How can I raycast gameobjects with specific layer ?

Hi everyone, I want to raycast gameobject that have "UI" layer. Currently, the official documentation seems to be clear about how I can do this. But, it just doesn't work. Here is my actual code :

 // Use this for initialization
     void Start ()
     {
         this.m_layer_ground = LayerMask.NameToLayer("UI");
     }
 
     // Update is called once per frame
     void Update()
     {
         if (Input.GetMouseButton(0))
         {
             print("Press Mouse Left Button");
 
             RaycastHit hit = new RaycastHit();
             Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 
             if (Physics.Raycast(ray, out hit, 1000f, this.m_layer_ground))
             {
                 print("Touch UI");
             }
         }   
     }

So, on Start function I get the good layer mask. Then, on Update function I wait for a mouse button held down. Then I create a ray from camera through screen point and finally I test if the ray collide a gameobject with "UI" layer with Physics.Raycast. I have several gameobject that have a "UI" layer on my scene and there are a box collider that cover there surface. But, when I click on one of these gameobject, I don't have any "Touch UI" on Unity console.

I don't see the problem...