How to cancel Finder search via keyboard?
It depends on your locale and how you started the search:


If you started the search with no window open, it's simply ⌘W (which closes the window)
If you started the search in an open window, it's ⌘[ (US)(or e.g. ⌘Ö (German)) which goes one step back to the previous window. The shortcut is usually visible in the menubar of the Finder > Go > Back.