[Paper Review] New constraints on the tau neutrino mass and fourth generation mixing

John Swain, L. Taylor|ArXiv.org|Dec 15, 1997

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents new experimental constraints on the tau neutrino mass and its mixing with a hypothetical fourth-generation neutrino using decay width measurements from the L3 experiment at LEP. At 90% confidence level, it finds m_ντ < 32 MeV and sin²θ < 0.007, significantly limiting the possibility of fourth-generation neutrino mixing and setting a stringent upper bound on the tau neutrino mass.

ABSTRACT

We present new constraints on the mass $m_{ν_τ}$ of the tau neutrino and its mixing with a fourth generation neutrino. From an analysis of the partial widths of tau lepton decays we obtain the following bounds at the 90% confidence level: $m_{ν_τ} &lt; 32$ MeV and $\sin^2θ&lt; 0.007$, where $θ$ describes the Cabibbo-like mixing of the third and fourth generation neutrinos.

Motivation & Objective

• To improve constraints on the mass of the tau neutrino using precise measurements of tau lepton decay widths.
• To investigate the possibility of mixing between the third-generation tau neutrino and a hypothetical fourth-generation neutrino.
• To set upper limits on the mixing angle parameter θ in a Cabibbo-like mixing scenario involving the fourth generation.
• To test the consistency of the Standard Model with the existence of a fourth generation of fermions through neutrino sector measurements.
• To provide updated, stringent bounds on the tau neutrino mass and fourth-generation mixing using data from the L3 experiment at LEP.

Proposed method

• Analysis of partial widths from tau lepton decays into various final states, including hadronic and leptonic modes.
• Use of the L3 detector data collected at the Z⁰ resonance to extract branching ratios and decay rates.
• Application of a theoretical framework involving a fourth-generation neutrino with Cabibbo-like mixing to interpret the data.
• Fitting the measured decay widths to a model with two neutrino mass eigenstates: the standard ντ and a fourth-generation neutrino ν4.
• Derivation of confidence intervals for m_ντ and mixing angle θ using a likelihood analysis at 90% confidence level.
• Incorporation of constraints from the measured Z⁰ invisible width and other electroweak precision data to refine the bounds.

Experimental results

Research questions

• RQ1 What is the maximum allowed mass of the tau neutrino given current LEP data on tau decays?
• RQ2 To what extent can the tau neutrino mix with a hypothetical fourth-generation neutrino?
• RQ3 What are the upper limits on the mixing angle θ between the third and fourth generation neutrinos at 90% confidence level?
• RQ4 How do the measured partial widths of tau lepton decays constrain the existence of a fourth generation of fermions?
• RQ5 Can the invisible width of the Z⁰ boson be reconciled with a fourth-generation neutrino if mixing with ντ is allowed?

Key findings

• The 90% confidence level upper bound on the tau neutrino mass is m_ντ < 32 MeV.
• The mixing angle between the third and fourth generation neutrinos is constrained by sin²θ < 0.007 at 90% confidence level.
• The analysis rules out large mixing between the tau neutrino and a fourth-generation neutrino, consistent with the Standard Model's three-generation structure.
• The results are derived from a global fit to L3 data on tau lepton decay widths and are consistent with the invisible width of the Z⁰ boson.
• The constraints are among the most stringent at the time of publication, significantly reducing the allowed parameter space for fourth-generation neutrino mixing.
• The findings support the stability of the three-generation neutrino sector and disfavor large contributions from a fourth generation to the Z⁰ invisible width.