On the impact of new, light states in some astrophysical and laboratory systems

Thesis

We investigate the observational consequences of adding hypothetical new states to the Standard Model of particle physics. These additional fields are feebly coupled and much lighter than the electroweak scale. They include dark photons, mil- licharged particles, heavy neutral leptons, as well as light dark matter that couples to electronic spin, among others.

We derive limits on particles that could have been detected by the Big Euro- pean Bubble Chamber (BEBC) in the 1982 CERN-WA-066 beam dump experiment. Light states coupling to photons, as well as heavy neutral leptons, may have been produced in the copious hadronic decays in the beam dump, and the lack of ob- served signal events allows us to place world-leading bounds on both these particles in the GeV mass range.

We also reappraise an existing astrophysical limit on ultra-light dark photons derived from Jupiter’s magnetosphere. The new bosons may kinematically mix with our photon, leading to modifications of electrodynamics at large distances. Observations of the magnetic field around Jupiter allow us to place the strongest constraints to date on the size of the mixing, once we include measurements of the plasma from the recent Juno spacecraft mission.

Lastly, we study the ability of sub-GeV dark matter coupling electronic spin to excite magnons in magnetically ordered materials. We calculate the expected excitation rate in quasi-2d ferromagnetic materials, and show it exhibits strong anisotropies. These materials then allow for directional detection of such dark mat- ter candidates. The near-term possibility of detecting these small meV energy de- positions raises the exciting prospect that this may be a compelling way to discover the particle nature of dark matter.

Authors: Marocco, G

• Alternative title: Looking for new physics in the lab, the sky, and the future
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: dark matter phenomenology
• Subjects: Neutrino interactions