[Paper Review] Invariant mass spectra of $ au^{-} ightarrow h^{-}h^{-}h^{+} u$ decays

Ian Michael Nugent|arXiv (Cornell University)|Jan 29, 2013

Particle physics theoretical and experimental studies6 references16 citations

TL;DR

This paper presents unfolded invariant mass spectra for four hadronic tau decays—τ⁻→π⁻π⁻π⁺ντ, τ⁻→K⁻π⁻π⁺ντ, τ⁻→K⁻π⁻K⁺ντ, and τ⁻→K⁻K⁻K⁺ντ—using 342 fb⁻¹ of e⁺e⁻ data from the BABAR experiment. Employing Bayesian unfolding and background subtraction, the study achieves high-resolution spectral distributions critical for measuring |Vus| and improving modeling of tau decays at the LHC and future B-factories.

ABSTRACT

Using 342$fb^{-1}$ of $e^{+}e^{-}$ annihilation data collected with the BABAR detector at the SLAC PEP-II electron-positron asymmetric energy collider operating at a center-of-mass energies near 10.58GeV, we present the preliminary measurements of the invariant mass distributions of $ au^{-} ightarrow \pi^{-}\pi^{-}\pi^{+} u$, $ au^{-} ightarrow K^{-}\pi^{-}\pi^{+} u$, $ au^{-} ightarrow K^{-}\pi^{-}K^{+} u$ and $ au^{-} ightarrow K^{-}K^{-}K^{+} u$, where events with $K_{S}^{0} ightarrow \pi^{-}\pi^{+}$ decays are excluded.

Motivation & Objective

• To measure high-resolution invariant mass spectra for four hadronic tau decay modes involving charged pions and kaons.
• To reduce detector-induced distortions in the invariant mass distributions using Bayesian unfolding.
• To subtract backgrounds from particle misidentification and additional π⁰ decays to isolate signal contributions.
• To validate the accuracy of the BABAR branching fraction measurements using control samples.
• To provide refined spectral data for improving theoretical modeling of tau decays in future B-factory and LHC experiments.

Proposed method

• Used 342 fb⁻¹ of e⁺e⁻ annihilation data collected at √s ≈ 10.58 GeV with the BABAR detector.
• Selected τ⁻→h⁻h⁻h⁺ντ decays by requiring the companion τ⁺ to decay leptonically.
• Applied particle identification to distinguish pions and kaons, excluding K⁰S→π⁺π⁻ decays.
• Performed arithmetic background subtraction for cross-feed from misidentified particles and additional π⁰ contributions.
• Employed Bayesian unfolding with signal Monte Carlo to correct for detector resolution and efficiency effects.
• Normalized all unfolded distributions to unity for direct comparison with simulation and tuning.

Experimental results

Research questions

• RQ1 How do the invariant mass distributions for τ⁻→π⁻π⁻π⁺ντ, τ⁻→K⁻π⁻π⁺ντ, τ⁻→K⁻π⁻K⁺ντ, and τ⁻→K⁻K⁻K⁺ντ decays appear after removing detector effects?
• RQ2 What is the contribution of cross-feed backgrounds from misidentified particles to each decay channel?
• RQ3 How significant is the contamination from events with an additional π⁰ in the final state for each decay mode?
• RQ4 To what extent do detector resolution and efficiency distort the true invariant mass spectra?
• RQ5 How well do the unfolded spectra agree with the generator-level Monte Carlo and the CLEO tune for Tauola?

Key findings

• The cross-feed background from misidentified particles was measured at 0.85±0.01 for τ⁻→π⁻π⁻π⁺ντ, 38.5±0.2 for τ⁻→K⁻π⁻π⁺ντ, 2.9±0.1 for τ⁻→K⁻π⁻K⁺ντ, and 27.7±3.0 for τ⁻→K⁻K⁻K⁺ντ.
• Additional π⁰ contamination was estimated at 3.6±0.3% for τ⁻→π⁻π⁻π⁺ντ, 2.3±0.4% for τ⁻→K⁻π⁻π⁺ντ, 0.4±0.1% for τ⁻→K⁻π⁻K⁺ντ, and <5.0% for τ⁻→K⁻K⁻K⁺ντ.
• Non-τ backgrounds contributed less than 0.5% in all channels, confirming their minor impact.
• The Bayesian unfolding procedure had minimal impact on the spectra, indicating good detector resolution.
• The unfolded spectra show good agreement with the generator-level Monte Carlo and the CLEO tune for Tauola 2.8.
• The results provide essential input for determining the strange and non-strange spectral density functions and for extracting |Vus| with improved precision.