Particle Physics

The Cherenkov Telescope Array Observatory (CTAO) is the upcoming next-generation ground-based very-high-energy (VHE) gamma-ray observatory. The CTAO will significantly advance the study of VHE gamma-rays through a combination of wider field of view,  substantially increased detection area, and superior angular and spectral resolution over an energy range extending from tens of GeV to hundreds of TeV. Full-sky coverage will be achieved using two independent Imaging Air Cherenkov Telescope (IACT) arrays: one in the northern hemisphere (Canary Islands, Spain) and one in southern hemisphere (Paranal, Chile). The CTAO will explore a wide range of science topics in high-energy astrophysics, including the origin of higher-energy cosmic rays, mechanisms for particle acceleration in extreme environments, and astroparticle phenomena that may extend the Standard Model of particle physics.  In this talk, I will outline the broad science potential of the CTAO and provide the CTAO’s current status and timeline. I will also describe the contributions of the CTAO-US collaboration to CTAO, including the development of an ultra-high resolution Schwarzschild-Couder telescope for VHE astronomy and the emergence of UV-band optical astronomy at the sub-100 micro-arcsecond angular scale.

The matter power spectrum on subgalactic scales is very weakly constrained so far. While inflation predicts a nearly scale-invariant primordial power spectrum down to very small scales, many new physics scenarios can lead to significantly different predictions, such as axion dark matter in the post-inflationary scenario, vector dark matter produced during inflation, early matter domination, kinetic misalignment axions, self-interacting dark matter, atomic dark matter, etc. Therefore, any successful measurement on the matter power spectrum tests inflation extensively and probes early universe dynamics and the nature of dark matter, making it a new frontier in cosmology and dark matter physics. We proposed observing fast radio bursts (FRB) with solar-system scale interferometry by sending radio telescopes to space, which allows us to greatly expand the sensitivity on the matter power spectrum from Mpc to AU scales. Two sightlines looking at the same FRB source can sample different regions of the Universe in the transverse direction and thus obtain an arrival time difference that depends on the matter power spectrum. Our calculations show that this setup will be sensitive to the scale-invariant power spectrum predicted by inflation on small scales and can also probe QCD axion miniclusters predicted in the post-inflationary scenario.

Dark matter particle interactions could imprint characteristic signals in cosmic-ray and multi-wavelength observations of the sky. The central challenge is to distinguish these signatures from similar spectra produced by standard astrophysical processes, such as the life and death of stars and the interactions of cosmic rays with interstellar material. The GAPS Antarctic balloon payload, en route for its initial flight in December 2024, is the first experiment optimized specifically for low-energy cosmic antideuterons, an essentially background-free signature of dark matter, as well as antiprotons in an unprecedented low-energy region and leading sensitivity to cosmic anithelium. In this talk, I will detail the novel GAPS detection technique, its flight instrument, and the potential impact of these measurements in the coming years. 

Primordial black hole (PBH) dark matter can be probed by "picolensing". Widely spatially separated gamma-ray detectors near Earth would observe parallax of an intervening PBH lens with respect to a cosmologically distant gamma-ray burst (GRB). This parallax can be of order the Einstein angle of the lens, resulting in differential magnification of the source as viewed from the two detectors. Simultaneous brightness measurements of the same GRB made by two detectors is sensitive to this effect. Two recent studies in the literature have shown this approach could be a promising way to search for PBH dark matter in part of the "asteroid mass window", roughly (few) * 1e-15 < M_{PBH} / M_{Sun} < (few) * 1e-11. In this talk, I will discuss some ongoing work to explore the robustness of this signal to various uncertainties not previously carefully accounted for: e.g., uncertainties in the transverse extent of the GRB emission region, its intensity profile, detector background rates, sensitivity of the projection to outlier GRB events, etc. I'll show that, while the large GRB source size uncertainties do degrade previous projections somewhat, it is still possible to probe most of the PBH DM asteroid mass window with a mission that employs two Swift/BAT-class detectors separated by a distance on the order of an AU. Depending on the total number of GRBs that such a mission ultimately observes, it may even be possible to robustly probe new subcomponent dark-matter parameter space at PBH masses above the window, potentially as high as (few) * 1e-6 M_{Sun}.

Big Bang Nucleosynthesis (BBN) is a powerful tool for probing both new physics and LCDM, and complements analyses utilizing the Cosmic Microwave Background (CMB) and results from particle experiment.  I will provide two examples of BBN probes of BSM models.  I will then discuss new kinds of analyses that can be performed with the recently-released fast and differentiable BBN code LINX.  In particular, LINX can be used to perform full BBN+CMB joint analyses at a level of sophistication that has never been achieved before, even in LCDM analyses.

Dark matter and neutrinos are elusive neutral matter filling up our Universe. The PandaX (Particle and Astrophysical Xenon) experiment, located in the China Jinping Underground Laboratory, is dedicated to searching for dark matter particles and studying the fundamental properties of neutrinos. The current running experiment, PandaX-4T, contains a sensitive time-projection-chamber with a 3.7-ton liquid xenon target. In this talk, after an overview, I will present recent results from PandaX-4T in dark matter direct detection, double beta decay of 136Xe, as well as solar neutrinos. I will also present a concrete plan for the next-generation xenon observatory in CJPL, PandaX-xT.

In this talk I will do three things. First, I will outline the conditions under which the interaction rate of inelastic processes with a system consisting of N targets scales as N^2. Second, I will present computations of interaction rates for several weakly interacting particles, including the Cosmic Neutrino Background and QCD axion dark matter, and will explain the underlying physics. Third, I will introduce new quantum observables that do not rely on net energy transfer, but can still extract these N^2 effects. This talk will not address a concrete experimental proposal, but the effects presented may point to a new class of table-top and ultra-low threshold particle detectors.

I discuss how quantum mechanical effects prevent violations of causality in low energy processes, even when faster-than-light propagation is possible. At the classical level, faster-than-light propagation can be used to build "time-machine" configurations that violate causality. However, low-energy quantum excitations propagating on these backgrounds lead to divergent backreaction through loop effects. These divergences fully probe the UV dynamics of the system, making it impossible to prepare and describe causality violating configurations in the regime of validity of effective field theory. In light of these results, I conclude that effective field theories with negative Wilson coefficients or Galileon-symmetric interactions are not in tension with causality, despite leading to faster-than-light propagation.

Hot viscous plasmas unavoidably emit a gravitational wave background, similar to electromagnetic black body radiation. In this talk we will discuss the contribution from hidden particles to the diffuse background emitted by the primordial plasma in the early universe. While this contribution can easily dominate over that from Standard Model particles, both are capped by a generic upper bound that makes them difficult to detect with interferometers in the foreseeable future. We will illustrate our results on the examples of axion-like particles and heavy neutral leptons. We will also discuss how this bound affects the previous estimates of gravitational wave backgrounds from particle decays out of thermal equilibrium.

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I will argue that there are quantum states of the field theories of general relativity and electromagnetism that we typically ignore, but have interesting phenomenological effects. These states amount to relaxing the constraint equations known as the Hamiltonian and momentum constraints in GR and Gauss’ law in EM. Turning off the Hamiltonian constraint sources non-dynamical parts of the metric which mimic a pressure-less dust, and thus these effects may be the explanation as to why we have inferred the existence of dark matter, both locally and cosmologically. Turning off the momentum constraints add additional velocity-dependent source terms to this effective dust, but these effects are not conserved and redshift quickly outside the horizon. Turning off the Gauss’ law constraint mimics a charge density that does not respond to electric forces, but follows geodesics, thus adding a charged component to the dust. The effects in electromagnetism may have interesting impacts on BBN, the baryon-photon fluid during and after recombination, galactic dynamics, and cosmic rays. If this new structure in the gravitational and electric fields explain dark matter, it forbids an early period of inflation and therefore requires a different explanation for density perturbations.

The axion is one of the most compelling new physics candidates, deriving many of its important properties from an approximate shift symmetry. In this talk, we will consider the general form of the axion coupling to photons in the presence of such a broken shift symmetry. We will show that the axion-photon in general becomes a non-linear monodromic function of the axion. The non-linearity is correlated with the axion mass and singularities in the axion-photon coupling are associated with cusps in the axion potential. We derive the general form of the axion-photon coupling for several examples including the QCD axion and show that there is a uniform general form for this monodromic function. The full non-linear profile of this coupling is phenomenologically relevant to the dynamics induced on axion domain walls/strings and other extended objects involving the axion.

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