A study of the reaction e(+)e(-)->mu(+)mu(-)gamma(ISR) at LEP and search for new physics at annihilation energies near 80 GeV

A study of the channel e(+)e(-) --> mu(+)mu(-)gamma ISR, where gamma ISR is an initial state radiation photon, is presented using data collected by the DELPHI experiment from 1991 to 1994. The total cross-section at effective annihilation: energies (root s') below the Z(0) peak is obtained by using the events with relatively hard initial state radiative photon(s) (E gamma > 1 GeV). The differential cross-section as a function of the muon polar production angle is also determined in order eo extract the forward-backward asymmetries for the reaction e(+)e(-) --> mu(+)mu(-) at energies root s' between 20 and 87 GeV. The ratio of the helicity cross-sections sigma LL+sigma RR/sigma RL+sigma LR, where the two subscripts stand for the helicities of the incoming e(-) and outgoing mu(-) respectively, is extracted from the differential cross-sections in order to test the Standard Model and to look for new physics near 80 GeV. No deviations from the Standard Model were found.