- What is the nature of the universe’s highest-energy particle accelerators, those responsible for accelerating cosmic rays to 1020 eV energy and beyond?
- What is the nature of the sources associated with a handful of astrophysical high-energy neutrinos with energies of 1012 to 1015 eV that have been observed by IceCube?
- What sources account for the diffuse gamma-ray emission with energies of 109 to 1012 eV observed by Fermi and 1013 to 1015 eV measured by Tibet-AS and LHAASO?
- What is the physical connection among the three messenger particles? Can we have a grand-unified picture for the fluxes of these three astroparticles?
- What astrophysical properties or phenomena cause some supermassive black holes (active galactic nuclei) to produce copious amounts of high-energy neutrinos, while others do not?
- Can we detect high-energy neutrinos and gamma rays from gravitational wave emitters such as compact binary mergers?
- What can we learn about the sources of the highest-energy cosmic rays by studying high-energy cosmic rays and neutrinos over the energy range 1012 to 1020 eV?
- What can we learn about fundamental physics by studying high-energy cosmic rays and neutrinos over the energy range 1012 – 1020 eV?
- To what extent are particles accelerated and neutrinos produced in violent cosmic events including gamma-ray bursts (GRBs), supernovae and hypernovae, tidal disruption events, and flaring episodes of magnetized neutron stars (magnetars) and supermassive black holes (active galactic nuclei)?
- What are the abundances of ultraheavy cosmic ray nuclei, and what do these tell us about the physics of ultrahigh-energy cosmic-ray sources, which can be active galactic nuclei, supernovae or binary neutron star mergers?
- How do cosmic rays propagate to Earth and what interactions do they experience along the way?
Discoveries and Milestones
- Multi-messenger connection:
Kohta Murase investigated the connection among neutrinos, gamma rays and cosmic rays. For extragalactic neutrinos, the grand-unification scenario for three messenger particles was proposed (Murase & Waxman 16 PRD, Fang & Murase 18 Nature Phys), and the necessity of hidden neutrino sources was shown (Murase et al. 2016 PRL). The Galactic neutrino flux, which is now observed by IceCube, was predicted based on the diffuse gamma-ray flux measured from the Galactic plane (Fang & Murase 21 ApJ).
- Supermassive black holes as cosmic-ray accelerators:
Kohta Murase and Peter Meszaros worked on high-energy neutrino and gamma-ray emission from active galactic nuclei (Murase et al. 20 PRL, Kheirandish et al. 21 ApJ, Kimura et al. 21 Nature Com.). The model explains neutrino emission from NGC 1068 as well as the all-sky neutrino flux. The group analyzed characteristics of blazar flares (Murase et al. 18 ApJ, Oikonomou et al. 19 MNRAS, Toomey et al. 20 MNRAS, Yoshida et al. 23 ApJ). Multimessenger signatures of supermassive black hole mergers were also revealed (Yuan et al. 20 PRD, Yuan et al. 21 ApJ).
- Violent explosions as cosmic-ray accelerators:
Peter Meszaros and Kohta Murase worked on high-energy emission from various transient phenomena. They showed that the observed ultrahigh-energy cosmic rays observed by Auger, including their heavy composition, can be explained by low-luminosity gamma-ray bursts or trans-relativistic supernovae (Zhang et al. 18 PRD, Zhang & Murase 19 PRD). The group also revealed TeV-PeV multimessenger emission from gamma-ray bursts (Carpio & Murase 20 PRD, Zhang et al. 21 ApJ, Murase et al. 22 ApJL, Zhang et al. 23 ApJL, Bhattacharya et al. 23 MNRAS). New models for high-energy neutrino and gamma-ray emission from supernovae (Murase 18 PRDR, Kheirandish & Murase 23 ApJL) and tidal disruption events (Senno et al. 17 ApJ, Guepin et al. 18 A&A, Murase et al. 20 ApJ, Reusch et al. 22 PRL) have been developed. The group also searched for high-energy counterparts of fast radio bursts and newborn magnetars (Murase et al. 16 MNRAS, 21 MNRAS).
- Neutrino physics and dark matter:
Kohta Murase showed that neutrinos from transients can be used as a probe of neutrino interactions beyond the Standard Model (Murase & Shoemaker 19 PRL, Eskenasy et al. 23 PRD, Carpio & Murase 23 JCAP). The group also worked on multimessenger constraints on heavy dark matter (Murase et al. 15 PRL, Cohen et al. 17 PRL, Das et al. 23 PRD). Murase and Peter Meszaros showed that neutron star binary mergers and fast radio bursts can be used as a powerful probe of the equivalence principle (Wei et al. 15 PRL, Shoemaker & Murase 18 PRD).
- With the NASA Cosmic Ray Energetics and Mass (CREAM) high-altitude balloon experiment, and its version deployed to the International Space Station (ISS-CREAM), Stephane Coutu has accumulated the world's definitive direct measurements of high-energy cosmic-ray nuclei from H to Fe. The elemental energy spectra lend support to the Galactic supernova shock acceleration origin of cosmic rays, but with some spectral features that are not fully explained.
- Prompt analysis and distribution of an IceCube + AMON electronic alert for the 22 Sep. 2017 high-energy neutrino event led directly to the association of this neutrino with the flaring supermassive black hole (blazar) TXS 0506+056. This source thus became the first candidate for an extraterrestrial high-energy neutrino source. The Penn State team provided the interpretation of the multimessenger data (Keivani et al. 18 ApJ, Murase et al. 18 ApJ, Petropoulou et al. 20 ApJ).
- Using machine learning techniques, Doug Cowen’s group has contributed to world-leading measurements of neutrino oscillation parameters using atmospheric neutrinos, and has identified the first high-purity sample of astrophysical tau neutrino events using IceCube data characterizing the astrophysical tau neutrino flux at median energies of about 1014 eV.
- With multiple projects aiming to detect the radio signature of ultra-energetic astrophysical neutrinos, Stephanie Wissel’s group have constrained the flux of neutrinos with energies >1019 eV with the ANITA and ARA experiments.
- Stephane Coutu is studying light isotopes of cosmic ray nuclei, including 9Be (stable) and 10Be (radioactive, with a 1.4 Myr lifetime), with the new NASA High Energy Light Isotope eXperiment (HELIX) high-altitude balloon project. These elements are produced as spallation residues of cosmic ray interactions during Galactic propagation, tracking secondary processes related to the production of antimatter, crucial in interpreting the positron measurements made by the AMS-02 experiment.
- The new Trans Iron Galactic Element Recorder for the International Space Station (TIGERISS) is under development for space deployment. With it, Stephane Coutu will be measuring the populations of ultraheavy cosmic ray nuclei up to lead, which have a factinating astrophysical origin in violent mergers of binary neutron stars or in supernova explosions.
- Doug Cowen's group is preparing for the deployment of the approved IceCube Upgrade that will greatly augment the project’s sensitivity to atmospheric neutrino oscillations The group is also involved in the design and construction of next-generation neutrino detectors at Brookhaven National Lab and on the Eos experiment at Berkeley using water-based liquid scintillator (WbLS), and in Europe on the LiquidO/CLOUD detector prototype using opaque WbLS. They remain involved in the WATCHMAN experiment, designed to use low-energy electron anti-neutrinos to monitor nuclear reactors at significant stand-off distances for nuclear non-proliferation purposes.
- Abe Falcone uses the VERITAS TeV gamma-ray telescope, combined with telescopes sensitive to lower energy bands, to study emission processes from particle acceleration at sites with astrophysical jets and shocks, such as blazars, X-ray/TeV binaries, and GRBs.
- Peter Meszaros is involved in theoretical calculations on high-energy emission from gamma-ray bursts and gravitational wave sources such as compact binary mergers and supermassive black hole binaries.
- Kohta Murase is investigating the multimessenger connection among high-energy astroparticles. He focuses on high-energy phenomena that occur in the vicinity of black holes and magnetars. He is also working on dark matter and neutrino physics beyond the Standard Model. The group is developing the AMES code that enables us to simulate multimessenger emission from various astrophysical objects.
- Stephanie Wissel is building the next generation of ultrahigh energy neutrino experiments using radio instrumentation. The RNO-G experiment in Greenland is expected to be the largest neutrino experiment in the world, covering 40 km2 and PUEO is the first balloon experiment (PUEO) selected as a NASA PIONEERS mission.
- All faculty are involved in the Astrophysical Multimessenger Observatory Network (AMON), which is a PSU initiative to collate cosmic messengers (photons, gamma rays, neutrinos, gravitational waves) from multiple worldwide observatories. This allows the detection of distant astrophysical transient phenomena and inform our understanding of the high-energy universe.
- Yu (Carl) Chen has developed new analysis techniques for the ISS-CREAM experiment flux analysis and has characterized the timing performance of the HELIX time-of-flight detectors, and now holds a postdoctoral appointment at UCLA to work on the GAPS balloon experiment and on CTA.
- Monong Yu has developed new machine learning algorithms for event classification in analyzing ISS-CREAM data, and now holds a postdoctoral appointment at the High-Energy Astrophysics Institute (IFEA) in Barcelona to work on the HERD experiment to deploy to the Chinese Space Station.
- Yuchieh Ku, working with engineering staff in the Detector Development Lab, has designed and tested a system to extend the frequency range of the PUEO experiment down to 50 MHz, enhancing the sensitivity to air showers produced by tau neutrinos and cosmic rays.
- Andrew Zeolla has studied the sensitivity of a mountaintop radio experiment BEACON to tau neutrinos and used neural networks to identify cosmic ray air showers in a prototype at the White Moutain Research Station Center in Bishop CA.
- Bryan Hendricks and Ryan Krebs have been heavily involved with installing and commissioning RNO-G in Greenland. Together with the DDL, they have developed novel firmware for advanced physics triggers and have designed and built hundreds of horizontally polarized antennas.
- Adam Baldoni built the data acquisition system for a 1-ton, ~50 PMT water-based liquid scintillator detector at BNL, and is currently implementing a more complex version of that system for the 5-ton, 200-PMT Eos detector at UC-Berkeley. He has contributed to the understanding of the data from the BNL detector that is verifying the expected performance of WbLS for low-energy particle detection and, in Eos, the ability to separate Cherenkov from scintillation light on an event-to-event basis.
- Garrett Wendel contributed to the development of a machine-learning-based reconstruction technique using "likelihood-free inference" on IceCube, has ported it to Eos and plans to port it to LiquidO/CLOUD. The technique provides the best vertex, energy and direction resolutions while at the same time requiring minimal manual tuning, enabling Garrett to perform studies of a wide variety of detector configurations in a short period of time, providing detector designers with crucial information at an early stage.
- Abhishek Das has worked on modeling of high-energy neutrino emission from active galactic nuclei with AMES.
- Stephane Coutu (Physics)
- Doug Cowen (Physics)
- Abe Falcone
- Derek Fox
- Peter Meszaros
- Kohta Murase (Physics)
- Stephanie Wissel (Physics)
- Adam Baldoni
- Abhishek Das
- Bryan Hendricks
- Yuchieh Ku
- Garrett Wendell
- Andrew Zeolla
- Institute for Gravitation and the Cosmos
- Center for Multimessenger Astrophysics
- Astrophysical Multimessenger Observatory Network
- IceCube Experiment
- Eos Experiment
- High Energy Light Isotope eXperiment
- Cosmic Ray Energetics And Mass
- Trans Iron Galactic Element Recorder on the International Space Station
- Detector Development Lab
- WATCHMAN Experiment