The Gates by John Connolly

The Gates - Eleven year-old Samuel Johnson witnesses his neighbors, the Abernathys, and another couple conducting a seance in their basement.  During the seance, all four of the adults are sucked into a bluish portal and come back changed.  Samuel recognizes that evil has befallen the neighborhood, but nobody will listen to him except his friends.  As Mrs. Abernathy summons other monsters to kill Samuel.  However, Samuel is either able to make friends with them or fight them off.  Nurd, a demon who has been banished to the wasteland, is teleported to earth separately from the demons who have taken over the Abernathy's bodies.  Meanwhile, at CERN in Switzerland, the Large Hadron Collider (LHC), the largest particle accelerator ever built, has thrown off a particle.  The problem is that the LHC is a contained environment that nothing should escape.