##### When & Where

The CTP Lunch Club meets at 12pm in the CTP Cosman seminar room every Friday (provided that there are sufficient speakers). A light lunch will be provided (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.

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, Nikhil Raghuram at srivat91-at-mit[dot]edu or Chih-Liang Wu at cliang-at-mit[dot]edu.

• September 30, 12pm
Michael Blake

Diffusion and Chaos in Incoherent Black Holes

In 2014 Hartnoll proposed a fundamental bound on diffusion constants Dv2/kBT$D \gtrsim \hbar v^2/k_B T$. In this talk I will discuss a class of holographic theories which saturate such a bound, where $v$vvvv$v$ is the velocity of the butterfly effect. These results suggest a novel connection between transport at strong coupling and quantum chaos.

• October 21, 12pm

Holographic hadronization, transport coefficients, and hard probes of N=4 super Yang-Mills plasma on the Coulomb branch

I study a rotating black 3-brane Type IIB supergravity background solution which in the extremal limit is dual to N = 4 super Yang-Mills theory on the Coulomb branch (cSYM) at zero temperature, while in the non-extremal limit is dual to N = 4 cSYM at finite temperature. After showing that the non-extremal background solution has two branches which correspond to large black hole that has positive specific heat, and small black hole that has negative specific heat and Hawking radiate, I show that the two branches are connected to each other by a second-order phase transition. I also show that the large black hole branch has qualitatively similar equation of state (EoS) to that of pure Yang-Mills theory on the lattice, while the small black hole branch has EoS that one would expect from a hadronizing thermal gauge theory (N = 4 cSYM), i.e., its entropy and energy densities decrease with temperature. Moreover, the bulk viscosity for the large black hole branch decreases with temperature and has a maxima around a critical temperature Tc while, for the small black hole branch, it increases with temperature. I show that the conductivity, jet quenching parameter, drag force, and momentum diffusion coefficients of the large black hole branch increase with temperature and asymptote to their conformal value while, for the small black hole branch, they decrease with temperature as one would expect in a hadronizing thermal gauge theory (N = 4 cSYM) where the color degrees of freedom are decreasing. Finally, by studying the Wilson loop (minimal surfaces) in the extremal limit, I show that the heavy quark-antiquark potential V(L) is given by Cornell potential. I also compute the quantized mass spectrum of the scalar and spin-2 glueballs in the extremal limit.

• November 4, 12pm
Ben Elder

Jet Fragmentation and Fractal Observables

Fragmentation functions have been used as a tool for calculations of exclusive cross sections in hadronic processes for over thirty years. While non-perturbative, their renormalization group evolution can be calculated in perturbation theory. We generalize the well known DGLAP equations to describe the evolution of a broader class of generalized fragmentation functions. Generalized fragmentation functions can be used to make exclusive measurements of a broad class of observables that we call fractal observables. We describe the space of these observables whose distributions evolve according to our generalization of the DGLAP equations. Fractal observables are in general not collinear safe, however this formalism allows us to calculate cross sections for them by absorbing the collinear singularities into the generalized fragmentation functions.

• November 18
Lina Necib

New angles on energy correlation functions

In this talk, I will present a new set of observables constructed to identify specific features within jets, as relevant for identifying hadronically decaying electroweak scale particles, or distinguishing quark and gluon jets. I will begin by introducing power counting, a method used to identify the parametric scaling of different observables, and show how it can be used to design powerful discriminants. Power counting can be used to understand the stability of observables, which plays an important role in experimental studies. I will then present N2, a new observable with improved discrimination in 2 prong signals, that is stable under mass and pT cuts. An alternate approach to stable observables is to use grooming, a technique used to reduce soft radiation in jets. I will use power counting to identify two more observables M2, and a modified D2, which are stable when applied on groomed jets. I will close by applying the same techniques to identify observables for 3 prong discrimination, and quark/gluon discrimination.

• December 9
Jasmin Brewer

Jet Shape Modification in Holographic Plasma

Jets in heavy ion collisions are among the most important probes of the properties of the quark gluon plasma, since they provide information about hard scatterings of partons with the medium. The energy loss of a jet in the medium is thought to be sensitively dependent on its width, making this an important observable. In this work, we compute the jet shape modification of an ensemble of jets in holography. We consider each jet to be represented holographically by a string in the dual gravitational theory, and construct an ensemble of jets in vacuum with initial energies and opening angles given by perturbative QCD. Here we improve upon previous work by solving the full string dynamics, which enables us to compute the full jet shape, and find that the jet shape in vacuum scales with opening angle at a given energy. We then model a heavy ion collision by propagating our ensemble of vacuum jets through an expanding, cooling droplet of strongly coupled plasma. We present results on the modification of the jet shape by the plasma and compare to recent CMS data.

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