Particle physics studies sub-atomic constituents of matter and the laws of their mutual interactions. Our current knowledge is embodied in a mathematical description called the Standard Model (SM), which has been extremely successful and has withstood generations of experimental tests. With the Higgs boson discovery at the Large Hadron Collider (LHC), all the predicted constituents of the SM have been observed. Remarkably, the measured Higgs boson mass falls into a narrow region where consistency of the SM does not require new particles up to a very high energy scales. Moreover a large parameter space of new physics above the electroweak mass scale beyond the SM description has been explored at the LHC, with no new physics found so far.
Yet the SM is clearly incomplete, fundamentally contradicting the emerging Standard Model of cosmology. A clear piece of evidence for phenomena beyond the SM description is the conclusion from cosmological observations that the matter density of the universe is dominated by Dark Matter (DM) rather than the "normal" SM matter. The canonical "WIMP" DM scenario involving heavy particles interacting via the weak force is now almost ruled out experimentally by the LHC, as well as dedicated direct and indirect WIMP detection experiments. Therefore the hidden sector DM scenario is increasingly considered as a natural possibility, which has recently given rise to a new active field of research. Major advances in the exploration of this new paradigm can be achieved by exploiting data sets collected by the existing “intensity frontier” experiments.
The hidden sector is postulated to consist of new particles with masses in the broad vicinity of the electroweak scale that do not interact via the known electroweak and strong forces, and interact feebly with ordinary matter through light mediator particles charged under both SM and hidden sector fields, or mixing with their SM counterparts. In addition to providing DM candidates, hidden-sector models lead to simple and predictive cosmology, and offer solutions for astrophysical and low-energy anomalies.
It is proposed to perform the first search for production of a short-lived axion-like particle (ALP) in charged-kaon decays (K+ --> pi+a), which decaying promptly into a di-photon final state, using world's largest data set of charged kaon decays collected by the NA62 experiment at CERN in 2016-18. New powerful constraint on the ALP phase space will be established; a possible ALP discovery would be a breakthrough in particle physics. To achieve credible results on the ALP search, it is mandatory to understand the main Standard Model background process K+ --> pi+ gamma gamma. Therefore a comprehensive study of this process is also proposed, including a re-analysis of the inputs to the description of this decay within Chiral Perturbation Theory, and the most precise measurement of this process to date. This measurement by itself represents an important test of the theories describing low-energy dynamics. These novel measurements are based on an existing data sample collected recently, thereby minimizing the associated risks.
