mystery tree

My son picked this leaf for a science project but I cannot figure out what it is. Massechusetts, USA is the region. Can anyone identify?

 

Poplar of some sort, probably one of the hybrids produced for timber growing.

 

I thought it might be a poplar but I can't find any examples of a poplar with a leaf so large. Not sure if it is a poplar.

 

I've seen hybrid Balsam Poplars with leaves 30cm long. The largest leaves are on the lead shoots at the top of the tree, often 3 or 4 times longer than leaves on low branches of the same tree.

 

Good theory then. What about an American Linden?

 

Nope, definitely not that! Wrong shape leaf - that has a lop-sided base and a less triangular apex.