Theory Seminar

Theory Seminar: BRST vs. EPR - The Maxwell Story

The question of whether entanglement entropy in gauge theories is BRST invariant has an odd answer: two different representatives of a BRST cohomology class have different entanglements, but the replica trick path integral commonly used to calculate entanglement is invariant under BRST transformations. After short introductions to entanglement in gauge theories and Hamiltonian BRST quantisation, I will explain why this is so and how it evades the usual arguments about the equivalence between the path integral and the Hamiltonian pictures. Finally, I will comment on the possibility of a prescription to "fix" the Hamiltonian calculation -- to make it BRST-invariant and equal to the answer given by the replica trick.

Date: 
Monday, November 27, 2017 - 2:00pm to 3:00pm
Location: 
CP-179
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Theory Seminar: CP-Symmetry and Baryon number violation in Nucleons on a Lattice

One of the main frontiers of Nuclear Physics is searching for violations of fundamental symmetries such as parity and time reversal. These symmetries have to be broken at the level above the Standard Model prediction, otherwise it will be hard to explain the observed amount of nuclear matter in the Universe. Measurements of the electric dipole moment (EDM) of nucleon, nuclei, and atoms are the most promising ways to observe CP-symmetry violations in the quark-gluon sector. Several experiments plan to improve the bound on the neutron EDM by two orders of magnitude in the next decade. Another vital condition for the baryogenesis is the violation of the baryon number, which, despite extensive several decade-long searches for proton decays and neutron oscillations, has never been observed. Interpreting these experimental limits in terms of fundamental particles and their interactions requires robust theoretical understanding of hadron structure. Thanks to mature numerical methods of solving QCD on a lattice, we can now investigate effects of non-Standard Model quark-gluon interactions on the properties of protons and neutrons. I will present our recent progress in calculations of nucleon EDM induced by quark-gluon color-electric dipole interaction (quark chromo-EDM) performed in QCD with physical masses of quarks and discretization preserving chiral symmetry. In addition, I will present results for the neutron-antineutron oscillation amplitudes and its implications for BSM phenomenology.

Date: 
Monday, November 20, 2017 - 2:00pm to 3:00pm
Location: 
CP-179
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Theory Seminar: Soft Black Hole Absorption Rates as Ward Identities

Recently, a number of exciting connections have been made between large gauge transformations (eg. BMS) and infrared physics (eg. Weinberg's soft graviton theorem). One of the more exciting explorations in this vein was Hawking-Perry-Strominger's (HPS) investigation of the consequences of these new symmetries for black hole physics. I will show very concretely that the Ward identity for the BMS-like large U(1) gauge transformations discussed by HPS fixes the low energy black hole absorption rate for photons. Time permitting, I will discuss broader implications and future extensions.

Date: 
Monday, October 2, 2017 - 2:00pm to 3:00pm
Location: 
CP-179
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Theory Seminar: Phenomenology of neutron-antineutron conversion

In the Standard Model (SM), baryon number – lepton number (B-L) is perfectly conserved.  Therefore, the observation of B-L violation reveals the existence of physics beyond the SM. Traditionally, given the severe experimental constraints on ∆B =1 processes, B-L violation with baryons is probed via neutron-antineutron oscillations, although this process suffers from quenching in the presence of external fields or matter. I will discuss another possibility, neutron-antineutron conversion, in which the ∆B=2 process appears with an external source. I will start with the Lorentz invariant B-L violating operators of lowest mass dimension and show how the appearance of constraints on the “arbitrary” phases in the discrete symmetry transformations help restrict the possible low energy neutron-antineutron transformation operators. Then I will discuss the connection between neutron-antineutron oscillations and conversions --- and how neutron-antineutron conversion can provide a complementary probe to neutron-antineutron oscillation experiments. Finally, I will discuss possible neutron−antineutron conversion proposals and explicitly show how neutron-antineutron conversion experiments can set limits on the scale of B − L violation.
Date: 
Monday, October 16, 2017 - 2:00pm to 3:00pm
Location: 
CP 179
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Theory Seminar: BCFW recursion relations for in Chern-Simons theories with fundamental matter

 
In this talk, I will discuss how BCFW recursion relations can be used to compute all tree-level scattering amplitudes in terms of 2 → 2 scattering amplitude in U(N) N = 2 Chern-Simons (CS) theory coupled to matter in fundamental representation. It is interesting to note that, even though the non-supersymmetric Chern-Simons theory coupled to fundamental fermions or bosons does not admit a good BCFW deformation, we can use the N = 2 supersymmetric recursion relations to write down a recursion relation for a non-supersymmetric fermionic theory. I will discuss the utility of higher point amplitudes from the point of proving dualities in the Chern-Simons theories coupled to matter.
Date: 
Friday, October 13, 2017 - 12:00pm to 1:00pm
Location: 
CP 179
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Theory Seminar: Understanding Black Hole Formation in String Theory

It is known that thermalization in a CFT corresponds to black hole formation in AdS space. For a system to thermalize it must interact. To address black hole formation in string theory we look for thermalization in the D1D5 CFT which has an AdS dual. Thermalization should occur through interactions caused by twist operators which deform the theory off of its free point. The twist operators can join and unjoin ‘strings’ in the CFT. No clear evidence of thermalization was identified at first order in the twist deformation. We therefore compute interactions at second order in the twist deformation with an initial excitation propagating on one of the strings. We consider transitions of the initial excitation to three lower energy excitations in the final state. This yields preliminary evidence for thermalization.
Date: 
Wednesday, October 11, 2017 - 1:00pm to 2:00pm
Location: 
CP 179
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Colloquium

Date: 
Friday, April 28, 2017 - 3:30pm to 4:30pm
Location: 
CP155
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Old and new scaling laws in quantum quench

Systems with time dependent couplings which interpolate between constant values and involve critical points are expected to display universal features. Recently, holographic methods have been used to understand some key aspects of such quantum quenches, which have applications to many areas of physics. This has led to insight into key aspects of well known scaling behavior for slow quenches which cross or approach critical points, viz. Kibble-Zurek scaling. In the opposite regime of fast quench, holographic methods have uncovered new scaling laws which have been subsequently shown to hold in general quantum field theories. This talk will review some of these exciting developments.

Date: 
Monday, September 12, 2016 - 2:00pm to 3:00pm
Location: 
CP179
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Theory Seminar: Asymptotic Symmetries and Gravitational Waves

Date: 
Monday, February 22, 2016 - 2:00pm to 3:00pm
Location: 
CP179
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Theory Seminar: Searching for Physics Beyond the Standard Model from Lattice QCD

After the discovery of the Higgs boson, physicists keep searching for New Physics in cosmic, energy and intensity frontiers. In the intensity frontier, a key stone is to provide precise Standard Model prediction by controlling the uncertainty from low energy QCD.

Lattice QCD has been developed to tackle the nonperturbative problems in low energy QCD. It has been demonstrated to be a powerful tool in dealing with the ''gold-plated'' observables such as the hadron spectra and decay constants. To meet the requirement of future experiments in the intensity frontier, we need to study more complicated system beyond gold-plated.

Rare kaon decay is such an example. The cleanliness of the Standard Model predication of the branching ratio for the rare kaon decays K^+ -> pi^+ nu nu-bar and K_L -> pi^0 nu nu-bar make them an ideal place to search for New Physics. The CERN NA62 experiment aims for an observation of O(100) events of K^+ -> pi^+ nu nu-bar decay and a 10%-precision measurement of the branching ratio. The KOTO experiment at J-PARC in Japan aims for first observation of the CP-violating decay K_L -> pi^0 nu nu-bar.

In this talk I will present an exploratory study of the long-distance contributions to the K+ -> pi+ nu nu-bar decay amplitude from first principles using lattice QCD. Such study is necessary to interpret future experiments at a 10% precision or better.

Date: 
Wednesday, December 16, 2015 - 1:30pm to 3:30pm
Location: 
CP179
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