[Paper Review] Search for lepton flavor violating τ- decays into ℓ- η, ℓ- η′ and ℓ- π0

Y. Miyazaki, I. Adachi+130 more|arXiv (Cornell University)|May 17, 2007

Particle physics theoretical and experimental studiesPhysics and Astronomy33 references38 citations

TL;DR

This study searches for lepton-flavor-violating τ⁻ decays into a charged lepton and a pseudoscalar meson (η, η′, π⁰) using 401 fb⁻¹ of data from the Belle experiment at KEKB. No signal is observed, and new upper limits on branching fractions at 90% confidence level are set, improving previous limits by factors of 2.3 to 6.3.

ABSTRACT

We have searched for lepton-flavor-violating τ decays with a pseudoscalar meson (η, η′ and π0) using a data sample of 401 fb-1 collected with the Belle detector at the KEKB asymmetric-energy e+ e- collider. No evidence for these decays is found and we set the following upper limits on the branching fractions: B (τ- → e- η) < 9.2 × 10-8, B (τ- → μ- η) < 6.5 × 10-8, B (τ- → e- η′) < 1.6 × 10-7, B (τ- → μ- η′) < 1.3 × 10-7, B (τ- → e- π0) < 8.0 × 10-8 and B (τ- → μ- π0) < 1.2 × 10-7 at the 90% confidence level. These results improve our previously published upper limits by factors from 2.3 to 6.3. © 2007 Elsevier B.V. All rights reserved.

Motivation & Objective

• To search for lepton-flavor-violating decays of the τ lepton into a charged lepton and a pseudoscalar meson (η, η′, π⁰).
• To improve the sensitivity to rare τ decays that violate lepton flavor universality.
• To set more stringent upper limits on branching fractions for τ⁻ → ℓ⁻P decays (P = η, η′, π⁰), where ℓ = e, μ.
• To test extensions of the Standard Model that predict such rare decays, such as models with flavor-changing neutral currents or heavy sterile neutrinos.
• To enhance the experimental constraints on new physics beyond the Standard Model through precision measurements of rare τ decays.

Proposed method

• Utilized a data sample of 401 fb⁻¹ collected with the Belle detector at the KEKB asymmetric-energy e⁺e⁻ collider.
• Performed a model-independent search for τ⁻ → ℓ⁻P decays (P = η, η′, π⁰) using kinematic reconstruction and particle identification techniques.
• Applied selection criteria based on invariant mass reconstruction of the final-state particles to isolate signal-like events.
• Used background estimation techniques based on control regions and Monte Carlo simulations to model combinatorial and continuum backgrounds.
• Calibrated signal and background efficiencies using simulated events and validated with control samples.
• Computed upper limits on branching fractions using a frequentist approach at the 90% confidence level, incorporating systematic uncertainties.

Experimental results

Research questions

• RQ1 What are the upper limits on the branching fractions for τ⁻ → e⁻η, τ⁻ → μ⁻η, τ⁻ → e⁻η′, τ⁻ → μ⁻η′, τ⁻ → e⁻π⁰, and τ⁻ → μ⁻π⁰ decays?
• RQ2 How do these new limits compare to previous experimental results in terms of sensitivity improvement?
• RQ3 What is the impact of these results on models predicting lepton flavor violation beyond the Standard Model?
• RQ4 Can the absence of signal in these decays be used to constrain specific new physics scenarios?
• RQ5 What is the significance of the observed data compared to the expected background in the signal regions?

Key findings

• No significant signal is observed for any of the six lepton-flavor-violating τ⁻ decay modes into pseudoscalar mesons.
• The upper limit on B(τ⁻ → e⁻η) is set at 9.2 × 10⁻⁸ at 90% confidence level.
• The upper limit on B(τ⁻ → μ⁻η) is set at 6.5 × 10⁻⁸ at 90% confidence level.
• The upper limit on B(τ⁻ → e⁻η′) is set at 1.6 × 10⁻⁷ at 90% confidence level.
• The upper limit on B(τ⁻ → μ⁻η′) is set at 1.3 × 10⁻⁷ at 90% confidence level.
• The upper limits improve upon previous results by factors ranging from 2.3 to 6.3, demonstrating enhanced sensitivity.