Two distinguishable non-interacting particles, each of mass m are in a one-dimensional infinite square well in the interval [0, a]. If x1 and x2 are position operators of the two particles, the expectation value 〈x1x2〉 in the state in which one particle is in the ground state and the other one is in the first excited state, is
1
\(\frac{1}{2} a^2\)
2
\(\frac{1}{2} \pi^2 a^2\)
3
\(\frac{1}{4} a^2\)
4
\(\frac{1}{4} \pi^2 a^2\)