Measurement of the Luminous-Region Profile at the PEP-II Ip, And Application to e ± Bunch-Length Determination

The three-dimensional luminosity distribution at the interaction point (IP) of the SLAC B-Factory is measured continuously, using e⁺e^-→ e⁺e^-, μ⁺μ^-events reconstructed online in the BABAR detector. The centroid of the transverse luminosity profile provides a very precise and reliable monitor of medium- and long-term orbit drifts at the IP. The longitudinal centroid is sensitive to variations in the relative RF phase of the colliding beams, both over time and differentially along the bunch train. The measured horizontal r.m.s. width of the distribution is consistent with a sizeable dynamic-β effect; it is also useful as a benchmark of strong-strong beam-beam simulations. The longitudinal luminosity distribution depends on the e^±bunch lengths and vertical IP β-functions, which can be different in the high- and low-energy rings. Using independent estimates of the β functions, we analyze the longitudinal shape of the luminosity distribution in the presence of controlled variations in accelerating RF voltage and/or beam current, to extract measurements of the e⁺and e^-bunch lengths.