Upper bound on the decay tau-->microgamma from the Belle detector

Area of Science:

• Particle Physics
• High Energy Physics
• Lepton Flavor Physics

Background:

• Lepton flavor violation (LFV) is a key area of investigation in particle physics.
• The Standard Model of particle physics predicts negligible LFV, making its observation a sign of new physics.
• The tau lepton offers a unique probe for LFV due to its large mass and short lifetime.

Purpose of the Study:

• To search for the lepton-flavor-violating decay tau --> mu gamma.
• To set an upper limit on the branching fraction of this decay, constraining theoretical models.

Main Methods:

• Analysis of a large dataset (86.3 fb^-1) collected by the Belle detector.
• Application of rigorous data selection and background rejection techniques.
• Statistical analysis to determine signal significance and set upper limits.

Main Results:

• No significant signal for the tau --> mu gamma decay was observed.
• An upper limit for the branching fraction was established at B(tau --> mu gamma) < 3.1 x 10^-7 at the 90% confidence level.

Conclusions:

• The absence of a signal places stringent constraints on theories beyond the Standard Model.
• This result contributes to the ongoing search for new physics phenomena through precision measurements of rare decays.