This series consists of talks in the areas of Particle Physics, High Energy Physics & Quantum Field Theory.
Soft theorems for the scattering of low energy photons and gravitons and cosmological consistency conditions on the squeezed-limit correlation functions are both understood to be consequences of invariance under large gauge transformations. I apply the same method used in cosmology -- based on the identification of an infinite set of "adiabatic modes" and the corresponding conserved currents -- to derive flat space soft theorems for electrodynamics and gravity.
I will discuss the properties, and constraints on, new light
particles, which appear in many extensions of the Standard Model. An
especially well motivated example is the QCD axion, and I will show how
its mass and couplings can be extracted at high precision. I will also
discuss its properties at finite temperature, and possible distinguishing
features if it makes up dark matter. More generally, strong constraints on
the couplings of new light particles to the Standard Model come from their
We describe a new solution to the strong CP problem inspired by the massless up quark solution. At high energies, QCD is embedded in a SU(3)xSU(3)xSU(3) model, with each matter generation charged under a different site. Instanton effects are unsuppressed at the scale of Higgsing to the SM diagonal QCD, and a set of anomalous U(1)_PQ symmetries removes the low-energy strong CP phase. A non-zero theta parameter is generated at loop level near current bounds. Similar models can also lead to a heavy axion solution to the strong CP problem.
The T2K experiment studies neutrino properties by producing a beam of muon neutrinos and sending them 295 km across Japan to the Super-Kamiokande detector. En route, neutrinos undergo a transmutation known as “neutrino oscillations” wherein they can transition to two other species or flavours, electron and tau neutrinos. Starting in 2014, T2K has run with an antineutrino beam to study the corresponding antineutrino oscillations and the possibility that a complex phase in the neutrino mass and flavour mixing may lead to differences in neutrino and antineutrino oscillations.
Lack of fine tuning in effective field theory does not ensure that a particular scenario is natural or even realizable in a UV complete theory of quantum gravity. Large field axion inflation appears natural from the effective field theory perspective, but I argue that it is tuned from a quantum gravity perspective. The argument is based on the Weak Gravity Conjecture (WGC), a conjectural universal feature of quantum gravity that is present in all known string theory examples.
Supposing there exists no new physics stabilizing the weak scale, the Standard Model Higgs potential exhibits a true vacuum at large field values, rendering the electroweak vacuum metastable (i.e., long lived relative to the age of the Universe). While this scenario need not preclude our current existence, it may not reconcile with a period of large(ish)-field inflation---large fluctuations in the Higgs field, induced by the inflationary energy density, can lead to the field locally sampling the unstable/true vacuum part of the potential, with potentially disastrous consequences.
In this talk, I will present a framework in which Weinberg's anthropic explanation of the cosmological constant problem also solves the hierarchy problem. The weak scale is selected by chiral dynamics that controls the stabilization of an extra dimension. When the Higgs vacuum expectation value is close to a fermion mass scale, the radius of an extra dimension becomes large, and develops an enhanced number of vacua available to scan the cosmological constant down to its observed value.
The fundamental constants of our universe may have been set to maximize the production of similar universes, through repeated parametric variation. In this context, I will advocate that by the time the maximum entropy producer in our universe has reached maximum complexity, the majority of its energy should be re-purposed towards the production of additional universes. This builds on elements of prior proposals, including cosmological natural selection, the nonsingular universe, and the causal entropic principle.
With the discovery of a Higgs like boson at LHC in 2012, and the lack of any statistically significant evidence of physics beyond the Standard Model so far,
In this talk I will propose a new mechanism for thermal dark matter freezeout, termed Co-Decaying Dark Matter. Multi-component dark sectors with degenerate particles and out-of-equilibrium decays can co-decay to obtain the observed relic density. The dark matter density is exponentially depleted through the decay of nearly degenerate particles, rather than from Boltzmann suppression. The relic abundance is set by the dark matter annihilation cross-section, which is predicted to be boosted, and the decay rate of the dark sector particles.