Related papers: Theoretical Predictions for Pionium Searches
The scattering of $\gamma \pi \to \pi \pi$ is studied using the axial anomaly, elastic unitarity, analyticity and crossing symmetry. Using the technique to derive the Roy's equation, an integral equation for the P-wave amplitude is obtained…
We use effective hadronic field theory to study in-medium properties of the $\phi$ meson. The dominant decay channels $\phi\toK^{+}K^{-}$ and $\phi\to\pi\rho$ are modeled using an SU(3) chiral Lagrangian with normal and abnormal parities.…
A fit of the experimental data to the total cross section of the process $\gamma\gamma->\pi^+\pi^-$ in the energy region from threshold to 2500 MeV has been carried out using dispersion relations with subtractions for the invariant…
We study the properties of axial vector mesons a1 and b1 as relativistic S-wave states which are predicted in the U~(12)-scheme, through the analyses of their radiative and pionic decays. Specifically, partial widths of the strong a1 (b1)…
We present and develop a general dispersive framework allowing us to construct representations of the amplitudes for the processes $P\pi\to\pi\pi$, $P=K,\eta$, valid at the two-loop level in the low-energy expansion. The construction…
The effective potential of the $O(N)$ model at large $N$ limit is reinvestigated with varying pion mass and temperature. For large pion masses and high temperatures, we find the phenomenologically favored vacuum, located on the upper branch…
I summarize the history of theoretical predictions of, and experimental attempts to measure, pion-pion (S-wave) scattering lengths. Recent measurements at CERN confirm Weinberg's 1966 prediction of the I=2 scattering length and Basdevant…
We present an extraction of the pion-nucleon ($\pi N$) scattering lengths from low-energy $\pi N$ scattering, by fitting a representation based on Roy-Steiner equations to the low-energy data base. We show that the resulting values confirm…
The collision-induced shift and broadening of selected dipole transition lines of pionic helium in gaseous helium at low temperatures up to $T=12$ K and pressure up to a few bar are calculated within variable phase function approach. We…
The single-pion production reactions $pp\to d\pi^+$, $pp\to np\pi^+$ and $pp\to pp\pi^0$ were measured at a beam momentum of 0.95 GeV/c ($T_p \approx$ 400 MeV) using the short version of the COSY-TOF spectrometer. The implementation of a…
We have developed a model for the N N --> N N pi pi reaction and evaluated cross sections for the different charged channels. The low energy part of those channels where the pions can be in an isospin zero state is dominated by N*…
A set of once subtracted dispersion relations with imposed crossing symmetry condition for the pipi D- and F-wave amplitudes is derived and analyzed. An example of numerical calculations in the effective two pion mass range from the…
In a previous work, we have reanalyzed the I=0 S-wave \pi\pi-scattering phase-shift through a new method of Interfering Breit-Wigner amplitudes, and have shown the existence of a light scalar, \sigma particle, accompanied by a negative…
The pi+pi- atom decays -in the ground state - predominantly into two neutral pions. We present a general expression for the corresponding decay width in the framework of QCD (including photons). It contains all terms at leading and…
The $\pi\pi$ scattering amplitude with regular analytical properties in the $s$ complex plane has been constructed. It describes simultaneously the data on the $\pi\pi$ scattering, $\phi\to\pi^0\pi^0\gamma$ decay and $\pi\pi\to K\bar K$…
The $S$- and $P$- wave phase shifts of low-energy pion-nucleon scatterings are analysed using Peking University representation, in which they are decomposed into various terms contributing either from poles or branch cuts. We estimate the…
From recent analysis of the $\pi\pi$ scattering amplitude, it has been claimed that there exists a broad and light $\sigma$ meson. However, if this meson really exists, it must also appear in other observables such as the pion scalar form…
A correction of the low energy theorem of the \gamma\to3\pi anomaly has been found. A_{3\pi}(0,0,0) and the cross section are calculated. Theory agrees with data. There is no new adjustable parameter.
We present technical details of the evaluation of the elastic pi-pi scattering amplitude to two loops in chiral perturbation theory. In particular, we elaborate on the renormalization procedure at the two-loop order and on the evaluation of…
We review the cross sections for low-energy $\pi^- \gamma$ reactions in the framework of chiral perturbation theory. Charged pion Compton scattering, $\pi^- \gamma\to \pi^-\gamma$, is considered up to one-loop order where the pion's…