Times: Wednesdays, 4:45-6:00 p.m. (but sometimes 9 a.m., depending on time zone)
To receive the announcements and the invitations to the video conference, please contact Zac Picker and subscribe to the Google group firstname.lastname@example.org. All subscribers will receive an invitation to the seminar as full participants.
Seminars can also be attended in-person at room PAB 4-708 of the Physics and Astronomy Building at UCLA.
Wednesday Oct. 4, 2023 4:45 PM PST
Christopher Cappiello (Queen's U, Kingston), Exciting dark matter
Wednesday Oct. 11, 2023 4:45 PM PST
Vincent S.H. Lee (Caltech), TBC, Gravitational waves and dark matter
Wednesday Oct. 18, 2023 4:45 PM PST
Stefano Profumo (UC Santa Cruz), TBC, PBHs (probably)
Wednesday, Oct. 25 2023 4:45 PM PST
Valerio De Luca (University of Pennsylvania), TBC, Superfluid dark matter and black holes
Wednesday, Nov. 1 2023 4:45 PM PST
Mehrdad Phoroutan Mehr (UC Riverside), TBC, Dark matter phenomenology
Wednesday, Nov. 8 2023 4:45 PM PST
Ellen Sirks (University of Sydney), TBC, cosmology or astronomy
Past seminars, 2022/2023
Wednesday June 7, 2023 4:45 PM PST
Elena Pinetti (Fermilab), Putting all the X in one basket: X-ray constraints on sub-GeV dark matter
I will focus on light dark matter particles, with a mass between 1 MeV and a few GeV. These particles can annihilate or decay into electron-positron pairs which can upscatter the low-energy fields in our Galaxy and produce X-ray emission. By using the X-ray data from XMM-Newton, Integral, Suzaku and NuStar, we derive strong constraints on MeV dark matter. In the decay scenario, our bounds are the strongest to date for masses above 100 MeV and improve up to 3 orders of magnitude upon existing limits. In the annihilation case, our constraints are the strongest available for dark matter masses above 180 MeV.
Wednesday May 31, 2023 4:45 PM PST
Xiaolong Du (UCLA and Carnegie Observatories), Axion Star Mergers and Enhanced Axion Dark Matter Decay
Axion stars are observed to form in simulations of dark matter (DM) halos consisting of axion-like particles. They are the ground states of the Schrödinger-Poisson equation (soliton solution) with the self-gravity balanced by the so-called “quantum pressure”. Axion stars are unstable above a critical mass, and can decay to either relativistic axions or photons, depending on the values of the coupling constants. The emitted photons heat the intergalactic medium and alter the epoch of reionization. I will present our study on two mechanisms by which solitons lead to enhanced DM decay: by plasma blocking of parametric resonance, and by major mergers leading to formation of super-critical solitons. I will discuss how we can compute the enhanced DM decay rate using the extended Press-Schechter formalism and Monte Carlo merger trees. I will also show the new constraint on axion-photon coupling constants for axion mass in the range 1e-14 eV - 1e-8 eV we obtained using the Planck measurement on the Thompson optical depth.
Wednesday May 17, 2023 4:45 PM PST
Jonah Hyman (UCLA), Catastrogenesis: Producing primordial black holes by string-wall annihilation
We propose a new scenario for the formation of asteroid-mass primordial black holes (PBHs): A global U(1) symmetry is broken spontaneously and then explicitly in the early Universe. This forms a network of cosmic strings and domain walls. In the process of catastrogenesis, this string-wall network is annihilated, producing axion-like particles, PBHs, and gravitational waves. PBHs produced through catastrogenesis could constitute all of the dark matter, and the gravitational waves produced through catastrogenesis could be visible in future interferometers.
Wednesday May 10, 2023 (9:00 am PST, online only!)
Sten Delos (Max Planck Institute for Astrophysics), "Primordial black holes and ultradense halos"
Primordial black holes (PBHs) form from large-amplitude initial density fluctuations and may comprise some or all of the dark matter. If PBHs have a broadly extended mass spectrum, or in mixed PBH-particle dark matter scenarios, the extreme density fluctuations necessary to produce PBHs also lead to the formation of a much greater abundance of dark matter minihalos that could form even before the matter-dominated epoch. Their early formation would make the density inside these halos extraordinarily high, leaving them susceptible to detection by microlensing and other approaches.
Wednesday May 3, 2023 (9:00 am PST, online only!)
Joseph Allingham (Technion), Towards a more holistic description of galaxy clusters: a joint Strong Lensing & X-ray galaxy cluster study
Strong gravitational Lensing allows to map the total (dark matter and baryonic) density within galaxy clusters; while X-ray and SZ effect observations unveil the Intra-Cluster Medium (ICM) baryonic physics. After presenting both methods, I will display the analytical relationship between both the DM and ICM fluids, under reasonable hypotheses (hydrostatic equilibrium, self-similar ICM temperature profile). I will then review such a joint analysis on cluster Abell S1063, and the new avenues it opens to study dark matter in clusters.
Wednesday March 1, 2023
Joshua Ziegler (University of Texas at Austin), "Peering into the Gap: Learning about Dark Matter from the Pair Instability Mass Gap"
Current models of stellar evolution predict a lack of black holes in the mass range 50-140 solar masses. We explore one way that introducing dark matter to this stellar evolution could influence this mass gap. In particular, given appropriate conditions, it is possible that the addition of dark matter may offer a way to produce black holes throughout this mass gap. In addition, we explore how dark matter could play a role in producing stellar evolution effects that could be observable.
Wednesday February 22, 2023
Sebastian Baum (Stanford), "Axion Clumps Meeting Neutron Stars"
Abstract: Axions are intriguing candidates for dark matter. Depending on the formation mechanism of axion dark matter, the axion field may exhibit substantial density fluctuations on small scales. These density fluctuations lead to the formation of self-gravitating clumps of axions, known as miniclusters and axion stars. In this talk, I will discuss these clumps and what is, and what is not, known about them, and how to, perhaps, find them. In one of the classical axion dark matter scenarios (where the Peccei-Quinn symmetry is broken after the end of inflation), most of the axion dark matter may be bound in such axion clumps. On the one hand, this makes "direct detection" type searches for axions such as ADMX more difficult since the ambient axion density might be much lower than the usual ~0.3 GeV/cm3 expectation. On the other hand, such axion clumps might offer new exciting possibilities for "indirect detection" of axions: if such an axion clump would encounter a neutron star, the axions could resonantly convert into radiophotons in the neutron star's magnetosphere. The signal would be a narrow spectral line, strongly anisotropic, and lasting a typical time scale of ~1 year for an axion minicluster to ~1 minute for an axion star.
Wednesday February 15, 2023
Minxi He (KEK), "Hot Spots Created by Primordial Black Holes"
Black holes evaporate by emitting Hawking radiation in vacuum or a thermal bath with lower temperature than the Hawking temperature. Evaporation accelerates as the black hole losses its mass through emission of particles and plays a significant role especially when the black hole mass is small. In the case of astrophysical black holes which are formed by collapse of stars, Hawking radiation is negligible. However, primordial black holes (PBHs) which form in the early Universe may have masses small enough for the evaporation to be important. In this talk, I will show how the Hawking radiation from small PBHs interact with the ambient plasm and get thermalized, which results in high-temperature region around a PBH, i.e. a hot spot. I will also briefly discuss its implication to cosmology.
Wednesday February 8, 2023
Alexander C. Ritter (Melbourne University), "Exploring the cosmological dark matter coincidence using infrared fixed points"
The asymmetric dark matter (ADM) paradigm is motivated by the apparent coincidence between the cosmological mass densities of visible and dark matter. However, most ADM models only relate the number densities of visible and dark matter, and do not motivate the similarity in their particle masses. One exception is a framework introduced by Bai and Schwaller, where the dark matter is a confined state of a dark QCD-like gauge group, and the confinement scales of visible and dark QCD are related by a dynamical mechanism utilising infrared fixed points of the two gauge couplings. In this seminar, I will discuss recent work where we built upon this framework by properly implementing the dependence of the results on the initial conditions for the gauge couplings in the UV. We then reassessed the ability of this framework to naturally explain the cosmological mass density coincidence, and identified features of the viable models that allow them to naturally relate the masses of the dark baryon and the proton.
Wednesday Febuary 1, 2023 (9:00 am PST, online only!)
Damiano F.G. Fiorillo (Niels Bohr Institutet, Københavns Universitet), "Strong Supernova 1987A Constraints on Bosons Decaying to Neutrinos"
Supernovae are an ideal testbed for the existence of feebly interacting particles, which can be produced in the dense and hot cores and escape the star. If the emission is so copious that it drains significant energy from the core, it would have shortened the duration of the neutrino burst observed from SN1987A at Kamiokande II and the Irvine-Michigan-Brookhaven (IMB) detectors. This allows to constrain novel particles using an energy-loss criterion. Here we show that, if the new particles are coupled to neutrinos, they lead to an additional distortion of the neutrino signal. Focusing on the example of Majoron-like particles, we show that they could have decayed outside the core, producing 100 MeV neutrinos. Using published and unpublished legacy data from Kamiokande II and IMB, we show that no such feature was observed in 1987, allowing us to constrain the new boson emission. Our constraints are more stringent than the ones coming from Big Bang Nucleosynthesis, and imply that the emission is 100 times smaller than the flux saturating the energy-loss criterion.
Wednesday January 25, 2023 (9:00 am PST, online only!)
Lorenzo Piga (INFN, Parma), "Effects of accreting Primordial Black Holes on the Cosmic Microwave Background"
In Piga et al. (2022), we developed a more realistic picture of accretion for Primordial Black Holes (PBHs) by accounting for the contribution of outflows. Such a model of accretion would affect the thermal history of the universe to an extent that can be probed with a number of cosmological observables such as the Cosmic Microwave Background (CMB) anisotropies. We have found that the presence of such outflows introduces an additional layer of uncertainty that needs to be taken into account when quoting cosmological constraints on the PBH abundance, with important consequences in particular in the LIGO-Virgo-KAGRA (LVK) observational window. An accurate modelling of the accretion mechanism is fundamental to test the consistency between cosmological constraints and astrophysical hints (such as Gravitational Waves (GWs)) in support of the PBH hypothesis.
Wednesday Jan 11, 2023 (4:45 pm PST)
Giovanni Pierobon (University of New South Wales), "Axions in the post-inflationary scenario: from strings to miniclusters"
In the scenario in which the QCD axion is born after inflation, the Univere is filled with a highly inhomogeneous scalar field that evolves in a nonlinear fashion. Understanding the eventual abundance and distribution of axionic dark matter in this scenario therefore requires dedicated numerical simulations. In this talk we will summarise our study on the complex dynamics of the axion field, in chronological order with cosmic time, from the scaling of cosmic strings, the formation of domain walls and axitons, and the gravitational formation of miniclusters. We discuss the numerical methods and potential problems involved with simulating post-inflationary axions and how comparing with generic axion-like particles (ALPs), the dark matter production changes substantially. We describe the distribution of dark matter axions well after matter-radiation equality and what are the implications for axion direct detection experiments
Wednesday November 9, 2022 (5:00 p.m P.T)
Joaquim Iguaz (LAPTh), "Are PBHs everything everywhere all at once? Astrophysical and cosmological signatures of PBHs"
In recent years, Primordial Black Holes (PBHs) have been presented as extremely versatile objects providing a unique probe of the early Universe, gravitational phenomena, high energy physics and quantum gravity. Of particular interest is the role of PBHs as a non-particle candidate for the dark matter (DM). Although most of the PBH DM parameter space is tightly constrained, the asteroid mass range is still potentially viable. The lower end is accessible via high-energy astrophysical probes, sensitive to their Hawking evaporation spectrum. In the first part of the talk, I will revisit the constraints on evaporating PBHs from both the isotropic X-ray and soft γ-ray background, and the diffuse soft γ-ray emission towards the inner Galaxy as measured by INTEGRAL, setting the strongest limit on PBH DM for masses up to 4×10^17 g. The interest for PBHs has also been revamped in the light of recent LIGO/Virgo measurements of coalescing black hole binaries with typical masses of tens of M_Sun. The best-motivated scenario for a sizable PBH contribution to such events invokes the QCD phase transition, which naturally enhances the probability to form PBH with masses of stellar scale. In the second part of the talk, I will reconsider the expected mass function associated not only to the QCD phase transition proper, but also the following particle antiparticle annihilation processes, and analyse the constraints on this scenario from a number of observations. We find that the scenario is not viable, unless ad hoc features in the power-spectrum are introduced by hand. Despite these negative results, we note that a future detection of coalescing binaries involving sub-solar PBHs has the potential to check the cosmological origin of SMBHs at the e± annihilation epoch, if indeed the PBH mass function is shaped by the changes to the equation of state driven by the thermal history of the universe.
Wednesday November 2, 2022 (5:00 p.m P.T)
Markus Mosbech (University of Sydney), “Probing dark matter microphysics with gravitational waves and cosmology”
Dark matter remains a mysterious component in our universe. In order to escape existing constraints, it must be at most weakly interacting, and has an upper bound on its allowed mass if it is a thermal relic. I will present a novel method of constraining the microphysics of dark matter using the observed gravitational wave signal, via the impact on structure formation. I will supplement this with forecasts for constraints from 21cm line intensity mapping, as with the next generation of observatories, these two signals may put the strongest limits yet on dark matter-neutrino scattering.
Wednesday October 26, 2022
Isaac Wang (Rutgers Univerity), “Electroweak Baryogenesis from a Naturally Light Singlet Scalar”
We discuss a minimal singlet-scalar extension to the Standard Model that achieves a strong first-order electroweak phase transition. The singlet can be naturally light because of an approximate shift symmetry and no extra hierarchy problem beyond that of the Standard Model Higgs is introduced. The baryon asymmetry of the universe may be explained by local electroweak baryogenesis arising from a coupling between the singlet and weak gauge boson. The predicted electron electric dipole moment is much below the current bound. Strong first order can be achieved from MeV-scale light scalar and small mixing with the Higgs. The viable parameter space can be probed by the observations of rare Kaon decay and the cosmic microwave background. A parity-symmetric model solving the strong CP problem is also discussed. The mixing angle is predicted for a scalar with a mass around 10 GeV or 10 MeV.