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Friday, January 29th 2016

12:30 pm:

Friday, February 5th 2016

12:30 pm:

In this talk we discuss two quantum mechanical problems, the

quartic double-well and the sine-Gordon potentials.

Feynman diagrams in the instanton background are used for the calculation

of the tunneling amplitude (the instanton density) in the three-loop

order. Unlike the two-loop contribution where all involved Feynman

integrals are rational numbers, we will show that in the three-loop case

they can contain irrational contributions as well.

Friday, February 12th 2016

12:30 pm:

Friday, February 19th 2016

12:30 pm:

Twin Higgs (TH) model gives a naturalness motivation to study the non-colored BSM particles, which usually have decay processes relating to the dark-hadronization and displaced signal. In this talk, I will use the exotic twin-quarks to discuss the unique collider phenomenology of the TH models. These quarks play a vital role in UV completing the TH model, and their decay products contain both SM particles and twin-hadrons, including twin-glueballs, mesons, and leptons. These twin-objects decay displacedly into SM b-quarks or leptons, and the striking signal allows the reach of twin UV-physics at the LHC and future collider. Interesting complimentarily constraints between collider and astrophysical search will also be discussed.

Friday, February 26th 2016

12:30 pm:

Friday, March 11th 2016

12:30 pm:

Friday, March 18th 2016

12:30 pm:

Friday, March 25th 2016

12:30 pm:

One of the most concrete implications of the discovery of the Higgs boson is that, in the absence of physics beyond the standard model, the long term fate of our universe can now be established through precision calculations. Are we in a metastable minimum of the Higgs potential or the true minimum? If we are in a metastable vacuum, what is its lifetime? To answer these questions, we need to understand tunneling in quantum field theory. As we delve into how tunneling works, we find many unusual elements: complex quantities which should be real, gauge-dependent quantities which should be physical, an hbar expansion which differs from the ordinary loop expansion, and ultraviolet degrees of freedom that don't decouple. This talk will discuss some of these elements and present some new perspectives on quantum tunneling.

Friday, April 1st 2016

12:30 pm:

In induced electroweak symmetry breaking models, the SM-like Higgs vev is triggered by a small hidden sector electroweak symmetry breaking tadpole. This structure decouples the physical Higgs mass from the quartic term in the Higgs potential, leading to interesting possibilities in UV completions of the Higgs sector as a composite pseudo-Nambu Goldstone boson (PNGB). This framework can allow the minimal ~v^2/f^2 tuning of some models to be evaded, and is particularly appealing in the case of the composite twin Higgs where neutral top partners cut off the leading contributions to the PNGB potential.

Friday, April 8th 2016

12:30 pm:

We overview the pattern of evolution of self-gravitating solitons in the Einstein gravity coupled to matter field. We consider transformation of the matric and links between the Skyrmions and monopoles in flat space-time and Bartnik-McKinnon solitons and hairy black holes. We also briefly discuss corresponding solitons in asymptotycally AdS spacetime

Friday, April 15th 2016

12:30 pm:

Over a huge mass range, from ~keV down to ~10^-22 eV, bosonic dark matter candidates can be described as oscillating classical fields. Through weak couplings to the Standard Model, they can induce in a variety of feint, time-oscillating classical signals. I will discuss ways to search for this dark matter “pulse”, including precision accelerometers, and the “Dark Matter Radio” being built at Stanford.

Friday, April 22nd 2016

12:30 pm:

The operator basis of an effective field theory (EFT) is the set of independent operators which contribute to scattering processes. We embark on the first systematic studies of operator bases, aiming to elucidate the structure underlying what is meant by "independent operators". We show that operator bases are organized by the conformal algebra, allowing us to systematically account for redundancies associated with the use of equations of motion and integration by parts. As a means to study the operator basis, we introduce a partition function defined to count operators weighted by their field content. We provide a matrix integral formula that allows us to compute this partition function. This allows us to solve an outstanding problem in EFTs: determining the number of independent higher dimension operators in a given EFT. This solution is applied to the Standard Model EFT, where we enumerate the operator content up to dimension fifteen. The physical definition and rich structure underlying operator bases is suggestive that more physical information can be pulled from the operator basis, and we give a few speculative thoughts along these lines.

Friday, April 29th 2016

12:30 pm:

Friday, May 6th 2016

12:30 pm:

Friday, September 9th 2016

12:30 pm:

Friday, September 16th 2016

12:30 pm:

We discuss information loss from black hole physics in

AdS_3, focusing on two sharp signatures infecting CFT_2 correlators at

large central charge c: 'forbidden singularities' arising from

Euclidean-time periodicity due to the effective Hawking temperature,

and late-time exponential decay in the Lorentzian region. We study an

infinite class of examples where forbidden singularities can be

resolved by non-perturbative effects at finite c, and we show that the

resolution has certain universal features that also apply in the

general case. Analytically continuing to the Lorentzian regime, we

find that the non-perturbative effects that resolve forbidden

singularities qualitatively change the behavior of correlators at

times t ∼ S_{BH}, the black hole entropy. This may resolve the

exponential decay of correlators at late times in black hole

backgrounds. By Borel resumming the 1/c expansion of exact examples,

we explicitly identify 'information-restoring' effects from heavy

states that should correspond to classical solutions in AdS_3. Our

results suggest a line of inquiry towards a more precise formulation

of the gravitational path integral in AdS_3.

Friday, September 23rd 2016

12:30 pm:

This talk consists of two parts. In the first part, we discuss indices of 5d superconformal field theories (SCFTs) with emphasis on indices of 1d operators. We construct an index for BPS operators supported on a ray in 5d SCFTs with exceptional global symmetries. We compute the E_n representations (for n=2,\dots,7) of operators of low spin, thus verifying that while the expression for the index is only SO(2n-2)\timesU(1) invariant, the index itself exhibits the full E_n symmetry (at least up to the order we expanded). The ray operators we studied in 5d can be viewed as generalizations of operators constructed in a Yang-Mills theory with fundamental matter by attaching an open Wilson line to a quark. For n\le 7, in contrast to local operators, they carry nontrivial charge under the \mathbb{Z}_{9-n}\subset E_n center of the global symmetry. In the second part, we discuss an on going project of bootstrapping 5d SCFTs. We consider the four point function of the 1/2 BPS operators. We discuss the crossing symmetry and the superconformal blocks of the four point function. We present some preliminary results on the bounds of the operator dimensions and OPE coefficients obtained from analyzing the crossing equation numerically.

Friday, September 30th 2016

12:30 pm:

During inflation, a scalar field with a shallow potential such as the Higgs field can develop a large vacuum expectation value (VEV) through quantum fluctuation. The relaxation of the scalar field from this large value to the minimum of its potential after inflation can lead to several interesting consequences in the Universe. In this talk, I will explore a possibility that the baryon number asymmetry of the Universe is generated via the Higgs field relaxation during reheating. I will also show that the same leptogenesis mechanism can be applied to other scalar fields explaining the matter-antimatter asymmetry of the Universe.

Friday, October 7th 2016

12:30 pm:

I will review recent developments in the non-linear theory of massive gravitons, or spin-2 fields. On de Sitter space, there exists a special value for the mass of a graviton for which the linear theory propagates 4 rather than 5 degrees of freedom. If a fully non-linear version of the theory exists and can be coupled to known matter, it would have interesting properties and could solve the cosmological constant problem. I will describe evidence for and obstructions to the existence of such a theory, and recent developments.

Friday, October 14th 2016

12:30 pm:

The observed small neutrino masses are one of the greatest mysteries in current theoretical particle physics. Many possible origins have been proposed so far, such as the see-saw mechanism, radiative corrections, or large extra dimensions. While all these models have been connected in some way to the Higgs condensate, we propose a substantially different mechanism based on nonperturbative gravity: assuming that gravity contains a topological theta-term analogous to the famous theta-term of QCD, we show that a neutrino condensate emerges and effectively generates the small neutrino masses. This neutrino mass generation mechanism implies numerous phenomenological consequences, such as the invalidity of the cosmological neutrino mass bound, enhanced neutrino-neutrino interactions, and neutrino decays.

Friday, October 21st 2016

12:30 pm:

A 6.8 sigma anomaly in the invariant mass distribution of e+e- pairs produced via internal pair creation in 8Be nuclear transitions has been reported recently by Krasznahorkay et al. in Phys. Rev. Lett. 116 (2016) 042501. The data can be explained by a 17 MeV vector gauge boson X produced in the transition of an excited beryllium state to the ground state, 8Be* -> 8Be X, followed by the decay X -> e+e-. We find that the gauge boson X can be associated with a new protophobic fifth force (i.e. with a coupling to protons suppressed compared to its coupling to neutrons) with a characteristic range of 10 fm and milli-charged couplings to first generation quarks and electrons. We show that such a protophobic gauge boson is consistent with all available experimental constraints and we discuss several ways to embed this new particle into an anomaly-free extension of the Standard Model. One of the most appealing theories of this type is a model with gauged baryon number, in which the new gauge boson kinetically mixes with the photon, and provides a portal to the dark matter sector.

Friday, October 28th 2016

12:30 pm:

Three dimensional gauge theories with eight and four supercharges are known to enjoy three-dimensional mirror symmetry — a duality which can be thought of a supersymmetric generalization of particle-vortex duality. In the supersymmetric world theory it interchanges Coulomb and Higgs branches of the two dual theories. Originally 3d mirror symmetry was proposed for certain type of N=2 quiver gauge theories. We shall provide a generalization to a larger class of models which includes more complicated quiver and theories with Chern-Simons terms.

Friday, November 4th 2016

12:30 pm:

A compelling idea in quantum gravity is that gravity is not in fact a fundamental force. Rather, in this view, gravity (and perhaps spacetime itself) is regarded as emerging from the coarse-graining of some as-yet-unidentified microscopic degrees of freedom. I will show that the first and second laws of thermodynamics, applied to the underlying degrees of freedom, give rise precisely to Einstein's equations and the null energy condition, respectively.

Friday, November 11th 2016

12:30 pm:

A fourth generation of chiral quarks and leptons is tightly constrained by LHC data. However, new fermions may exist if their mass is not generated by the Higgs field. Such "vectorlike quarks” are searched at the LHC under the assumption that they decay into a quark and a boson. I will show that other, more exotic decays of the vectorlike quarks may be the dominant ones. Furthermore, new gauge bosons may undergo cascade decays via vectorlike fermions, leading to novel collider signatures.

Friday, November 18th 2016

12:30 pm:

Understanding the nature of dark matter is an open question of central importance to particle physics and cosmology. In this talk, I discuss a model where the dark matter is a fermion charged under a dark U(1) gauge symmetry and its interactions are mediated by a massive dark photon. I will summarize the current status in the search for such a dark sector. The main focus of this talk is on the non-perturbative effects in particular dark matter bound states, which could have strong impact on the interpretation of existing experimental results and leads to new channels for the future search.

Friday, November 25th 2016

12:30 pm:

Friday, December 2nd 2016

12:30 pm:

The discrimination of quark- and gluon-initiated jets is a topic being actively explored by the ATLAS and CMS collaborations, with potential applications to physics searches at the LHC. In this talk, I shall give a brief overview of observables sensitive to quark and gluon jet sub-structure, the current understanding and challenges in predicting them using Monte Carlo tools, and how some of those challenges can be overcome using data-driven templates being derived by the LHC experiments. I shall then illustrate the potential impact of quark-gluon tagging in physics searches, taking the example of gluino pair production at the LHC.

Friday, December 9th 2016

12:30 pm:

In this informal talk I will summarize various projects at the frontier of the conformal bootstrap, including studying 4-point functions of fermions, stress-energy tensors, and chiral multiplets in superconformal theories. The resulting bootstrap constraints both make contact with known theories and also reveal mysterious features that may give the first pieces of evidence for new previously unknown conformal field theories.

Friday, December 16th 2016

12:30 pm:

With the discovery of a Higgs boson, the Standard Model of Particle Physics is complete. However, there is still much to learn about the Standard Model and we may yet expect new physics. This talk will discuss the various ways in which new physics can appear in measurements of processes involving Higgs bosons. There will be a particular focus on double Higgs production. Measurement of this process is important for determining the shape of the scalar potential and the mechanism of electroweak symmetry breaking in the Standard Model. I will give an overview of new physics that can appear in this channel, including resonant production in a simple extension of the Standard Model.

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