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We report on RXTE observations of the microquasar XTE J1550-564 during a ~70day outburst in April-June 2000. We study the evolution of the PCA+HEXTEspectra over the outburst. The source transited from an initial Low Hard State(LS), to an Intermediate State (IS), and then back to the LS. The source showsan hysteresis effect similar to what is observed in other sources, favoring acommon origin for the state transitions in soft X-ray transients. The firsttransition occurs at a ~ constant 2-200 keV flux, which probably indicates achange in the relative importance of the emitting media. The second transitionis more likely driven by a drop in the mass accretion rate.  In both LS, the spectra are characterized by the presence of a strongpower-law tail (Compton corona) with a variable high energy cut-off. During theIS, the spectra show the presence of a ~0.8 keV thermal component (accretiondisk). We discuss the apparently independent evolution of the two media, andshow that right after the X-ray maximum on MJD 51662, the decrease of thesource luminosity is due to a decrease of the power-law luminosity, at aconstant disk luminosity. This, together with the detection of radio emission(with a spectrum typical of optically thin synchrotron emission), may suggestthat the corona is ejected and further detected as a discrete radio ejection.Comment: Accepted for publication in ApJ. 9 pages, 4 figures, abstract  abridge

Spectral Evolution of the Microquasar XTE J1550−564 over Its Entire 2000 Outburst