Particle theory

Abstract:

Abstract:

Scattering amplitudes provide insight into the all-orders structure of gauge theories. Particularly rich is their non-planar sector, where new and interesting physical phenomena appear. This thesis aims to push the boundaries of perturbative scattering amplitudes, with special emphasis on calculations in massless Quantum Chromodynamics. We review the colour and helicity decomposition of gauge theory amplitudes, as well as the structure of their ultraviolet and infrared divergences. We then discuss the application of state-of-the-art methods to the computation of all four-point three-loop scattering amplitudes in Quantum Chromodynamics. As an immediate consequence we both verify the structure of infrared divergences and extract the gluon Regge trajectory at the corresponding perturbative order. We further describe the computation of five-gluon scattering at the two-loop order, with special emphasis on the multi-scale complexity of this process. Finally, we explore the idea of simplifying the integrand representation of gauge theoretic scattering amplitudes by leveraging their highly-constrained infrared structure. We provide a proof-of-concept application to two-loop four- gluon scattering amplitudes.

Abstract:

Solar flares convert magnetic energy into thermal and non-thermal plasma energy, the latter implying particle acceleration of charged particles such as protons. Protons are injected out of the coronal acceleration region and can interact with dense plasma in the lower solar atmosphere, producing mesons that subsequently decay into gamma rays and neutrinos at O(MeV-GeV) energies. We present the results of the first search for GeV neutrinos emitted during solar flares carried out with the IceCube Neutrino Observatory. While the experiment was originally designed to detect neutrinos with energies between 10 GeV and a few PeV, a new approach allowing for a O(GeV) energy threshold will be presented. The resulting limits allow us to constrain some of the theoretical estimates of the expected neutrino flux.

Abstract:

The IceCube Collaboration has observed a high-energy astrophysical neutrino flux and recently found evidence for neutrino emission from the blazar TXS 0506+056. However, the source or sources of most of the observed flux remains uncertain. Through-going muons produced by muon-neutrinos are ideal to search for point-like neutrino emission from astrophysical sources because their arrival direction can be resolved with an angular resolution $\leq1^\circ$. Here, an unbinned search for steady point-like neutrino sources is performed based on eight years of IceCube data measured between 2009 and 2017. Compared to previous searches, this search includes an improved event selection and reconstruction and it is optimized for point-like neutrino emission with the same flux-characteristics as the observed astrophysical muon-neutrino flux. The result is an improvement in flux sensitivity of ~35% assuming an $E^{-2}$ spectrum. The sensitivity on the muon-neutrino flux is at a level of $E^2 \mathrm{d} N /\mathrm{d} E = 3\cdot 10^{-13}\,\mathrm{TeV}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$. No new evidence for neutrino sources is found in a full sky scan and in an a priori candidate source list. Furthermore, no significant excesses above background are found from populations of sub-threshold sources. The implications of the non-observation for potential source classes are discussed.