KICP Friday Noon Seminars: 2020
DateTalk TitleSpeaker
January 10, 2020New mass windows and detection prospects for primordial black hole dark matterNicholas Orlofsky, University of Wisconsin-Madison
January 17, 2020Understanding neutrino background implications in LXe-TPC dark matter searches using 127Xe electron capturesDylan J Temples, Northwestern Universtiy / Fermilab
January 24, 2020The Origin of Multiphase Galaxy OutflowsEvan E Schneider, Princeton University
January 31, 2020X-ray Search for Axions from Nearby Isolated Neutron StarsRaymond T Co, University of Michigan
February 7, 2020Heavy neutrinos and their role in the early UniverseJessica M Turner, Fermilab
February 14, 2020A New Look at the Migdal EffectTien-Tien Yu, University of Oregon
February 21, 2020Near Future of Dark Matter Searches: Go Big, or Go LowCarmen Carmona Benitez, Pennsylvania State University
February 28, 2020The Inner Milky Way: Our New Closest LI(N)ERDhanesh Krishnarao, University of Wisconsin-Madison
March 13, 2020 cancelledCosmological constraints on ultralight axionsRenee A Hlozek, University of Toronto
April 10, 2020 cancelledTBDNicholas L Rodd, UC Berkeley
May 8, 2020 cancelledMetastable Nuclear Isomers as Dark Matter AcceleratorsHarikrishnan Ramani, UC Berkeley
May 22, 2020Multi-messenger Astrophysics: Probing Compact Objects with Cosmic ParticlesKe Fang, Stanford University
May 29, 2020Nucleons, Electrons, and Pasta: Discovering Dark Matter by Reheating the Neutron Star SoupNirmal Raj Krishna, TRIUMF
June 5, 2020Dark Photon Oscillations in our Inhomogeneous UniverseHongwan Liu, Princeton University and New York University

New mass windows and detection prospects for primordial black hole dark matter
January 10, 2020 | ERC 401 | 12:00 PM | KICP Seminar
Nicholas Orlofsky, University of Wisconsin-Madison

Recent developments have opened up new possible windows where primordial black holes (PBHs) can make up all of dark matter (DM). In the first half of the talk, I will describe how microlensing of X-ray pulsars could help to probe the asteroid-mass window. In the second half, I will discuss how stable (nearly) extremal PBHs with masses below traditional evaporation bounds could populate all of DM. These extremal PBHs could give rise to energetic signals when present-day binaries merge.

Understanding neutrino background implications in LXe-TPC dark matter searches using 127Xe electron captures
January 17, 2020 | ERC 401 | 12:00 PM | KICP Seminar
Dylan J Temples, Northwestern Universtiy / Fermilab

Dark matter searches using dual-phase xenon time projection chambers (LXe-TPC) rely on the discrimination between electronic recoils (background) and nuclear recoils (signal) based on the ratio of ionization electrons to scintillation photons produced by the interaction in the liquid xenon. This discrimination is calibrated at low energies using tritium β-decays. However, neutrino and Compton scatters from inner-shell electrons of xenon atoms result in the emission of Auger electrons and x-rays in addition to the primary recoiling electron, and thus have a different event topology than β-decays and valence-shell electron recoils. Due to their low kinetic energy and large numbers, these secondary particles can deposit larger amounts of energy within a small radius, which is uncharacteristic of valence electron recoils and is more akin to nuclear recoils. This affects the profile of the neutrino-electron scattering background in a way that is, so far, uncalibrated and unaccounted for in LXe-TPC dark matter searches, and presents the possibility of a false discovery claim. To investigate the significance of this effect, electrons capture decays of 127Xe are used to simulate the vacancies produced by inner-shell e-ν scatters in the Xenon Electron-recoil L-shell Discrimination Analyzer (XELDA) detector. The 127Xe source produces a high-purity sample of inner-shell vacancies accompanied by an Auger cascade that can easily be isolated from the prompt γ's emitted in the decay. In this talk, I will present preliminary results from a cross-calibration of the XELDA detector using both 127Xe EC-decays and 3H β-decays. The observed discrepancy reduces the efficiency with which neutrino-induced electron recoils can be rejected in large-scale LXe-TPC dark matter searches, such as LUX-ZEPLIN and DARWIN.

The Origin of Multiphase Galaxy Outflows
January 24, 2020 | ERC 401 | 12:00 PM | KICP Seminar
Evan E Schneider, Princeton University

Star-forming galaxies are often observed to host outflows - gas that is flowing away from the galaxy in phases ranging from cold molecular clouds to hot X-ray emitting plasma. While these multiphase outflows are routinely observed, theoretically constraining their origin and evolution has proven difficult. Explaining the prevalence and velocities of the cool ionized phase (T~10^4 K) in particular poses a challenge. In this talk, I will discuss a potential dual origin for this cool gas. Through a series of extremely high-resolution simulations run with the GPU-based Cholla code, I will show that in high star formation surface density systems, dense disk gas can be pushed out by the collective effect of clustered supernovae, explaining the low-velocity material. Subsequent shredding and mixing of these clouds creates gas with intermediate densities and temperatures that is prone to radiative cooling, allowing momentum to transfer between phases and producing high velocity cool gas. In addition to explaining the nature of outflows themselves, these multiphase winds could potentially be a source of the cool photo-ionized gas that is found in abundance in galaxy halos.

X-ray Search for Axions from Nearby Isolated Neutron Stars
January 31, 2020 | ERC 401 | 12:00 PM | KICP Seminar
Raymond T Co, University of Michigan

We performed a search for axion-like particles (ALPs) in the nearby isolated neutron stars and interpreted the recently observed excess in X-ray by an axion model. These ALPs can be produced from the hot cores of the neutron stars at temperatures around 10^8 K and be converted into X-ray photons within the magnetosphere of the neutron stars. The dominant production modes are the nucleon bremsstrahlung and Cooper pair-breaking-formation processes. These ALPs and the produced photons carry energies of order the core temperature or the superfluid gaps and are therefore much hotter than the surface temperature of around 10^6 K. X-ray dim isolated neutron stars are excellent targets due to low background in the signal range of 1-100 keV. We fit the axion model to an X-ray excess in 2-8 keV observed by XMM-Newton and Chandra and discuss prospects of confirming or ruling out the axion interpretation by future measurements of NuSTAR.

Heavy neutrinos and their role in the early Universe
February 7, 2020 | ERC 401 | 12:00 PM | KICP Seminar
Jessica M Turner, Fermilab

I will discuss the role of heavy neutrinos in the early Universe and how they can generate the observed matter anti-matter asymmetry, the electroweak scale and light neutrino masses. In addition, I will discuss a novel mechanism of baryogenesis which proceeds via a time-varying operator.

A New Look at the Migdal Effect
February 14, 2020 | ERC 401 | 12:00 PM | KICP Seminar
Tien-Tien Yu, University of Oregon

Searches for sub-GeV dark matter rely on sensitivity to small ionization signals. These signals can arise from dark matter-electron scattering, in which the scattered electron produces the ionization signal. However, the signal can also arise in dark matter-nucleus scattering through a process known as "Migdal" scattering. In this talk, I will review the theory behind "Migdal" ionization and demonstrate the connection between dark matter-electron scattering and Migdal scattering. As a concrete example, I will compare the two dark matter processes for a dark photon mediator.

Near Future of Dark Matter Searches: Go Big, or Go Low
February 21, 2020 | ERC 401 | 12:00 PM | KICP Seminar
Carmen Carmona Benitez, Pennsylvania State University

The identification of dark matter is presently one of the greatest challenges in science, fundamental to our understanding of the Universe. There are a number of experiments and R&D projects planned in the near future aiming to directly detect dark matter particles, and they largely fall into two categories: larger iterations of previous experiments; and novel techniques that seek to drive sensitivity towards low mass particles. One of the former, the LUX-ZEPLIN (LZ) experiment has grown out of its two precursors with the goal of constructing a next generation dark matter detector at Sanford Underground Research Facility (SURF) in South Dakota, with 7 tonnes of fully active liquid xenon. LZ has been designed to explore much of the parameter space available for WIMP models, with excellent sensitivity for WIMP masses between a few GeV and a few TeV. In this talk I will present an overview of the LZ detector design, current project status and timeline.
As for the latter, we explore the use of a novel technique, the Snowball chamber, in the search for low-mass dark matter. This chamber uses supercooled water as the target, employing an exotic phase transition of metastable water in a similar fashion to a bubble chamber in reverse, but with enhanced low energy threshold (sub-keV) and background discrimination as a function of thermodynamic conditions. I will discuss the potential of this new technology to drastically expand detector sensitivity in the sub-GeV range, opening up a new parameter space currently out of reach.

The Inner Milky Way: Our New Closest LI(N)ER
February 28, 2020 | ERC 401 | 12:00 PM | KICP Seminar
Dhanesh Krishnarao, University of Wisconsin-Madison

Recently, we discovered diffuse ionized gas associated with the bar of the Milky Way ~1-2 kpc from Galactic Center (The Tilted Disk) that exhibits optical emission line ratios characteristic of Low Ionization (Nuclear) Emission Regions [LI(N)ERs] in other galaxies. This makes the inner Milky Way the closest example of a LI(N)ER in the universe and gives us a unique opportunity to study and constrain many individual sources of ionization with spatially resolved, multi-wavelength observations. Additionally, continued studies of large samples of nearby galaxies can provide a face-on perspective towards understanding the distribution of the ISM near bars and the impact of bars on the ISM. I will share our current understanding of the ionizing radiation field throughout the Milky Way as constrained through optical line observations with the Wisconsin H-Alpha Mapper (WHAM) and compare with findings from extragalactic integral field observations of Milky Way Analogs from SDSS MaNGA. In particular, the unique ionization mechanisms and anomalous optical line ratios surrounding bars will be shown and future steps towards constraining these mechanisms using simulations and new observations in the Milky Way will be discussed. Bridging the Milky Way with extragalactic systems allows for the power of statistics to provide insight on the formation and evolution of The Galaxy through cosmic time while also extrapolating high resolution observations from within to explain extragalactic trends and relations.

Cosmological constraints on ultralight axions
March 13, 2020 cancelled | ERC 401 | 12:00 PM | KICP Seminar
Renee A Hlozek, University of Toronto


Ultra-light axions of mass around 10^{-22} eV are a promising dark matter candidate well motivated by high energy physics. Axions of these mass are well probed by cosmological observations. I'll discuss current constraints on ultralight dark matter, focusing on the cosmic microwave background and large scale structure. I'll discuss how future improvements in measurements of the CMB from experiments like the Simons Observatory and CMB Stage 4 will open a window on this dark sector.

April 10, 2020 cancelled | ERC 401 | 12:00 PM | KICP Seminar
Nicholas L Rodd, UC Berkeley


Metastable Nuclear Isomers as Dark Matter Accelerators
May 8, 2020 cancelled | ERC 401 | 12:00 PM | KICP Seminar
Harikrishnan Ramani, UC Berkeley


Inelastic dark matter and strongly interacting dark matter are poorly constrained by direct de- tection experiments since they both require the scattering event to deliver energy from the nucleus into the dark matter in order to have observable effects. We propose to test these scenarios by searching for the collisional de-excitation of meta-stable nuclear isomers by the dark matter particles. The longevity of these isomers is related to a strong suppression of '?''?'- and '?''?'-transitions, typically inhibited by a large difference in the angular momentum for the nuclear transition. The collisional de-excitation by dark matter is possible since heavy dark matter particles can have a momentum exchange with the nucleus comparable to the inverse nuclear size, hence lifting tremen- dous angular momentum suppression of the nuclear transition. This de-excitation can be observed either by searching for the direct effects of the decaying isomer, or through the re-scattering or decay of excited dark matter states in a nearby conventional dark matter detector setup. Existing nuclear isomer sources such as naturally occurring 180mTa, 137mBa produced in decaying Cesium in nuclear waste, 177mLu from medical waste, and 178mHf from the Department of Energy storage can be combined with current dark matter detector technology to search for this class of dark matter.

Multi-messenger Astrophysics: Probing Compact Objects with Cosmic Particles
May 22, 2020 | Zoom Room (online) | 12:00 PM | KICP Seminar
Ke Fang, Stanford University

The study of compact stellar remnants such as black holes and neutron stars is an important component of modern astrophysics. Recent observations of the first neutron star merger event and an active galactic nucleus as a potential high-energy neutrino source open a new way to study compact objects using multi-messengers. The key to coordinated detection and interpretation of multiple messenger signals, namely, electromagnetic radiation, cosmic rays, neutrinos, and gravitational waves, is to understand the link between the messengers. We try to answer this question from both theoretical and observational perspectives. We study high-energy particle propagation in the vicinity of magnetar-powered transients and black hole jets using numerical simulation. We also investigate analysis frameworks aiming to exploit data across multiple wavelengths and messengers. We close the talk by overlooking the future of Multi-messenger Astrophysics, in light of upcoming facilities such as SWGO and IceCube-Gen2, as well as new questions brought by recent observations.

Nucleons, Electrons, and Pasta: Discovering Dark Matter by Reheating the Neutron Star Soup
May 29, 2020 | Zoom Room (online) | 12:00 PM | KICP Seminar
Nirmal Raj Krishna, TRIUMF

I present a largely model-independent probe of dark matter interactions with nucleons and electrons. 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. I treat neutron star capture of dark matter by scattering (a) in the various layers of the well-understood stellar crust, on nucleonic and nuclear constituents, which include non-spherical "pasta" phases, (b) in the less understood stellar core, on nucleons and muons using non-relativistic kinematics, and on electrons using relativistic kinematics. I show that the (non-)observation of dark kinetic heating of neutron stars would overcome several limitations of terrestrial searches for dark matter, and advance challenging frontiers by orders of magnitude.

Dark Photon Oscillations in our Inhomogeneous Universe
June 5, 2020 | Zoom Room (online) | 12:00 PM | KICP Seminar
Hongwan Liu, Princeton University and New York University

The dark photon is a well-motivated extension of the Standard Model which can mix with the regular photon. This mixing is enhanced whenever the dark photon mass matches the primordial plasma frequency, leading to resonant conversions between photons and dark photons at specific redshifts. These conversions can produce observable cosmological signatures, including distortions to the cosmic microwave background blackbody spectrum. Until recently, constraints on the dark photon based on these effects have been derived assuming a homogeneous universe. In this talk, I will review the physics of photon/dark-photon mixing, and introduce a new analytic formalism that can account for the inhomogeneous distribution of matter in our universe. Using this formalism, we obtain new limits on the dark photon mixing parameter, significantly correcting and extending earlier results.