  Feng-Yun-Zhang have proved a function field analogue of the arithmetic
Siegel-Weil formula, relating special cycles on moduli spaces of unitary
shtukas to higher derivatives of Eisenstein series. We prove an extension of
this formula in a low-dimensional case, and deduce from it a Gross-Zagier style
formula expressing intersection multiplicities of cycles in terms of higher
derivatives of base-change $L$-functions.
