀) production in pp reactions are investigated within a boson-exchange model indicating that the f₀-meson might hardly be detected in these collisions in the K(anti)K or ππ decay channels whereas the exclusive channel pp→da₀⁺ looks very promising. (author)

[en] In this talk we will present our work investigating the light scalar mesons. We begin with the lightest states (masses less than 1 GeV), which are needed within our framework to agree with experiment, and note that they have the quantum numbers of a nonet. Analysis of the isoscalar mixing pattern suggests that this nonet has a non-trivial qqq-barq-bar component. We extend our study to consider the conventional qq-bar scalar candidates above 1 GeV and propose a simple mixing mechanism to explain some surprising features thereof

[en] When simultaneously analysing experimental data on the processes ππ → ππ, KK-bar, ηη in the channel with the vacuum quantum numbers, definite indications of the QCD nature of the f0 resonances below 1.9 GeV are obtained. An assignment of the scalar mesons below 1.9 GeV to lower nonets is proposed. An approach, using only first principles (analyticity and unitarity) and the uniformizing variable, is applied

[en] In the energy range from 1.05 to 1.38 GeV, the upper limits for the cross sections of e⁺e⁻ → f₀(600)γ, f₀(980)γ, f₀(1350)γ, and f₂(1270)γ → π⁰π⁰γ processes have been established. Measurements are performed in accordance with the statistics gathered in experiment with a spherical neutral detector (SND) on the VEPP-2M e⁺e⁻ collider. The resultant upper limits vary from 6 to 42 pb depending on the energy and the model.

[en] The results of anti-pp-annihilation measurements with Crystal Barrel detector are presented. Light meson spectroscopy is stressed. The evidence for exotic states like Glue-Balls or Hybrids is shown. (author)

[en] The analysis of the experimental data of Crystal Barrel Collaboration on the pp-bar annihilation in flight with the production of mesons in the final state resulted in a discovery of a large number of mesons over the region 1900-2400 MeV, thus allowing us to systematize quark-antiquark states in the (n, M2) and (J, M2) planes, where n and J are radial quantum number and spin of the meson with the mass M. The data point to linear meson trajectories, with a universal slope. Relying on these data and recent K-matrix analysis a nonet classification is performed. In the scalar-isoscalar sector, the broad resonance state f0(1200-1600) is superfluous for the qq-bar classification, i.e. it is an exotic state. The ratios of coupling constants for the transitions f0 → ππ, KK-bar, ηη, ηη' point to the gluonium nature of the broad state f0(1200 - 1600). The lightest pseudoscalar glueball is also discussed

[en] A series of new scalar and tensor mesons has been discovered at LEAR, mostly in the Crystal Barrel experiment. One of these states, the f₀(1500), has unusual properties: a comparatively narrow width and a decay pattern which seems incompatible with SU(3) relations based on the OZI rule. In a naive quark model a s anti(s) state should be expected at this mass but the f₀(1500) partial decay width to anti(K)K is small. This leads to the conclusion that the state is supernumerous and must interpreted as the ground-state scalar glueball mixing with nearby quarkonia. Alternative interpretations are however not yet ruled out. (author)

[en] Measurements or 90% confidence upper limits are found, as follows, for branching fractions averaged over charge conjugate states (b) and CP violating charge asymmetries (a) of charged Β decays with the final state K⁺π^-π⁺. The first error given is statistical; the second is systematic and the third, where provided, is that due to uncertainties in the model of resonance contributions used. b(B⁺ → K*⁰π⁺,K⁺π^-) = (10.3 ± 1.2 ± 0.7 ± 1.8) x 10-⁶; b(B⁺ → f₀(980)K⁺, f₀ → π⁺π^-) = (9.2 ± 1.2 ± 0.6_-2.6^+2.2) x 10^-6; 6b(B⁺ → D⁰π⁺, D⁰ → K⁺π^-) = (184.6 ± 3.2 ± 9.7) x 10^-6; b(B⁺ → ρ⁰K⁺, ρ⁰ → π⁺π^- < 6.2 x 10^-6; b(B⁺ → K⁺π^-π⁺ non-resonant) < 15.2 x 10^-6; b(B⁺ → higher K*⁰π⁺, higher K*⁰ → K⁺π^-) = (25.1 ± 2.0_-1.8-4.2^+1.7+10.3) x 10^-6, b(B⁺ → higher fK⁺, higher f → π⁺π^-) < 10.7 x 10^-6; a(B⁺ → K*⁰π⁺) = -0.05 ± 0.12 ± 0.02; a(B⁺ → f₀(980)K⁺) = -0.05 ± 0.12 ± 0.02 where 'higher K*⁰' means any combination of K₀*⁰(1430), K₂*⁰(1430), K₁*⁰(1680) and 'higher f' means any combination of f₂(1270), f₀(1370) and f₀(1430). These analyses were performed using 61.6 million BB-bar pairs produced by the PEP-II collider at SLAC and recorded by the BABAR detector. (author)