PhD Defense by Iryna Boiarska

Title: Chasing feebly interacting particle

The Standard Model of particle physics  successfully describes all known elementary particles. Its theoretical predictions have been confirmed by numerous measurements. Deviations from the Standard Model predictions in some measurements still hold the status of "anomalies'' (have low statistical significance or have not been independently confirmed).

At the same time, there are several firmly established phenomena that cannot be described within the Standard Model: neutrino masses and oscillations; the nature of dark matter; the mechanism of generation of baryon asymmetry of the Universe. To explain them, we need to extend the Standard Model, probably by adding new particles.

There are two possible reasons why we have not detected such particles yet: either they are too heavy to be produced at modern colliders, or they are too feebly interacting to be detected. This thesis is devoted to the latter option.

The thesis covers several classes of feebly interacting particles} (FIPs) — neutrino and scalar. Specifically, for the neutrino portal, the bounds of the past CERN experiment (CHARM) have been re-interpreted and demonstrated to be several orders of magnitude stronger for FIPs with the masses  O(1 GeV) than reported by the collaboration. For the scalar portal, the phenomenology of GeV-scale scalars has been revised, and new scalar production channels have been explored.

Finally, the sensitivity of the new experiment, FASER, towards the scalar portal has been revisited. It has been demonstrated that the second phase of the FASER experiment is able to probe scalars produced in the decays of Higgs bosons in the mass range 40-60 GeV

To summarize, this thesis advances our knowledge about feebly interacting particles beyond the state of the art and proposes new ways of their searches at Intensity Frontier experiments.