Particle Physics

Any quantum field theory can be thought of as arising from a perturbed UV conformal field theory, suggesting that information about the full RG flow is encoded in the original CFT. I will discuss ongoing work developing new methods for extracting this information to study strongly-coupled IR dynamics. This method uses a UV basis of conformal Casimir eigenstates to construct the Hamiltonian, which is then truncated at some maximum Casimir eigenvalue and diagonalized to approximate the low energy spectrum of the IR theory. After presenting a general framework which can be applied to CFTs in any number of dimensions, I will then focus on the specific example of scalar field theory, which can be used to study strongly-coupled systems like the 2D and 3D Ising models. Finally, I will discuss the possible application of this framework to gauge theories, including QCD.

In the framework of the ordinary seesaw model with right-handed neutrinos (and nothing else) we show that the total lepton number violating decay of the Higgs doublet into a right-handed neutrino and a standard model lepton can successfully account for the baryon asymmetry of the Universe. This is possible thanks to thermal effects shortly before the sphalerons decouple.

We derive in the framework of soft collinear effective field theory (SCET) a Lagrangian describing the t-channel exchange of Glauber quarks, which are incorporated through fermionic potential operators in the effective theory. The Wilson line structure of the operators, which is derived from matching calculations and the symmetries of the effective theory, describe additional soft and collinear emissions from a fermionic t-channel exchange in the forward scattering limit to all orders. Lightcone singularities in the fermionic potential are regulated using a rapidity regulator, whose corresponding renormalization group evolution gives rise to the quark reggeization at amplitude level.

Since the Higgs, the LHC has offered no discoveries despite a sweeping hunt for new resonances. However, many well-motivated new physics models would not turn up in resonance searches, but could instead be uncovered in existing and forthcoming LHC data, by searching for novel angular distributions of Standard Model particles. I will show that in the upcoming years at the LHC, (1) through radiative corrections, dark matter can turn up in dilepton scattering angles in the parametric regions where it would elude conventional missing energy-based searches; the scattering patterns can reveal dark matter's mass, self-conjugation property, spin, and chirality of interactions, (2) through tree level interference, leptoquarks may emerge in dilepton scattering angles long before they would in dedicated direct searches; these angular measurements would also overtake the excellent sensitivities of low-energy precision measurements of atomic parity violation, and (3) Higgs Yukawa couplings to light quarks of the size of the bottom Yukawa could be measured using the Higgstrahlung scattering angle.

Sub-GeV dark matter is a theoretically motivated but largely unexplored paradigm of dark matter. In this talk, I will discuss recent work on the direct detection of sub-GeV dark matter through dark matter-electron scattering. I will present some motivated models that can be probed with these techniques as well as projections for current and near-term noble liquid, semiconductor, and scintillator experiments. Finally, I will discuss some new techniques that may allow us to more robustly discriminate between dark matter signatures and background.

Utilizing the Fermi measurement of the gamma-ray spectrum toward the Galactic Center, we derive some of the strongest constraints to date on the dark matter (DM) lifetime in the mass range from hundreds of MeV to above an EeV. Our profile-likelihood based analysis relies on 413 weeks of Fermi Pass 8 data from 200 MeV to 2 TeV, along with up-to-date models for diffuse gamma-ray emission within the Milky Way. We model Galactic and extragalactic DM decay and include contributions to the DM-induced gamma-ray flux resulting from both primary emission and inverse-Compton scattering of primary electrons and positrons. For the extragalactic flux, we also calculate the spectrum associated with cascades of high-energy gamma-rays scattering off of the cosmic background radiation. We argue that a decaying DM interpretation for the 10 TeV-1 PeV neutrino flux observed by IceCube is disfavored by our constraints. We interpret the results in terms of individual final states and in the context of simplified scenarios such as a hidden-sector glueball model.

Just like how milli-electric charged particles can exist, so can milli-magnetic charged particles.  We review simple ways of evading the standard quantization arguments and why there are no model independent constraints on magnetically charged particles, milli-charged or not.  We then provide the first ever model independent bounds coming from magnetar cooling arguments.

Recently a new solution to the hierarchy problem was proposed which makes use of the cosmological evolution of a light scalar field, a scanner, instead of symmetry or anthropic arguments to select a small Higgs mass. In the original proposal this scanner field could be the QCD axion and thus such class of solution became known as ``relaxion’’. Two central features required of the relaxion are a reason for why a small Higgs mass is special from the scanner’s viewpoint and a mechanism fo the scanner to dissipate its energy in order to stop in a value associated with small Higgs mass. In this talk we propose a novel mechanism that achieves both goals and opens new possibilities for the relaxion. We show that particle production can create an effective friction force for the relaxion and show how it can be used to ``select’’ the TeV scale as a special energy scale from reasonable initial conditions. This allows for the scanning to happen much faster than in previous relaxion models, which usually require very large amounts of inflation, and also allows for the scanning to take place after inflation has ended.

A very close supernova explosion could have caused a mass extinction

of life in Earth. In 1996, Brian Fields, the late Dave Schramm and the
speaker proposed looking for unstable isotopes such as Iron 60 that could
have been deposited by a recent nearby supernova explosion. A group from
the Technical University of Munich has discovered Iron 60 in deep-ocean
sediments and ferromanganese crusts due to one or more supernovae that
exploded O(100) parsecs away about 2.5 million years ago. These results
have recently been confirmed by a group from the Australian National
University, and the Munich group has also discovered supernova Iron 60 in
lunar rock samples and in microfossils of magnetotactic bacteria. 
This colloquium will discuss the data and their interpretation
in terms of supernova models, and the possible implications for
life on Earth.

Cold dark matter provides a remarkably good description of cosmology and astrophysics. However, observations connected with small scales might be in tension with this framework. In particular, structure formation simulations suggest that the density profiles of dwarf spheroidal galaxies should exhibit cusps, in contrast to observations. I will show that Fermi repulsion can explain the observed cored density profiles in dwarf galaxies for sub-keV fermionic dark matter.  While in conventional dark matter scenarios, such sub-keV thermal dark matter would be excluded by free streaming bounds, I will argue that these constraints are ameliorated in models with dark matter at lower temperature than conventional thermal scenarios. Finally, I will outline a class in which the dark matter typically has a lower temperature than the thermal expectation, dubbed Flooded Dark Matter, and discuss aspects of model building.