Abstract: The discovery of the Higgs boson added a fundamental piece to the Standard Model, but it also opened new questions about the nature of electroweak symmetry breaking and possible connections to hidden sectors. A broad class of models predicts new resonances below the Higgs mass, despite the mainstream interest in the high energy regime, such as light scalars/pseudoscalars, or dark mediators that could evade traditional search strategies. In this seminar, I will discuss the experimental challenges and novel techniques used in ATLAS to probe this low mass regime, where large Standard Model backgrounds and trigger limitations demand creative solutions. I will highlight a biased selection of searches and discuss ongoing efforts to extend sensitivity to unexplored regions of parameter space.

Speaker Bio: Luis is a Juan de la Cierva postdoctoral researcher member of the ATLAS Collaboration at CERN. His research focuses on photon physics, Higgs boson measurements, and searches for light resonances below the Higgs mass. He earned his PhD at Université Paris Cité, where he worked on diphoton resonance searches and photon isolation performances under high pileup conditions. He is currently involved in searches for light particles with LHC in exotic Higgs boson decays to better understand potential portals to physics beyond the Standard Model as well as precision measurements of the Higgs boson differential cross-section in the diphoton channel.