def gamma_CT_fun(x,*p):
    """ gamma/U0=f(CT), i.e. x=Ct
    INPUTS:
       x = CT value
       p : list of parameters of fitting function
    NOTE: returns gamma_t/U0! 
    """
    return -2*(p[0] * x + p[1] * x**2 + p[2] * x**3 + p[3] * x**4)