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Thursday, January 10th 2019

Friday, January 25th 2019

12:30 pm:

Friday, February 1st 2019

12:30 pm:

I will discuss a class of models in which thermal dark matter is lighter than an MeV. If dark matter thermalizes with the Standard Model below the temperature of neutrino-photon decoupling, constraints from measurements of the effective number of neutrino species are alleviated. This framework motivates new experiments in the direct search for sub-MeV thermal dark matter and light force carriers.

Friday, February 8th 2019

12:30 pm:

The particle that makes up the dark matter of the universe could be an axion. A significant fraction of the axion dark matter could be in the form of bound Bose-Einstein condensates of axions. The condensates are called "axion stars" if they are bound by gravity and "oscillons" if they are self-bound. There may be aspects of axion dark matter in which the effects of axion stars and oscillons have not been fully taken into account.

Friday, February 15th 2019

12:30 pm:

A variety of experimental efforts are currently underway to detect ultra-light dark photon dark matter — a spin-1 particle dark matter candidate with mass below 1 eV. However, dark photon dark matter has a notorious production problem: it is challenging to write down a model that yields the correct relic abundance of non-relativistic dark photons. In this talk I will discuss how dark photon dark matter is created from a network of near-global, Abelian-Higgs cosmic strings. These strings are expected to survive in the universe today, and their motions create a stochastic gravitational wave noise.

Friday, February 22nd 2019

12:30 pm:

Friday, March 1st 2019

12:30 pm:

Halometry---mapping out the spectrum, location, and kinematics of nonluminous structures inside the Galactic halo---can be realized via effects that variable weak gravitational lensing induces on the proper motions of stars and other luminous background sources. Modern astrometric surveys provide unprecedented positional precision along with a leap in the number of cataloged objects. Astrometry thus offers a new and sensitive probe of collapsed dark matter structures over a wide mass range, from one millionth to several million solar masses. It opens up a window into the spectrum of primordial density fluctuations with very small comoving wavenumbers, scales hitherto poorly constrained.

I will outline several detection strategies for dark matter substructure based on time-domain weak gravitational lensing, after summarizing existing techniques and constraints. I will present preliminary results from ongoing analyses based on Gaia's second data release. Finally, I will show that minimal models of axion-like dark matter naturally produce dense small-scale structures which can probed by the aforementioned astrometric lensing techniques.

Friday, March 8th 2019

12:30 pm:

I will present a mechanism of leptogenesis

which is based on the vacuum CP-violating phase transition. This approach

differs from classical thermal leptogenesis as a specific seesaw model, and

its UV completion, need not be specified. The lepton asymmetry is generated

via the dynamically realised coupling of the Weinberg operator during the

phase transition. This

mechanism provides strong connections with low-energy neutrino experiments.

Friday, March 15th 2019

12:30 pm:

Friday, March 22nd 2019

12:30 pm:

Friday, March 29th 2019

12:30 pm:

We consider the quantum partition function for a system of quantum spinors and then derive an equivalent (or dual) classical partition function for some scalar degrees of freedom. The coupling between scalars is non-trivial (e.g. a model on 2-sphere configuration space), but the locality structure of the dual system is preserved, in contrast to the imaginary time formalism. We also show that the measure of integration in the classical partition function can be formally expressed through relativistic Green's functions which suggests a possible mechanism for the emergence of a classical space-time from anti-commutativity of quantum operators.

Friday, April 5th 2019

12:30 pm:

The conventional misalignment mechanism can explain axion dark matter only in a limited mass range and face various difficulties for vector dark matter that we refer to as dark photons. For the former case, we provide a dynamical explanation for an initial condition near the hilltop or the bottom of the potential, which allows for a much wider mass range. Additionally, we propose new dark matter production mechanisms for axions and dark photons via parametric resonance and tachyonic instability, respectively. These ideas expand the parameter space to the regions of interest for the extensive experimental searches.

Friday, April 12th 2019

12:30 pm:

We first review the zero-form/one-form ’t Hooft anomaly matching conditions, using the example of the charge-Q massless Schwinger model. This is the simplest quantum field theory with such anomalies and their manifestation can be seen from different points of view. We will then argue that the Q=2 case and its various generalizations are relevant for the physics of domain walls in four-dimensional super-Yang-Mills theory and adjoint QCD and will show how various aspects of bulk/domain wall interactions found earlier via string theory can be seen in a purely field theoretic setting.

Friday, April 19th 2019

12:30 pm:

I will review the Weak Gravity Conjecture (WGC) and related ideas (such as the Swampland Distance Conjecture and “tower” versions of the WGC), which aim at characterizing generic properties of quantum gravity theories that can be used to diagnose when a low-energy effective field theory cannot be completed into a consistent gravitational theory. As one application, I will explain how arguments based on these conjectures suggest that a theory of quantum gravity requires the photon to be exactly massless.

Friday, April 26th 2019

12:30 pm:

Friday, May 3rd 2019

12:30 pm:

I will present a status report for the study of confining strings. First, I will focus on confining strings in massive adjoint two-dimensional QCD, where analytical progress is most straightforward to achieve. These theories provide an interesting example of interacting relativistic models with infinite (quantum Boltzman) statistics. Then I will describe lessons learnt from this exercise for confining strings in higher dimensions and discuss relations between confining strings, 2d gravity and TTbar deformation.

Friday, May 10th 2019

12:30 pm:

Friday, May 17th 2019

12:30 pm:

Friday, September 6th 2019

12:30 pm:

Friday, September 13th 2019

12:30 pm:

Understanding flux compactifications of string theory is important for both phenomenology and the AdS/CFT correspondence. However, the presence of fluxes greatly complicates both the physics and mathematics of such solutions and not much is known about them in general. In my talk I will give an overview of the work done in an upcoming paper in which we address the question of what properties generic N=1 D=4 Minkowski compactifications have. Such questions are most naturally formulated in the language of exceptional generalised geometry (EGG), where it is known that such solutions are described by an SU(7) structure on the generalised tangent bundle. In my talk I will spend some time introducing the formalism of EGG before describing what an SU(7) structure implies for the geometry. We will find that the structure has properties very reminiscent of complex structures in conventional geometry. Guided with this intuition we will be able to provide a method for analysing the moduli of these solutions in a systematic way, giving explicit examples for G_{2} manifolds, GMPT compactifications and Calabi-Yau manifolds. Moreover, we will see that we can give an explicit expression for the superpotential and Kahler potential of the lower dimensional effective theory in terms of the generalised tensors defining the SU(7) structure.

Friday, September 20th 2019

12:30 pm:

I will show that in many cases spontaneous symmetry breaking leads to the emergence of a higher-form symmetry that has a mixed anomaly with the original symmetry. The converse is also true: these mixed anomalies can be shown to require the existence of a massless particle that non-linearly realizes the symmetry. This perspective leads to an extension of Landau's paradigm for classifying phases that distinguishes for example both sides of the BKT transition and certain fractional quantum hall phases; it also streamlines the hydrodynamic treatment of symmetry-broken phases. I will mostly focus on the case where the group is U(1), but will also discuss certain extensions to non-abelian or discrete groups.

Friday, September 27th 2019

12:30 pm:

I present a new approach for explaining the smallness of the observed cosmological constant. While we do assume a large number of patches with different CC's, in our mechanism the patches with a large CC will be short lived and undergo a crunch after a short time. This mechanism has falsifiable experimental predictions, and also solve the measure problem of the usual anthropic approach.

Friday, October 4th 2019

12:30 pm:

Gravitational probes of Dark Matter (DM) structures on sub-galactic scales constitute an interesting tool to shed light on DM properties in a model-independent way. It has been recently pointed out that variable weak gravitational lensing effects on the motion of background stars can be used to probe non-luminous structures inside the Milky Way halo. I will describe one possible detection strategy that relies on velocity templates, and targets collapsed DM structures in the mass range from million to billion solar masses. The data analysis techniques will be discussed in detail with an application to the second data release of Gaia.

Friday, October 11th 2019

12:30 pm:

Astrophysical observations spanning dwarf galaxies to galaxy clusters indicate that dark matter densities in their central regions are much more diverse than predicted in the prevailing cold dark matter theory. In this talk, I will show that self-interacting dark matter can provide a unified solution to a number of long-standing puzzles in astronomy and astrophysics, including the diverse rotation curves of spiral galaxies, dark matter distributions in Milky-Way satellite galaxies and density cores in galaxy clusters.

Friday, October 18th 2019

12:30 pm:

The Wilsonian paradigm suggests universality of quantum field theory in the infrared. Interestingly, it also suggests universality of quantum mechanics (d=1 quantum field theory) in the ultraviolet. What, then, lies in the landscape of the infrared of quantum mechanics, and what are the ways in which the ultraviolet can be modified? I will focus on a particular deformation of quantum mechanics inspired by the T-Tbar deformation of two-dimensional quantum field theory. In the context of holography, this deformation will correspond to placing Jackiw-Teitelboim gravity in a finite box. I will also present an equivalent description of this deformation in terms of coupling to worldline gravity.

Friday, October 25th 2019

12:30 pm:

In the first part of this talk I will describe how type 1a supernovae (SN) can be used to constrain the interactions of heavy dark matter (DM), which may heat a white dwarf (WD) sufficient to trigger runaway fusion and ignite a SN. Based on the existence of long-lived WDs and the observed supernovae rate, we put new constraints on ultra-heavy DM candidates that produce high energy SM particles in a WD. This rules out supersymmetric Q-ball DM in parameter space complementary to terrestrial bounds. We also constrain DM which is captured by WDs and forms a self-gravitating DM core. Such a core may form a black hole that ignites a supernovae via Hawking radiation, or which causes ignition via a burst of annihilation during gravitational collapse. It is also intriguing that these DM-induced ignition scenarios provide an alternative mechanism of triggering supernovae from sub-Chandrasekhar mass progenitors. In the second part of the talk, I will show how superconducting RF cavities can significantly improve the sensitivity of "light shinning through walls" searches for axion-like particles (ALPs). Our setup uses a gapped toroid to confine a static magnetic field and prevent quenching of the superconductor. Such a search has the potential to probe axion-photon coupling down to g ~ 2 x 10^-11 GeV^-1, comparable to future optical and solar searches.

Friday, November 1st 2019

12:30 pm:

The cosmological collider allows one to measure, through non-analytic correlations of inflationary density perturbations, three parameters of a heavy particle during inflation: spin, coupling, and mass. To date though, the cosmological collider literature focuses solely on models which have these three parameters as independent variables, and there is thus essentially no differentiating power between the different models. We propose the first models where relations can be established between different measurable parameters, and therefore the first that can be conclusively distinguished from other models. These models provide the most minimal signatures of the Standard Model at the cosmological collider depending on a single coupling between Standard Model fermions and the inflaton, making the cosmological collider a new tool to look for physics beyond the Standard Model.

In this talk, I will mainly discuss how these new measurements could suggest the existence of a high-energy second minimum in the Higgs potential. If the Higgs field resided in this new minimum during inflation and was brought back to the electroweak vacuum by thermal corrections during reheating, the Standard Model particles would leave their imprints on the bispectrum. I will focus on the SM fermions, whose dispersion relation can be modified by the coupling to the inflaton, leading to an enhanced particle production during inflation even if their mass during inflation is larger than the Hubble scale. This results in a large non-analytic contribution to non-Gaussianities. Measuring the contributions from two fermions would allow us to compute the ratio of their masses, and to ascribe the origin of the signal to a new Higgs minimum.

I will also touch upon the observable signature in the case of a stable low energy Higgs minimum, where fermion production induces electroweak symmetry breaking dynamically. The production of fermions stops when the Higgs expectation value and hence the fermion masses become too large, suppressing fermion production. The dynamical equilibrium between these processes gives the SM fermions masses that are uniquely determined by their coupling to the inflation, leaving a distinct cosmological collider physics signal.

Friday, November 8th 2019

12:30 pm:

Dark matter structures are expected to exist over a large range of scales, and their properties and distribution can strongly correlate with the underlying particle physics. In this talk, I will describe two separate methods to statistically infer the properties of dark matter substructure using (astrometric)-weak and strong lensing observations, respectively. In the first part of the talk, I will describe how the motion of subhalos in the Milky Way induces a correlated pattern of motions in background celestial objects---known as astrometric weak lensing---and how global signatures of these correlations can be measured using the vector spherical harmonic decomposition formalism. These measurement can be used to statistically infer the nature of substructure, and I will show how this can be practically achieved with future astrometric surveys and/or radio telescopes such as WFIRST and the Square Kilometer Array. Next, I will describe a novel method to disentangle the collective imprint of dark matter substructure on extended arcs in galaxy-galaxy strong lensing systems using likelihood-free (or simulation-based) inference techniques. This method uses neural networks to directly estimate the likelihood ratios associated with population-level parameters characterizing substructure within lensing systems. I will show how this method can provide an efficient and principled way to mine the large sample of strong lenses that will be imaged by future surveys like LSST and Euclid to look for signatures of dark matter substructure. I will emphasize how the statistical inference of substructure using these techniques can be used to stress-test the Cold Dark Matter paradigm and probe alternative scenarios such as scalar field dark matter and enhanced primordial fluctuations.

Friday, November 15th 2019

12:30 pm:

We consider the Standard Model (SM) with the addition of a U(1)′ gauge symmetry and a complete fourth family of quarks and leptons which are vector-like with respect to the full SU(3)C×SU(2)L×U(1)Y×U(1)′ gauge symmetry. The model provides a unified explanation of experimental anomalies in (g−2)μ as well as b→sℓ+ℓ− decays. We find good fits to the deviations from the SM, while at the same time fitting all other SM observables. The model includes a new Z′ gauge boson, a U(1)′-breaking scalar, and vector-like leptons all with mass of order a few 100 GeV. It is consistent with all currently released high energy experimental data, however, it appears imminently testable with well designed future searches. Also precision flavor experiments, especially more accurate direct determinations of CKM matrix elements, would allow to probe the best fit points. This talk is based arXiv:1906.11297 published in PRD and working in progress.

Friday, November 22nd 2019

12:30 pm:

Heavy neutral leptons are a well motivated extension of the Standard Model and offer a generic portal to hidden sectors. In this talk, I will discuss the latest ideas in the search for heavy neutrinos with potential exotic partners (e.g., hidden vector or scalars). These non-minimal scenarios have been proposed to explain longstanding experimental anomalies, such as the LSND and MiniBooNE results, and have a rich phenomenology at short baselines. I will show that many of these models are in fact ruled out by current data, and highlight new ways to test the remaining allowed cases with near-future experiments such as MicroBooNE.

Friday, November 29th 2019

12:30 pm:

Friday, December 6th 2019

12:30 pm:

I present a dark sector scenario based on a slice of 5d AdS space. Such a construction, also motivated as the AdS dual of a composite dark sector, naturally leads to models in which parts of the dark sector vanish at high 4d momentum. This AdS "opacity" arises from the dressing of propagators by bulk interactions. Exotic dark sector signatures are possible from laboratory to cosmological scales, including long-range forces with non-integer behaviour, non-standard momentum losses, and periodic signals at colliders.

Friday, December 13th 2019

12:30 pm:

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