Resonance Chiral Lagrangian Research Articles

Triple-product asymmetry in the radiative two-pion tau decay

In this work, we perform a detailed study of the τ− → π−π0γν decay process within the resonance chiral theory. We pay special attention to the triple-product asymmetry in the τ− → π−π0γν process. The minimal resonance chiral Lagrangian and the odd-intrinsic parity resonance operators are simultaneously included to calculate the decay amplitudes. Various invariant-mass distributions in the π−π0, π−γ and π0γ systems are studied and they reveal different resonance dynamics. We further predict the intriguing nonzero triple-product asymmetry distributions, which may provide useful guidelines for future experimental measurements conducted at the Belle-II and super tau-charm facilities.

Lightest pseudoscalar exchange contribution to light-by-light scattering piece of the muon g − 2

Lightest pseudoscalar (P=π0,η,η′) exchange contribution to the light-by-light (LbL) scattering piece of the muon anomaly, aμ=(gμ−2)/2, has been evaluated using a resonance chiral Lagrangian (RχL). Best description of pion transition form-factor (TFF) data is obtained with only tiny violations of one of the relations in the minimal consistent set of short-distance constraints on the anomalous RχL couplings. η(′)TFF, predicted in terms of the πTFF and the η−η′ mixing, are in good agreement with measurements. With this input, we obtain aμP,LbL=(10.47±0.54)⋅10−10, consistent with the reference determinations in the literature, albeit with smaller error.

19pAL-10 Tau and Hadron decay with intrinsic parity violation in Resonance Chiral Lagrangian

19pAL-10 Tau and Hadron decay with intrinsic parity violation in Resonance Chiral Lagrangian

Study of the tau meson decay modes with Monte Carlo generator TAUOLA. Status and perspectives

In the last two years substantial progress for the simulation of the process: τ−→π−π+π−ντ by the Monte Carlo generator TAUOLA was achieved. It is related to a new parameterization of the corresponding hadronic current based on the Resonance Chiral Lagrangian and the recent availability of the unfolded distributions from the BaBar Collaboration analysis for all invariant hadronic masses. The theoretical model parameters were fitted to the one-dimensional distributions provided by the BaBar Collaboration and results of the fit are discussed. A set of the hadronic currents for other final states with two and three pseudo-scalars is also installed in TAUOLA and the preliminary results for fitting K+K−π− and π0π− to BaBar and Belle data are presented.

Confronting theoretical predictions with experimental data; fitting strategy for multi-dimensional distributions

After developing a Resonance Chiral Lagrangian (RχL) model to describe hadronic τ lepton decays [18], the model was confronted with experimental data. This was accomplished using a fitting framework which was developed to take into account the complexity of the model and to ensure the numerical stability for the algorithms used in the fitting. Since the model used in the fit contained 15 parameters and there were only three 1-dimensional distributions available, we could expect multiple local minima or even whole regions of equal potential to appear. Our methods had to thoroughly explore the whole parameter space and ensure, as well as possible, that the result is a global minimum. This paper is focused on the technical aspects of the fitting strategy used. The first approach was based on re-weighting algorithm published in [17] and produced results in around two weeks. Later approach, with improved theoretical model and simple parallelization algorithm based on Inter-Process Communication (IPC) methods of UNIX system, reduced computation time down to 2-3 days. Additional approximations were introduced to the model decreasing time to obtain the preliminary results down to 8 hours. This allowed to better validate the results leading to a more robust analysis published in [12].

Resonance chiral Lagrangian currents and experimental data forτ−→π−π−π+ντ

In this paper we document the modifications introduced to the previous version of the Resonance Chiral Lagrangian current ({\it Phys.Rev.} {\bf D86} (2012) 113008) of the $\tau^\pm \to \pi^\pm \pi^\pm \pi^\mp \nu_\tau$ decay which enable the one dimensional distributions measured by the BaBar collaboration to be well modeled. The main change required to model the data is the addition of the $\sigma$ resonance. Systematic errors, theoretical and experimental ones, limitations due to fits of one dimensional distributions only, and resulting difficulties and statistical/systematic errors for fitted parameters are addressed. The current and fitting environment is ready for comparisons with the fully exclusive experimental data. The present result for $\tau^\pm \to \pi^\pm \pi^\pm \pi^\mp \nu_\tau$ is encouraging for work on other $\tau$ decay modes and Resonance Chiral Lagrangian based currents.

Hadron structure inτ→KKπντdecays

We analyse the hadronization structure of both vector and axial-vector currents leading to tau -> KKpi nu_tau decays. At leading order in the 1/Nc expansion, and considering only the contribution of the lightest resonances, we work out, within the framework of the resonance chiral Lagrangian, the structure of the local vertices involved in those processes. The couplings in the resonance theory are constrained by imposing the asymptotic behaviour of vector and axial-vector spectral functions ruled by QCD. In this way we predict the hadron spectra and conclude that, contrarily to previous assertions, the vector contribution dominates by far over the axial-vector one in all KKpi charge channels.

Tests of the naturalness of the coupling constants in ChPT at order p6

We derive constraints on combinations of O(p6) chiral coupling constants by matching a recent two-loop calculation of the πK scattering amplitude with a set of sum rules. We examine the validity of the natural expectation that the values of the chiral couplings can be associated with physics properties of the light resonance sector. We focus, in particular, on flavor symmetry breaking of vector resonances. A resonance chiral Lagrangian is constructed which incorporates flavor symmetry breaking more completely than was done before. We use πK unsubtracted sum rules as tests of the modelling of the resonance contributions to the chiral couplings. In some cases the O(p6) couplings are found not to be dominated by the resonance contributions.

Strangeness-changing scalar form factors

We derive expressions for strangeness-changing scalar form factors, which incorporate known theoretical constraints both at low and high energies. Their leading behaviour in the limit of a large number of colours is calculated from the resonance chiral Lagrangian, and a set of short-distance constraints on the scalar resonance couplings is obtained, imposing the form factors to vanish at infinite momentum transfer. The important effects of meson rescattering are taken into account by exploiting unitarity and analyticity. Making use of previous results on S-wave Kπ scattering [1] , and a coupled-channel dispersion-relation analysis, the Kπ, Kη and Kη′ scalar form factors are calculated up to 2 GeV. These scalar form factors are a key ingredient in the determination of the corresponding scalar spectral function which is important in the extraction of the mass of the strange quark from QCD sum rules as well as hadronic τ decays.