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Simulation of tau decays, ambiguities and anomalous couplings

Zbigniew Was, Ananya Tapadar, J. M. John, S. Banerjee

TL;DR

The paper addresses the challenge of simulating tau decays and radiative corrections in e^+e^- collisions with potential New Physics, using the KKMC/TAUOLA/PHOTOS framework and exploring spin- and frame-dependent observables. It presents a feasibility study of adapting KKMC to e^+e^- → π^+π^-γ via reweighting μ^+μ^-γ events, and analyzes factorization into bremsstrahlung and Born-like kinematics using Mustraal and Collins-Soper frames, including low-energy validation relevant to Belle II. Key contributions include demonstrating that Mustraal can reproduce Born-like distributions at Belle II energies, extending HepMC3 event records with helicity weights and polarimetric vectors, and enabling external reweighting for anomalous couplings without regenerating events, together with an examination of β asymmetries in τ decays that probe hadronic current models. The work enhances flexible, model-agnostic exploration of radiative corrections and New Physics scenarios directly on existing event samples, supporting precision tau physics and future collider analyses.

Abstract

From the perspective of low energy tau decays and radiative corrections in decays, not much has changed since the last, Tau23 conference. Also, TAUOLA, tau decay library, and PHOTOS for radiative corrections in decays have not changed much, neither for QED nor for New Physics processes application. Progress was in the domain of flexibilities for applications and in the domain of New Physicists. There was progress with use of exclusive exponentiation (KKMC Monte Carlo) to evaluate quality of tau pair formation and decay factorization, with Collins-Soper and Mustraal frames. Ambiguities for New Physics or other processes could be thus addressed. Associated projects of KKMC f77 and KKMCee C++, are presented: (i) $π^+ π^-$ pair production at Belle II energies, interesting input for Pi_gammagamma(s), (ii) attribution of helicity like labels for taus in simulated HepMC3 format events, (iii) also tau polarimetric vectors prepared for user application of New Physics.

Simulation of tau decays, ambiguities and anomalous couplings

TL;DR

The paper addresses the challenge of simulating tau decays and radiative corrections in e^+e^- collisions with potential New Physics, using the KKMC/TAUOLA/PHOTOS framework and exploring spin- and frame-dependent observables. It presents a feasibility study of adapting KKMC to e^+e^- → π^+π^-γ via reweighting μ^+μ^-γ events, and analyzes factorization into bremsstrahlung and Born-like kinematics using Mustraal and Collins-Soper frames, including low-energy validation relevant to Belle II. Key contributions include demonstrating that Mustraal can reproduce Born-like distributions at Belle II energies, extending HepMC3 event records with helicity weights and polarimetric vectors, and enabling external reweighting for anomalous couplings without regenerating events, together with an examination of β asymmetries in τ decays that probe hadronic current models. The work enhances flexible, model-agnostic exploration of radiative corrections and New Physics scenarios directly on existing event samples, supporting precision tau physics and future collider analyses.

Abstract

From the perspective of low energy tau decays and radiative corrections in decays, not much has changed since the last, Tau23 conference. Also, TAUOLA, tau decay library, and PHOTOS for radiative corrections in decays have not changed much, neither for QED nor for New Physics processes application. Progress was in the domain of flexibilities for applications and in the domain of New Physicists. There was progress with use of exclusive exponentiation (KKMC Monte Carlo) to evaluate quality of tau pair formation and decay factorization, with Collins-Soper and Mustraal frames. Ambiguities for New Physics or other processes could be thus addressed. Associated projects of KKMC f77 and KKMCee C++, are presented: (i) pair production at Belle II energies, interesting input for Pi_gammagamma(s), (ii) attribution of helicity like labels for taus in simulated HepMC3 format events, (iii) also tau polarimetric vectors prepared for user application of New Physics.
Paper Structure (5 sections, 2 equations, 4 figures)

This paper contains 5 sections, 2 equations, 4 figures.

Table of Contents

  1. Introduction
  2. On feasibility of using KKMC Monte Carlo for $e^+e^- \to \pi^+\pi^- \gamma$.
  3. Factorizing distributions.
  4. Validating in low energy limit; Mustraal frame when $m_\tau/E >>0$
  5. Applications

Figures (4)

  • Figure 1: (a)Ratio of: reweighted $\pi^+\pi^-$ invariant mass to generated $\mu^+\mu^-$ invariant mass. (b) The $\cos\theta$ distribution of $\mu^+\mu^-$ direction. Only hard photons events selected. Mustraal frame used (to be explained later). Shape of Born distribution recovered. Hard scattering angle $\theta$ is defined in the muon pair rest frame ( Mustraal or Collins Soper). (c) The same $\cos\theta$ distribution of $\mu^+\mu^-$, but reweighted to $\pi^+\pi^-$, as previously, only hard photons events selected.
  • Figure 2: Low energy validation plots: comparison of effective $\cos\theta$ distributions obtained with Collins--Soper and Mustraal frames with semi-analytical distributions of Born level distribution merged with KKMCee invariant mass generated spectrum.
  • Figure 3: The figures are made at CM energy = 10.58 GeV and provide benchmark, both for our stand-alone and KKMCee algorithms. The attribited helicity-likes are presented.
  • Figure 4: Validation result. With stand-alone tool, we have reconstructed plot of Banerjee:2022sgf.