Winter 2018

Dark Matter could reside in a hidden sector with gauge structure similar to the Standard Model. In particular, the hidden sector may include a non-Abelian gauge interaction with confinement scale around 100 MeV, similar to our QCD. Dark mesons, counterparts of the usual pions, kaons etc., can then play the role of dark matter. Such particles may experience strong number-changing self-interactions, similar to the 2K<->3pi scattering familiar in QCD. Intriguingly, such self-interactions can naturally produce a thermal relic abundance of dark mesons consistent with observations. In this talk we will explore two variations of this basic scenario, “Strongly-Interacting Massive Particle” (SIMP) and “Elastically-Decoupling Relic” (ELDER). We will discuss the basic features of each scenario, explicit models in which they may be realized, and their experimental signatures.  

We describe a new approach for quantum cosmology based on computational complexity.

By defining a cosmology as a space-time containing a vacuum with specified properties (for example small cosmological constant) together with rules for how time evolution will produce the vacuum, we can associate global time in a multiverse with clock time on a supercomputer which simulates it. We argue for a principle of ``limited computational complexity" governing early universe dynamics as simulated by this supercomputer, which translates to a global measure for regulating the infinities of eternal inflation.  We also give various definitions of the computational complexity of a cosmology, and argue that there are only a few natural complexity classes.  Based on joint work with Frederik Denef, Brian Greene and Claire Zukowski, and the preprint arXiv:1706.06430 .

Quantum quench - a sadden change of system Hamiltonian - provides a rich and tractable framework to access dynamics of thermalization of a quantum isolated system. When the chnge of Hamiltonian is fast but not instantaneous (so called fast quenches) the dynamics often can be described in terms of the UV fixed point, thus leading to universal predictions. We employ conformal perturbation theory to calculate the behavior of various quantities (one and two point functions) during and after the quench. Furthermore, by calculating the energy fluctuations after the quench we argue that at late times the system will thermalize, provided it satisfies the Eigenstate Thermalization Hypothesis. 

Dark matter (DM) comprises approximately 27% of the energy in the observable universe. Its properties, such as its mass and interactions, remain largely unknown. Unveiling the properties of DM is one of the most important tasks in high energy physics. For the past few years, motivated by possible new physics at the electroweak scale, many DM experiments have looked for DM with mass at O(100) GeV. This is not the only possibility, however. Large chunks of parameter space supported by other well-motivated models remain to be carefully studied. Exploring these regimes requires creative ideas and advanced technologies. I will first talk about the novel  proposal on using superconductor as the target material for DM direct detection. This setup has the potential to lower the direct detection mass threshold from few GeV to keV, consequently probing the warm dark matter scenario. Then I will present a recent proposal utilizing the Gravitational Wave (GW) experiments, i.e.  LIGO and LISA, to search for ultra-light dark photon dark matter. We show these GW experiments can go well beyond existing constraints and probe large regions of unexplored parameter space. Both proposals are under serious investigation by experimental groups and likely to be carried out in the near future.

New singlet scalar bosons have broad phenomenological utility and feature prominently in many extensions of the Standard Model. Such scalars are often taken to have Higgs-like couplings to SM fermions in order to evade stringent flavor bounds, e.g. by assuming Minimal Flavor Violation (MFV), which leads to a rather characteristic phenomenology. Here we describe an alternative approach, based on an effective field theory framework for a new scalar that dominantly couples to one specific SM fermion mass eigenstate. A simple flavor hypothesis, similar in spirit to MFV, ensures adequate suppression of new flavor changing neutral currents. We consider radiatively generated flavor changing neutral currents and scalar potential terms in such theories, demonstrating that they are often suppressed by small Yukawa couplings, and also describe the role of CP symmetry. We further demonstrate that such scalars can have masses that are significantly below the electroweak scale while still being technically natural, provided they are sufficiently weakly coupled to ordinary matter. In comparison to MFV, our framework is rather versatile since a single (or a few) desired scalar couplings may be investigated in isolation. We illustrate this by discussing in detail the examples of an up-specific scalar mediator to dark matter and a muon-specific scalar that may address the muon anomalous magnetic moment discrepancy.

There seem to be no good examples of UV complete theories that have low-lying massive higher spin states isolated by a large gap, despite the relative ease of constructing effective field theories describing such states.  We discuss constraints from analytic dispersion relations and subluminality of eikonal scattering that may help to explain this and provide insight into the possible interactions of massive higher spins.

Dark matter orchestrates the expansion of the universe and the development of the cosmic web of structure, yet its identity is unknown.  We know that dark matter molds luminous matter into galaxies, yet the microphysical processes that govern its own creation and evolution remain a mystery.  Despite its cosmic importance, the nature of dark matter remains one of the biggest unsolved problems in fundamental physics.  However, it is one that may be solved with the tools of astronomy.  In this talk, I will show how astronomical observations have shaped our understanding of the microphysical properties of dark matter.  I will discuss the exciting prospects for a new generation of astronomical facilities to enable measurements of dark matter physics.

In recent times we have learned that if in QFT, the constraint of locality and/or Lorentz invariance are lifted, the patterns of symmetry breaking are far richer than in local relativistic field theories.  Recently we have studied some (conformal) field theories with global symmetries in the sector where the value of the global charge Q is large.  We find that the low energy excitations in this sector are described by a particular form of the non-relativistic Goldstone theorem.  We also provide heuristic arguments that the effective theory describing such sector contains an effective coupling constant suppressed by powers of the large charge.  The comparison of our heuristic arguments with "exact" results (lattice MonteCarlo) are remarkably good.