Abstract
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.
Highlights
Direct and indirect evidence for physics beyond the Standard Model (BSM) can be achieved at Future Circular Collider (FCC)-ee via a combination of high precision measurements, where new physics could manifest as small deviations with respect to the SM predictions, and via searches, both focusing on specific BSM models, or in a model-independent way, targeting final state signatures or sensitive areas of the phase space
The ensemble of rare decays of heavy-flavoured particles is formed by flavour-changing neutral current (FCNC) decays with either photons or leptons in the final state
The performance sketched by the detector developments of the International Linear Collider (ILC), a proposed linear e+e− collider in Japan [8], would need to be enhanced to allow for these measurements
Summary
J. Plus (2021) 136:1056 of the Future Circular Collider (FCC) integrated program towards ≥ 100 TeV proton-proton collisions in the same infrastructure [1]. In addition to an essential and unique program of high-precision Standard Model (SM) measurements, FCC-ee offers powerful opportunities for the discovery of new phenomena. Direct and indirect evidence for physics beyond the Standard Model (BSM) can be achieved at FCC-ee via a combination of high precision measurements, where new physics could manifest as small deviations with respect to the SM predictions, and via searches, both focusing on specific BSM models, or in a model-independent way, targeting final state signatures or sensitive areas of the phase space. Two broad lines are proposed and highlighted for dedicated exploration at FCC-ee: rare and suppressed decays, and long-lived particles that could give rise to distinct physics signatures
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