Hunt for rare processes and long-lived particles at FCC-ee

In this essay, we discuss the possibilities and associated challenges concerning beyond the Standard Model searches at FCC-ee, such as rare decays of heavy-flavoured particles and long-lived particles. The Standard Model contains several suppression mechanisms, which cause a given group of processes to happen rarely, resulting in rare decays. The interest in these decays lies in the fact that the physics beyond the Standard Model does not need to be affected by the same suppression mechanism and therefore can naturally manifest in these decays. Their interest is reinforced by the recent report of several measurements of b-flavoured rare decays, showing deviations with respect to the Standard Model predictions. We will show how the FCC-ee project has unique capabilities to address these scientific questions and will consider the related detector design challenges to meet. Another group of processes discussed are those that produce new particles with relatively long lifetimes that travel substantial distances inside the detectors before decaying. Models containing long-lived particles can give answers to many open questions of the Standard Model, such as the nature of dark matter, or the neutrino masses, among others, while providing an interesting experimental complement to mainstream searches. Long-lived particles often display unique experimental signatures, such as displaced tracks and vertices, “disappearing” tracks, or anomalously charged jets. Due to this, they are affected by very low background levels but in exchange, they often require dedicated reconstruction algorithms and triggers. The discovery of any of the discussed cases would have a critical impact in High Energy Physics, and FCC-ee could provide a unique experimental opportunity to explore them. Moreover, the searches proposed here could motivate an out-of-the-box optimization of the experimental conditions that could bring in innovative solutions, such as new, possibly very large tracking detectors; or cutting-edge reconstruction algorithms that would boost the FCC-ee reach for unusual final states.

Atomki anomaly in gauged U(1)R symmetric model

The Atomki collaboration has reported that unexpected excesses have been observed in the rare decays of Beryllium nucleus. It is claimed that such excesses can suggest the existence of a new boson, called X, with the mass of about 17 MeV. To solve the Atomki anomaly, we consider a model with gauged U(1)R symmetry and identify the new gauge boson with the X boson. We also introduce two SU(2) doublet Higgs bosons and one singlet Higgs boson, and discuss a very stringent constraint from neutrino-electron scattering. It is found that the U(1)R charges of the doublet scalars are determined to evade the constraint. In the end, we find the parameter region in which the Atomki signal and all experimental constraints can be simultaneously satisfied.

Long-lived light neutralinos at Belle II

We consider light neutralinos of mass about 1 GeV, produced from τ lepton rare decays at Belle II, in the context of R-parity-violating (RPV) supersymmetry. With large and clean samples of τ leptons produced at the Belle II experiment, excellent sensitivity to such light neutralinos with the exotic signatures of displaced vertices is expected. We focus on two benchmark scenarios of single RPV operators, $$ {\lambda}_{311}^{\prime }{L}_3{Q}_1{\overline{D}}_1 $$ λ 311 ′ L 3 Q 1 D ¯ 1 and $$ {\lambda}_{312}^{\prime }{L}_3{Q}_1{\overline{D}}_2 $$ λ 312 ′ L 3 Q 1 D ¯ 2 , which induce both the production and decay of the lightest neutralino. For the reconstruction of a displaced vertex, we require at least two charged pions in the final states. We perform Monte-Carlo simulations for both signal and background events, and find that Belle II can explore regions in the parameter space competitive with other probes. In particular, for the $$ {\lambda}_{311}^{\prime } $$ λ 311 ′ scenario, it can put limits up to two orders of magnitude stronger than the current bounds.