Search for New Physics in First and Second Generation Quark Yukawa Couplings through Rare Exclusive Decays of the Observed Higgs Boson

Following the discovery of a Higgs boson with a mass of about 125 GeV,a detailed set of property measurements has confirmed that it plays acentral role in the spontaneous breaking of the electroweak symmetry.Nevertheless, its role in the generation of fermion mass, inparticular of the first and second generation, is still unclear. Inthe Standard Model (SM) this is implemented in an ad hoc manner throughYukawa interactions, and many beyond-the-SM theories offer richphenomenology and exciting prospects for the discovery of New Physics in this sector.This project will attack - for the first time - in asystematic and comprehensive way the experimentally most unconstrainedsector of the SM: the couplings of the light-quarks (up, down, charmand strange) to the Higgs boson, including possible flavour-violatinginteractions. The rare exclusive Higgs boson decays to ameson and a photon or Z boson, which is a novel and unique approach, will be searched for with the ATLASdetector at the CERN Large Hadron Collider (LHC). At the same time,an extensive set of measurements of analogous rare exclusive decays ofthe W and Z bosons will be performed, further enhancing the scientificvalue of the proposed research programme.The expected branching ratio sensitivity of 10^{-6} for the Higgsboson decays, and 10^{-9} for the W and Z boson decays will probeviable New Physics models, and in several cases will reach and surpass theSM predictions. This project will lead to a profound extensionof the ATLAS and LHC physics output, going beyond what was previouslyconsidered possible. It will open a new line of research in the Higgssector, providing relevant input to many different areas of frontierresearch, including particle cosmology and planning for possiblefuture particle physics facilities.