Aspects of higher-point functions in BCFT$_d$

Abstract: We study three-point correlation functions of scalar operators in conformal field theories with boundaries and interfaces. We focus on two cases where there are one bulk and two boundary operators (B$\partial\partial$), or two bulk and one boundary operators (BB$\partial$). We perform a detailed analysis of the conformal blocks in different OPE channels. In particular, we obtain the bulk channel conformal blocks of the BB$\partial$ three-point functions for arbitrary exchanged spins in a series expansion with respect to the radial coordinates. We also study examples of such three-point functions in the simplest holographic dual where the $AdS_{d+1}$ space contains a brane filling an $AdS_d$ subspace. Such a setup arises in top-down models with probe branes and is also relevant for the functional approach to boundary and interface CFT correlators. We systematically study the Witten diagrams in this setup both in position space and in Mellin space. We also discuss in detail how to decompose these Witten diagrams into conformal blocks.