Abstract : The measurement of the W boson mass is a key issue of LEP2. In the
W+ W- -> q1 q2bar q3 q4bar channel, a large systematic error comes from Bose-Einstein
correlations, which could induced a non-independent fragmentation of the two W.
This thesis deals with the measurement of these correlations in W boson pair decays.
We will focus on the measurement of such correlations between pions from different
The standard model theory and the ALEPH experiment are described in the two
first chapters. The analysis requires a selection of W+ W- -> q1 q2bar l nu_l events, which
is presented in chapter three. The W+ W- -> q1 q2bar q3 q4bar and W+ W- -> q1 q2bar tau nu event
selections are also described in this part. The different phenomenological models of
Bose-Einstein correlations are reviewed in chapter four, with their adjustment on
the ALEPH data recorded at $\sqrt(s)$ = 91 GeV. The model predictions are compared
to results of measurements done in W+ W- decays observed at energies of collisions
of 172, 183 and 189 GeV. Bose-Einstein correlations between pions coming from
different W in the W+ W- -> q1 q2bar q3 q4bar channel are disfavoured by 2.7 standard deviations.