KEKB is a double-ring electron/positron collider with a horizontal crossing angle. The crab cavities achieved an effective head-on collision and gained a higher specific luminosity. For a crabbing collision, tune spectra of a colliding bunch were observed on a spectrum analyzer. The beam-beam spectrum showed strong nonlinear resonant phenomena. Considering this nonlinearity, the coherent beam-beam tune shift was measured as a function of the bunch current. It was confirmed that the vertical beam-beam parameter estimated from the coherent beam-beam tune shift agreed with the value obtained from a bunch-by-bunch luminosity monitor. The estimated vertical beam-beam parameter attained a saturation level of approximately 0.05, which is termed a beam-beam limit. We found that the bunch current corresponding to the beam-beam limit was considerably lower than that used in usual operations.
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