Helicity-flip bremsstrahlung and the measurement of CP-violating form factors in polarized e⁺e⁻ collisions

Abstract

Certain momentum correlations in the production and subsequent decay of heavy fermion pairs in e⁺e⁻ collisions have increased sensitivity to CP-violating electric and "weak" dipole form factors of the fermion in the presence of longitudinal polarization of the e⁺e⁻ beams. However, unless the polarizations of e⁺ and e⁻ are equal and opposite, collinear initial-state radiation accompanied by helicity flip results in a CP-invariant contribution to these and acts as a background, and must be calculated and subtracted in order to isolate the CP-violating contribution due to the dipole form factor. We calculate this background contribution to such correlations, to order α, in e⁺e⁻→τ⁺τ⁻ and e⁺e⁻tt̅. For the anticipated luminosities at τ-charm factories and the SLC, for the τ, and at a future linear collider with a center-of-mass energy of 500 GeV for the top quark, these backgrounds are smaller than the statistical background and may be safely neglected.