2. A neutral pion in flight in the lab with energy E, decays into two photons of energies E1,2. 1. Find a relation between the energy difference squared of the two pho-\tons, A² = (E₁-E₂)², and sin², where is the opening angle between their trajectories. \text { 2. Use the fact that } \Delta^{2} \geq 0 \text { in deten ine the minimal opening angle. } 3. Show that the maximal opening angle is max = π (i.e., show that it corresponds to a possible outcome of the process), and use this to find the maximal energy difference between the two photons.