OPTIMAL OBSERVABLES FOR NEW-PHYSICS SEARCH AT LEP2

New observables sigma(+/-) for the process e(+)e(-) --> mu(+)mu(-) all ow one to get more direct information on additional Z' boson effects t han what is obtained from the canonical ones, sigma and A(FB). Their d eviations from the Standard Model predictions have very specific energ y dependences, which are precisely determined by SM parameters. At ene rgies varying from TRISTAN to LEP2, one can uniquely predict the signs of Delta sigma(+/-) induced by a Z' as well as the locations of their extrema and zeros. This unambiguous energy correlation could be quite useful in distinguishing effects due to Z' exchange from those caused by other new physics sources. Furthermore, there are two energy point s, root s(+) similar or equal to 78 GeV and root s(+) similar or equal to 113 GeV, where SM as well as the deviations sigma(+/-)(SM) attain their minimum values or vanish. These points could be very favourable for a search for new physics beyond the SM and beyond Z' effects. (C) 1997 Elsevier Science B.V.