Cal. Pen. Code § 1096
A defendant in a criminal action is presumed to be innocent until the contrary is proved, and in case of a reasonable doubt whether his or her guilt is satisfactorily shown, he or she is entitled to an acquittal, but the effect of this presumption is only to place upon the state the burden of proving him or her guilty beyond a reasonable doubt. Reasonable doubt is defined as follows: "It is not a mere possible doubt; because everything relating to human affairs is open to some possible or imaginary doubt. It is that state of the case, which, after the entire comparison and consideration of all the evidence, leaves the minds of jurors in that condition that they cannot say they feel an abiding conviction of the truth of the charge."
Ca. Pen. Code § 1096