How to detect collision in ARcore cloud anchor?

i trying to detect touch colliion after instantiate my prefab in ARcore cloud anchor in unity.Please find solution for the same .
I trying with this below:-
Ray raycast = Camera.main.ScreenPointToRay(touch.position);
RaycastHit raycastHit;
if (Physics.Raycast(raycast, out raycastHit))
Debug.Log(“Inside ray”);
if ( == “f1”)
Debug.Log(“f1. collider”);

            if (raycastHit.collider.tag == "f1")
                Debug.Log("tag. hit");


// not working…

you could check out this Unity tutorial to learn about raycasting: