def Getp2(M_0, M_1, M_2):
    """
    Consider a two-body decay :math:`M_0\\rightarrow M_1M_2`. In the rest frame of :math:`M_0`, the momentum of
    :math:`M_1` and :math:`M_2` are definite.

    :param M_0: The invariant mass of :math:`M_0`
    :param M_1: The invariant mass of :math:`M_1`
    :param M_2: The invariant mass of :math:`M_2`
    :return: the momentum of :math:`M_1` (or :math:`M_2`)
    """
    M12S = M_1 + M_2
    M12D = M_1 - M_2
    p = (M_0 - M12S) * (M_0 + M12S) * (M_0 - M12D) * (M_0 + M12D)
    return p / (4 * M_0 * M_0)