noise of zero mean and variance?
The Box–Muller method uses two independent random numbers distributed uniformly on $(0,1)$, then uses such two uniformly distributed numbers to produce values with normal distribution. 

Zero-mean indicates the average value of the detected signal at each time point is zero. In digital signal processing, zero often refers to $2^{n-1}$ where $n$ is the bit number of digitizer. For example, if you have a 16-bit signal ($65536$), then $32768$ represent the zero in double, and those numbers that less than $32768$ are negative.