À            Some Physics Results from ALEPH          À
 
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W boson Mass
Above the threshold at about 160 GeV, W bosons are pair produced in e+e- collisions. From the measured decay products, the energy and momentum of the W boson is reconstructed and its mass is derived. The average over all decay channels and with data at several energies is (until 1998) :

MW=80.41±0.05 GeV


Number of particle families (with light neutrinos)

     From a comparison of Standard Model predictions to the observed cross section e+e- -> Hadrons at different centre-of-mass energies (the "Z lineshape") the number of particle families (with light neutrinos) has been determined to be

Nn=2.982±0.013.
The mass of the Z particle has been determined to be
MZ=91.188±0.003 GeV.


Strength of the strong interaction, as
 
The theory of the strong interaction, QCD,  predicts a decreasing strength
of the strong interaction with increasing energy. 
ALEPH has measured as  at low energy equivalent to the mass of the tau lepton ( Þ mt=1.77 GeV) and at high energy at the mass of the Z boson  (Þ mZ=91.19GeV):
as(mt)=0.332+-0.002   and       as(mZ)=0.119+-0.004
The observed decrease with increasing energy agrees with the QCD expectation.