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Friday, January 20th 2017

12:30 pm:

Friday, January 27th 2017

12:30 pm:

Despite being associated with particles of zero rest mass, electromagnetic and gravitational waves do not travel solely on the null cone in generic curved spacetimes. (That is, light does not always propagate on the light cone.) This inside-the-null-cone propagation of waves is known as the tail effect, and finding novel ways of understanding it in the strong field regime near a black hole may find applications for modeling the gravitational signals sought after by next-generation space-based detectors such as LISA

Friday, February 3rd 2017

12:30 pm:

One of the typical low-temperature phases of fermionic matter is

described by a phenomenological theory called Landau Fermi liquid.

I will be discussing the first proposed exactly solvable

microscopic theories which behave at low energies as Landau Fermi liquid.

These theories are the large-N three-dimensional Chern-Simons-fermion

and Thirring models. I will demonstrate how the three-dimensional

large-N power allows one to obtain exact results, at any value of the coupling

constant, for a number of interesting observables.

Friday, February 10th 2017

12:30 pm:

Indirect detection is one of the major ways to search for dark matter. However, backgrounds have been a major problem for these searches. In this talk, I will introduce two new techniques to distinguish signal from background. Firstly, I will show how telescopes with ~ 0.1% energy resolution can exploit the Doppler shift of sharp photon features arising from dark matter interactions and separate the signal from background. The technique is general and I will give an example of this search strategy with the 3.5 keV line. In the second half of my talk, I will show how limits from the searches for very high energy photons can be used to constrain dark matter interactions. Using this observable, I will constrain very heavy dark matter which is very difficult to constrain using other means.

Friday, February 17th 2017

12:30 pm:

The LIGO detection of gravitational waves has opened a new window on

the universe. I will discuss how the process of superradiance,

combined with gravitational wave measurements, makes black holes into

nature's laboratories to search for new light bosons, from axions to

dark photons. When a bosonic particle's Compton wavelength is

comparable to the horizon size of a black hole, superradiance of these

bosons into bound "Bohr levels" extracts energy and angular momentum

from the black hole. The occupation number of the levels grows

exponentially and the black hole spins down. One candidate for such an

ultralight boson is the QCD axion with decay constant above the GUT

scale. Current black hole spin measurements disfavor a factor of 30

(>100) in axion (vector) mass; future measurements can provide

evidence of a new boson. Particles transitioning between levels and

annihilating to gravitons may produce thousands of monochromatic

gravitational wave signals, and turn LIGO into a particle detector.

Friday, February 24th 2017

12:30 pm:

Friday, March 3rd 2017

12:30 pm:

Detection of tensor mode fluctuations at the largest cosmological scales is often expected to provide a robust evidence of inflation and to fix the inflationary energy scale. Such direct connection is however applicable only when the tensor perturbations are originated from their quantum fluctuations during inflation and are effectively decoupled from other energy contents. We consider a case exceptional to this lore. When an SU(2) gauge field is present and is prevented from diluting away by a coupling to a pseudo-scalar, the configuration of the SU(2) vev preserves the background isotropy, and on this geometry, the SU(2) perturbations introduce an additional tensor mode. The substantial growth experienced by this mode is then transferred to the true tensor fluctuations of the metric already at the linear level. While these fields are energetically subdominant to the inflaton, their perturbations can be considerably larger than the standard amplitude. We discuss several potential constraints on the mechanism and demonstrate that detectable tensor mode signals even for low energy inflation.

Friday, March 10th 2017

12:30 pm:

Supernova 1987A created an environment of extremely high temperatures and nucleon densities. The rough agreement between predictions of core collapse models and observations of a "neutrino burst" provide an opportunity to set bounds on a wide range of theories of new physics. We present new bounds on dark sector models, incorporating finite-temperature effects on the production and trapping for the first time, and also utilizing a more realistic model of the high-mixing parameter space than in previous work. This has dramatic effects for the landscape of such models.

Friday, March 17th 2017

12:30 pm:

Friday, March 24th 2017

12:30 pm:

Axions are attractive candidates for theories of large-field inflation that are capable of generating observable primordial gravitational wave backgrounds. These fields enjoy shift symmetries that protect their role as inflatons from being spoiled by coupling to unknown UV physics. This symmetry also restricts the couplings of these axion fields to other matter fields. At lowest order, the only allowed interactions are derivative couplings to gauge fields and fermions. These derivative couplings lead to the biased production of fermion and gauge-boson helicity states during and after inflation. I will describe some recent work on reheating in axion-inflation models that are derivatively coupled to Abelian gauge-fields and fermion axial-currents. The biased production of fermion helicity-states in these theories has interesting phenomenological implications for leptogenesis, while gauge preheating in this class of models can be extremely efficient, and can result in cosmologically relevant magnetic fields.

Friday, March 31st 2017

Friday, April 7th 2017

12:30 pm:

The LIGO observatory has recently reported several detections of gravitational waves from the coalescence of binary black holes. We consider the extraordinary possibility that the detected events involving heavier masses are mergers of primordial black holes making up the dark matter in the Universe. We will describe various ways of testing this proposition once more gravitational wave data is gathered, survey some of the existing constraints and present a novel probe of massive compact dark matter in the relevant mass range based on strong gravitational lensing of fast radio bursts. We will conclude with a summary of the observational prospects to test the proposed scenario over the next decade.

Friday, April 14th 2017

12:30 pm:

Matching a full theory onto an effective field theory (EFT) by integrating out heavy fields is often useful for connecting low-energy phenomenology to high-scale physics. I introduce a new formulation of one-loop matching in terms of "covariant diagrams," which, unlike conventional Feynman diagrams, preserve gauge covariance and thus simplify calculations. Starting from a set of simple rules, I will demonstrate the use of covariant diagrams with examples. I will also discuss an interesting universal structure of one-loop effective Lagrangians revealed by such calculations.

Monday, April 17th 2017

2:00 pm:

Cosmic inflation is an impressively successful paradigm in our understanding of the early Universe. Its embedding into particle physics is however still an open question. In this talk, I will present a `complete' particle physics model of inflation. Based on the implementation of D-term hybrid inflation in strongly coupled supersymmetric gauge theories, this setup overcomes all the usual problems of D-term hybrid inflation, including the consistent generation of an Fayet-Iliopoulos term in supergravity and the cosmic string problem. The D term may be associated with a gauged U(1)_B-L, so that the end of inflation spontaneously breaks B-L in the visible sector, setting the stage for neutrino mass generation and leptogenesis.

Friday, April 28th 2017

12:30 pm:

I will present two scenarios where standard searches for SUSY at the LHC are not very efficient because the mass splitting between the LSP and the NLSP is small and therefore there is not a lot of MET in the events. I will then use different observables to be able to improve the reach of the LHC in this kind of situations.

Friday, May 5th 2017

12:30 pm:

I will review the motivation for and construction of a theory of Lorentz-invariant massive gravity which is free of the Boulware-Deser ghost. After giving an overview of observational constraints on massive gravity, I will describe in more detail spherically symmetric solutions which exhibit the Vainshtein screening mechanism, which is required to avoid the van Dam-Veltman-Zakharov discontinuity. I will show how these solutions can be used to study fifth force effects in the solar system, as well as scalar radiation from binary pulsars.

Friday, September 8th 2017

12:30 pm:

A cosmological magnetic field of nG strength on Mpc length scales could be the seed magnetic field needed to explain observed few microG large-scale galactic magnetic fields. I first briefly review the observational and theoretical motivations for such a seed field, two galactic magnetic field amplification models, and some non-inflationary seed field generation scenarios. I then discuss an inflation magnetic field generation model. I conclude by mentioning possible extensions of this model as well as potentially observable consequences.

Friday, September 15th 2017

12:30 pm:

Friday, September 22nd 2017

12:30 pm:

The large-scale structure of our universe (the distribution of galaxies on very large-scales for instance) contains a wealth of information about the origin, evolution, and matter content of the universe. Extracting this information relies crucially on understanding how galaxies and other biased objects trace the large-scale matter distribution. In a universe such as our own, with both cold dark matter and massive neutrinos, or in alternative cosmologies with clustered quintessence, this problem is much more complicated. I will discuss new tools that my group has developed to study gravitational evolution in cosmologies with multiple fluids, the novel signatures we have identified including a new probe of neutrino mass, and the broader implications for models of large-scale structure.

Friday, September 29th 2017

12:30 pm:

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.

Friday, October 6th 2017

12:30 pm:

The Veneziano amplitude is a very useful tool to model meson scattering in confining gauge theories, with many attractive features, namely providing an example of amplitudes with a clear linear Regge trajectory. But this formulation is incomplete- it effectively only exist in theories where the coupling never runs. We expect the Regge trajectory to bend away from linearity due to the running of the coupling at higher energies: we propose a way of doing so, using string theory and the holographic duality, in order to improve on a long-standing result of Makeenko and Olesen.

Friday, October 13th 2017

12:30 pm:

Friday, October 20th 2017

12:30 pm:

Dark Matter (DM) provides strong evidence for physics beyond the Standard Model (SM). Arguably, rather than suggesting a specific mass scale for New Physics, it may point to a dark sector, weakly-coupled to the SM, as hinted at by the comparable abundances of dark matter and visible baryons. In the past few years, a program of new experiments has expanded DM searches far beyond the WIMP paradigm to include new hidden forces and matter. While this program has made impressive progress, there are considerable challenges that must be overcome to fully explore the viable dark sector scenarios over a wide range of mass scales. In this talk, I will discuss on the one hand the unprecedented opportunity to test electroweak scale dark sectors using high-energy experiments, and in particular the high-luminosity LHC. On the other hand, I will present new search strategies to test lighter dark sectors at low-energy high-intensity experiments, with a special emphasis on existing and planned experiments at Fermilab.

Friday, October 27th 2017

12:30 pm:

The central region of Supernovae are one of the hottest and densest regions in the Universe. Due to the high temperatures, particles with masses below hundreds of MeV can be copiously produced if they have non-negligible couplings to the Standard Model. In this talk I will show that in a wide range of dark sector models, the dark matter flux from past Supernovae could be sufficiently large enough to be detected in dark matter direct detection experiments.

Friday, November 3rd 2017

12:30 pm:

In this talk, I study all translationally and rotationally invariant local theories involving massless spin 2 and spin 1 particles that mediate long range forces, allowing for general energy relations and violation of boost invariance. Although gauge invariance is not a priori required to describe non Lorentz invariant theories, I first establish that locality requires a type of `soft gauge invariance'. Then by taking the soft graviton limit in scattering amplitudes, I prove that in addition to the usual requirement of universality of couplings, the special relativistic energy formula is also required and must be exact. I contrast this to the case of theories with only spin 1 particles, where, although one can still derive charge conservation from locality, special relativity can be easily violated. I provide indications that the entire structure of relativity can be built up from spin 2 in this fashion. This talk is based on Phys. Rev. D 96, 084048 (2017) arXiv:1704.05071.

Friday, November 10th 2017

12:30 pm:

A fundamental question in hadron and nuclear physics is how the mass scale for protons and other hadrons emerges from QCD, even in the limit of zero quark mass. I will discuss a new approach to the origin of the QCD mass scale and color confinement based on "light-front holography", a formalism which relates the bound-state amplitudes in the fifth dimension of AdS space to the boost-invariant light-front wavefunctions describing the structure of hadrons in physical space-time. The result is a set of PoincarĂ¨- invariant bound-state wave equations which incorporate quark confinement and successfully predict many observed spectroscopic and dynamical features of hadron physics, such as linear Regge trajectories with identical slope in both the radial quantum number and internal orbital angular momentum. Generalizing this procedure using superconformal algebra, leads to a unified Regge spectroscopy of meson, baryon, and tetraquarks, including remarkable supersymmetric relations between the masses of mesons and baryons of the same parity. The pion, although composite, is massless for zero quark mass, One also can predict hadronic observables such as structure functions, transverse momentum distributions, and the distribution amplitudes defined from the hadronic light-front wavefunctions. The analytic behavior of the QCD coupling controlling quark and gluon interactions at large and small distances is also determined. The result is an effective coupling defined at all momenta with a transition mass scale which sets the interface between perturbative and nonperturbative hadron dynamics. I will also briefly discuss how conformal constraints lead to the elimination of the renormalization scale ambiguity for perturbative QCD calculationsâ€‹

Friday, November 17th 2017

12:30 pm:

Electroweak baryogenesis is an appealing scenario in which the baryon

asymmetry of the universe is generated during the electroweak phase

transition. However, in the standard model it fails because the phase

transition is not first order. And even if it were, the amount of

CP-violation is also not enough to generate the observed asymmetry. In

this talk, I discuss how these problems can be solved in the context

of composite Higgs models. The electroweak phase transition can be

sufficiently strong if it happens simultaneously with the confining

phase transition of the composite Higgs sector. Furthermore, enough

CP-violation for successful electroweak baryogenesis can arise if the

Yukawa couplings vary during the phase transition.

Friday, November 24th 2017

12:30 pm:

Friday, December 1st 2017

12:30 pm:

In this talk I will argue that general relativity may be viewed as a useful limit of quantum mechanics with many degrees of freedom, very much like thermodynamics is a useful limit of classical mechanics with many degrees of freedom. First, I will construct statistical ensembles of ket-vectors using spatially covariant dual field theories with a metric tensor playing the role of a conjugate thermodynamic variable to the so-called information tensor (which is related to both Fisher matrix and Fubini-Study metric). Secondly, I will analyze evolution of the ensembles of ket-vectors to argue that an approximate space-time covariance of the dual field theories can be achieved if certain quantum computational complexities are minimized. And finally, I will show that minimization of a non-equilibrium entropy production can lead to the Einstein-Hilbert dynamics of the metric tensor for a particularly simple and highly symmetric form of the Onsager tensor.

Friday, December 8th 2017

12:30 pm:

Anomalies in the CMB on large angular scales could find an explanation in terms of pre-inflationary physics or intrinsic statistical anisotropies. However, due to cosmic variance it is difficult to conclusively test many of these ideas using the primary cosmic microwave background (CMB) alone. In this talk, I will outline a program to place stringent observational constraints on theories that predict ultra-large scale structure or statistical anisotropies using the Sunyaev Zel'dovich effect and tracers of large-scale structure. These methods will become accessible with next-generation CMB experiments and planned galaxy surveys. I will forecast constraints on novel models in the scalar and tensor sectors. The prospect of better observations makes it important to deliver precise predictions on the theory side. I will end by briefly describing efforts to connect pre-inflationary physics to observation using numerical relativity.

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