Exposing the dead cone effect with jet substructure techniques

The dead cone is a well-known effect in gauge theories, where radiation from a charged particle of mass $m$ and energy $E$ is suppressed within an angular size of $m/E$. This effect is universal as it does not depend on the spin of the particle nor on the nature of the gauge interaction. It is challenging to directly measure the dead cone at colliders, however, since the region of suppressed radiation either is too small to be resolved or is filled by the decay products of the massive particle. In this paper, we propose to use jet substructure techniques to expose the dead cone effect in the strong-force radiation pattern around boosted top quarks at the Large Hadron Collider. Our study shows that with $300/\mathrm{fb}$ of 13--14 TeV collision data, ATLAS and CMS could obtain the first direct evidence of the dead cone effect and test its basic features.