Particle Physics

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. Finally, I'll present a simple model that realizes co-decaying dark matter. 

We discuss how to formulate a quantum field theory of dark energy interacting with dark matter. We show that the proposals based on the assumption that dark matter is made up of heavy particles with masses which are very sensitive to the value of dark energy are strongly constrained. Quintessence-generated long range forces and radiative stability of the quintessence potential require that such dark matter and dark energy are completely decoupled. However, if dark energy and a fraction of dark matter are very light axions, they can have significant mixings which are radiatively stable and perfectly consistent with quantum field theory. Such models can naturally occur in multi-axion realizations of monodromies. The mixings yield interesting signatures which are observable and are within current cosmological limits but could be constrained further by future observations.

Dark matter can be a thermal relic exponentially lighter than
the weak scale without being exponentially weakly coupled. I will present
three mechanisms to obtain light thermal dark matter with sizable
self-interactions and couplings to the Standard Model.

An unbroken U(1)' is a minimal possibility for a dark matter self interaction, and may even be associated with dark matter stability. However, such an interaction faces incredibly strong constraints due to collective plasma effects, which dominate over 2-to-2 scattering by an order-of-magnitude of orders-of-magnitude. I will discuss the physics of these collective effects, and show preliminary results of simulation. The constraint of such a self interaction is estimated to be nearly as weak as gravity. 

Effective field theories (EFT) are everywhere in particle physics. Given an EFT, the first question we ask is “what are all the operators consistent with the symmetries and degrees of freedom at a particular expansion order? In this talk I will show how this question can be attacked, and often answered, using an object called a Hilbert series.

Astrophysical observations spanning dwarf galaxies to galaxy clusters indicate that dark matter halos are less dense in their central regions compared to expectations from collisionless dark matter N-body simulations. Using detailed fits to dark matter halos of galaxies and clusters, we show that self-interacting dark matter may provide a consistent solution to the dark matter deficit problem across all scales, even though individual systems exhibit a wide diversity in halo properties. 

The High-Energy community is only now in the process of fully appreciating the opportunities the LHC provides by producing electroweak-scale resonances beyond threshold. On the one hand this is reflected by changing from the so-called 'kappa framework’ to effective operators and on the other hand by studying Higgs and gauge boson production in processes with large momentum transfer. Accessing more exclusive phase space regions will allow to either discover New Physics or improve Higgs-boson couplings measurements. I will discuss implications of and tools necessary for these measurements, focusing on Higgs boson and Dark Matter phenomenology.

I explain the hierarchy problem of the standard model of particle physics, and discuss some of the ideas which have been put forward to resolve it. I then show that a specific class of theories, built around a framework known as neutral naturalness, can help address this problem while remaining consistent with all current experimental tests. I explain that while certain theories in this class give rise to striking signals, others are extremely difficult to test, and require a detailed study of the properties of the Higgs boson. I consider the implications of these results for the Large Hadron Collider, and for future experimental programs.

Light sterile neutrinos are predicted in many theories beyond the Standard Model and may be hinted at in short-baseline data. However cosmological data seems to rule out these neutrinos. Intriguingly, this tension is ameliorated when these new neutrinos are self-interacting. I will explore the impact of this self-interaction on their evolution in the early universe and on the spectrum and flavor of IceCube's ultrahigh energy neutrinos.

The venerable proton continues to play a central role in fundamental particle physics.   Neutrinos scatter from protons in neutrino oscillation experiments, Weakly Interacting Massive Particles (WIMPs) are expected to scatter from protons in dark matter searches, and electrons or muons are bound by protons in precision atomic spectroscopy.   Our understanding of the proton is an obstacle to the success of next generation experiments hoping to discover CP violation in the lepton sector and determine the neutrino mass hierarchy, discover the particle nature of dark matter, or reveal new interactions such as those that violate lepton universality.  In this talk I present (i) an overview of the current state of knowledge in the neutrino sector, and theoretical advances that will determine a crucial missing ingredient in the predicted signal process of neutrino-nucleus scattering at long baseline neutrino oscillation experiments (ii) the first complete calculation of the scattering cross section of a proton on a static electroweak source, which determines WIMP-nucleus scattering rates at underground direct detection experiments and (iii) the status of the proton radius puzzle, whose most "mundane" resolution requires a 5 standard deviation shift in the value of the Rydberg constant.   I describe how each of these problems has spurred the development of powerful new methods in effective quantum field theory.

I will discuss the recent LHC excess in the di-photon distribution at an invariant mass of 750 GeV.  Various explanations in terms of weakly coupled and strongly coupled physics will be presented. Possible connection with Dark Matter will also be discussed.

Understanding the microscopic nature of dark matter (DM) is one of the most outstanding problems facing modern physics.  There is to-date no evidence for non-gravitational interactions of DM with the rest of the Standard Model and also no hint for any particular DM mass.  My talk with focus on new techniques to search for GeV-TeV scale weakly-interacting DM by looking for DM annihilating in the cosmos into cosmic rays such as gamma-rays and neutrinos. These potential signs of new physics are easily confused with standard but poorly understood astrophysical backgrounds, such as populations of dim point sources (PSs) like millisecond pulsars.  I will present new methods to characterize Galactic and extra-Galactic PS populations, which have complicated DM searches in these regions.  I will show that accounting for unresolved PSs in Fermi gamma ray data leads both to new constraints on the dark sector along with some unexpected surprises.  I will conclude by briefly describing a new idea for a laboratory experiment able to probe ultra-light axion DM, with masses some 20 orders of magnitude smaller than the weak scale.