Disk relations for tree amplitudes in minimal coupling theory of gauge field and gravity

KLT relations on $S_2$ factorize closed string amplitudes into product of open string tree amplitudes. The field theory limits of KLT factorization relations hold in minimal coupling theory of gauge field and gravity. In this paper, we consider the field theory limits of relations on $D_2$. Though the relations on $D_2$ and KLT factorization relations hold on worldsheets with different topologies, we find the field theory limits of $D_2$ relations also hold in minimal coupling theory of gauge field and gravity. We use the $D_2$ relations to give three- and four-point tree amplitudes where gluons are minimally coupled to gravitons. We also give a discussion on general tree amplitudes for minimal coupling of gauge field and gravity. In general, any tree amplitude with $M$ gravitons in addition to $N$ gluons can be given by pure-gluon tree amplitudes with $N+2M$ legs.