Peter Buck's psychedelic guitar intro gives a tantalizing taste of the new-model R.E.M. — like Jefferson Airplane circa Surrealistic Pillow covering Sonic Youth's "Eric's Trip." A track from the eagerly awaited Collapse Into Now (due in March), "Discoverer" has the widescreen presence of R.E.M. classics like New Adventures in Hi-Fi. But when Michael Stipe bellows, "Hey, baby, this is not a challenge," you don't believe it.