Particle Physics

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.

I will demonstrate that SL(2,Z) duality is a property of all low-energy effective Abelian theories with electric or magnetic charges  The duality will be verified at one loop by comparing the amplitudes in the case of an electron and the dyon that is its SL(2,Z) image, and I will show that it can be extended order by order in perturbation theory.  I will discuss how the duality generically breaks down at high energies, and show how the results apply to the Seiberg-Witten theory.

In this talk, I discuss several application of semileptonic B-meson form factors.  Topics include the determination of $|V_{ub}|$ and $|V_{cb}|$, hints for new physics in semitauonic decays, and Standard-Model predictions for flavor-changing-neutral-current processes: $B\to P\nu\bar{\nu}$ and $B\to P\ell^+\ell^-$, where $P$ denotes a pion or kaon.
I will also cover some details of the underlying lattice-QCD calculations at a nontechnical level.

The Weak Gravity Conjecture (WGC), in its original form, says that given an abelian gauge theory there should be at least one charged particle whose charge is bigger than its mass in Planck units. This has surprisingly powerful implications for the possibility of large-field inflation. In this talk I will explore some of the arguments linking the WGC to inflation before taking a closer look at a different question: which version of the WGC should we be trying to prove? I suggest that the right version to focus on is much stronger than the original WGC, and requires a sufficiently light particle to exist in every representation of the gauge group. I will present some evidence for this statement from both weakly-coupled string theory and semiclassical GR. This talk is based on work with Ben Heidenreich and Tom Rudelius (arXiv:1506.03447, 1509.06374, and further work in progress).

Axions, having a perturbative shift symmetry, can have masses much smaller than other types of particles in a technically natural way. Ultralight axions (ULAs) with m~10^{-22} eV are attractive dark matter candidates with novel properties that distinguish them from cold dark matter (CDM). A single ULA with a GUT scale decay constant provides the correct relic density without fine-tuning. Quantum gravitational effects are expected to break continuous global symmetries, and may spoil the axion potential. However, if the axion global symmetry is an accidental symmetry descending from an exact discrete symmetry, then the problematic higher dimensional operators can be forbidden to very high order. I will discuss the astrophysical and cosmological phenomenology of ULAs that makes them attractive, and methods to distinguish them from CDM observationally. I will also discuss a two-axion model which solves the strong CP problem and in addition possesses a ULA that may be detectable via ~month period nuclear spin precession in an experiment such as CASPEr-Wind. Given time, I may discuss other experimental searches for axion-like particles.

Asymptotically AdS spacetimes with reflecting boundary conditions represent a natural setting for studying superradiant instabilities of rotating or charged black holes. In the first part of this talk, I prove that all asymptotically AdS black holes with ergoregions in dimension d ≥ 4 are linearly unstable to gravitational perturbations. This proof uses the canonical energy method of Hollands and Wald in a WKB limit. In the second part of the talk, I consider a charged Reissner-Nordstrom-AdS black hole---which is superradiantly unstable to charged scalar field perturbations at the linear level---and study the full *nonlinear* evolution of the instability. In this special case, the instability occurs even for spherically symmetric perturbations, which simplifies the analysis and allows for the use of numerical general relativity simulations. Our results show that nonlinear backreaction causes the black hole to lose charge and mass to the scalar field as the instability proceeds. Eventually, higher scalar field harmonics become nonsuperradiant, and they are reabsorbed into the black hole. The final state is described by a “hairy” black hole, surrounded by a scalar condensate in the fundamental (lowest) mode. I discuss implications of this work on the original problem of the rotating black hole superradiant instability.

We propose the discovery of the electroweak monopole as the final test of the standard model. Unlike the Dirac's monopole in electrodynamics which is optional, the electroweak monopole must exist within the framework of the standard model because the $U(1)_{em}$ becomes non-trivial. We estimate the mass of the monopole to be around 4 to 7 TeV, and expect the production rate to be relatively large, $(1/\alpha_{em})^2$ times bigger than the WW production rate. This implies that the MoEDAL detector at LHC could have a real chance to detect it.  

If the dark matter is made up of a bosonic particle, it can be ultralight, with a mass potentially much below 1 eV. Well-known DM candidates of this type include pseudoscalars like the QCD axion, and vectors such as hidden photons kinetically mixed with the Standard Model. Moduli, even-parity scalars with nonderivative couplings to the SM, can also be light dark matter. I will show that they cause tiny fractional oscillations of SM parameters, such as the electron mass and the fine-structure constant, in turn modulating length and time scales of atoms. Rods and clocks, used in gedanken experiments in relativity, have since transformed into actual precision instruments. The size of acoustic resonators and the frequency of optical clocks can now be measured to 1 part in 10^22 and 10^18, respectively, and thus constitute sensitive probes of moduli. 

In this talk, I will give an overview of the parameter space of modulus dark matter, and discuss the sensitivity of the proposed experiments compared to existing constraints from fifth-force tests.

A simple way to trivially satisfy precision-electroweak and flavor constraints in composite Higgs models is to require a large global symmetry breaking scale, f > 10 TeV. This leads to a tuning of order 10^-4 to obtain the observed Higgs mass, but gives rise to a 'split' spectrum where the strong-sector resonances with masses greater than 10 TeV are separated from the pseudo Nambu-Goldstone bosons, which remain near the electroweak scale. To preserve gauge-coupling unification (due to a composite top quark), the symmetry breaking scale satisfies an upper bound f < 100-1000 TeV, which implies that the resonances are not arbitrarily heavy and may be accessible at future colliders. Furthermore, by identifying dark matter with a pseudo Nambu-Goldstone boson, the smallest coset space containing a stable, scalar singlet and an unbroken SU(5) symmetry is SU(7) / SU(6) x U(1). Interestingly, this coset space also contains a metastable color-triplet pseudo Nambu-Goldstone boson that can decay via a displaced vertex when produced at colliders, leading to a distinctive signal of unnaturalness.