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Fall 2015

Seminars take place on Wednesdays 12:00-1:00pm in Randall Laboratory #3481
Organizer: Bibhushan ShakyaUri Kol

Achilleas Porfyriadis (Harvard): " Kerr/CFT in the sky" |  Sep 16 

Astronomical observation suggests the existence of near-extreme Kerr black holes in the sky. Properties of diffeomorphisms imply that dynamics of the high-redshift near-horizon region of near-extreme Kerr are governed by an infinite-dimensional conformal symmetry. This symmetry may be exploited to analytically compute a variety of potentially observable processes. In this talk I will show how we compute the gravitational radiation emitted by a small compact object that orbits in the near-horizon region and plunges into the horizon of such a near-extreme black hole.

Slides

 Jason Evans (U Minnesota): "Pure Gravity Mediation" |  Sep 23

Because the Higgs mass is rather large for weak scale supersymmetry breaking, it is likely that the soft masses of the MSSM are well beyond the weak scale.  A rather well motivated way of explaining this is found in models such as pure gravity mediation.  The base component of pure gravity mediation is a non singlet which breaks supersymmetry.  This forbids gravity mediated gaugino masses and the dominant contribution to the gaugino masses comes from anomalies.    As it turns out, the simplest model of pure gravity mediation, with universal soft masses, can meet all experimental constraints. In this seminar, I will review the simplest model or pure gravity mediation and then discuss some interesting variations. The first variation I will discuss is non-universal soft Higgs masses. Non-universal Higgs soft masses opens up the parameter space significantly because the constraints on tan beta are relaxed.  Also, if non-universal Higgs soft masses are allowed, pure gravity mediation can be made consistent with a Peccei-Quinn symmetry which can explain the size of theta QCD. The axion of this Peccei-Quinn symmetry can be the dark matter as well. Since pure gravity mediation models often give too little dark matter this may explain this short coming of the model. Another way to realize a viable dark matter candidate in these models is through the addition of  vector states.  These additional vector states will modify the gaugino masses in such away that the bino can be the LSP and can co-annihilate with other guaginos reducing the dark matter density to acceptable levels. These vectors states also generate a non-universality of the Higgs soft masses through running relaxing the constraints on tan beta even if the Higgs soft masses are universal at the GUT scale.  Lastly, I will discuss proton decay in pure gravity mediation with non-universal Higgs soft masses.  Because non-universal Higgs soft masses allow the other soft masses to be pushed lower, the proton decay of these models for a minimal SU(5) GUT could have a proton lifetime  just beyond the reach of planned experiments.

Slides

Thomas Dumitrescu (Harvard): "Anomalies, Renormalization Group Flows, and the a-Theorem in Six Dimensions" |  Sep 30

The a-Theorem is a general constraint on unitary RG flows in even dimensions. I will describe the status of the a-Theorem in six dimensions, and prove it for a large class of supersymmetric flows. I will then explain how to compute the a-anomaly in six-dimensional SCFTs by relating it to the R-symmetry and gravitational anomalies of these theories. This leads to additional checks of the a-Theorem, and an interesting puzzle for theories with free gauge fields in the deep IR.

Slides

Eder Izaguirre (Perimeter): "The vector portal: a window to a dark sector" |  Oct 7

An emerging paradigm in particle physics is the possibility that new matter resides in its own sector — a Dark Sector (DS) —  connected to the Standard Model via a portal. In this talk I will focus on a well-motivated example of such a scenario: the vector portal. I will discuss two distinct phases of the theory. In one, matter in the DS is uncharged under the known forces and is a viable candidate for light Dark Matter. In the other phase of this framework, matter in the DS can instead acquire a non-quantized electromagnetic charge. Finally, I describe proposals for new small-scale experiments to sharply test the different phases of the vector portal. 

Slides

Chris Beem (IAS): "Algebraic structures in superconformal field theories" |  Oct 14

The bootstrap approach to conformal field theory adopts an algebraic framework based on the operator product expansion to study strongly interacting CFTs. In general this algebraic structure is not analytically tractable. In contrast, in theories with a sufficient amount of supersymmetry, there turn out to be operator subalgebras that are quite tractable. From these one may determine a large amount of information about the spectrum of supersymmetric operators and their three point couplings. I will describe the situation for superconformal field theories in three, four, and six dimensions.

Christoph Uhlemann (U Washington): "Supersymmetric flavors on curved space and a precision test of AdS/CFT" |  Oct 21

Quantum field theory on curved space has long been studied for its interesting phenomenology, and more recently also as a means to obtain non-perturbative results in supersymmetric theories. In this talk I will describe the holographic dual for N=4 SYM coupled to massive N=2 flavors on spaces of constant curvature. I will discuss a topology-changing phase transition on S^4 and confront holographic computations with exact field theory results obtained using supersymmetric localization.

Slides

John Kehayias (Vanderbilt): "The Old Made New: Nonlinear Sigma Models" |  Oct 28

This talk will give an overview of some new results on an old topic we are all familiar with: the good ol’ nonlinear sigma model (NLSM). The structure (as a group or topology) of the nonlinearly realized symmetry leads to charge quantization of a U(1) (i.e. hypercharge) without a monopole or Grand Unified Theory. We can extend this result to larger groups and find relations between the quantum numbers and a nontrivial quantization condition (e.g. relating charge and SU(2) representations). There is rich phenomenology and model building which can be done. Studying NLSMs in supergravity necessarily leads to flat directions and light fields. This is an ideal setting for models of high scale inflation, the Higgs, or an axion.

Bert Vercnocke (University of Amsterdam): "Anti-branes, fluxes and black holes" |  Nov 4

This talk will be about non-supersymmetric solutions of string theory. I will start with reviewing the most recent activity on anti-branes in flux backgrounds and string vacua. Then I will show the strong analogy with non-supersymmetric black holes and their microstates. I will expand on the fruit we can get from this cross-polination.

arXiv references: 1309.0692, 1312.6889, 1411.3720

Slides

Eric Kuflik (Cornell): "Displaced Physics at the LHC: R-Parity Violation and Displaced Higgs" |  Nov 11

I will discuss two theoretical motivations for displaced objects at the LHC: supersymmetric models with R-parity violation and models of neutral naturalness. Leading experimental constraints on supersymmetric models with R-parity violation (RPV) and a long-lived lightest superpartner (LSP) will be presented as well as constraints on a displaced Higgs boson. I will discuss the strengths and limitations of LHC Run I displaced objects searches and projections for LHC Run II.

Slides

Prashant Saraswat (Maryland/ Johns Hopkins): "Towards a No-Lose Theorem for Naturalness" |  Nov 18

We derive a phenomenological "no-lose theorem" for naturalness up to the TeV scale, which applies when quantum corrections to the Higgs mass are canceled by perturbative BSM particles (top partners) due to to some symmetry. In particular we focus on neutral top partner scenarios, including but not limited to the Twin Higgs model. We classify all perturbative neutral top partner structures in order to compute their irreducible low-energy signatures at proposed future lepton and hadron colliders, as well as the irreducible tunings suffered in each scenario. Central to our theorem is the assumption that SM-charged BSM states appear in the UV completion of neutral naturalness, which is the case in all known examples. We find that proposed future colliders probe any such scenario of naturalness with tuning of 10% or better unless the top partner multiplicity is very high. This provides strong model-independent motivation for both new lepton and hadron colliders, which in tandem act as discovery machines for general naturalness. Realizing a concrete scenario which avoids our arguments while still lacking experimental signatures remains an open model-building challenge.

Slides

William Witczak-Krempa (Harvard): "Universal corner entanglement in CFTs " |  Dec 2

The structure of entanglement can yield new physical insights into many-body quantum systems. Here, I’ll describe key properties of the entanglement entropy of CFTs in 2+1d. In particular, we’ll see that sharp corners in the entangling surface contribute a regulator-independent function that depends non-trivially on the corner opening angle. I’ll argue that in the smooth limit this function yields the 2-point function of the stress tensor. This sheds light on recent cutting edge simulations of the quantum critical Ising, XY and Heisenberg models. I’ll also present a new lower bound for this function. I will then generalize to Rényi entropies, which yields a simple procedure to extract the thermal entropy using corner entanglement of the groundstate alone. Connections will be made to CFTs in 1+1d and 3+1d, as well as to Lifshitz theories.

Slides

 Asher Berlin (U Chicago): "New Directions in Dark Matter Direct Detection" |  Dec 9

Most models of WIMPs that possess sizable tree-level interactions with the electroweak bosons of the Standard Model are under ever increasing pressure from the relentless Moore's law like progression of today's direct detection experiments. This has led many to consider various scenarios in which the WIMP-nucleon scattering is suppressed at leading order. While much progress has been done in this regard, there remain many interesting avenues to explore. In this talk, I will summarize ongoing work in this direction within the context of the MSSM and other frameworks and discuss the viability to observe/rule-out a few of these types of models.

Slides