Fall 2015: organized by Tzu-Chieh Wei, Federico Bonetti, and Sam McDermott

12/10 Thur 2:30pm Jonathan Heckman (UNC at Chapel Hill)

Title: The Big List of Little Strings

12/7 Mon 1:00pm Alexander Turbiner (Simons Center)

Title: 3-body quantum Coulomb problem: where we are

Abstrract:
Current status of 3-body Coulomb problem (negative hydrogen ion
H-,helium atom, lithium ion, H2+ ion etc) will be described.Major
emphasis will be given to a question of stability vs Coulomb charge and
to analytic structure of the ground state energy. The existence of two
critical charges with associated square-root and essential
singularities, respectively, is predicted. The 2nd excited,
weakly-bound state of H- is predicted as a result.

12/3 Thur 2:30pm Po-Yao Chang (Rutgers)

Title: Surface and vortex states in topological nodal superconductors

Besides
topological band insulators, which have a full bulk gap, there are also
gapless phases of matter that belong to the broad class of topological
materials, such as topological semimetals and nodal superconductors.We
study examples of gapless topological phases, focusing in particular on
nodal superconductors, such as nodal noncentrosymmetric superconductors
(NCSs). We compute the surface density of states of nodal NCSs and
interpret experimental measurements of surface states. In addition, we
investigate Majorana vortex-bound states in both nodal and fully gapped
NCSs using numerical and analytical methods. We show that different
topological properties of the bulk Bogoliubov-quasiparticle wave
functions reflect themselves in different types of zero-energy
vortex-bound states. In particular, in the case of NCSs with tetragonal
point-group symmetry, we find that the stability of these Majorana zero
modes is guaranteed by a combination of reflection, time-reversal, and
particle-hole symmetries. Finally, by using a dimensional reduction
procedure from higher-dimensional topological insulators and
superconductors, we derive a classification of topologically stable
Fermi surfaces in semimetals and nodal lines in superconductors.

12/2 Wed 2:30pm Simeon Hellerman (Tokyo U., IPMU)

Title: Quantum Information Theory of the Gravitational Anomaly

Abstract:
Recent papers have appeared describing certain peculiarities of
measures of quantum-mechanical entanglement in local quantum field
theories with nonvanishing gravitational anomaly. We show that
these peculiarities follow from the fact that the Hilbert space of a
gravitationally anomalous QFT simply does not ever tensor factorize at
all into Hilbert spaces in complementary regions, and thus the defining
assumption on which the concept of quantum entanglement is based,
always fails in gravitationally anomalous theories. We show the
breakdown of all known definitions of entanglement when the
gravitationally anomaly is present, and give a general proof that no
such definition exists, based on a series of tight logical connections
between tensor factorizations, boundary conditions, and the
gravitational anomaly.

11/23 Mon 2:00pm Lorenzo Di Pietro (Weizmann)

Title: “Universal Properties of Cylinder Partition Functions”

11/18 Wed 2:30pm Joint HET/RIKEN/YITP Seminar

David Shih (Rutgers)

Title: "Natural SUSY Today"

11/11 Thur 2:30pm Matt Low (IAS)

Title: Composite Spin-One Resonances at the LHC

11/4 Thur 2:30pm Ramis Movassagh (IBM)

Title: A counterexample to the area law for quantum matter

Abstract:

Entanglement
is a quantum correlation which does not appear classically, and it
serves as a resource for quantum technologies such as quantum
computing. The area law says that the amount of entanglement between a
subsystem and the rest of the system is proportional to the area of the
boundary of the subsystem and not its volume. A system that obeys an
area law can be simulated more efficiently than an arbitrary quantum
system, and an area lawprovides useful information about the low-energy
physics of the system. It was widely believed that the area law could
not be violated by more than a logarithmic factor (e.g. based on
critical systems and ideas from conformal field theory) in the system’s
size. We introduce a class of exactly solvable one-dimensional models
which we can prove have exponentially more entanglement than previously
expected, and violate the area law by a square root factor. We
also prove that the gap closes as n^{-c}, where c \ge 2, which rules
out conformal field theories as the continuum limit of these models. It
is our hope that the mathematical techniques introduce herein will be
of use for solving other problems.

(Joint work with Peter Shor).

References:

Phys. Rev. Lett. 109, 207202

http://arxiv.org/abs/1408.1657

11/3 Wed 2:30pm Matteo Baggioli (Barcelona IFAE)

Title: "Solid applications of massive gravity"

11/2 Mon 2:00pm Dhritiman Nandan (Humboldt University)

Title: Hidden structures in form factors of N+4 sYM

10/29 Thur 2:30pm Prateek Agrawal (Harvad)

Title: The Cosmological Constant Problem in Scalar Gravity

10/22 Thur 2:30pm Juven Wang (Harvard/IAS)

Title: Geometric phase, string braiding statistics and spacetime surgery

Abstract:

Topological
invariances are useful tools to classify and characterize distinct
states of condensed matter. Berry phase is one of such
invariances, which can be generalized to the non-Abelian geometric
matrix when energy eigenstates degenerate. For topological
orders, specifically the many-body quantum states with bulk energy gap
and with topology-dependent ground state degeneracy, the geometric
matrix encodes the braiding statistics of energetic excitations. Apart
from Berry phase, we also use the wave function overlap to derive the
geometric matrix. We study string and particle statistics of 2+1 and
3+1 spacetime dimensions. We utilize aspects of both quantum topology
and spacetime topology to derive new formulas analogous to Verlinde’s
via the spacetime surgery. This provides new insights for higher
dimensional topological states of matter.

If there is extra time
left, I will give a brief survey on the similar notion of geometric
matrix for symmetry-protected topological states (SPTs), the SPT
invariants and its field theory representation.

10/21 Wed 2:30pm Olaf Hohm (MIT)

Title: Introduction into Exceptional Field Theory

10/14 Wed 2:30pm Simone Giombi (Princeton U)

Title: Sphere Free Energy and the epsilon Expansion

Abstract:
I will discuss the dimensional continuation of the sphere free energy
in conformal field theory. This provides a smooth interpolation between
a-anomaly coefficients in even dimensions and F-values in odd
dimensions, and suggests a generalization of the existing a-theorems
and F-theorems to continuous dimensions. As concrete applications, I
will explain how to use the Wilson-Fisher epsilon expansion to find the
value of F for the 3d Ising CFT and related O(N) symmetric models, and
for the conformal phase of the 3d QED with N_f massless fermions.

10/12 Mon 2:30pm Roman Orus (U Mainz)---special seminar

Title: Entanglement, tensor networks, and topological quantum order

Abstract:
Topological order in a 2d quantum matter can be determined by the
topological contribution to the entanglement Renyi entropies. However,
when close to a quantum phase transition, its calculation becomes
cumbersome. In this talk I will show how topological phase transitions
in 2d systems can be much better assessed by multipartite entanglement,
as measured by the topological geometric entanglement of blocks.
Specifically, I will present an efficient tensor network algorithm
based on Projected Entangled Pair States (PEPS) to compute this
quantity for a torus partitioned into cylinders, and then use this
method to find sharp evidence of topological phase transitions in 2d
systems with a string-tension perturbation. When compared to tensor
network methods for Renyi entropies, this approach produces almost
perfect accuracies close to criticality and, on top, is orders of
magnitude faster. Moreover, I will show how the method also allows the
identification of Minimally Entangled States (MES), thus providing a
very efficient and accurate way of extracting the full topological
information of a 2d quantum lattice model from the multipartite
entanglement structure of its ground states. If time allows I will also
present briefly other ongoing projects at our group involving the use
of tensor networks to study large-spin Kagome quantum antiferromagnets,
1d symmetry-protected topological order, continuous unitary
transformations, and (1+1)d lattice gauge theories.

10/8 Thur 2:30pm Travis Maxfield (U Chicago)

Title: A Landscape of Field Theories

Abstract:
Studying a quantum field theory involves a choice of space-time
manifold and a choice of background for any global symmetries of the
theory. We argue that many more choices are possible when specifying
the background. In the context of branes in string theory, the
additional data corresponds to a choice of supergravity tensor fluxes.
We propose the existence of a landscape of field theory backgrounds,
characterized by the space-time metric, global symmetry background and
a choice of tensor fluxes. As evidence for this landscape, we study the
supersymmetric six- dimensional (2, 0) theory compactified to two
dimensions. Different choices of metric and flux give rise to distinct
two-dimensional theories, which can preserve differing amounts of
supersymmetry.

10/1 Thur 2:30pm Yiming Zong (YITP)

Title: Gravothermal evolution of galactic dark matter halos with velocity-dependent self-interactions

Abstract
:

9/30 Wed 2:30pm David Cosower (IAS & Saclay)

Title: Cross-Order Relations from Maximal Unitarity

Abstract:

9/24 Thur 2:30pm Alexandros Anastasiou (Imperial College)

Title: Gravity as the square of Yang-Mills

Abstract
: A recurring theme in the attempts of understanding the quantum theory
of gravity is the idea of “Gravity as the square of Yang-Mills”. This
involves the tensoring of the multiplet content of two super-Yang-Mills
theories to obtain the multiplet content of a supergravity theory. A
complete understanding of this correspondence requires studying how
both global and local symmetries originate from the corresponding
Yang-Mills factors. In the first part of the talk I will show how we
can construct the global (U-duality) supergravity symmetries out of the
corresponding global Yang-Mills ones (R-symmetries) in all spacetime
dimensions between 3 and 10. In the second part I will demonstrate how
the local symmetries of the gravity theory (in linearised
approximation) can also be derived from local gauge invariance of the
Yang-Mills factors. This will be done through a specific example in 4
spacetime dimensions.

9/23 Wed 2:30pm Jelle Hartong (U Brussels)

Title: Holographic Reconstruction of 3D Flat Space-Time from Null Infinity

Abstract: Over the recent years a lot of progress has been made in extending holographic dualities (at the gravity level) to space-times that are not asymptotically AdS. Space-times such as Lifshitz, warped AdS and Schroedinger geometries (relevant for holographic applications to condensed matter systems and to Kerr/CFT) have to a greater or lesser extent been put on a firm holographic foundation in the regime where the bulk dynamics is described by the Einstein-Hilbert action plus matter. An important role in these developments has been played by non-relativistic symmetry groups and novel types of boundary geometries such as Newton-Cartan geometries. In this talk it will be shown that also asymptotically flat space-times can be studied holographically using similar methods. I will show that the boundary geometry at future null infinity is described by a so-called Carrollian geometry and that one can define a boundary energy-momentum tensor whose associated conserved currents span the infinite dimensional BMS_3 algebra.

9/17 Thur 2:30pm Harikrishnan Ramani (YITP)

Title: The WW story

Abstract:Run 1 of the LHC has reported routine excesses(around 20%) in WW measurements both at 7 and 8 TeV.Accounting for higher order QCD effects through pT resummation can explain some of the discrepancy. The CMS experiment obtained good theory-experiment agreement partly due to accounting for higher order corrections calculated by our group. A closely related approach; jet veto resummation has also had success explaining this excess. In this seminar I will talk about the WW experiment set up, transverse momentum resummation and reweighting methods. I shall also compare the two resummation procedures(transverse momentum and jet veto resummation).

9/10 Thur 2:30pm Zhibai Zhang (CityTech CUNY)

Title: Black Rings in Supergravity

Abstract:
Black rings are five-dimensional black holes with non-spherical event
horizons. Their existence indicates that the black hole uniqueness
theorems of four dimensions do not apply in five dimensions, and that
the classification of black holes in higher dimensions is more rich. In
this talk, I will give a brief review of the general properties of
black rings and their construction, present new black ring solutions in
supergravity and discuss their thermodynamics. I will also discuss some
possible future directions.

9/3 Thur 3:30pm (note special time) Tzu-Chieh Wei (YITP)

Title: Hamiltonian quantum computer in one dimension

Abstract:
There are several approaches for quantum computation (QC), such as the
standard circuit model, adiabatic QC, measured-based QC, etc. In
addition, quantum computation can be achieved by preparing an
appropriate initial product state of qudits and then letting it evolve
under a fixed Hamiltonian. The readout is made by measurement on
individual qudits at some later time. Feynman was the first to propose
such a Hamiltonian quantum computer, but his Hamiltonian involves
4-body interaction among qubits that are not geometrically local. Can a
Hamiltonian quantum computer be constructed even in a one-dimensional
geometry with short-ranged (i.e. nearest few neighbors)
interaction? It turns out that it is possible with a nearest-neighbor
(i.e., 2-local) interaction, but the individual spins need to have at
least 8 levels (i.e., the local Hilbert space has dimension d=8).
Here we consider the compromise between the locality k and the local
dimension d in one spatial dimension for universal Hamiltonian quantum
computers. As the locality k increases, it is expected that the minimum
required d can decrease. In particular, for 3-local interactions (k=3)
how much lower than 8 can the local dimension d? We provide a
construction that uses d=5, i.e. spin-2. The resulting Hamiltonian is,
however, not translationally invariant. When one imposes translation
invariance, the required d will need be higher. We also provide another
construction with d=8 translationally invariant Hamiltonian that is
universal (in the sense of quantum computation). One implication of our
results is that simulating generic 1D chains of spin-2 particles
belongs to the complexity class BQP and is fact BQP-complete.

* Only basic quantum mechanics is used in this talk and no background on quantum computation is required

8/26 Wed 2:30pm Michael Spillane (YITP)

Title: Thermal Corrections to Renyi Entropies on S^{d-1}

Abstract:
Interest in entanglement entropy and its generalization R\'{e}nyi
entropy is growing due to its applications in a variety of fields in
physics, from quantum computation and phase transitions to black hole
physics and holography. In this talk I will discuss the effects
of a small finite temperature on both measures of entanglement.
The primary focus will be on calculating corrections for a free fermion
on an n-sphere. Via conformal maps and a Boltzmann expansion of
the density matrix, the corrections can be related to a specific two
point function on a conical space. This work expands on previous work
in Refs. arXiv:1407.1358 and arXiv:1411.6505.