Past events

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.

November 17, 2021 (4:45 p.m P.T) Seminar
Shirley Li (Fermilab), “Neutrino-Nucleus Scattering in Neutrino Oscillation Experiments”

November 10, 2021 (4:45 p.m P.T) Seminar
Harikrishnan Ramani (Stanford University), “Detection of terrestrial millicharges”

November 3, 2021 (9:00a.m P.T) Seminar
Carlos Blanco (Princeton University), “New Directions in Dark Matter Direct Detection”

Wednesday, October 27th, 2021 (9:00a.m P.T) Seminar
Alexander Millar (Stockholm University), “Dark photon limits: a cookbook”

Wednesday, October 20th, 2021 (9:00a.m P.T) Seminar
Weishuang Linda Xu (UC Berkeley), “Cosmological Constraints on Light (but Massive) Relics”

Wednesday, October 13th, 2021 (9:00a.m P.T) Seminar *POSTPONED*
Gilly Elor (Mainz Institute for Theoretical Physics), “Mesogenesis ”

Wednesday, October 6th, 2021 (4:45p.m P.T) Seminar
Anupam Ray (Tata Institute), “Unravelling the mystery of Dark Matter with Black Holes”

Wednesday, June 16th, 2021 (4:45p.m P.T) Seminar
Rebecca Leane (SLAC), “Detecting Dark Matter in Planets”

Wednesday, June 2nd, 2021 (9:00a.m P.T) Seminar
Gabriele Franciolini (University of Geneva), “Primordial Black Holes and Gravitational Wave Observations”

Wednesday, May 26th, 2021 (4:45p.m P.T) Seminar
Anna Simpson (UCLA), “Gravitational Waves from ALP Cosmic String-Wall Networks”

Wednesday, May 19th, 2021 (4:45p.m P.T) Seminar
Hongwan Liu (Princeton University & NYU), “New Aspects of Vector-Portal Dark Matter”

Wednesday, May 12th, 2021 (9:00 a.m P.T) Seminar
Rachel Houtz (Durham University), “Light Dark Matter through Resonance Scanning”

Wednesday, May 5th, 2021 (4:45p.m P.T) Seminar
Brian Shuve (Harvey Mudd College), “Baryogenesis and Dark Matter from Freeze-In”

Wednesday, April 28th, 2021 (4:45p.m P.T) Seminar
Mudit Jain (Rice University), “CMB birefringence from ultra-light axion string networks”

Wednesday, April 14th, 2021 (*9:00am* P.T): Seminar
Anna Suliga (Niels Bohr Institute), “Physics beyond the Standard Model in astrophysical environments”

Wednesday, March 17th, 2021 (4:45pm P.T): Joshua Foster (University of Michigan), “Searching for Dark Matter with 20 Years of Blank Sky Data”

Wednesday, March 10th, 2021 (9:00am P.T): Lena Funcke (Perimeter Institute), “How gravity can shape the low-energy frontier of particle physics”
Wednesday, March 3rd, 2021 (4:45pm P.T): Clara Murgui (Caltech), “Gauging Baryon and Lepton numbers.”
Wednesday, February 24, 2021 (9:00am P.T): Zahra Tabrizi (Virginia Tech), “Constraints on Effective Field Theory from neutrino experiments”
Wednesday, February 17, 2021 (4:45pm P.T): Ciaran O'Hare (University of Sydney), “Directional dark matter detection and the Cygnus project”
Wednesday, February 10, 2021 (9:00am P.T): Samuel Witte, (University of Amsterdam (GRAPPA)), “Non-Standard Observables from Black Hole Superradiance”
Wednesday, February 3rd, 2021 (4:45pm PT): Christopher Cappiello (Ohio State University), “Probing the Landscape of Heavy, Strongly Interacting, and Composite Dark Matter.”
Wednesday, January 27th, 2021 (4:45p.m): Djuna Croon (TRIUMF), “New physics and the black hole mass gap.”
Wednesday, January 20th, 2021 (4:45p.m): Keisuke Harigaya (Institute for Advanced Study), “Axion Kinetic Misalignment and Axiogenesis”
Wednesday, January 13th, 2021 (Special time: 9am): Sunny Vagnozzi (University of Cambridge), “The trouble with spatial curvature”
Wednesday, December 9th, 2020 (4:45 p.m.): Julia Gehrlein (Brookhaven), “CP-Violating Neutrino Non-Standard Interactions in Long-Baseline-Accelerator Data”
Wednesday, December 1, 2020 (4:45 p.m.): Journal Club John Terry (UCLA), “Global analysis of the Sivers functions at NLO+NNLL in QCD”
Wednesday, November 24, 2020 (4:45 p.m.): Laura Sberna (Perimeter Institute), "Dirty gravitational-wave physics"
Wednesday, November 18, 2020 (4:45 p.m. 45 minute talk*): Philip Lu (UCLA), "Constraining Primordial Black Holes with Dwarf Galaxy Heating"
Wednesday, October 21, 2020 (4:30 p.m.): Journal club meeting Manibrata Sen (UC Berkeley/Northwestern University), "keV sterile dark matter and extra radiation"
Wednesday, October 28, 2020 (4:30 p.m.): Journal club meeting William DeRocco (Stanford), "Muons in supernovae: implications for the axion-muon coupling"
Wednesday, October 21, 2020 (4:30 p.m.): Journal club meeting Graciela Gelmini, "Gravitational wave signatures from discrete flavor symmetries" Literature: arXiv:2009.01903 Philip Lu, "Sterile neutrinos in non-standard cosmologies", Literature: arXiv:1909.13328
Wednesday, October 14, 2020 (4:30 p.m.): Journal club meeting We will restart the weekly meeting with two short presentations: Alexander Kusenko, "News about primordial black holes". Literature: arXiv:2001.09160, arXiv:2008.12456 Edoardo Vitagliano, "In-medium effects in scalar and scalar mediated direct detection". Literature: arXiv:2006.13909, arxiv:2006.06836

Monday, March 9 (3:00 p.m.): Brian Shuve (Harvey Mudd)
"Baryogenesis and Dark Matter from Freeze-In"

We propose a simple model in which the baryon asymmetry and dark matter are created via the decays and inverse decays of color-triplet scalars, at least one of which must be in the TeV mass range. Singlet fermions produced in these decays constitute the dark matter. The singlets never reach equilibrium, and their coherent production, propagation, and annihilation generates a baryon asymmetry. We find that that the out-of-equilibrium condition and the dark matter density constraint typically require the lightest scalar to be long-lived, giving good prospects for detection or exclusion in current and upcoming colliders. In generalizing the leptogenesis mechanism of Akhmedov, Rubakov and Smirnov, our model expands the phenomenological possibilities for low-scale baryogenesis.

Monday, March 2 (3:00 p.m.): Yanou Cui (UC-Riverside)

"Cosmic Archaeology with Gravitational Waves from (Axion) Cosmic Strings"

Many motivated extensions of the Standard Model of particle physics predict the existence of cosmic strings. Gravitational waves (GWs) originating from the dynamics of the resulting cosmic string network have the ability to probe many otherwise inaccessible properties of the early universe. In this study we show how the frequency spectrum of a stochastic GW background (SGWB) from a cosmic string network can be used to probe Hubble expansion rate of the early universe prior to Big Bang Nucleosynthesis (BBN). We also demonstrate that current and planned GW detectors have the potential to detect such GW signals. The potential SGWB from global/axion strings will also be discussed which may provide a new probe for axion-like dark matter models. Furthermore we will show that in contrary to the standard expectation, cosmic strings formed before inflation could regrow back into horizon and leave imprints, with GW bursts potentially being the leading signal.

Wednesday, February 19 (4:30 p.m.): Eisuke Sonomoto (ICRR and Kavli IPMU, University of Tokyo)

Oscillon of Axion-like Particle

Abstract: Axion-Like Particle (ALP) is predicted from string theory in the broad mass range and the local configuration of a real scalar field, called oscillon, is often produced In ALP potentials. In this talk, I will discuss the ALP oscilon formation and its effect on cosmology. First, I will talk about the formation of the long-lived oscillons of ultra-light ALP based on our recent paper [arXiv: 1909.10805]. Next, I will talk about the primordial black hole formation from oscillons of heavy ALP based on the proceeding work.

Friday, February 21 (1:00 p.m.): Irene Tamborra (Niels Bohr Institute)

* Special seminar: location 3-121 Knudsen

Beyond the Standard Model Physics and Astrophysical Neutrinos

Abstract: Neutrinos offer an ideal window on physics beyond the Standard Model. I will discuss how the mixing of active neutrinos with their sterile counterparts with keV mass may have a potential major impact on the physics of core-collapse supernovae and outline a few examples of how the high-energy neutrino events of astrophysical origin observed by the IceCube Neutrino Telescope already place tight bounds on non-standard physics scenarios.

Wednesday, February 12 (4:30 p.m.): Jorinde Van de Vis (DESY)

"An effective approach to electroweak baryogenesis"

Abstract: The asymmetry between matter and antimatter is one of the big outstanding questions of particle physics and cosmology. In this talk, I will argue that electroweak baryogenesis is an interesting mechanism to generate the baryon asymmetry. The new physics needed for electroweak baryogenesis come into play at a relatively low scale and can thus be tested in experiment. I will address three issues related to Electroweak baryogenesis. First I will explain whether electroweak baryogenesis can be studied in the model-independent framework of the Standard Model Effective Field Theory. Second, I will compare the contributions to the value of the asymmetry resulting from CP-violating interactions of different Standard Model particles. Third, I will discuss the validity of the so-called vev-insertion approximation, that is used to compute the value of the baryon asymmetry.

Wednesday, January 8 (4:30 p.m.): Sekhar Chivukula and Elizabeth Simmons (UCSD)

"Scattering Amplitudes and Sum Rules for Massive Spin-2 States"

Spin-2 Kaluza-Klein states arise in many theoretical contexts, from string theory and AdS/CFT to phenomenological models of particle physics and dark matter; they are also the focus of experimental searches at the LHC. A key question that influences their phenomenology and the range of validity of any effective field theories built around them is how quickly scattering amplitudes involving these modes grow with energy. The faster they grow, the lower the energy at which the amplitudes violate partial-wave unitarity, rendering the theory inconsistent. We have performed the first calculation of the 2 à 2 scattering amplitudes for longitudinal spin-2 KK modes and shown that, contrary to claims in the literature, an intricate web of cancellations reduces the overall growth rate of the scattering amplitudes to being merely linear in s, as consistent with the behavior of 5D gravity. This holds both in theories based on a flat extra dimension and those, like Randall-Sundrum models, with a warped extra dimension. We have also demonstrated how to encode the conditions responsible for the cancellations as sum rules relating the couplings of the different KK states. This provides a more intuitive understanding of why some of the cancellations occur and highlights some key differences between theories with flat and warped extra dimensions. Our talk will summarize this work and indicate further directions for exploration.

Monday, May 20 (3:00 PM), 2019: Noemie Globus (NYU/CCA)

"The origin of the ultra-high energy cosmic-ray dipole"

Abstract: Although their astrophysical sources remain a mystery, new measurements brought by experiments such as the Pierre Auger Observatory and Telescope Array, have radically improved our knowledge of the ultra-high energy cosmic-rays (UHECRs). I will review the current observational status (spectrum, composition, and arrival directions) and present new results on the interpretation of the first 5 sigma anisotropy in the UHECR sky: a “dipole” for UHECRs with energies above 8 EeV, reported in 2017 by the Pierre Auger Observatory.

Wednesday, May 8 (4:30 PM), 2019: Slava Turyshev (JPL, Caltech, UCLA)

"Direct Multipixel Imaging of an exo-Earth with a Solar Gravitational Lens Telescope"

Nature has presented us with a very powerful “instrument” that we have yet to explore and learn to use. This instrument is the Solar Gravitational Lens (SGL), which results from the ability of the gravity field of the Sun to focus light from faint, distant targets. In the near future, a modest telescope could operate on the focal line of the SGL and, using the enormous magnification power of the Lens, could provide high-resolution images and spectroscopy of a habitable exoplanet. We discuss the imaging properties of the SGL, when the image occupies many pixels in the region near the optical axis. We discuss a mission to the SGL focal region that could provide us with direct, multi-pixel, high-resolution images and spectroscopy of a potentially habitable Earth-like exoplanet. Based on our initial studies, we find that such a mission could produce (100×100) pixels images of “Earth 2.0” at distances up to 30pc with spatial resolution of ~100 km on its surface, enough to see its surface features. We address some aspects of mission design and spacecraft requirements, as well as capabilities needed to fly this mission in the next two decades.

Monday, May 6 (3:00 PM), 2019: Harald Fritzsch (LMU Munich)

"Flavor Mixing of Quarks and Neutrinos "

The mass matrices of the quarks and leptons have three "texture zeros", as expected e.g. in Grand Unified Theories. The flavor mixing angles are functions of the fermion masses. For the quarks the results are in good agreement with the experimental results. The small neutrino masses are due to the "see-saw"-mechanism. We calculate the values of the three Majorana neutrino masses: 1.4 meV - 9 meV - 51 meV. The effective neutrino mass, relevant fo the neutrinoless double beta decay, is about 25 times smaller than the present limit from the experiments.

Wednesday, May 1 (4:30 PM), 2019: Kev Abazajian (UC Irvine)

"The Saga of Indirect Dark Matter Searches in Photons Split by a Million in Energy: Interpretations of X-ray lines and Gamma-ray Bumps"

I will discuss the results leading to our current understanding of the Galactic Center Excess in gamma-rays detected by the Fermi-LAT telescope, and its interpretations as a dark matter or astrophysical source. The signal has been identified with millisecond pulsars early on, but recent results find that the identification with point sources is not robust. However, new detailed modeling of astrophysical sources in the Galactic Center (GC) also has led to the GC as a top constraint on annihilating dark matter. I will also discuss the status of a candidate decaying dark matter signal in a 3.5 keV X-ray line. It has been seen in several observations of clusters of galaxies, galaxies and the diffuse background, and not seen in a few observations where it is expected. I will discuss the particle physics and galaxy formation implications of the signal.

Wednesday, April 24 (4:30 PM), 2019: Smadar Naoz (UCLA)

"Small Scale Structure Formation Without Dark Matter"

The Λ-Cold Dark Matter (ΛCDM) cosmological model has had great success in describing structure formation and the evolution of the Universe and reproduce many observational phenomena. However, it seems that there are still many discrepancies at small scales. I will focus on two main long-standing challenges to the ΛCDM model. One is the formation of globular clusters with little to no dark matter component; and the second is the so-called “missing satellite problem,” which refers to the overabundance of dark matter sub-halos predicted by ΛCDM models compare to the number of observed dwarf galaxies. I will show how globular clusters can form naturally whenever there is a relative stream velocity between baryons and dark matter. The stream velocity causes a phase shift between linear modes of baryonic and dark matter perturbations, which translates to a spatial offset between the two components when they collapse. I will show how baryonic clumps can form outside the virial radii of their counterpart dark matter halos. I will also show that these objects can form stars and that their observed properties are consistent with today's globular clusters. This new channel for the formation of globular clusters naturally accounts for the dark satellites, which are globular clusters dark matter counterparts, that are deprived of gas.

Monday, April 8 (3:00 PM), 2019: Nassim Bozorgnia (Durham)

"Probing the local dark matter velocity distribution"

The local dark matter velocity distribution is an important input in the analysis of data from dark matter direct detection experiments. Uncertainties in this distribution complicate the interpretation of direct detection results, and prevent a precise determination of the particle physics properties of dark matter. High resolution hydrodynamic simulations of galaxy formation have recently become possible and provide important information on the properties of the dark matter halo. I will discuss the local dark matter velocity distribution of Milky Way-like galaxies extracted from state-of-the-art hydrodynamic simulations, and explore possible correlations between the stellar and dark matter velocity distributions. In particular, I will discuss if there is a subset of stars which trace the local dark matter distribution in various simulated Milky Way-like galaxies.

Monday, April 1 (3:00 PM), 2019: Cristiano Germani (U. Barcelona)

"Primordial Black Holes as Dark Matter Candidates"

In this talk I will overview the physics of primordial black holes formation from single field inflationary models and discuss their statistical distribution.

Monday, March 4 (3:00 PM), 2019: Kallia Petraki (NIKHEF,LPTHE)

"Bound states in dark matter phenomenology"

In the most traditional dark matter scenarios, dark matter is hypothesised to possess only contact-type interactions. However, there is motivation to consider dark matter coupled to force carriers much lighter than itself. I will discuss some of the intricacies that arise in these scenarios, with emphasis on non-perturbative effects, in particular the formation of bound states and their phenomenological implications.

Monday, February 4 (3:00 PM), 2019: Sam Witte (Univ. of Valencia)

"Indirect Searches for Axion Dark Matter with Radio Telescopes"

In this talk I will discuss recent proposals to indirectly search for axion dark matter using radio telescopes. After reviewing the properties and expectations of axion dark matter, I will present a detailed comparison of the advantages and drawbacks of three distinct methods: non-resonant axion-photon conversion in large scale magnetic fields, resonant axion-photon conversion in the magnetospheres of neutron stars, and the simulated decay of the axion to two photons.

Monday, February 6 (4:30 PM), 2019: Simeon Bird (UC-Riverside)

"Massive Neutrinos in Galaxy Clusters and Did LIGO Detect Dark Matter?"

I'll present new efficient and accurate techniques for including massive neutrinos in N-body simulations, using a linear response (to the cold dark matter) approximation for the neutrinos. I'll discuss the potential for massive neutrinos to resolve some cosmological tensions within CMB observations galaxy clusters, using new efficient and accurate techniques for including massive neutrinos in N-body simulations. Then I'll discuss emerging events related to the idea that the merging black holes detected by LIGO are primordial and a component of the dark matter.

Monday, January 28 (3:00 PM), 2019: Graham White (TRIUMF)

"Beyond vanilla electroweak baryogenesis"

Electroweak baryogenesis (EWB) is a minimal, testable framework for explaining why there is more matter than anti-matter. Recent limits on permanent electric dipole moments greatly constrain most realizations of the vanilla scenario. This motivates looking at extensions to the framework. Such extensions can be organized around the Sakharov conditions - one can modify the efficiency of B violation, modify the type of CP violating sources or modify how the departure from equilibrium occurs. I explore minimal ways of realizing such extensions to the EWB framework while demonstrating their testability.

Friday, November 2 (11:00 AM), 2018: Edoardo Vitagliano (Max Planck Institute for Physics)

"Hiding blazars with decaying alps"

A new exciting era has begun with the development of multi-messenger astronomy. I will present a case study in which multi-messenger, multi-wavelength observations are exploited to indirectly probe fundamental physics beyond the Standard Model, making use of data from neutrino telescopes (IceCube), gamma-ray satellites (Fermi-LAT) and sounding rockets equipped with infrared cameras (CIBER). This extends the already flourishing multi-messenger astronomy tools. Indeed, the measurement of the diffuse background spectrum at 0.8-1.7 micron from the CIBER experiment has revealed a significant excess of the Cosmic Infrared Background (CIB) radiation compared to the theoretically expected spectrum. I will discuss the hypothesis that decays of axion-like particle (ALP) could explain this excess, which attenuates the diffuse TeV gamma-ray flux and alleviates the tension between the detected neutrino and gamma ray fluxes.

Friday, August 24 (11:00 AM), 2018: Adam Falkowski (CNRS, Univ. Paris-Sud)

"Stronger 21cm absorption from charge sequestration"

The unexpectedly strong 21cm absorption signal detected by the EDGES experiment suggests that the baryonic gas at the end of the dark ages was colder than predicted within the standard cosmological scenario. I will introduce one novel mechanism to lower the baryon temperature after recombination. The model includes a stable, negatively-charged particle with a significant cosmological abundance, such that the universe remains charge-neutral but the electron and proton numbers are no longer equal. The deficit of electrons after recombination results in an earlier decoupling of the baryon and CMB temperatures, and thus in a colder gas at the cosmic dawn. I will discuss the phenomenological constraints and possible smoking guns of this scenario.

Monday, April 23 (11:00 AM), 2018: Hiroyuki Sagawa (ICRR, University of Tokyo)

"Recent results of the Telescope Array experiment on ultra-high energy cosmic rays"

The Telescope Array (TA) is the largest ultra-high-energy cosmic ray (UHECR) detector in the Northern Hemisphere, which explores the origin and nature of UHECRs. TA is located in Utah, consisting of an array of scintillator detectors on the ground to sample the footprint of the air shower induced by UHECRs and telescopes to measure the fluorescence and Cerenkov light of the air shower. TA determines the energy spectrum and chemical composition of the primary particles, and also searches for anisotropy of arrival directions of UHECRs and their sources. We have found evidence of anisotropy of UHECRs in the northern sky. Here the recent measurements for spectrum, composition and anisotropy and the status of the TA extension will be presented.

Monday, April 23 (3:00 PM), 2018: Benjamin Grinstein (UCSD)

"Dark Particle Interpretation of the Neutron Decay Anomaly"

There is a long-standing discrepancy between the neutron lifetime measured in beam and bottle experiments. We propose to explain this anomaly by a dark decay channel for the neutron, involving a dark sector particle in the final state. If this particle is stable, it can be the dark matter. Its mass is close to the neutron mass, suggesting a connection between dark and baryonic matter. In the most interesting scenario a monochromatic photon with energy in the range 0.782 MeV – 1.664 MeV and branching fraction 1% is expected in the final state. We construct representative particle physics models consistent with all experimental constraints.

Monday, April 16, 2018: Nathaniel Craig (UCSB)

"New tools for solving hierarchy problems"

Hierarchies among observed parameters have long been a driver of physics beyond the Standard Model, but conventional explanations for these hierarchies are under increasing pressure from null experimental tests. In this talk I’ll survey a variety of new approaches to solving hierarchy problems within and beyond the Standard Model, including protection via accidental continuous symmetries arising from discrete symmetries; the generation of exponential hierarchies by analogy with condensed matter systems; and controlled violations of effective field theory. I’ll discuss possible applications of these ideas to dark matter and the electroweak hierarchy problem.

Monday, April 2, 2018: Josef Pradler (Institute of High Energy Physics, Austrian Academy of Sciences)

"Signatures of dark matter (well) below the GeV-scale"

I will discuss experimental and observational signatures of dark matter with mass at and very well below the GeV scale. The first signature regards scattering on nuclei and electrons in dark matter direct detection experiments. I will show how smaller but irreducible signal components can be tapped to dramatically extend the low-mass reach of existing searches. The second signature regards dark matter decay into dark radiation states. I will comment on the experimental detectability of dark radiation, and further show that 21 cm astronomy becomes a probe of very light dark radiation when its converted into ordinary photons before reionization. An explanation of the EDGES result is offered.

Wednesday, January 10, 2018: Mariana Carrillo Gonzalez (University of Pennsylvania), "Interacting dark sectors and the mildly non-linear regime"
[* special date, time: 4:30 PM]

In this talk, I will present both theoretical and observational results on an interacting dark sector. Firstly, I will introduce the field theoretical models that underlie the fluid description. The construction of these models leads to a consistent field theory for an interacting dark sector that behaves exactly like the coupled fluids at both the level of linear perturbations and the mildly non-linear regime. I will show that the quantum corrections are under control and that caustics do not appear at quasi-linear scales. Assuming the fluid description, observational constraints on these models are obtained. Here, I will focus on robust constraints that can be obtained using only mildly non-linear scales. This is done by combining lensing, clustering of galaxies, and the Cosmic Microwave Background (CMB) data sets.

Wednesday, October 4, 2017: Nirmal Raj (Notre Dame), "Dark Fires in the Sky: Model-Independent Dark Matter Detection via Kinetic Heating of Neutron Stars"
[* special date, time: 4:30 PM]

I present a largely model-independent probe of dark matter-nucleon interactions. Accelerated by gravity to relativistic speeds, local dark matter scattering against old neutron stars deposits kinetic energy at a rate that heats them to infrared blackbody temperatures. The resulting radiation is detectable by next generation telescopes such as James Webb, the Thirty Meter Telescope, and the European Extremely Large Telescope. While underground direct detection searches are not (or poorly) sensitive to dark matter with sub-GeV masses, higher-than-weak-scale masses, scattering with strong cross-sections, scattering below neutrino floors, spin-dependent per-nucleon scattering below per-nuclear cross-sections, velocity-dependent scattering, and inelastic scattering for inter-state transitions exceeding O(100 keV), the (non-)observation of dark kinetic heating of neutron stars should advance these frontiers by orders of magnitude. Popular dark matter candidates previously suspected elusive, such as the thermal Higgsino, may be discovered.

Monday, June 12, 2017: Tim Tait (UC Irvine), "Recent Developments in the Search for Dark Matter at Colliders"
[* special time: 11 AM]

Abstract: I will discuss recent work related to searching for production of dark matter at high energy colliders such as the LHC, including both the theoretical constructs used to frame the results of such searches, the new experimental picture post-Moriond, and some discussion of how these results fit into the broader picture of particle physics searches for dark matter.

Monday, June 5, 2017: Jessica Turner (IPPP, Durham), "Creating the Baryon Asymmetry from Lepto-Bubbles"

Abstract: The origin of the asymmetry between matter and anti-matter of the Universe remains one of the most important open questions in particle physics and cosmology. We propose a novel mechanism, the lepto-bubbles mechanism, to dynamically generate this observed asymmetry. The particle that undergoes the phase transition is a SM-singlet scalar which couples to the lepton number violating Weinberg dimension-5 operator. The phase transition provides an out of equilibrium condition and the time dependent complex couplings of the scalar to the Weinberg operator are CP-violating. Therefore, the CPPT mechanism satisfies Sakharov's conditions without the introduction of the right-handed neutrinos or the see-saw mechanism. We calculate the lepton asymmetry produced by the phase transition using the closed time path formalism.

Monday, May 22, 2017: Hai-Bo Yu (UC Riverside), "Explaining the Diverse Rotation Curves of Spiral Galaxies"

Abstract: The rotation curves of spiral galaxies exhibit a diversity that has been difficult to understand in the cold dark matter (CDM) paradigm. In this talk, I will show that the self-interacting dark matter (SIDM) model provides excellent fits to the rotation curves of a sample of galaxies with asymptotic velocities in the 25 to 300 km/s range that exemplify the full range of diversity. We only assume the halo concentration-mass relation predicted by the CDM model and a fixed value of the self-interaction cross section. The impact of the baryons on the SIDM halo profile and the scatter from the assembly history of halos as encoded in the concentration-mass relation can explain the diverse rotation curves of spiral galaxies. I will also discuss other smoking-gun signatures of SIDM in astrophysical observations.

Monday, March 13, 2017: Shigehiro Nagataki (RIKEN), "Astrophysical Big Bangs"

Abstract: Our group focuses on unveiling the many mysteries surrounding astrophysical explosive phenomena such as supernovae and gamma ray bursts from a theoretical point of view. Supernovae and gamma ray bursts are believed to be the most powerful explosions in our Universe, and yet very little about their explosion mechanisms is known to mankind. Members of our group use frontier physics in their daily routine to attack these problems in high-energy astrophysics. In many cases, we seek power from the quickly advancing supercomputing resources in Japan to make progress in our investigations. These 'astrophysical big bangs' continue to fascinate us with their unknown physics and puzzling astronomical phenomena (e.g. gravitational waves, r-process nucleosynthesis, particle acceleration, high-energy neutrinos, ultra high-energy cosmic rays, gamma-ray emission, etc), and they are commonly regarded as the most representative manifestations of the extreme Universe. Through our theoretical and computational endeavors, we strive to reveal the complete pictures of these violent explosions and provide state-of-the-art physical interpretations and predictions for future observations by the next-generation astronomical observatories. We are more than passionate to co-operate with fellow RIKEN researchers and all other interested groups in Japan and the globe, and together we will realize the Utopia for researchers in RIKEN. In this talk, I would like to introduce our research activities, together with short introduction of a new program in RIKEN "Interdisciplinary Theoretical and Mathematical Science Program (iTHEMS)"

Monday, February 27, 2017: Mark Wise (Caltech), "Dark Matter Bound States and Indirect Dark Matter Signals"

Abstract: Dark Matter and its antiparticle may form bound states analogous to the positronium states that an electron and positron form. I discuss how taking such bound states into account changes (in some regions of parameter space) our expectations for the dark matter annihilation rate at the time of recombination and today.

Monday, March 6, 2017: Valerie Domcke (APC, Paris), "Probing the early Universe with gravitational waves"

Abstract: Gravitational waves are unique messengers to explore the very early universe, probing energy ranges far beyond the reach of photon or even neutrino astronomy. The holy grail in this context is the stochastic gravitational wave background of cosmic inflation, which would shed light on the microphysics of inflation as well as on the entire subsequent cosmological history. In the simplest model of inflation this signal is however beyond the reach of current and planned gravitational wave interferometers. After reviewing this standard picture, I will discuss how modifications of this standard scenario can be a real game-changer, boosting the primordial gravitational wave signal into the range accessible by experiments such as eLISA and LIGO/VIRGO.

Monday, February 13, 2017: Mu-Chun Chen (UC Irvine), "Discrete (Family) Symmetries and Origin of CP Violation"

Abstract: The origins of the flavor mixing and CP violation remain a mystery in particle physics. The discovery of non-zero neutrino masses leads to yet another puzzle: why the neutrino masses are so small when compared to other fermions, and why two of the three neutrino mixing angles are so large when compared with their quark counterpart. Furthermore, CP violation in the Standard Model is insufficient to explain the observed cosmological baryon number asymmetry. On the other hand, the recent observation of a large value for the third neutrino mixing angle implies good future experimental prospects for discovering a new source of CP violation in the neutrino sector. This new CP violation source may be relevant for the dynamical generation of the cosmological matter-antimatter asymmetry.

In this talk, I will discuss how these outstanding questions in particle physics can be addressed by new physics beyond the Standard Model. In particular, I will focus on models based on discrete family symmetries which give rise to realistic masses and mixing angles of all observed fermions, including the neutrinos, with a significantly reduced number of parameters. I will point out a novel interesting possibility that for certain discrete symmetries, CP violation can be entirely group theoretical in origin. Specifically, we show that physical CP transformations always have to be class-inverting automorphisms of G. We also show that certain operations that have been dubbed generalized CP transformations in the recent literature do not lead to physical CP conservation. It is found that discrete groups can be classified into three types, each with different implications for CP violation.

Tuesday, March 1, 2016: Tomohiro Fujita (KIPAC, SLAC) "Large-scale magnetic fields can explain the baryon asymmetry of the universe"

Abstract: Helical magnetic fields in the primordial Universe can produce the observed amount of baryon asymmetry through the chiral anomaly without any ingredients beyond the Standard Model of particle physics. While they generate no B — L asymmetry, the generated baryon asymmetry survives the spharelon washout effect, because the generating process remains active until the electroweak phase transition. We show that the baryon asymmetry of our Universe can be explained, if the present large-scale magnetic fields indicated by the blazar observations have a negative helicity and existed in the early Universe before the electroweak phase transition.

Tuesday, April 5, 2016: Jiang-Hao Yu (University of Massachusetts Amherst) "Hidden sector shining the light: gamma-ray excess and diphoton excess"

Abstract: I will talk about how the hidden sector dark matter produces visible photon signatures. In the first half of my talk, I will focus on a CP violating hidden sector and its explanation on the GeV Gamma-ray excess. In the second half, I will present how a hidden sector and its UV completion explain the 750 GeV diphoton resonance.

Monday, April 18, 2016: Flip Tanedo (UC Irvine) "Dark Earthshine"

Abstract: I propose a search for dark matter annihilating into dark photons at the center of the Earth, focusing on the present and future reach of IceCube and space-based telescopes. I highlight the effect of low-velocity enhancements to dark matter capture and kinematic features that offer an opportunity for a smoking-gun signal. I also present prospects for an analogous search for dark photons from the center of the sun which may be detected by AMS-02, extending earlier work by modeling the effect of the solar wind.

Tuesday, May 17, 2016: Keisuke Harigaya (UC Berkeley) "A composite model for the 750 GeV diphoton excess"

Abstract: We study a simple model in which the recently reported 750 GeV diphoton excess arises from a composite pseudo Nambu-Goldstone boson (hidden pion) produced by gluon fusion and decaying into two photons. The model introduces an extra hidden gauge group at the TeV scale with a vectorlike quark in the bifundamental representation of the hidden and standard model gauge groups. We discuss the collider phenomenology and cosmology of hidden pions and other hidden resonances. We discuss possible theoretical structures above the TeV scale, e.g. conformal dynamics and supersymmetry, and their phenomenological implications.

Monday, May 23, 2016: Tim Tait (UC Irvine) "Evidence for a Photophobic 5th force?"

Abstract: Recently a 6.8σ anomaly has been reported in the opening angle and invariant mass distributions of e+e− pairs produced in 8Be nuclear transitions. The data are explained by a 17 MeV vector gauge boson X that is produced in the decay of an excited state to the ground state, 8Be∗ → 8BeX, and then decays through X→e+e−. The X boson mediates a fifth force with a characteristic range of 12 fm and has milli-charged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons. The photophobic X boson may also alleviate the current 3.6σ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment. I will discuss the experimental results concerning the nuclear transition, the details of a particle physics model which can hope to explain it while satisfying other constraints, and future experiments which can help shed light on the situation.

Find out more about the TEPAPP research group.