Observation of the dynamic beta effect at the Cornell Electron-Positron Storage Ring with the CLEO detector

Using the silicon strip detector of the CLEO experiment operating at the Cornell Electron-Positron Storage Ring (CESR), we have observed that the horizontal size of the luminous region decreases in the presence of the beam-beam interaction from what is expected without the beam-beam interaction. The dependence on the bunch current agrees with the prediction of the dynamic beta effect. This is the first direct observation of the effect.