How to hide foreground particles that goes under the floor

Hi guys
I m having a problem with compositing a scene where I animated a bouncing ball and added aaparticle sytem to it. Particles are supposed to fall on the ground and vanish as they goes down to the plane. When I am rendering it as a single layer, everything is fine. However if I am rendering the scene with two layers, 1. Foreground( bouncing ball with particles) and 2. Background( floor and walls), it renders the particles that are hidden under the floor also. So in the composition I couldn’t find a way to hide them.
So guys what are the possible solution of this.
Many thanks…:slight_smile:
Attached image is for demonstration purpose. Actual scene was still rendering so I am putting this image…

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In Blender Internal renderer you can assign a special transparent material (mask) to an object that shows only background. Make sure that the Alpha value is set to 0.