K\rightarrow \pi\pi e^+e^- decays and chiral low-energy constants

The branching ratios of the measured decay and of the still unmeasured decay are calculated to next-to-leading order in chiral perturbation theory (CHPT). Recent experimental results are used to determine two possible values of the combination of weak low-energy couplings (LECs) from the chiral Lagrangian. The values obtained are compared to the predictions of theoretical approaches to weak counterterm couplings to distinguish between the two values. Using the favoured value of the combination and taking into account additional assumptions suggested by the considered models, one obtains the branching ratio of the second decay as a function of the unknown combination of weak low-energy couplings. Finally, using values of the individual LECs derived from a particular model, one predicts the branching ratio of the decay. Received: 27 February 2001 / Published online: 11 May 2001     

The radiative leptonic decays D^0 \to e^+ e^-\gamma,\mu^+\mu^-\gamma in the standard model and beyond

We present a calculation of the rare decay modes and in the framework of the standard model. For the short distance part we have derived QCD corrections to the Wilson coefficients involved, including C₉. The latter is found to be strongly suppressed by the corrections, leading to diminished values for the branching ratios in the 10^-10 range. Within SM the exclusive decays are dominated by long distance effects. Non-resonant contributions are estimated using heavy quark and chiral symmetries to be at the level of , compared to the contributions arising from , with . The total SM branching ratio is predicted to be in the range (1-. We also consider contributions coming from MSSM with and without R parity conservation. The effects from MSSM are significant only for the R parity violating case. Such contributions enhance the branching ratio to , based on appropriately allowed values for C₉ and C₁₀. This selects as a possible probe of new physics. Received: 25 October 2002 / Published online: 24 January 2003     

\tau\rightarrow\omegA3\pi\nu decays

A theoretical study of the anomalous decay mode is presented. The theoretical value of the branching ratio of agrees well with the data. The branching ratio of is predicted. It is found that the vertices of and play a dominant role in these two decay modes. CVC is satisfied, and there is no adjustable parameter. Received: 30 July 2001 / Published online: 21 November 2001     

Dipole moments of the tau neutrino via the process {e^{+}e^{-}\rightarrow \nu\bar\nu \gamma} in a 331 model

The limits on the anomalous magnetic and electric dipole moments of the ?? _?? through the reaction $e^{+}e^{-}\rightarrow \nu \bar \nu \gamma$ at the Z ₁-pole, and in the framework of a 331 model are obtained. The results are based on the data reported by the L3 Collaboration at LEP. We find that the bounds are almost independent of the mixing angle ? of the model in the allowed range for this parameter.     

Gauge invariance, infrared/collinear singularitiesand tree level matrix element for e^ + e^- \to \nu_e \bar{\nu}_e \gamma \gamma

One of the necessary steps in constructing a high-precision option of KKMC, a Monte Carlo program for the high-precision simulation of fermion pair production at LEP and Linear Collider energies, was to make a careful study of the appropriate matrix elements calculated from QED and the complete standard model. In particular, the installation of the double bremsstrahlung matrix element for the process into the scheme of coherent exclusive exponentiation (CEEX) was necessary. In the CEEX scheme one has to define an extrapolation and/or reduction procedure to enable the use of the matrix elements for kinematical configurations with a large number of outgoing particles. The process under study is particularly interesting because of the gauge cancellation of contributions for photon emission from incoming fermion lines and t-channel W. The QED U(1) gauge properties require terms of the triple and quartic gauge couplings to be taken into consideration as well. A natural separation of the complete amplitude into gauge invariant parts was found and is among the main results of the paper. Each part has a well defined physical interpretation, which after partial integration over phase space provides infrared singular, leading-log, next-to-leading-log, and other terms. Contributions related to the triple and quartic gauge coupling of W (extracted with the help of an expansion around the contact W-interaction) have been ordered as well. The separation can be of broader interest; it originates from the rigorous calculation of matrix elements; it visualizes, in the language of spin amplitudes, the properties of factorization necessary for the common multi-process picture. For example, the multiple photon algorithm of PHOTOS, based on the parton shower-like approach, profits from similar considerations. These somewhat speculative aspects of the calculation will be mentioned in the paper as well. Received: 24 April 2005, Revised: 22 July 2005, Published online: 11 October 2005 Supported in part by the EU grant MTKD-CT-2004-510126, in patnership with the CERN Physics department, and Polish State Committee for Scientific Research (KBN) grant 2 P03B 091 27 for years 2004-2006.