TAUOLA update for decay channels with e+e− pairs in the final state

Abstract

With the arrival of high luminosity B-factories like the Belle II experiment, τ decay measurements have become more precise than ever, allowing rarer processes to be explored, and finer details of τ decays to be studied. These are important to understand the spectrum of intermediate particles produced in τ decays. Therefore Monte Carlo generators, like the TAUOLA program, have to facilitate precision analysis as well as confront new models that constantly emerge with the availability of high statistics experimental data. New decay channels and models may lead to large variation of matrix elements size within the available phase space, as in the case when e+e− pairs are present in final state. It requires appropriate presampler of the phase space generator and a proper documentation to help users introduce their own models. While releasing a new update, it is important for the TAUOLA Monte Carlo library to maintain the general structure of the previous versions to preserve backward compatibility. The aim is to minimize changes from the user perspective. This paper presents a demonstration of new models implementation facilitated by the current update. Program summaryProgram Title: tauola-bbbCPC Library link to program files:https://doi.org/10.17632/3hyw3f7v32.1Licensing provisions: GPLv2Programming language:FORTRAN/C++Journal reference of previous version: Comput. Phys. Comm., 232:220–236, 2018Does the new version supersede the previous version?: NoReasons for the new version: Meeting the needs of present day experiments.Summary of revisions: Phase space presamplers enriched with the possibility to generate narrow peaks of e+e− pairs, if present in matrix elements of 4- and 5-body decay channels and due to electromagnetic or New Physics interactions. The examples of physical models are implemented for: τ−→π−e−e+ντ, τ−→ν¯μμ−e−e+ντ and τ−→ν¯ee−e−e+ντ channels, which are used for the collected new algorithm validations.Nature of problem: Present day experiments, in particular Belle II, have the capability to measure extremely rare, not yet measured τ decays, as well as precise measurements of known decay channels. They require a Monte Carlo generator capable of generating all desired decays. They also need the possibility to modify and test new models for such decays. At the same time the default initialization of the event generator should contain decent modeling of known τ decays.Solution method: The new version supplements previous publications on TAUOLA. Phase-space presamplers and new decay channels with additional light lepton pairs are introduced.Additional comments including restrictions and unusual features: TAR BALL of the future versions of the program may become available through:http://wasm.web.cern.ch/wasm/Welcome.html; its sub-pages http://tauolapp.web.cern.ch/tauolapp/ or http://wasm.web.cern.ch/wasm/f77.html