Upcoming Theory Seminars*
January 22
Speaker: Yasunori Nomura, UC Berkeley
Title: "The Multiverse---Particle Physics, Cosmology, and Quantum Gravity"
January 28
Speaker: Peter Boyle, University of Edinburgh
Title: “Precision physics from Lattice QCD”
Abstract:
The increasing precision of Lattice QCD enables the contribution the
best theoretical input for a number constraints on CKM physics. I
discuss in particular the Kl3 semileptonic form factor and the standard
model and beyond standard model Kaon bag parameters. Precise Lattice QCD
simulations are the result of improved theoretical approaches for
renormalisation of lattice operators, state of the art numerical
algorithms and powerful computers. I discuss recent advances in each of
these areas
January 30
Speaker: Yuri Kovchegov, The Ohio State University
Title: “Saturation Physics and Rapidity Correlations”
Abstract:
We will present an overview of the recent progress in our understanding of strong interactions in high energy scattering. We will demonstrate how in high energy scattering the wave functions of hadrons and nuclei are densely packed with gluons and quarks leading to a new phenomenon of parton saturation. The transition to the saturation regime is described by the saturation scale, which has a dimension of momentum and can be large in high energy scattering, making the strong coupling small, and allowing for a universal perturbative description of a host of hadronic and nuclear scattering processes. In particular we will discuss the prediction of the saturation physics for di-hadron correlations in high energy collisions. We show that multi-parton interactions inherent in the saturation approach lead to long-range rapidity correlations between the hadrons which have identical near- and away-side "ridge" structure. We compare this prediction to the recent data from the p+Pb run at the LHC.
February 4
Speaker: Frederik Denef, Institute for Theoretical Physics, K.U.Leuven
Title: "Controlling quantum cosmology"
Abstract:
Holographic dualities have allowed the construction of UV complete,
nonperturbative models of quantum gravity in spacetimes with negative
cosmological constant. In contrast, despite suggestive formal
similarities, very little progress along these lines has been made for
their positive cosmological constant counterparts, or more generally in
the construction of complete models of quantum gravity in an expanding
universe. I will review some of the attempts of the past, what the
obstacles are to the construction of holographic duals, and how the
first explicit, recent proposal for such a dual --- the Sp(N) model of
Anninos, Hartman and Strominger, conjectured to be dual to 4d higher
spin gravity with positive cosmological constant --- manages to
circumvent these issues. I will then describe our recent and ongoing
work exploring whether this model indeed makes sense beyond perturbation
theory, more precisely in its interpretation as providing an exact
expression for the "wave function of the (Vasiliev) universe". At this
point the evidence is mixed but intriguing. I will also outline a
related line of work, addressing the question how the characteristic
"evolutionary tree" of macroscopically distinct geometries generated by
quantum fluctuations and inflation can be extracted from the late time
wave function. Inspired by ideas developed in the theory of spin
glasses, we propose to consider certain distance distributions on state
space.
February 25
Speaker: Sergei Dubovsky, New York University
Title: "Evidence for a new particle on the worldsheet of the QCD flux tube"
Abstract:
We propose a new approach for the calculation of the spectrum of
excitations of QCD flux tubes. It relies on the fact that the worldsheet
theory is integrable at low energies. With this approach, energy levels
can be calculated for much shorter flux tubes than was previously
possible, allowing for a quantitative comparison with existing lattice
data. The improved theoretical control makes it manifest that existing
lattice data provides strong evidence for a new pseudoscalar particle
localized on the QCD fluxtube - the worldsheet axion.
March 4
Speaker: Piljin Yi, Korean Institute for Advanced Studies (KIAS)
Title: "Constructive Wall-Crossing & Seiberg-Witten"
Abstract:
In string theory, geometry and quantum states are sometimes tightly
connected. The wall-crossing problem is one example, where
(non-)existence of so-called calibrated cycles can be figured out by
solving relatively simple class of Schroedinger problems, or vice versa.
We start with a one-slide review of the Kontsevich-Soibelman (KS)
solution to the mathematical side of the wall-crossing problem and then
proceed to direct and comprehensive physics counting of BPS states in
the context of Seiberg-Witten theory. We point out several subtleties,
missed by previous such attempts, and provide a universal index formulae
from the semiclassical soliton viewpoint. Newly illuminated are how
field theory index, called protected spin character, is related to the
equivariant index of the low energy quantum mechanics of such BPS
states, and also the origin of rational invariant from quantum Bose and
Fermi statistics of identical particles. In the end, this physical and
constructive approach can be shown to be equivalent to the proposed KS
solution.
March 11
Speaker: Rachel Rosen, Columbia University
Title: "Massive Gravity and Beyond: A Particle Physics Approach to Modified Gravity"
Abstract:
The discovery of General Relativity (GR) required a leap of insight,
from the equivalence principle and general coordinate invariance to a
fully non-linear theory governing the dynamics of spacetime. Yet
remarkably, by applying the tenets of relativistic quantum field theory
to gravity one can arrive systematically at the same theory of GR. In
this talk I will review this approach to gravity. I will show how this
same approach can also be used to develop consistent non-linear
modifications of GR in which the graviton has a small mass, as well as
consistent theories of multiple interacting spin-2 fields. These
theories improve our understanding of the interplay between gravity and
particle physics and provide new approaches to solving the cosmological
constant problem.
March 25
Speaker: Xu Feng, High Energy Accelerator Research Organization (KEK)
Title: "The neutral pion decay and the chiral anomaly on the lattice"
Abstract:
We perform a nonperturbative calculation of the pion-to-two-photon
transition form factor and the associated decay width using lattice QCD.
The amplitude for a two-photon final state, which is not an eigenstate
of QCD, is extracted through a Euclidean time integral of the relevant
three-point function. We utilize the all-to-all quark propagator
technique to carry out this integration. We execute the calculation
using the overlap fermion formulation, which ensures the exact chiral
symmetry on the lattice and produces the chiral anomaly through the
Jacobian of the chiral transformation. We calculate the form factor and
decay width with a comprehensive estimate of various systematic errors,
except for a possible discretization effect. Our results reproduce the
predication of the ABJ anomaly in the chiral limit and also agree with
the PrimEx experimental measurement at the physical pion mass.
April 1
Speaker: Ami Katz, Boston University
Title: "Model Independent Direct Detection"
Abstract:
I will describe a model-independent approach to parameterizing possible
dark-matter (DM) interactions with nucleons. The idea is to require
only terms consistent with Galilean invariance and the rules of quantum
mechanics. This general framework leads to novel ways with which nuclei
can interact with the DM.
Besides the standard spin-dependent and spin-independent interactions,
there are three more types of interaction, sensitive to nuclear
properties of targets which were not considered previously. Moreover,
interference effects between operators lead to a rich parameter space. I
will report on the latest bounds on this parameter space, focusing on
the complementarity of different experiments.
April 8
Speaker: Rajan Gupta, Los Alamos National Lab
Title: "Exploring TeV scale physics in decays of [ultra]cold neutrons”
Abstract:
Possible novel scalar and tensor interactions at the TeV scale lead to
observable consequences in the decay distribution of neutrons. Such high
precision experiments complement direct searches at the LHC. The
biggest uncertainty in interpreting current proposed experiments and
bounding the scale of these new interactions is the calculation of the
scalar and tensor charges of the nucleon. This talk will motivate the
physics and describe the status of lattice QCD calculations being done
to calculate these charges with the desired precision.
April 15
Speaker: Christopher Kelly, Columbia University
Title: "Progress Towards the First Measurement of Direct CP-Violation in K->pi pi Decays From First Principles"
Abstract:
The direct violation of the CP symmetry, an essential component for
describing the asymmetry between matter and antimatter in the universe,
was first observed in the late 1990s in the decays of a kaon into two
pions. Since that time the experimental measure of CP violation in this
channel has become quite precise. However until recently it has not been
possible to calculate this quantity directly from the Standard Model as
it receives large contributions from QCD in the hadronic regime in
which perturbative calculations are not possible. This is unfortunate
because these decays are highly sensitive to Beyond the Standard Model
sources of CP violation, and a comparison between the Standard Model
prediction and the measured value could potentially lead to the
discovery of new physics. Now, using lattice QCD, and combining decades
of theoretical and computational developments, such a calculation has
become feasible.
The RBC & UKQCD collaboration have recently published the first
calculation of the K->pi pi decay amplitude in the I=2 channel. I
will discuss the techniques used for this calculation and then describe
our progress towards the more difficult task of measuring the decay in
the I=0 channel, which represents the last hurdle before a full ab
initio value for the measure of CP-violation can be obtained.
April 22
Speaker: Philip Schuster, Perimeter Institute
Title: "(Why) Is Helicity Lorentz-Invariant? Part 1: Introduction"
Abstract:
Massless particle states carry integer or half-integer spin about the
momentum axis, or "helicity". Lorentz symmetry allows for helicity
states to mix under boosts (like massive particle polarizations); such
mixing is not understood theoretically and is not obviously well
constrained by experiment. This possibility is historically known as
"continuous spin" and this talk provides an informal introduction to the
subject. We present evidence that "continuous spin" particles (CSPs)
can interact with matter via scattering amplitudes that approach
familiar scalar, electromagnetic, or gravitational ones in a high energy
(and/or non-relativistic) correspondence limit. Such interactions also
appear thermodynamically safe.
This talk will focus on the motivation for CSPs, their kinematics, and
properties of consistent amplitudes. We close by identifying some
directions for discovering a full interacting theory of CSPs or proving
that such theories can't exist. A 2nd talk on Tuesday will elaborate
on CSP thermodynamics and on a field theory description of CSPs.
April 23 (Tuesday, 831 Pupin 1:00PM)
Speaker: Natalia Toro, Perimeter Institute
Title: "(Why) Is Helicity Lorentz-Invariant? Part 2: Thermodynamics and Field Theory"
Abstract:
Massless particle states of integer or half-integer "helicity" can, in
general, mix under Lorentz boosts (like massive particle
polarizations). This possibility, historically known as "continuous
spin" particles (CSPs), is neither well understood theoretically nor
obviously well constrained by experiment. This talk is a continuation
of the introduction to CSPs given in Part 1 on Monday. We will examine
the physical consequences of the "helicity correspondence" in which
simple CSP amplitudes approach scalar, electromagnetic, or gravitational
amplitudes in a high-energy limit. In particular, we elaborate on the
thermodynamics of matter coupled to CSPs at high temperatures and
related observational constraints. We also present a gauge field theory
description of CSPs, and comment on some puzzles regarding how this
theory is related to the scattering amplitudes discussed previously.
April 29
Speaker: Raman Sundrum, University of Maryland
Title: "Metaphor for Dark Energy"
Abstract:
At the beginning of the twentieth century, there arose two distinct
means of extending Newton's Law of Gravity and the Equivalence Principle
to the relativistic regime. Of course one was General Relativity. The
other was Nordstrom's theory of scalar gravity, improved further by
Einstein and Fokker as a theory of curved spacetime. This theory
ultimately failed observational tests of relativistic gravity, but it
has come up in other guises in theoretical physics over the decades. I
will describe this remarkable theory and update it into the era of
quantum mechanics and string theory, and point out that it can provide a
simpler "laboratory" for thinking through some tough puzzles of real
gravity. In particular I show that scalar gravity has a strikingly
faithful version of the cosmological constant problem, satisfying the
same no-go "theorems" of real gravity, and yet there is an elegant
solution in terms of a subtle form of evolving "dark energy" that can be
understood in standard quantum field theory.
May 6
Speaker: Jeremy Sakstein, University of Cambridge
Title: "Detecting Modified Gravity in the Stars"
Abstract:
Screened Scalar-Tensor gravity such as chameleon and symmetron theories
allow order one deviations from General Relativity on large scales
whilst satisfying all local solar-system constraints. A lot of recent
work has therefore focused on searching for observational signatures of
these models and constraining them. If these models are to be viable
then our own solar system is necessarily screened, however, this may not
be the case for stars in Dwarf Galaxies, which can exhibit novel and
unique phenomena. These new effects can be exploited to produce
constraints that are far more competitive than laboratory and
cosmological tests and in this talk, I will describe some recent and
ongoing work using these phenomena to place new constraints.
* Please visit the Theory Seminar site for updates to the schedule.
