High Energy Theory Lunchtime Seminar

semester, 2015

Thursday, January 22nd 2015
12:15 pm:
No Seminar

Thursday, January 29th 2015
Speaker: James Unwin (Notre Dame U.)
Subject: High Scale Supersymmetry and F-theory GUTs

In the absence of New Physics at the TeV scale, GUTs still provide a
good motivation for supersymmetry at higher scales. Notably, it is
typically non-trivial to UV complete GUTs into string theory, but one
promising possibility is found in F-theory. I shall argue, therefore,
that considerations from string theory should play a central role in
the construction of such models of high scale supersymmetry.
Specifically, F-theory GUTs lead to calculable UV threshold
corrections to the running of the gauge couplings, which can in
principle improve the precision of unification. I will examine the
prospect of precision unification in models of F-theory High Scale
Supersymmetry and the experimental constraints on this model
from the non-observation of proton decay. Further, I will discuss
to what extent the proton lifetime can be extending due to the
localization of X,Y gauge bosons in higher dimensions.

Thursday, February 5th 2015
Speaker: Simon Knapen (UC Berkeley / LBL)
Subject: The Orbifold Higgs

We introduce and systematically study an expansive class of "orbifold Higgs" theories in which the weak scale is protected by accidental symmetries arising from the orbifold reduction of continuous symmetries. The protection mechanism eliminates quadratic sensitivity of the Higgs mass to higher scales at one loop (or more) and does not involve any new states charged under the Standard Model.

Thursday, February 12th 2015
Speaker: Yang Bai (U. Wisconsin)
Subject: Towards Understanding IceCube High Energy Neutrinos

The IceCube collaboration has recently observed high energy neutrinos in the 30 TeV to 2 PeV range. The flux is much above the atmospheric neutrino background and requires new sources to explain its origin. In this talk, I will provide a list of potential explanations with a focus on decaying dark matter and point-like sources.

Thursday, February 19th 2015
Speaker: Scott Davies, UCLA
Subject: Enhanced Cancellations and Ultraviolet Finiteness in Supergravity

In recent years, there has been much progress in understanding the ultraviolet properties of supergravity amplitudes. Through symmetry arguments and direct computations, supergravity has been found to be much tamer in the ultraviolet than previously thought. In this talk, I will discuss a new type of cancellation known as enhanced cancellation that renders a supergravity amplitude finite. Such cancellations cannot be understood through any symmetry argument based on a local covariant diagrammatic formalism. I will provide several examples where we observe enhanced cancellations, showing finiteness in supergravity amplitudes where naively divergence was expected. Understanding these enhanced cancellations through a means other than direct computation remains an open problem.

Thursday, February 26th 2015
12:15 pm:
Speaker: Jesse Thaler (MIT)
Subject: What is Sudakov Safety?

For almost four decades, infrared and collinear (IRC) safety has been the guiding principle for determining which jet observables can be calculated using perturbative QCD. Now in the LHC era, new jet substructure observables have emerged which are IRC unsafe, yet still calculable using perturbative techniques. In this talk, I explain the origin of these "Sudakov safe" observables and show how they blur the boundary between perturbative and nonperturbative aspects of QCD.

Thursday, March 5th 2015
Speaker: Marco Bochicchio (INFN, Rome)
Subject: An asymptotic solution of large-N QCD, and of large-N n=1 SUSY YM

We find an asymptotic solution for two-, three- and multi-point correlators of local gauge-invariant operators, in a lower-spin sector of massless large-N QCD (and of n=1 SUSY YM), in terms of glueball and meson propagators, in such a way that the solution is asymptotic in the ultraviolet to renormalization-group improved perturbation theory, by means of a new purely field-theoretical technique that we call the asymptotically-free bootstrap, based on a recently-proved asymptotic structure theorem for two-point correlators. The asymptotically-free bootstrap provides as well asymptotic S-matrix amplitudes in terms of glueball and meson propagators. Remarkably, the asymptotic S-matrix depends only on the unknown particle spectrum, but not on the anomalous dimensions, as a consequence of the LSZ reduction formulae. Very many physics consequences follow, both practically and theoretically. In fact, the symptotic solution sets the strongest constraints on any actual solution of large-N QCD (and of n=1 SUSY YM), and in particular on any string solution.

Thursday, March 12th 2015
Speaker: Marc Sher, College of William & Mary
Subject: The Instability of the Standard Model Vacuum

With the discovery of the Higgs boson in 2012, the basic parameters of the Standard Model are known. At large scales of O(10^11 GeV), the quartic coupling constant of the Higgs potential becomes negative, leading to the Standard Model vacuum being metastable. Calculations of the decay rate show that the present vacuum will live much longer than the age of the Universe. I will review the features of the effective Higgs potential that lead to the instability and describe how to calculate the decay rate. I will then show that Planck scale operators, which one might naively expect to be negligible given that the instability scale is of factor of 10^8 below the Planck scale, in fact can have dramatic changes on the decay rate.

Thursday, March 19th 2015
Speaker: No Seminar, SPRING BREAK

Thursday, March 26th 2015
12:15 pm:
Speaker: Clay C√≥rdova (Harvard)
Subject: TBA

Thursday, April 2nd 2015
12:15 pm:
Speaker: Chris Brust (Perimeter Institute)
Subject: Resurgent SUSY, Accidental SUSY and String Theory

I will discuss the mechanism of accidental SUSY, the notion of having non-supersymmetric RG flows ending on supersymmetric fixed points, from the point of view of QFT, RS and string theory. I will also discuss the mechanism's utility in BSM model building, specifically as a way of UV-completing natural SUSY in the model of "resurgent SUSY", solving both the little and big hierarchy problems. I will argue that a Z2 orbifold of the Klebanov-Strassler solution exhibits accidental SUSY, and furthermore is a prototype for the strong sector that should be present in resurgent SUSY.

Thursday, April 9th 2015
12:15 pm:
Speaker: David McGady (Princeton)
Subject: Constructing massless S-matrices

I will discuss several aspects of modern treatments of the on-shell S-matrix. In particular, I will give an inductive proof of on-shell recursion relations --- due to Britto, Cachazo, Feng, and Witten (BCFW) --- to graviton amplitudes in four dimensions. Unlike previous proofs of applicability in the literature, this proof relies exclusively on on-shell reasoning. As a corollary of this analysis, we show that the formerly mysterious ``bonus'' scaling of graviton amplitudes under large BCFW-shifts follows simply from Bose symmetry. I will also discuss a way to extract many beautiful, classic, no-go theorems from straightforward analysis of four-point scattering amplitudes.

Thursday, April 16th 2015
12:15 pm:
Speaker: Francis-Yan Cyr-Racine (Caltech/JPL)
Subject: A Particle Physicist's Nightmare: What If Dark Matter is Truly Dark?

Despite being ubiquitous throughout the Universe, the fundamental physics governing dark matter remains a mystery. Current dark matter searches implicitly assume that dark matter couples at some level to ordinary baryonic matter via Standard Model interactions. However, it remains possible that dark matter interacts so weakly with ordinary matter that it will escape detection with current and near-future technology. This particle physicist's nightmare forces us to consider other possible probes of dark matter physics. For instance, the evolution of cosmological density fluctuations on small causal length scales in the early epochs of the Universe is a very sensitive probe of the fundamental physics of dark matter. Studying the astrophysical structures that resulted from the gravitational collapse of fluctuations on these small scales can thus yield important clues about physical processes that took place in the dark sector in the early Universe. Today, most of these structures are locked-in deep inside the potential wells of massive galaxies, making the study of their properties difficult. Fortunately, due to fortuitous alignments between high-redshift bright sources and us, some of these galaxies act as spectacular strong gravitational lenses, allowing us to probe their inner structures. In this talk, we present a new statistical analysis formalism to extract information about mass substructures inside lens galaxies and discuss what this can tell us about dark matter physics.

Thursday, April 23rd 2015
12:15 pm:
Speaker: Natsumi Nagata (Minnesota)
Subject: Direct search for electroweak-charged dark matter

We discuss a class of fermionic dark matter candidates which are charged under
the SU(2)_L gauge interactions, and evaluate their scattering cross section
with a nucleon, which is an important quantity for dark matter direct
detection experiments. Such a fermionic dark matter particle interacts with
quarks and gluons through one- and two-loop processes, respectively.
We will find that the resultant scattering cross sections lie around O(10^{-(46-48)}) cm^2, which exceed the neutrino background and thus can be reached in
future direct detection experiments.

Thursday, April 30th 2015
12:15 pm:
Speaker: Erich Poppitz (Toronto)
Subject: Confining strings on R^3xS^1

I will begin with a brief and qualitative (but self-contained) overview of the semiclassically calculable small-S^1 confining dynamics in deformed Yang-Mills theory and QCD(adj) (including N=1 super Yang-Mills). I will then study confining strings in these theories, contrasting their properties with those of similar theories exhibiting abelian and nonabelian confining regimes, and will discuss unsolved problems for the future.

Thursday, May 7th 2015
12:15 pm:
Speaker: Surjeet Rajendran (UC Berkeley)
Subject: Cosmological Relaxation of the Electroweak Vacuum

A new class of solutions to the electroweak hierarchy problem is presented that does not require either weak scale dynamics or anthropics. Dynamical evolution during the early universe drives the Higgs mass to a value much smaller than the cutoff. The simplest model has the particle content of the standard model plus a QCD axion and an inflation sector. The highest cutoff achieved in any technically natural model is 10^8 GeV

Friday, September 11th 2015
There will be no seminar this week.

Friday, September 18th 2015
Speaker: John Terning, U. California, Davis
Subject: Experimental Probes of Vacuum Energy

Friday, September 25th 2015
Speaker: Richard Holman (CMU)
Subject: Open Effective Theories and Stochastic Inflation

What is the correct description of the effective theory for long wavelength modes in inflation? I'll argue that the standard EFT tools do not apply here and that a new description based on open effective theories is required. I'll discuss how this is related to the Stochastic Inflation program and how decoherence takes place in inflation.

Friday, October 2nd 2015
Speaker: Lance Dixon, SLAC/Stanford
Subject: Bootstrapping Amplitudes and Wilson Loops in Planar N=4 Super-Yang-Mills Theory

Friday, October 9th 2015
Speaker: Matthew Walters (BU)
Subject: Black Hole Thermality from Conformal Field Theory

I will discuss recent work studying universal properties of gravity in anti-de Sitter (AdS) spacetime from the perspective of conformal field theory (CFT). Using the AdS/CFT correspondence, scattering with black holes can be rephrased in terms of correlation functions involving operators with large scaling dimension. I will present new methods for calculating these correlation functions in 2d CFTs which make the connection to black holes and gravity manifest. In particular, I will show that operators with large scaling dimension create a thermal background with the correct Hawking temperature, with interactions that can be described using new on-shell diagrams. I will then discuss the connection with eigenstate thermalization and the possibility of generalizing these results to theories in higher dimensions.

Friday, October 16th 2015
Speaker: David Curtin (Maryland)
Subject: Towards a No-Lose Theorem for Naturalness

Theories of neutral naturalness can realize untuned solutions to the hierarchy problem while avoiding LHC current exclusion bounds on colored top partners. Their unusual signatures demonstrate the range of phenomena that can connected to stabilizing the electroweak scale, motivating searches for displaced vertices, exotic Higgs decays and emerging jets. I will give an overview of how to constrain these theories experimentally, and present recent work toward deriving a phenomenological model-independent 'no-lose theorem' for perturbative solutions to the hierarchy problem which rely on top partners, including neutral naturalness. This lends strong model-independent motivation to build both proposed future lepton and 100 TeV colliders, since both are needed as discovery machines of general naturalness.

Friday, October 23rd 2015
Speaker: Jessie Shelton, U. Illinois, Urbana-Champaign
Subject: Boosting Dark Matter Indirect Detection Signals with Black Holes

Friday, October 30th 2015
Speaker: Yonatan Kahn (Princeton)
Subject: The Goldstone and Goldstino of Supersymmetric Inflation

I construct an effective field theory (EFT) for the minimal degrees of freedom of supersymmetric inflation. These can be viewed as the goldstone of spontaneously broken time translations, and the goldstino of spontaneously broken SUSY, both of which are tied together in an interesting way through the structure of the SUSY algebra. I will outline some phenomenological consequences of the leading-order Lagrangian, including a modified goldstino/gravitino dispersion relation and a time-dependent gravitino mass phase. Finally, I will describe schematically the possible contributions of goldstino loops to inflationary correlators.

Friday, November 6th 2015
Speaker: Joe Bramante (Notre Dame)
Subject: Dark Matter Ignition of Type Ia Supernovae

Recent studies of low-redshift type Ia supernovae indicate that half explode from sub-Chandrasekhar mass progenitor white dwarfs. This talk explains how PeV mass asymmetric dark matter would ignite sub-Chandrasekhar white dwarfs, and form star-destroying black holes in old galactic center pulsars.

Friday, November 13th 2015
Speaker: James Wells, University of Michigan
Subject: Precision Analysis after the Higgs boson discovery

After the Higgs boson discovery we are able to refocus our efforts on precision analysis in search of new physics contributions now that the Standard Model and its parameters are known well. Some questions I will address concern self-consistent precision analysis frameworks, new physics expectations for deviations, and comparisons with experimental sensitivities for currently running and future facilities.

Friday, November 20th 2015
Speaker: Seyda Ipek, Fermilab
Subject: Baryogenesis via pseudo-Dirac Fermion Oscillations

In order to explain the baryon asymmetry of the Universe, we need extra CP violation, and an out-of-equilibrium process. Supersymmetric theories with a global U(1) symmetry solve the SUSY flavor and CP problems. They also have pseudo-Dirac gauginos with particle--antiparticle oscillations. If the gauginos decay out-of-equilibrium, and if there is CP violation in the oscillations, can they produce the baryon asymmetry? You should come and learn!

Friday, November 27th 2015
There will be no seminar this week: University closed for Thanksgiving

Friday, December 4th 2015
Speaker: Francesco D'Eramo (UC Santa Cruz)
Subject: Freeze-In Dark Matter with Displaced Signatures at Colliders

Freeze-in is a general and calculable mechanism for dark matter production in the early universe. Assuming a standard cosmological history, such a framework predicts metastable particles with a lifetime generically too long to observe their decays at colliders. In this talk, I will consider alternative cosmologies with an early matter dominated epoch, and I will show how the observed abundance of dark matter is reproduced only for shorter lifetimes of the metastable particles. Famous realization for such a cosmology are moduli decays in SUSY theories and inflationary reheating. Remarkably, for a large region of the parameter space the decay lengths are in the displaced vertex range and they can be observable at present and future colliders. I will conclude with an example of DFSZ SUSY theories where this framework is realized.

Friday, December 11th 2015
Speaker: Gilly Elor (MIT)
Subject: Hidden Sector Dark Matter: The Galactic Center Gamma-Ray Excess and Indirect Detection Constraints.

If dark matter is embedded in a non-trivial hidden sector, it may annihilate and decay to lighter hidden sector states which subsequently decay to Standard Model particles. While remaining agnostic to the details of the hidden sector model, our framework - with annihilations followed by cascading hidden sector decays - captures the generic broadening of the spectrum of secondary particles (photons, neutrinos, electron-positrons, and antiprotons) relative to the case of dark matter annihilating directly to Standard Model particles. I will detail how such scenarios can explain the apparent excess in GeV gamma-rays identified in the central Milky Way, while evading bounds from direct detection experiments. Additionally I will describe how indirect constraints on dark matter annihilation limit the parameter space for such cascade/multi-particle decays. In particular I will describe an investigation of the limits from the cosmic microwave background by Plank, the Fermi measurements of photons from the Milky Way Dwarf Spheroidal Galaxies, and positron data from AMS-02. Generally the bound from the Fermi dwarfs is the most constraining for annihilations to photon-rich final states, while AMS-02 is most constraining for electron and muon final states; however in certain instances the CMB bounds overtake both, due to their approximate independence of the details of the hidden sector cascade.

Friday, December 18th 2015
There will be no seminar this week.

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