Monica Jinwoo Kang (Caltech): Finding isomorphic superconformal field theories | 1/24/2024
When do two different looking quantum field theories describe the same physics? This is essentially asking when the quantum field theories are isomorphic. In the case of topological quantum field theories, there are sometimes a way to determine them via topological invariants. For a superconformal field theory, what would be the minimal set of “invariants” to determine when they are isomorphic? I will discuss some approaches to this question in the context of superconformal field theories in four and six dimensions. Utilizing 4d class S theories that also admits 6d (1,0) SCFT origins, I will explain how a certain class of 4d N=2 SCFTs, which a priori look like distinct theories, can be shown to describe the same physics. I will further explain how the 6d (1,0) origin sheds light on the 3d duality.
Taro Brown (UC Davis): Probing poles at infinity using on-shell diagrams | 1/31/2024
Locality and unitary forces scattering amplitudes to factorize when taking the momentum of one of the external particles to zero. This factorization has proven very useful for recursion relations for amplitudes at high multplicities. The recursion can break down, however, when the amplitude contains a pole at infinity. In this talk we are going to make modest step towards a prescription of “unitarity at infinity”. We do this by studying on-shell diagrams, which are on-shell gauge invariant objects that appear as cuts of loop integrands in the context of generalized unitarity and serve as building blocks for amplitudes in recursion relations. In the dual formulation, they are associated with cells of the positive Grassmannian. We will describe on-shell diagrams in N<4 supersymmetric Yang-Mills (SYM) theory and show that there exists a diagrammatic operation that corresponds to sending one of the momenta to infinity.
Katie Fraser (Harvard): Axion Interactions with Topological Defects | 2/07/2024
Given that axions are both a promising candidate to solve problems in the Standard Model and are ubiquitous in quantum gravity, it is crucial to accurately determine their signatures. In this talk, we discuss how the axion's compact field space leads to interesting interactions with topological defects, specifically monopoles and strings. In the case of monopoles, we show that, due to the Witten effect, axions interacting with abelian gauge fields generate a potential for the axion from loops of magnetic monopoles, and discuss a simple phenomenological example where this potential is the dominant contribution to the axion mass. In the case of strings, we discuss superconductivity from massless chiral excitations along the string. We show that bulk fermions do not need to become massless in the core of the string for there to be trapped massless excitations, and explore the counterintuitive phase structure of these zero modes, which become less localized to the string as the mass is increased.
Jason Evans (Shanghai Jiaotong): Effect of Ultralight Dark Matter on g-2 of the Electron | 2/14/2024
If dark matter is ultralight, the number density of dark matter is very high and the techniques of zero-temperature field theory are no longer valid. The dark matter number density modifies the vacuum giving it a non-negligible particle occupation number. For fermionic dark matter, this occupation number can be no larger than one. However, in the case of bosons the occupation number is unbounded. If there is a large occupation number, the Bose enhancement needs to be taken into consideration for any process involving particles which interact with the dark matter. Because the occupation number scales inversely with the dark matter mass, this effect is most prominent for ultralight dark matter. In fact, the Bose enhancement effect from the background is so significant for ultralight dark matter that, the correction to the anomalous magnetic moment is larger than experimental uncertainties for an effective coupling of of order 10^{-16-- -17} for a mass of order 10^{-20} eV if the dark matter is a dark photon or axion like particle. Furthermore, the constraint scales linearly with the dark matter mass and so new significant constraints can be placed on the dark matter mass all the way up to about 10^{-14} eV. Future experiments measuring g-2 will probe even smaller effective couplings.
Jonathan Sorce (MIT): Position-space analyticity and modular flow | 2/21/2024
I will describe ongoing work on the thermodynamics of quantum fields in far-from-equilibrium states. The key tool is modular flow, a nonstandard time-evolution map defined relative to a choice of state, which makes that state "look thermal." Famously, the modular flow for the Minkowski vacuum in the Rindler wedge is a geometric boost, which is one way of stating the Unruh effect. In this talk, I will outline a characterization of the settings in which modular flow is geometrically local, i.e., a complete list of "generalized Unruh effects" in arbitrary spacetimes and for arbitrary quantum field theories. The arguments involve analytic manipulations of position-space correlators, which may be of independent interest to those of you working on amplitudes.
Nick Geiser (UM): Recent progress on Coon amplitudes: the N-point amplitude and an exact field-theory limit | 3/13/2024
In the last two years, Coon amplitudes have received a burst of renewed interest in the context of the modern S-matrix bootstrap program. The four-point Coon amplitude was first discovered in 1969 by D.D. Coon and is a deformation of string theory’s famous Veneziano amplitude with a free deformation parameter q. At q = 1, Coon amplitudes become tree-level open string amplitudes. Recently, several groups have studied the low-energy expansion of Coon amplitudes, the unitarity properties of Coon amplitudes, various extensions and generalizations of Coon amplitudes, and possible physical models realizing the Coon spectrum. In this seminar, I will survey these recent developments along with some new results on N-point Coon amplitudes. By studying the N-point Coon amplitude, we will discover a particular limit which reproduces the tree-level amplitudes of a particular field theory with an infinite set of non-derivative single-trace interaction terms. This correspondence is the first definitive realization of the Coon amplitude (in any limit) from a field theory described by an explicit Lagrangian.
Dawid Brzeminski (U Maryland): Dynamical equilibration of dark matter and baryon energy densities | 3/20/2024
The near equality of the dark matter and baryon energy densities is a remarkable coincidence, especially when one realizes that the baryon mass is exponentially sensitive to UV parameters in the form of dimensional transmutations. We explore a new dynamical mechanism, where in the presence of an arbitrary number density of baryons and dark matter, a scalar adjusts the masses of dark matter and baryons until the two energy densities are comparable. In this manner, the coincidence is explained regardless of the microscopic identity of dark matter and how it was produced. This new scalar causes a variety of experimental effects such as a new force and a (dark) matter density dependent proton mass.
Evita Verheijden (Harvard): Cryptographic Censorship | 4/03/2024
The classical formulation of the weak cosmic censorship conjecture (WCCC) – the statement that singularities resulting from gravitational collapse are generically hidden behind event horizons – is most probably false. However, I will argue that there is compelling evidence that some version of it should be true in quantum gravity. Working towards a quantum gravitational formulation of the WCCC, I will prove “Cryptographic Censorship”, a theorem that provides a general condition for the formation of event horizons in AdS/CFT: sufficiently (pseudo)random boundary dynamics. I will also provide a classification of sizes of singularities, and show that “large”, “classical” singularities – the ones that the WCCC should rule out – are compatible with sufficiently (pseudo)random dynamics. Thus, if such singularities are indeed described by (pseudo)random dynamics, then they cannot exist in the absence of event horizons.
Cari Cesarotti (MIT): Physics Potential at a future Muon Collider | 4/10/2024
A clear outcome of Snowmass 2021 and now the US P5 report was the community support for R&D towards a future muon collider. In this talk we will discuss the general physics program that becomes available to the community during the construction and completion of the future collider. We will review not only the main challenges and advantages of such a collider compared to other possibilities, but also the projected reach of several specific models. Additionally, we consider the physics possibilities at the necessary demonstrator facilities along the way. For example, a beam dump would be an economical and effective way to increase the discovery potential of the collider complex in a complementary regime.
