The CTP Lunch Club meets every Friday at noon in the Cosman seminar room, 6C-442 (provided that there are sufficient speakers). Pizza will be provided.

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. The goal is learning, and to encourage participation, the seminars will be for students only.

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 organizers, Ben Elder at belder13-at-mit[dot]edu and Yinan Wang at wangyn-at-mit[dot]edu.

- February 19, 12pm

Wilke van der Schee

Evolution of the jet opening angle distribution in holographic plasma

Jets are particularly interesting probes to study the formation and evolution of quark-gluon plasma, as created in heavy ion collisions. In this talk I will briefly review attempts to describe the jet evolution in holography, and will argue that essential progress can be made by studying also the substructure of the jet, in particular the evolution of the opening angle. This is illustrated in a simple holographic model, where we start with energy and angular distributions from pQCD. We identify two competing effects: (1) all individual jets become wider, and (2) the distribution of final jets becomes narrower, since narrow jets suffer less energy loss and are thereby more likely to survive. Both effects can be visible in experimental data.

- February 26, 12pm

Yotam Soreq

Probing the Higgs and New Physics with Isotope Shift

We propose to use precision measurement of isotope shift in atomic clock transitions to probe physics beyond the standard model as well as to probe the Higgs boson coupling the the electron and up and down quarks. We show that the attractive Higgs force between nuclei and their bound electrons induces measurable non-linearities in a King plot of two isotope shifts. We present an experimental method which, given state-of-the-art accuracy in frequency comparison, competes with and potentially surpasses the Large Hadron Collider in bounding the Higgs-to-light-fermion couplings. Better knowledge of the latter is an important test of the Standard Model which could lead, besides the establishment of new physics above the weak scale, to an alternative understanding of the flavor puzzle. Moreover, we derive the reach for generic new physics above the GeV scale at the effective field theory level, as well as estimate the limits on possible new spin-independent forces mediated by sub-! GeV states coupled to electrons and neutrons. We also study the weak force and show that isotope shifts could provide strong constraints on the Z couplings to valence quarks, which complement precision observables at LEP and atomic parity violation experiments.

- March 4, 12pm

Hongwan Liu

Did Dark Matter End the Dark Ages?

The injection of electromagnetically interacting particles into the universe by dark matter annihilations or decays must have an effect on the ionization and thermal history of the universe. In this talk, I will outline how these effects are quantified, and what this can tell us about the properties of dark matter. In particular, I will talk about the possible role of dark matter in bringing about reionization, the last major phase transition of gas in the universe.

- March 11, 12pm

Arjun Bagchi

Tensionless strings and related fun things

I will talk about the formulation of the tensionless limit of closed bosonic string theory from the point of view of worldsheet symmetries.

- March 18, 12pm

Elton Zhu

How to solve constraint satisfaction problems with quantum algorithms?

Recently, Quantum Approximate Optimization Algorithm (QAOA) was proposed as an algorithm for finding approximate solutions to some constraint satisfaction problems with bounded degree. The algorithm is motivated by quantum adiabatic algorithm, but behaves very differently. I will briefly describe the problems, with motivation from anti-ferromagnetic Ising Model on random lattice, and show how QAOA has lead to a much better understanding of such problems.

- April 1, 12pm

Anand Natarajan

Quantum Non-Local Games and the Local Hamiltonian Problem

The study of quantum non-local games lies at the intersection of physics, computer science, and math. In this talk, I'll explain why the setting of non-local games is motivated by both fundamental questions in quantum foundations and complexity theory, as well as practical applications to cryptography and verification of quantum systems. I'll give a proof of a classic result in the area called the Tsirelson bound, which is both simple and illustrative of techniques used in later works. Finally, I'll discuss the quest to find a game that can only be won if the players share the ground state of a quantum many-body system; this is part of a broader research program called "quantum PCP." The last part is based on the preprint 1512.02090 joint with Thomas Vidick.

- April 8, 12pm

Aditya Pathak

Small x resummation in DIS using effective field theory

Small x in DIS refers to a kinematic regime where the election and the proton undergo near forward scattering, and can be viewed as the case where, in the center of mass frame, the colliding particles are deflected very slightly in direction perpendicular to their motion. This corresponds to the case where the Mandelstam variables have the hierarchy t<< s. One finds logs of the ratio t/s ~ x^2 in the perturbation expansion of the cross section, where x is the DIS kinematic variable. These logs are distinct from the case of the logs which appear after renormalization, and resumming these logs using the BFKL equation, and at the same time consistently resumming the usual logs from renormalization using the DGLAP equation, has been an active area of research for over past twenty years. In this talk I will go through the first step towards solving this problem using the framework of effective field theory. I will derive the leading log x resummation result using the framework of Soft Collinear Effective Theory with operators for Glauber exchange and compare it with the result which appeared almost twenty years ago in the literature.

- April 22, 12pm

Tarek Anous

The fractional quantum hall effect explained simply by a simple person (bis)

The fractional quantum hall effect was discovered in 1982, an integer year. I will attempt to explain how fractions play a role. Since I am no expert, hopefully one of {1/3,2/3,2/5,3/7...} of the lecture will be understood by the audience. Viewer discretion is advised.

- April 29, 12pm

Nikhil Raghuram

An Introduction to F-theory

F-theory is a powerful method for constructing and analyzing string vacua. My talk will try to motivate and introduce F-theory in an accessible. After giving some general background, I will describe how ideas from Type IIB string theory lead to F-theory. I will then describe some of the techniques important in F-theory, particularly those for constructing models with various gauge groups and matter representations. The discussion will conclude with a brief overview of current research areas involving F-theory.

- May 6, 12pm

Nick Rodd

Heavy Dark Matter Annihilation at One-loop

Calculating accurate indirect detection rates for heavy dark matter annihilation requires a careful treatment of two effects: Sommerfeld enhancement and large electroweak Sudakov logarithms. I will review how effective field theory techniques can be used to deal with these effects and in particular resum the large logs. I will then describe recent work to extend these results to higher order. Part of this involved calculating a number of one-loop diagrams in the theory and I will outline some of the interesting features of the calculation beyond what might appear in a standard QFT course.

- May 13, 12pm

Jasdeep Bains

Cosmological Time Crystals

In standard FRW cosmologies, homogenous scalar fields usually redshift with their amplitudes approaching some finite value. The presence of a Hubble term ensures that a minimum energy configuration is reached. In most cases this is a static configuration. However, in some recent work, a class of relativistic scalar field models has been discovered in which the so called “cosmic time crystal” behaviour manifests itself as an indefinitely oscillating field characterised by a finite amplitude. Such models were found to exist in expanding FRW spacetimes despite the presence of Hubble friction; a somewhat remarkable behaviour. This is a new step in a different direction to ordinary ghost condensate models, since in this new class of ghost condensates, there is an explicit time dependence in the stress-energy tensor. In this talk, I will very briefly present just some of the main features of this discovery.

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