• Axion-like particles
• Belle II
• Dark Matter
• QCD axion
• sensitivity

The Standard Model (SM) of particle physics is the theory used to describe the constituent of matter and the fundamental forces acting between them. It is capable of describing a great deal of physical phenomena, but today it is believed to be a non-complete theory. In fact it fails to give explanations to open questions, such as the strong CP problem and the nature of dark matter. The strong interaction, described by quantum chromodynamics, allows a lagrangian term not invariant under a CP. The term is porportional to an angular parameter θ, which could take any value in the [0, 2π] range. The measure of the neutron electric dipole moment allows to easure the value of θ, and the most recente measuraments give an extremely tight upper limit: θ < 10−11 . It is not clear why this value should be so small, and the problem goes under the name of strong CP problem. One solution, possibly the most promising one, is given by the model of Peccei-Quinn: a new field, arising as the pseudo Goldstone boson of a broken global symmetry, cancels out the term in the lagrangian, thus justifying the small value of θ. The new field is called axion field. The other longstanding problem in the SM concerns the nature of dark matter. This type of matter, which does not interact electromagnetically, has been proven to exist from cosmological observations (for example the galaxies rotational curves or the lensing effect) and it is believed to make up 25% of the matter in the universe. Since it has not been observed directly, it must have very feeble interactions with the SM sector. Search for dark matter nature involves a variety of physics sector, including particle physics. Over the years, many different particles have been considered as dark matter candidates, among which particles known as axion like particles (ALPs). In the first part of my thesis work, I’ve studied the heoretical and phenomenological implications of two classes of axion like particle (ALP) models.In particular I have focused on: 1) “visible” QCD axion at the MeV scale with flavor non-universal coupling to SM fermions such that the resulting axion is pion-phobic and able to evade a variety of very stringent phenomenological bounds (quarkonia decay, kaon decays, electron beam dump experiments); 2) Axion-like particles as dark matter candidate with tree-level flavor-violating
couplings. In particular I have focused on a new scenario where lepton flavor violation decays of SM leptons are directly responsible for the production of axion dark matter in the early universe via the freeze-in mechanism. Both models give rise to a very interesting phenomenology that can be tested at experiments measuring lepton flavor violation such as Belle II, MEG2, Mu3e etc. The second part of the thesis is dedicated on the experimental aspects, in particular testing these models with the Belle II experiment. The interesting channel for the “visible” QCD model is e+e−→γa→γe+e−, for which I have made a study of the experimental sensitivity on the
coupling parameter between axions and electrons entering the cross section of the process e+e−→γa. The upper limit on the sensitivity on the coupling parameter has been then compared to those coming from the recast of the dark photons search made by KLOE and BaBar. The results have given a sensitivity of order (1/GeV) – (0.9/GeV) for the axion mass ranging from 15 MeV to 30 MeV. It is noteworthy that, even though it is a preliminary result, the limit lowers the recedent bound imposed by the BaBar recast. For what concerns this QCD axion model, possible future developments from a simulation point of view could be : improve the analysis of the simulation, and implement a correct model of the physical process in the MadGraph simulator.
For the ALP dark matter model, lepton flavor violating channels are of particular interest. Since Belle II is also a tau factory, I studied the lepton flavor violating decay of the tau to lepton plus invisible. For this channel there is an analysis from which I got the experimental results. I have calculated the ratio between the Br(τ → l+ invisible)/Br(τ → lνν) and compared it with the experimental bound given from Belle II, both for an electron and a muon as final state lepton. The experimental result is still several orders of magnitude larger than the ratio expected for the specific model studied. For this ALP model the future prospects would be to implement the search in other experiments specialized in lepton-flavor violating processes, as Mu3e. Axion-like particles represent a versatile model with possible implications in various standard model scenarios, two of which are described in this thesis. They have been hypothesized with mass and coupling parameters covering a huge range of values, and thus have a very rich and complicated phenomenology. The study of their phenomenology and the existing exclusion limits are key to further understand which existing experiments are the most promising ones where to reveal these particles.