Yellow Flower

Any ideas about this plants name? It grew to about 8 feet tall, like a small tree and had yellow flowers that looked like bluebonnets? They were clustered together I'll take a better picture tomorrow, but here is a closeup of a flower. (with an ant)

 

Senna.

 

Thank you for replying.

 

Much better Google Image Search result comes from searching "cassia".

http://images.google.com/images?svnum=10&hl=en&lr=&q=cassia