N = 4 super Yang-Mills is a highly symmetric quantum field theory that physicists use as a model system for exploring ideas in quantum gravity and string theory. The researchers study interactions between small and large excitations of the theory. The small, or light, excitations are super gravitons, which can be thought of as particle-like ripples of spacetime. The large, or heavy, excitations are giant gravitons: extended brane-like objects that arise when a graviton carries a very large amount of angular momentum. The goal is to understand how the light excitations behave in the presence of these heavy objects.

To study this, the authors analyse a four-point correlation function containing two light (L) and two heavy (H) operators, known as an LLHH correlator. These correlators are notoriously difficult to calculate directly because doing so requires detailed knowledge of how the supergravity fields couple to the giant graviton and its fluctuations. The key idea of the paper is to treat the pair of heavy operators as a zero-dimensional defect. This reformulates the four-point function as a two-point function of light probes in the presence of the defect, allowing bootstrap techniques to be applied.

Using this defect framework together with bootstrap techniques, the authors compute all strong-coupling four-point functions involving two maximal giant gravitons and two supergravitons of arbitrary dimension. They also uncover a partially broken higher-dimensional hidden conformal symmetry that organizes these correlators. Finally, they determine the leading interaction-induced energy shifts for a complete class of double-particle states associated with the defect and argue that the defect picture provides the natural description of heavy-light correlators more generally. This establishes a powerful new framework for studying giant gravitons, non-planar effects, and aspects of quantum gravity and strongly coupled quantum field theories.

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Bootstrap and amplitudes: a hike in the landscape of quantum field theory by Henriette Elvang (2021)