Polarized ep collisions as probes for CP violation from beyond the standard model

We consider the deep-inelastic scattering of transversely polarized electrons with protons at DESY HERA. We consider an asymmetry defined as the difference between the number of final electrons (for neutral-current events) or jets (for charged-current events) appearing out of each half of the detector, relative to the plane defined by the direction of the electron spin and the beam axis. A nonvanishing asymmetry defined in this way indicates a breakdown of time-reversal (or CP) symmetry. Using a general effective-Lagrangian analysis, we compute, in a model-independent way, the asymmetry to be expected for T violation from beyond the standard model. If fermion masses are neglected we show that this asymmetry is dominantly sensitive to CP violation in the electroweak gauge-boson-lepton couplings and provides information about these couplings that is not available through other experiments, such as bounds on lepton electric dipole moments or μ-decay asymmetries. Depending on assumptions about the size of underlying couplings, experiments at HERA can be sensitive to new physics at scales as high as several TeV.