Subject: a Measurement of the Mass of the Tau Lepton.
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Abstract

Title:

a Measurement of the Mass of the Tau
Lepton.

Authors:

Jones, Lawrence Aston

Affiliation:

CALIFORNIA INSTITUTE OF TECHNOLOGY.

Journal:

Thesis (PH.D.)--CALIFORNIA INSTITUTE OF
TECHNOLOGY, 1995.Source: Dissertation Abstracts International, Volume:
56-01, Section: B, page: 0301.

Publication Date:

00/1995

Category:

Physics: Elementary Particles and High
Energy

Origin:

UMI

Abstract Copyright:

(c) 1995: UMI Company

Bibliographic Code:

1995PhDT........15J

Abstract
For a conventional charged lepton l under the
Standard Model, the electronic branching ratio B _sp{l}{e}, lifetime tau_{l},
mass m _{l}, the dimensionless weak coupling constant g_{l} and the W-boson mass
m_{W} are related by B_sp{l}{e}/ tau_{l} = g_sp{l} {2}g_sp{e}{2}m_sp
{l}{5}/6144pi^3m_sp {W}{4} up to small radiative and electroweak corrections.
This implies the following relationship among the above parameters for the tau
and mu<=ptons: g_sp{tau}{2}/g _sp{mu}{2} = m_sp {mu}{5}B_sp{tau
}{e}tau_mu/m_sp {tau}{5}B_sp {mu }{e}tau_tau. In 1992, world averages for the
above quantities yielded g_sp{tau}{2}/g _sp{mu}{2} = 0.941 +/- 0.025, implying a
2.4 standard deviation disagreement with lepton universality. A measurement of
the tau^+ tau^- production cross section in the region most sensitive to the tau
mass--a few MeV around threshold--provides the opportunity to measure the tau
mass with greatly improved precision. This thesis presents such a measurement
made using the Beijing Spectrometer at the Beijing Electron Positron Collider. A
search was conducted for threshold assuming a parameterized form for the tau^+
tau^- cross section. After each search point a likelihood was generated using
all the information from the search so far. This identified the energy where it
would be most efficient to search next. The mass value is obtained from a fit to
the energy dependence of the cross section. The measurement is independent of
the nu_tau mass. A likelihood-driven search was carried out at BES based on emu
events. The data has been re-analysed to take into account the additional decay
modes ee, epi, mu mu, mupi, and pi pi. This new analysis yields m_ tau =
1776.87_sp{-0.22-0.18}{+0.20+0.22 } MeV. Using contemporary values for B_sp
{tau}{e} and tau_tau and the result above, the ratio of the weak coupling
constants becomes g _sp{tau}{2}/g_sp {mu}{2} = 1.001 +/- 0.012 , and lepton
universality is supported.

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