  We develop a boundary field theory approach to electron spin resonance in
open $S=1/2$ Heisenberg antiferromagnetic chains with an effective staggered
field. In terms of the sine Gordon effective field theory with boundaries,we
point out the existence of boundary bound states of elementary excitations, and
modification of the selection rules at the boundary. We argue that several
"unknown modes" found in electron spin resonance experiments on KCuGaF$_6$ [I.
Umegaki et al., Phys. Rev. B 79, 184401 (2009)] and Cu-PM [S. A. Zvyagin et
al., Phys. Rev. Lett. 93, 027201 (2004)] can be understood as boundary
resonances introduced by these effects.
