arXiv subjects

Juerg Gasser

Publications and source records attributed to Juerg Gasser.

9 recordsLinked to original sources

arXiv2018

Solving integral equations in $η\to 3π$

A dispersive analysis of $η\to 3π$ decays has been performed in the past by many authors. The numerical analysis of the pertinent integral equations is hampered by two technical difficulties: i) The angular averages of the amplitudes need to be performed along a complicated path in the complex plane. ii) The averaged amplitudes develop singularities along the path of integration in the dispersive representation of the full amplitudes. It is a delicate affair to handle these singularities properly, and independent checks of the obtained solutions are demanding and time consuming. In the present article, we propose a solution method that avoids these difficulties. It is based on a simple deformation of the path of integration in the dispersive representation (not in the angular average). Numerical solutions are then obtained rather straightforwardly. We expect that the method also works for $ω\to 3π$.

hep-ph↗

arXiv2011

Cusps in K --> 3pi decays: a theoretical framework

Based on the analysis of 6.031*10^7 K+- --> pi0 pi0 pi+- decays, the NA48/2 collaboration has recently determined the S-wave pi-pi scattering lengths a_0-a_2 with high precision. In addition, the scattering length a_2 has been independently measured, although less precisely so. The present article discusses in detail one of the theoretical frameworks used in the data analysis.

hep-ph↗

arXiv2009

Relations between SU(2)- and SU(3)-LECs in chiral perturbation theory

Chiral perturbation theory in the two--flavour sector allows one to analyse Green functions in QCD in the limit where the strange quark mass is considered to be large in comparison to the external momenta and to the light quark masses m_u and m_d. In this framework, the low--energy constants of SU{2}_R \times SU{2}_L depend on the value of the heavy quark masses. For the coupling constants which occur at order p^2 and p^4 in the chiral expansion, we worked out the dependence on the strange quark mass at two--loop accuracy, and provided analogous relations for some of the couplings c_i which are relevant at order p^6. This talk comments on the methods used, and illustrates implications of the results obtained.

hep-ph↗

arXiv2009

Integrating out strange quarks in ChPT: terms at order p^6

Chiral perturbation theory in the two-flavour sector allows one to analyse Green functions in QCD in a limit where the strange quark mass is considered to be large in comparison to the external momenta and to the light quark masses. In this framework, the pertinent low-energy constants depend on the value of the heavy quark masses. In a recent article, we have worked out, for the coupling constants which occur at order p^4, the dependence on the strange quark mass at two-loop accuracy. Here, we provide analogous relations for some of the couplings which are relevant at order p^6. To keep the calculations somewhat reasonable in size, we consider only those 28 couplings which enter the Green functions built from vector- and axial-vector quark currents in the chiral limit. This provides the matching for 27 linear combinations of the 28 couplings.

hep-ph↗

arXiv2008

Isospin breaking in Kl4 decays

Data on Ke4 decays allow one to extract experimental information on the elastic pi pi scattering amplitude near threshold, and to confront the outcome of the analysis with predictions made in the framework of QCD. These predictions concern an isospin symmetric world, while experiments are carried out in the real world, where isospin breaking effects - generated by electromagnetic interactions and by the mass difference of the up and down quarks - are always present. We discuss the corrections required to account for these, so that a meaningful comparison with the predictions becomes possible. In particular, we note that there is a spectacular isospin breaking effect in Ke4 decays. Once it is taken into account, the previous discrepancy between NA48/2 data on Ke4 decays and the prediction of pi pi scattering lengths disappears.

hep-ph↗

arXiv2006

Cusps in K --> 3 pi decays

The pion mass difference generates a pronounced cusp in K --> 3 pi decays. As has recently been pointed out by Cabibbo and Isidori, an accurate measurement of the cusp may allow one to pin down the S-wave pi pi scattering lengths to high precision. Here, we present and illustrate an effective field theory framework that allows one to determine the structure of this cusp in a straightforward manner. The strictures imposed by analyticity and unitarity are respected automatically.

hep-ph↗

arXiv2006

Revisiting γγ->π^+π^- at low energies

We complete the recalculation of the available two-loop expressions for the reaction γγ->ππin the framework of chiral perturbation theory. Here, we present the results for charged pions. The cross section and the values of the dipole polarizabilities agree very well with the earlier calculation, provided the same set of low-energy constants (LECs) is used. With updated values for the LECs at order p^4, we find for the dipole polarizabilities (α_1-β_1)_{π^\pm} = (5.7\pm 1.0) 10^{-4} fm^3, which is in conflict with the experimental result recently reported by the MAMI Collaboration.

hep-ph↗

arXiv2005

Low-energy photon-photon collisions to two loops revisited

In view of ongoing experimental activities to determine the pion polarizabilities, we have started to recalculate the available two-loop expressions in the framework of chiral perturbation theory, because they have never been checked before. We make use of the chiral Lagrangian at order p^6 now available, and of improved techniques to evaluate the two-loop diagrams. Here, we present the result for the neutral pions. The cross section for the reaction gamma+gamma->pi0+pi0 agrees with the earlier calculation within a fraction of a percent. We present analytic results for the dipole and quadrupole polarizabilities, and compare the latter with a recent evaluation from data on gamma+gamma->pi0+pi0.

hep-ph↗

arXiv2002

Eta decays at and beyond $\mathbf{p^4}$ in Chiral Perturbation Theory

An overview is given of Chiral Perturbation Theory and various applications to eta decays. In particular, the main decay $η\to3π$ is discussed at the one-loop level, and estimates of the higher order corrections are given. The importance of $p^6$ and higher order effects in double Dalitz and $η\toπ^0γγ$ decays is pointed out. In all these cases the need for new experimental results is stressed.

hep-ph↗