High Energy Physics Seminar Archive

Abstract: Axion-like dark matter whose symmetry breaking occurs after the end of inflation predicts enhanced primordial density fluctuations at small scales. This leads to dense axion minihalos (or miniclusters) forming early in the history of the Universe. Condensation of axions in the minihalos leads to the formation and subsequent growth of axion stars at the cores of these halos. If, like the QCD axion, the axion-like particle has attractive self-interactions there is a maximal mass for these stars, above which the star rapidly shrinks and converts an O(1) fraction of its mass into unbound relativistic axions. This process would leave a similar (although in principle distinct) signature in cosmological observables as a decaying dark matter fraction, and thus is strongly constrained. We place new limits on the properties of axion-like particles that are independent of their non-gravitational couplings to the standard model. I will also introduce possible photon signatures from axinovae.

Abstract: In this talk, I’ll summarize the status of dark matter direct detection as it stands today and where it’s headed in the next ~10 years. While the experimental program to directly detect light dark matter is proceeding full steam ahead, the theoretical one is at a crossroads. I will review the status of both, highlighting the obstacles theories of sub-GeV dark matter must overcome. I will detail three such benchmarks direct detection experiments are exploring, focusing in particular on our latest “Dark Sink Dark Matter” which has immediate implications for SENSEI and other ongoing experiments.

Abstract: The fact that the cosmic abundances of baryonic and dark matter are so similar, despite their very different astrophysical behavior, is taken as a serious hint from Nature. We propose a rich dark sector parallel in structure to that of the standard model, and related to it via a Z₂ exchange symmetry. A WIMP baryogenesis mechanism is mirrored in the dark sector; small breakings in the Z₂ symmetry result in the dark matter being made of dark neutrons, which are the lightest dark baryons. An extra-dimensional realization of our model in the far UV naturally explains the hierarchical couplings of both sectors as well as the requisite Z₂-breakings. We carefully trace the cosmic history of both sectors, paying particular attention to serious pitfalls inadequately considered in previous literature, such as light relics and dark big bang nucleosynthesis. Finally, we briefly discuss potential experimental signatures.

Abstract: The process of Big Bang Nucleosynthesis (BBN) is a crucial test of cosmology. In this talk, I will describe a new code for predicting the primordial elemental abundance due to BBN. This code takes advantage of JAX, a machine learning framework, to enable fast and differentiable predictions of elemental abundances. This allows us to put BBN calculations on the same level of rigor and ease-of-use as cosmic microwave background analyses, taking nuclear rate uncertainties fully into account. The differentiable nature of the code will also allow the use of more sophisticated and efficient methods of parameter estimation beyond e.g. traditional MCMC techniques.

Abstract: Cosmological production and propagation of heavy particles during inflation can leave a distinctive signature in the non-Gaussianity of primordial density fluctuations. This presents a unique opportunity for the direct detection of particles as heavy as the inflationary Hubble scale (H ≲ 1013 GeV) through precision cosmology. However, the window of observability is often restricted to masses in a small region around H. In this talk, I will discuss a mechanism (dubbed “chemical potential”) for heavy complex scalar fields that can extend this window up to 60H. The mechanism utilizes the large kinetic energy of the inflaton to enhance particle production, and can impart potentially observable non-gaussianity, f_NL~ O(0.01-10). 

Abstract: Many new physics models that address one or more shortcomings of the SM predict additional particle content beyond the SM. I will discuss a few minimal extensions to the SM fermion and scalar sectors. First, I will argue that the new fermions must necessarily be vectorlike under the SM gauge group, and consider the prospects for one of the simplest non-trivial fermionic extension: weak isosinglet charged vectorlike leptons. I will show how an electron-positron collider, including a near future Higgs/top factory, can act as a discovery machine for these particles that seem to pose a much more difficult challenge at (future) hadron colliders. Then, I will consider the possibility of the “depleted Higgs boson” with a universal coupling suppression and an invisible decay width. One or both of these depletion factors can naturally arise in a large class of theories, by way of additional singlet scalars that mix with the Higgs boson. I will discuss the present status of the depleted Higgs boson and argue that in many cases the precision study of the Higgs boson is more powerful than searches for the extra scalar states, given the slate of next-generation experiments that are on the horizon. Finally, many researchers think that LHC limits on superpartner masses negatively affect the viability of supersymmetric gauge coupling unification. I will show through a high-precision analysis that that is not the case, even for standard minimal models of supersymmetry. I will argue that precision unification together with the observed Higgs mass constraint favors the superpartner masses that are in the range of several TeV and beyond. Furthermore, I will highlight regions of parameter space where a Higgsino or a wino can reproduce the thermal dark matter abundance.

Abstract: Within the standard cosmology --- and indeed even within most commonly studied modified cosmologies --- the history of the universe effectively consists of a sequence of single-component epochs with only brief transition periods between them.  However, many extensions of the Standard Model predict the existence of towers of unstable states.  Such towers often lead to a form of "cosmic stasis" in the early universe wherein the relative cosmological abundances of different energy components remain unchanged over an extended period, even as the universe expands.  The emergence of stasis is not a consequence of fine-tuning in cosmologies of this sort; rather, stasis turns out to be a global attractor toward which the universe naturally evolves for a broad range of initial conditions.  In this talk, I shall review the general conditions under which stasis emerges in such scenarios and discuss particular realizations of this phenomenon, such as those involving towers of Kaluza-Klein states or populations of primordial black holes with an extended mass spectra.  I shall also examine some of the potential implications of a stasis epoch for the evolution of primordial density perturbations and the growth of structure, for the primordial gravitational-wave background, for dark-matter production, and even for the age of the universe.

Abstract: We explore the "Elusive Universe," where dark matter, neutrinos, and gravity sculpt our observed nature, leaving many open questions. Our work focuses on opening up new directions to answer these puzzles. I will present two approaches:

1) Using precision astrometry and space mission data to study general relativity, dark matter, cosmic neutrinos, and fifth forces [1, 2, 3].

2) Utilizing quantum sensors mounted on spacecraft to study ultralight dark matter gravitationally bound to the sun, inspired by Deep Space Atomic Clocks (DSACs) and Parker Solar Probe [4].

These are highly interdisciplinary efforts, highlighted by the DOE Office of Science. The approaches can also be applied to studying interstellar objects, primordial black holes, Planet Nine, topological defects, dark nuggets, and strongly interacting dark matter.

[1] Tsai et al., https://arxiv.org/abs/2309.13106 (2023, OSIRIS-REx result)
[2] Tsai et al., https://arxiv.org/abs/2210.03749 (2022)
[3] Tsai et al., https://arxiv.org/abs/2107.04038, JCAP (2023)
[4] Tsai et al., https://arxiv.org/abs/2112.07674, Nature Astronomy volume 7, pages 113–121 (2022)

Abstract: We explore the ramifications of natural supersymmetry (natSUSY) frameworks for upcoming experiments at the high-luminosity Large Hadron Collider (HL-LHC). Specifically, we scrutinize the production and subsequent decay modes of heavy SUSY Higgs bosons, both neutral and charged, as well as stop pairs and electroweakino pairs, within the context of natSUSY. The study highlights the importance of decay hierarchy for accurate interpretation of LHC data. A detailed examination reveals that the dominant decay modes of heavy winos to Standard Model bosons—W, Z, or h—alongside light higgsinos with weak-scale masses, emerge as a unique signature of the natSUSY paradigm. The investigation delineates both the discovery and exclusion limits for these heavy SUSY particles, thereby offering critical insights into the viability and constraints of natural SUSY models in the forthcoming LHC runs.

Abstract: We study the solar emission of light dark sector particles that self-interact strongly enough to self-thermalize. The resulting outflow behaves like a fluid that accelerates under its own thermal pressure to highly relativistic bulk velocities in the solar system. Compared to the ordinary noninteracting scenario, the local outflow has at least ~10^3 higher number density and correspondingly at least ∼10^3 lower average energy per particle. We show how this generic phenomenon arises in a dark sector composed of millicharged particles strongly self-interacting via a dark photon. The millicharged plasma wind emerging in this model has novel yet predictive signatures that encourage new experimental directions. This phenomenon demonstrates how a small step away from the simplest models can lead to radically different outcomes and thus motivates a broader search for dark sector particles.

Abstract: Super-horizon (iso)curvature perturbations can arise from statistical fluctuations of bubbles in a first-order phase transition (PT). The perturbations follow a generic power-law scaling in PT parameters. Even if the PT occurs in a secluded dark sector, the resulting curvature perturbation is constrained by cosmological data. I will present existing and future bounds on the dark radiation energy released associated with a dark-sector PT. For PTs that happen between photon temperatures from eV to keV, this new constraint already surpasses the standard ∆Neff bound. Future constraints on the primordial curvature perturbation may extend the bound to the 100 MeV scale.