Particle theory

Abstract:

Galaxy clusters represent excellent laboratories to search for Axion-Like Particles (ALPs). They contain magnetic fields which can induce quasi-sinusoidal oscillations in the X-ray spectra of AGNs situated in or behind them. Due to its excellent energy resolution, the X-ray Integral Field Unit (X-IFU) instrument onboard the Athena X-ray Observatory will be far more sensitive to ALP-induced modulations than current detectors. As a first analysis of the sensitivity of Athena to the ALP-photon coupling $g_{a \gamma \gamma}$, we simulate observations of the Seyfert galaxy NGC1275 in the Perseus cluster using the SIXTE simulation software. We estimate that for a 200ks exposure, a non-observation of spectral modulations will constrain ${g_{a\gamma\gamma}\lesssim1.5\times10^{-13}\rm{GeV}^{-1}}$ for $m_a \lesssim 10^{-12} \rm{eV}$, representing an order of magnitude improvement over constraints derived using the current generation of satellites.

Abstract:

The swampland program of delineating the space of effective field theories consistent with quantum gravity appears similar to the bootstrap program of delineating the space of quantum field theories consistent with conformal symmetry. With this in mind we rewrite the effective field theory of the Large Volume Scenario in AdS space solely in terms of $R_{AdS}$, in a form suitable for holographic analysis. This rewritten EFT takes a remarkably universal (and previously unnoticed) form, which is uniquely determined in the large-volume limit up to terms suppressed by $\mathcal{O} \left( 1/\ln R_{AdS} \right)$, with no reference to any of the fluxes, brane or instanton configurations that enter the microphysics of moduli stabilisation. The putative dual 3d CFT will have two low-lying single trace scalars, an even-parity scalar $\Phi$ dual to the volume modulus with $\Delta_{\Phi} = \frac{3}{2}\left( 1 + \sqrt{19} \right) \simeq 8.038$ and an odd-parity scalar $a$ dual to the volume axion with $\Delta_a = 3$. On the AdS side the higher-point interactions are likewise uniquely determined. As the AdS theory is both subject to swampland constraints and holographically related to a CFT, we argue that holography will lead to a `bootland' --- a map between swampland constraints on the AdS side and bootstrap constraints on the CFT side. We motivate this with a discussion of swampland quantum gravity constraints on the axion decay constant in the $\mathcal{V} \to \infty$ limit and the $\langle \Phi \Phi a a \rangle$ 4-point function on the CFT side.

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.