  (abridged) We describe XMM-Newton Guaranteed Time observations of a sample of
eight high redshift (0.45<z<0.62) clusters. The goal of these observations was
to measure the luminosity and the temperature of the clusters to a precision of
\~10%, leading to constraints on the possible evolution of the
luminosity--temperature relation, and ultimately on the values of the matter
density, Omega_M and, to a lesser extent, the cosmological constant Omega_L.
The clusters were drawn from the SHARC and 160 Square Degree (160SD) ROSAT
surveys. Here we describe our data analysis techniques and present, for the
first time with XMM-Newton,Lx-Tx relation. For each of the eight clusters in
the sample, we have measured total bolometric luminosities, performed
beta-model fits to the radial surface profiles and made spectral fits to a
single temperature isothermal model. We describe data analysis techniques that
pay particular attention to background mitigation. Characterizing the Lx-Tx
relation as Lx = L_{6} (T/6keV)^{alpha},we find L_{6}=16.8 +7.6/-5.2 10^{44}
erg/s and alpha=2.7 +/-0.4 for a EdS H=50 cosmology at a typical redshift z
=0.55. Comparing with the low redshift study by Markevitch, assuming L-T to
evolve as (1+z)^A, we find A=0.68 +/-0.26 for the same cosmology and A=1.52
+0.26/-0.27 for a concordance cosmology. We conclude that there is now evidence
from both XMM-Newton and Chandra for an evolutionary trend in the L-T relation.
Our observations lend support to the robustness and completeness of the SHARC
and 160SD surveys.
