Times: Wednesdays, 5:00-6:15 p.m.

To receive the announcements and the invitations to the video conference, please contact Zac Picker and subscribe to the Google group tepapp-seminar@lists.ucla.edu. All subscribers will receive a zoom invitation to the seminar as full participants, or are welcome to join us in room PAB 4-708 of the Physics and Astronomy Building at UCLA.

Seminars marked 'in person' indicate the speaker will be physically at UCLA (rather than on zoom).

Spring 2024

Wednesday, Apr. 17 2024 In person
Huangyu Xiao (Fermilab) Detecting Axion Dark Matter with Black Hole Polarimetry

The axion or axion like particle (ALP), as a leading dark matter candidate, is the target of many on-going and proposed experimental searches based on its coupling to photons. However, indirect searches for axions have not been as competitive as direct searches that can probe a large range of parameter space. In this talk, I will introduce the idea that axion stars will inevitably form in the vicinity of supermassive black holes due to Bose-Einstein condensation, enhancing the axion birefringence effect and opening up more windows for axion indirect searches. The oscillating axion field around black holes induces polarization rotation on the black hole image, which is detectable and distinguishable from astrophysical effects on the polarization angle, as it exhibits distinctive temporal variability and frequency invariability. We show that the polarization measurement from Event Horizon Telescope can set the most competitive limit on axions in the mass range of 10^-21-10^-16 eV.

Wednesday, Apr. 24 2024 In person
Yifan Lu (UCLA)TBC, Feeding plankton to whales

Wednesday, May. 1 2024 In person
Alex Laguë (University of Pennsylvania, University of Toronto) TBC, ultralight DM simulations


Wednesday, May. 8 2024 In person
Shirley Li (UC Irvine) TBC, Supernovae neutrinos

Wednesday, May. 15 2024
Gabriela Sato-Polito (John Hopkins)TBC, Pulsar timing arrays


Wednesday, May. 22 2024, In person
Arnab Dasgupta (University of Pittsburgh) Baryon Asymmetry and dark matter


Winter Term 2024

Wednesday, Feb. 7 2024
Ciaran O'Hare (University of Sydney), Axion dark matter in the solar neighbourhood

Axions are an increasingly popular dark matter candidate with a flourishing experimental campaign now poised to discover them if they exist. In one of the general classes of cosmological production scenarios—namely the scenario in which the axion is born after inflation—it has been known for some time that dark matter is expected to inherit ultra-small-scale inhomogeneities from dynamics taking place at the QCD scale. These inhomogeneities eventually collapse into small-scale structures called miniclusters or minihalos, which have potentially drastic consequences for experimental efforts to detect the axion on Earth. In this talk, I will outline some of the work going into making a prediction for the degree of substructure in the axion dark matter distribution and attempt to quantify the extent to which prospects for detecting axions experimentally are hindered, or potentially doomed, due to the formation of miniclusters.

Wednesday, Feb. 14 2024, In person
Isabelle John (Stockholm University), Understanding Dark Matter with Cosmic Rays and Stars at the Galactic Center

Dark matter may annihilate into final states including positrons, which would appear as peaked signatures in the local cosmic-ray positron flux. Using a refined model of the energy losses experienced by the positrons during propagation, I will show that contributions from dark matter can be clearly distinguished from pulsars, which also produce highly-energetic positrons. This makes the cosmic-ray positron flux a promising place to search for dark matter signals. Dark matter may also change the evolution of stars. At the Galactic Center, the dark matter density is very high, and dark matter can sufficiently accumulate and annihilate inside the star. I will present results on how this extra energy affects the stellar evolution, can disrupt stars and even prevent their formation.

Friday, Feb. 23 2024, 10:30 AM (TEP Journal Club)
Yuval Grossman The Neutrino Force

Wednesday, Mar. 6 2024 In person
Muping Chen (UCLA), Primordial Black Hole Sterile Neutrinogenesis: Sterile Neutrino Dark Matter Production Independent of Couplings

Sterile neutrinos are well-motivated and actively searched for hypothetical neutral particles that would mix with the Standard Model active neutrinos. They are considered prime warm dark matter (DM) candidates, typically when their mass is in the keV range, although they can also be hot or cold DM components. We discuss in detail the characteristics and phenomenology of sterile neutrinos that minimally couple only to active neutrinos and are produced in the evaporation of early Universe primordial black holes (PBHs), a process we called ``PBH sterile neutrinogenesis". Contrary to the previously studied production mechanisms, this novel mechanism does not depend on the active-sterile mixing. The resulting sterile neutrinos have a distinctive spectrum and are produced with larger energies than in typical scenarios. This characteristic enables sterile neutrinos to be WDM in the unusual 0.3 MeV to 0.3 TeV mass range, if PBHs do not matter-dominate the Universe before evaporating. When PBHs matter-dominate before evaporating, the possible coincidence of induced gravitational waves associated with PBH evaporation and astrophysical X-ray observations from $\nu_s$ decays constitutes a distinct signature of our scenario.

Autumn Term 2023

Wednesday, Nov. 8 2023 4:45 PM PST
Ellen Sirks (University of Sydney), Galaxy clusters: giant dark matter particle colliders

Galaxy clusters are the largest gravitationally bound structures in the Universe. Because of their high density and local velocity dispersion, they are ideal environments for probing the nature of dark matter. The specific properties of dark matter can have great effects on both clusters as a whole as well as on the galaxies residing in them. For example, if the self-interaction cross-section is non-zero, such effects include (but are not limited to) offsets in merging clusters, rounder cluster haloes, modified gravitational lensing, subhalo evaporation, and the flattening of density profiles. In this talk I will present my work studying some of the effects of self-interacting dark matter on simulated galaxy clusters. In addition, I will make forecasts for the balloon-borne telescope SuperBIT, which flew from April 15 to May 25, 2023.

Wednesday, Nov. 1 2023 4:45 PM PST
Mehrdad Phoroutan Mehr (UC Riverside), Constraints on Dark Matter Models with Astrophysical Observations

In this talk, I will discuss the constraints on dark matter properties with compact astrophysical objects. I will first present indirect detection constraints of self-interacting dark matter models using gamma-ray measurements from the M87 galaxy. This model predicts robust dark matter self-interactions, thermalizing the inner halo. Consequently, the density profile surrounding the supermassive black hole of M87 becomes shallower compared to that in the cold dark matter model. We find that the expected signal strength is below the current gamma-ray constraints for both s- and p-wave annihilation, even in the presence of Sommerfeld enhancement. Then, I will discuss dark matter searches with exoplanets. Dark matter particles can be captured by exoplanets, leading to observable signals, such as anomalous heating and potential destruction. Lastly, I will discuss constraints on inelastic dark matter from neutron stars, direct detection experiments, and the observed relic abundance of dark matter

Wednesday, Oct. 25 2023 4:45 PM PST
Valerio De Luca (University of Pennsylvania), Superfluid dark matter around black holes

The theory of superfluid dark matter is based on sub-eV, self-interacting, bosons which may undergo Bose-Einstein condensation at the center of galaxies, thus creating a superfluid homogeneous core. We show how the superfluid density profile changes when massive black holes sit within these environments, giving rise to dark matter spikes whose slopes depend on the bosons self-interactions. Finally, we discuss the role of dynamical friction in the evolution of black hole binaries moving within the superfluid.

Wednesday Oct. 18, 2023 4:45 PM PST
Stefano Profumo (UC Santa Cruz), Rogue Worlds Meet the Dark Side

In the age of gravitational wave astronomy and direct black hole imaging, the possibility that some of the black holes in the universe have a primordial, rather than stellar, origin, and that they might be a non-negligible fraction of the cosmological dark matter, is quite intriguing. I will discuss a few ongoing projects, including (perhaps not in this exact order) (1) understanding how current and future microlensing surveys may have detected, or will soon detect, a mix of rogue planets and, potentially, light black holes of non-stellar origin, (2) what to expect if, and whether to expect that, a nearby light black hole was exploding right now, (3) how self-interacting dark matter can lead to primordial-looking late-forming black holes, and (4) whether microstructure black holes (originally proposed by Picker and Kusenko) can produce exotic forms of antimatter.

Wednesday Oct. 11, 2023 4:45 PM PST
Vincent S.H. Lee (Caltech), Probing Dark Matter with Pulsar Timing Arrays and Gravitational Wave Detectors

Pulsar timing arrays (PTAs) and gravitational wave detectors can serve as valuable tools in the detection of dark matter. Dark matter substructure within the Milky Way Galaxy can induce gravitational pulls on pulsars, leading to observable deviations in pulsar timings. We demonstrate that dark matter models predicting enhanced power on small scales are potentially within the reach of future PTA experiments, such as the Square Kilometer Array (SKA). This includes a class of post-inflationary QCD axion models known as axion miniclusters. We also discuss pipelines for searching for dark matter signals in PTA datasets and recent results from the 15-year data release of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which has reported evidence of a stochastic gravitational background. Furthermore, dark matter particles with a mass on the kilogram scale can also produce signals in laser interferometry-based gravitational wave experiments. We discuss the signals and detection prospects from current and future gravitational wave experiments, such as LIGO and the Einstein Telescope.

Wednesday Oct. 4, 2023 4:45 PM PST
Christopher Cappiello (Queen's U, Kingston), The Morphology of Exciting Dark Matter and the Galactic 511 keV Signal

More than 50 years after it was first observed, the 511-keV gamma ray line at the Galactic Center remains unexplained, and numerous dark matter models have been proposed to account for this signal. Perhaps the simplest is the annihilation of light dark matter into electron-positron pairs, but this model faces challenges due to cosmological bounds on the required dark matter mass range. An alternative model, which avoids these constraints, is exciting dark matter (XDM). In this model, collisions between TeV-scale dark matter particles excite the dark matter to a state that can then decay back to the ground state, releasing an electron-positron pair. We study the morphology of the 511-keV signal produced by both the annihilating and exciting dark matter models, and compare the resulting flux with the most recent INTEGRAL data. We perform the first full statistical analysis of the exciting dark matter model, and find that it reproduces the observed morphology significantly better than the annihilating dark matter case.

Find out more about the TEPAPP research group.