Particle Physics

The correlation functions of large-scale fluctuations are crucial observables in modern cosmology. These boundary correlators contain rich information about the bulk dynamics and can be used to probe physics at the inflation scale. There have been considerable efforts in the analytical study of inflation correlators in recent years. In this talk, I will introduce the basic structure of inflation correlators and several analytical methods we have developed for their computation. In particular, I will introduce the method of partial Mellin-Barnes (PMB) representation. With this method, we can largely solve the massive tree correlators analytically. At the loop level, we use PMB to prove a factorization theorem for the nonanalytical part of inflation correlators, which produces a characteristic nonlocal signal. Finally, we can recover the full correlator starting from this nonanalytical part using a dispersion integral.

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Strongly coupled confining theories are well-motivated in many BSM frameworks. The early universe cosmological history of these theories provides possibilities for observable signals. These theories undergo confinement deconfinement phase transition in the early universe, which can result in gravitational wave signals, observable in upcoming experiments. Using AdS/CFT, these theories have been studied in the Randall-Sundrum framework, and various quantitative aspects of the phase transition have been calculated. In the models that have been considered, the rate of transition from the deconfined phase to the confined phase is very small and leads to a period of supercooling. This enhances the gravitational wave signal, but presents a tension between a low confinement scale and fitting to the standard picture of BBN. In this talk, I will briefly review the calculations leading to these conclusions, and argue that some of the issues are specific to the simplified models that have been studied. I will present two modifications that are expected on general grounds, motivated by including strong IR effects systematically. Such effects change the results significantly. In particular, new qualitative features appear which have been missed in previous investigations. I will briefly comment on the phenomenological implications and open questions. The talk will be based on 2309.10090 and 2401.09633.

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In the presence of radiation from bright astrophysical sources at radio frequencies, axion dark matter can undergo stimulated decay to two nearly back-to-back photons, meaning that bright sources could have faint counterimages in other parts of the sky. The counterimages will be spectrally distinct from backgrounds, taking the form of a narrow radio line centered at half the axion mass with a spectral width determined by Doppler broadening in the dark matter halo. In essence, axions behave as an imperfect monochromatic mirror. The morphology of the induced images can be nontrivial, with blurring due to the geometry of the source and image as well as spatial smearing due to the galactic kinematics of axion dark matter. I will show that the axion decay-induced counterimages of galactic sources may be bright enough to be detectable with archival data from CHIME and other ongoing or planned radio surveys. CHIME therefore can run as a competitive axion experiment simultaneously with other science objectives, requiring no new hardware.

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Besides providing a possible explanation to the strong CP problem and dark matter, the QCD axion possesses a rich cosmology. For example, if PQ breaking occurs after inflation, then axion cosmic strings form. Near the QCD phase transition, every axion string become attached to a domain wall which pull on the strings and cause the string-wall network to decay. While every string becomes attached to a domain wall, it is possible, though rare, to form an enclosed domain wall that is not attached to any axion string. These enclosed domain walls collapse under their own self tension, compressing a large amount of energy into a small volume and thereby potentially forming a primordial black hole. In this talk, I will discuss the abundance of enclosed domain walls, their dynamics of collapse, the efficiency of black hole formation, and their relic abundance. For sufficiently large axion decay constants, there may be an observable gravitational lensing signal at future lensing telescopes, especially in models with axion-like-particles.

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Particle production can occur in a curved spacetime like, for example, in the case of thermal emission of particles from black holes, otherwise known as Hawking radiation. The Unruh effect is a quantum field theory result that is closely connected to Hawking radiation. It states that accelerated observers associate a thermal bath of particles to the vacuum state of inertial observers. The Unruh effect has been given special attention because contrary to black hole evaporation, it is a prediction made in a flat (Minkowski) spacetime and therefore can be, in principle, tested in the laboratory. Recently, we have investigated the connection between the Unruh effect and classical radiation for a uniformly accelerated particle. This link seems counter-intuitive since the former is a purely quantum effect while the latter is a classic one. Nonetheless, we find that using a full quantum field treatment of the radiation exchanged by an accelerated charge with the surrounding Unruh thermal bath, the resultant power reduces at tree-level to the usual Larmor formula. The results are also consistent with the observation made by Unruh and Wald which states that the emission of a photon in the inertial frame corresponds to the emission or absorption of a photon in the accelerated frame. The fact that the derivation makes the link between the Unruh effect and the Larmor radiation from a uniformly accelerated charged particle clearer will perhaps help in resolving some of the controversies that have surrounded the Unruh effect since its discovery.

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Theories which spontaneously break spacetime parity can solve the strong CP problem. They usually have few free parameters and are therefore very predictive, but their landscape remains quite unexplored. I will present a construction based on a complete mirror copy of the standard model, linked to our world by colored portal fields. Those induce the partial spontaneous breaking of the color groups, yielding a vanishing theta angle at low energies. The lightest BSM fields could be found at colliders, and are either colored (pseudo-Goldstone or vector) bosons, or some of the vectorlike fermions predicted by parity. The lightest of the latter can actually play the role of thermal dark matter in our model, unlike what was previously found in similar constructions.

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This talk is virtual, but will also be broadcast in the Sky Room. 

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A cosmological network of axion strings in our Universe today may leave its imprint on the polarization pattern of the cosmic microwave background radiation through the phenomenon of axion-string-induced birefringence. I will explain how this signal arises, discuss how it depends on the properties of the string network and the axion-photon coupling, describe how existing measurements of anisotropic birefringence place constraints on axion strings, and discuss how the non-Gaussian nature of this signal could be leveraged in searches with future data.

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