### Rigid Body

Two identical particles of the mass $M$ are connected to the two ends of a rigid massless rod of the length $a$. The system initially rotates around the center-of-mass with the angular velocity $\omega$. One of the particles encounters a third one (with the same mass) at rest, which momentarily sticks to it. What is the angular velocity of the rotation around the center-of-mass after the collision ? (No gravity.)