(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.
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