Particle Physics

From dark matter to the strong CP problem to the dynamics behind the weak scale, a variety of observations make for a compelling case that the Standard Model is an incomplete description of subatomic physics. Yet none of these puzzles provides unambiguous guidance on how we should proceed to find what comes next.

I will argue that this state of affairs calls for a multi-directional strategy in our quest for physics Beyond-the-Standard-Model. Only a combination of new theoretical developments and original ideas, confronted with the vast array of experiments at our disposal, will provide us with the big picture we need to move beyond.

Models of Dark Matter (DM) can leave unique imprints on the Universe's small scale structure by boosting density perturbations on small scales. We study the capability of Pulsar Timing Arrays to search for, and constrain, subhalos from such models. The models of DM we consider are ordinary adiabatic perturbations in ΛCDM, QCD axion miniclusters, models with early matter domination, and vector DM produced during inflation.

If we live in a supersymmetric world, SUSY has to be broken at some (high) scale. I will show how the presence of a SUSY-breaking hidden sector can lead to gravitational wave (GW) signals at future interferometers. I will focus on first order phase transitions that can occur along the pseudomodulus universally related to SUSY breaking. Current bounds on the superpartners are compatible with GW signals at future interferometers, while the observation of a GW signal from a SUSY-breaking hidden sector would imply superpartners within the reach of future colliders.  Along the way, I will analyze the new field theoretical features of the phase transition along the pseudomodulus direction. These allow us to pinpoint what are the necessary requirements for a SUSY-breaking hidden sector to lead to strong GW signals.

Axion cosmic strings have for some time been considered a potential source of enhancement of axion dark matter production, and have been the subject of extensive simulations (for references, see out in recent years).  But axion strings are rather peculiar entities.  This talk will explore some aspects of these objects, and suggest that they are not likely to play a distinguished role in early universe cosmology.

Detection mechanisms for low mass bosonic dark matter candidates, such the axion or hidden photon, leverage potential interactions with electromagnetic fields, whereby the dark matter (of unknown mass) on rare occasion converts into a single photon. Current dark matter searches operating at microwave frequencies use a resonant cavity to coherently accumulate the field sourced by the dark matter and a near standard quantum limited (SQL) linear amplifier to read out the cavity signal. To further increase sensitivity to the dark matter signal, sub-SQL detection techniques are required. Here we report the development of a novel microwave photon counting technique and a new exclusion limit on hidden photon dark matter. We operate a superconducting qubit to make repeated quantum non-demolition measurements of cavity photons and apply a hidden Markov model analysis to reduce the noise to 15.7 dB below the quantum limit, with overall detector performance limited by a residual background of real photons. With the present device, we perform a hidden photon search and constrain the kinetic mixing angle to  ≤ 1.68×10−15 in a band around 6.011 GHz (24.86 μeV) with an integration time of 8.33 s. This demonstrated noise reduction technique enables future dark matter searches to be sped up by a factor of 1300. By coupling a qubit to an arbitrary quantum sensor, more general sub-SQL metrology is possible with the techniques presented in this work.

The gravitational coupling of nearby massive bodies to test masses in a gravitational wave (GW) detector cannot be shielded, and gives rise to 'gravity gradient noise’ (GGN) in the detector. In this talk, I will discuss how any GW detector using local test masses in the Inner Solar System is subject to GGN from the motion of the field of 10^5 Inner Solar System asteroids, which presents an irreducible noise floor for the detection of GW that rises exponentially at low frequencies. This severely limits prospects for GW detection using local test masses for frequencies below (few) x 10^{-7} Hz. At higher frequencies, I’ll show that the asteroid GGN falls rapidly enough that detection may be possible; however, the incompleteness of existing asteroid catalogs with regard to small bodies makes this an open question around microHz frequencies, and I’ll outline why further study is warranted here. Additionally, I’ll mention some prospects for GW detection in the ~10 nHz-microHz band that could evade this noise source.

This talk will be split into two distinct halves: The first half will be based on the paper arxiv:2007.03662 and suggest that an interplay between microscopic and macroscopic physics can lead to an undulation on time scales not related to celestial dynamics. By searching for such undulations, the discovery potential of light DM search experiments can be enhanced.
The second half will look at some currently unpublished work into finding all the semi-simple subalgebras of u(48) which contain the SM. Such algebras (in a loose sense of the term) form GUTs and studying them has relevance to family unification, proton decay etc. Although there has been previous work into the classification of GUTs, we believe this is the first this broad question has been answered.

In recent years, there has been growing interest in cosmological first-order phase transitions in view of gravitational wave observations with space interferometers such as LISA. However, there is only limited understanding on the bubble dynamics and the gravitational wave signals arising from ultra-supercooled transitions (in which the released energy dominates the plasma energy, i.e., near-vacuum transitions), due to the highly relativistic nature of the transition.

In this talk, I introduce some approaches to understand the dynamics and the gravitational wave signals of ultra-supercooled first-order phase transitions:

(1) These transitions proceed with the propagation and collision of highly relativistic fluid profiles involving shock waves. I introduce an approach to construct an effective description of the propagation of such relativistic profiles (1905.00899).

(2) I present an approach to extend the existing model of gravitational wave production and calculate the gravitational wave signals analytically (1707.03111).

In the conventional weakly-interacting massive particle (WIMP) paradigm the late-time density of dark matter (DM) is set by the rate of two-body annihilations, but there has been considerable recent interest in exploring alternative DM scenarios where other interactions control the final abundance. I will show that by fully exploring the parameter space of a simple, weakly-coupled dark sector, we can find a rich set of novel pathways which lead to the observed relic density of DM. In particular, we can identify and characterize a general class of mechanisms in which the DM relic abundance is determined by processes controlling the thermal coupling of the DM and Standard Model (dubbed the KINetically DEcoupling Relic -- KINDER), generalizing previously-studied special cases of this behavior.

One of the puzzles that the newly data-rich fields of cosmology and astrophysics are most advantaged to tackle is the nature of the dark sector.  In particular, a dark sector that thermalizes with the SM bath at some epoch has present-day observable properties that are directly tied to early-universe interactions. In this talk I survey some recent work on detecting different types of thermal dark particles by leveraging different cosmological and astrophysical datasets.  I discuss the utility of each of these different probes, the implication for particle theories, and prospects for the future. 

Axion-like particles (ALPs) are one of the most attractive solutions to the dark matter issue. In this talk, I will discuss new ideas in the search for ALPs in astrophysical structures. In particular, I will focus on the emission associated with their decay into photons. The discussion will involve different ALP mass ranges and wavelength bands.

Rolling scalar fields play an important role in understanding cosmology within a particle physics framework. Coupling a rolling scalar field to light degrees of freedom gives rise to a thermal friction which, if large enough, induces a thermal bath. In the context of inflation the presence of such a thermal bath has compelling consequences as it significantly alters the usual observables, leading to a suppression of the tensor-to-scalar ratio r and a unique prediction for non-gaussianities. In my talk, I will illuminate why the axion of a non-Abelian gauge group is the ideal candidate for generating the thermal friction and how it sets the stage for a minimal setup of warm inflation, as well as a potential solution to the Hubble tension.