Graduate Lunch Club
Spring 2007 schedule of talks
Fridays, 12:00p.m - 1:00pm. Pizza will be delivered early
Center for Theoretical Physics, building NE25
4th floor, CTP seminar room 4-107
Massachusetts Institute of Technology
Christiana Athanasiou and Nabil Iqbal
The CTP Lunch Club meets at 12 noon in the CTP seminar room every Friday (provided that there are sufficient speakers). A light lunch will be provided begining at 11:50am (usually pizza, however some other options may be explored).
The seminars are designed for graduate students and should be accessible to all students. First year students are particularly encouraged to attend so that they may learn about research being performed in the CTP.
Workshops are aimed at teaching students about particular areas of research rather than presenting cutting edge research. The goal is learning, and to encourage participation, faculty are asked not to attend the workshop portion of these seminars.
Email notification of the club will be sent to the ctp-all, ctp-postdocs and ctp-students email lists as appropriate.
If you wish to speak, or have suggestions about speakers and/or possible workshop topics, please contact the organizer listed above.
Neel order, quantum spin liquids and quantum criticality in two
This talk is concerned with the possibility of a direct second order
transition out of a collinear Neel phase to a paramagnetic spin liquid
in two dimensional quantum antiferromagnets. Contrary to conventional
wisdom, we show that such second order quantum transitions can
potentially occur to certain spin liquid states popular in theories of
the cuprates. We provide a theory of this transition and study its
universal properties in an $\epsilon$ expansion. The existence of such a
transition has a number of interesting implications for
spin liquid based approaches to the underdoped cuprates. In particular
it considerably clarifies existing ideas for incorporating
antiferromagnetic long range order into such a spin liquid based
Anthropics: A Guide for the Perplexed
A major problem facing modern theoretical physics is that it looks like certain
parameters of our theories need to be tuned with enormous precision in order to
get a Universe that looks anything like our own. A possible explanation that
has recently attracted a lot of attention and controversy is the Anthropic
Principle (AP): perhaps there are many universes were the parameters take
different values, and we naturally happen to live in one of the rare universes
that can sustain intelligent life.
In my talk I will broadly discuss the philosophy, history, and physics of
as it applies to cosmology and particle/nuclear physics. Is the AP the same or
the opposite of "intelligent design"? Has this line of thinking been useful to
science in the past? What does the AP assume are the necessary conditions for
intelligent life to form? Under what scenarios would it succeed or fail? Why
has its popularity surged so spectacularly in recent years? How is it invoked
by string theorists in the context of flux compactification? How is it
connected to inflation? Should physicists be paid to think about this stuff?
My own attitude towards the usefulness of the Anthropic Principle is one
guarded skepticism. I will try to keep the discussion as constructive and
non-confrontational as possible. The talk will be aimed at graduate students
who know basic particle physics and General Relativity but who are bewildered
by talk of landscapes and multiverses.
The crystallography of three-flavor quark matter
Cores of neutron stars feature matter at large baryonic densities. I
will try to convince you that matter in these conditions may very well
exist as a crystalline color superconductor, meaning a color
superconductor whose condensate varies periodically in space. After
describing the phases in detail, I will explain that these phases are
remarkable because they are very rigid, while at the same time they are
superfluid. This might have implications on aspects of neutron star
phenomenology. Looking ahead, I will describe the phenomenon called
pulsar glitch and show how these glitches may be explained by the
presence of a crystalline color superconducting core.
Actions, Boundary Terms, and AdS/CFT
The action provides an important link between classical and quantum physics.
Modern treatments of quantum theories are based on the path integral
formulation, where all field configurations consistent with the boundary
conditions contribute according to the exponential of their action. In the
semiclassical approximation, where Planck's constant becomes very small, the
path integral is dominated by stationary points of the action and we recover
classical physics. In this talk I will point out some shortcomings of
familiar gravitational actions when we try to use them in a path integral. I
will discuss various proposals for addressing these problems, and illustrate
how they work using some simple examples from the AdS/CFT correspondence.
A new reality: B --> pi pi annihilation in SCET
B-decays to two light mesons provide a wealth of information about the strong
force and quark flavor transitions. Some channels are loop- dominated allowing
the possibility of significant contributions from physics beyond the Standard
Model (SM). It has been suggested that the data has hints of new physics, but
progress on the SM prediction is still underway. I'll report on our recent
calculation of the leading "annihilation" contributions to the B --> M1 M2
amplitudes with the soft-collinear effective theory. Our work eliminates a
possible SM explanation for the data.
Penguin Loops for Nonleptonic B decays in the Standard Model
I will present the calculation of order alpha_s penguin contributions
for B to pi pi and related decays from charm quark, up quark and
magnetic penguin loops. Our calculation include term proportional to
the largest Wilson coefficients in both NDR and HV renormalization
schemes for the amplitudes that are leading order in Lambda/m_b and
chirally enhanced. I will also discuss the implication of our
calculation on the theoretical explanation of the large phase
observed in these decays.
String Compactifications on non-Calabi-Yau Manifolds
Although Calabi-Yau compactifications have been the industry standard
in string theory for many years, they are clearly a very limited subset
of possible string compactifications. I'll discuss a relatively new
generalization of Calabi-Yau's, namely, manifolds with SU(3)-structure.
These manifolds retain many of the nice features of Calabi-Yau's, yet
allow for more interesting types of supersymmetry breaking in four
dimensions. I won't assume any knowledge of string theory, and my talk
will surely be more accessible and fun than the almost concurrent talk
by Radu Roiban at Harvard on N=8 supergravity.
An Introduction to Solitons and Oscillons
In nonlinear field theories there can arise coherent, localized
solutions of the equations of motion, which can be thought of as
superpositions of waves held together by their own self-interactions. I
will begin by introducing the well-known theory of static solutions of
this kind, known as solitons. I will then describe recent work on
oscillatory solutions, known as oscillons or breathers, about which much
less is known.
On the possible finiteness of N=8 supergravity
Recent work has shown that N=8 supergravity is possibly less
divergent than previously thought, and this has led to the speculation that N=8
supergravity may actually be perturbatively UV finite. In this journal club
talk I will attempt to review aspects of this subject.
Non-perturbative renormalization of the chromomagnetic operator
in Heavy Quark Effective Theory
After reviewing some facts about renormalization and scheme
conversions, I will give a short introduction to Heavy Quark Effective
Theory and its formulation in a lattice regularization. In HQET the
dimension-five chromomagnetic operator gives the leading contribution to
the B-B^* mass splitting in the bottom mesons. I will present a
non-perturbative calculation of its renormalization. This brings the
predicted mass splitting in significantly better agreement with
experiment than the use of perturbative renormalization.
Introduction to black hole thermodynamics
I provide an introduction to the subject of black hole
thermodynamics. While the presentation will be elementary, some aspects
of current research will be discussed.
ADHM Construction and Instanton Calculus
Over the past few years there has been big progress in studying
instanton effects in gauge theory. I will start by explaining the basics
of instanton effects and proceed to reviewing the ADHM construction of
instantons, which plays central roles in many instanton calculations. I
would also like to comment shortly on the localization theorem, which
makes the exact instanton summation possible.
Introduction to 21cm Cosmology
The redshifted 21cm hyperfine line of neutral hydrogen provides the only
direct, three-dimensional view of the cosmic structure formation in the high-
redshift universe. In this talk, I will start by reviewing the fundamental
physics of 21cm hyperfine transition, then discuss the thermal history of the
inter-galactic medium and the power spectrum of 21cm line at various epochs. I
will also review a number of radio interferometers currently being planned or
constructed, and discuss how accurately we can constrain cosmological
parameters with the measurements of 21cm power spectrum.