# Graduate Lunch Club

##### Spring 2007 schedule of talks

##### When

Fridays, 12:00p.m - 1:00pm. *Pizza will be delivered early*

##### Location

Center for Theoretical Physics, building NE25

4th floor, CTP seminar room 4-107

Massachusetts Institute of Technology

##### Organizers

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.

##### February 16

*Neel order, quantum spin liquids and quantum criticality in two
dimensions*

**Pouyan Ghaemi**

MIT

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

approach.

##### February 23

*Anthropics: A Guide for the Perplexed*

**Alejandro Jenkins **

MIT

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
the AP

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
of

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.

##### March 2

*The crystallography of three-flavor quark matter*

**Rishi Sharma**

MIT

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.

##### March 9

*Actions, Boundary Terms, and AdS/CFT*

**Robert McNees**

*Brown University*

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.

##### March 16

A new reality: B --> pi pi annihilation in SCET

**Chris Arnesen**

*MIT*

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.

##### March 23

Penguin Loops for Nonleptonic B decays in the Standard Model

**Ambar Jain**

*MIT*

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.

##### April 6

String Compactifications on non-Calabi-Yau Manifolds

**Brian Wecht**

*MIT*

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.

##### April 13

An Introduction to Solitons and Oscillons

**Noah Graham**

*Middlebury College*

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.

##### April 20

On the possible finiteness of N=8 supergravity

**Henriette Elvang**

*MIT*

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.

##### April 27

Non-perturbative renormalization of the chromomagnetic operator

in Heavy Quark Effective Theory

**Harvey Meyer**

*MIT*

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.

##### May 4

Introduction to black hole thermodynamics

**Daniel Grumiller**

*MIT*

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.

##### May 11

ADHM Construction and Instanton Calculus

**Sanefumi Moriyama**

*MIT*

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.

##### May 18

Introduction to 21cm Cosmology

**Yi Mao**

*MIT*

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.