One of the most intriguing and challenging issues in astrophysics involves the investigation of astrophysical sources that can subsequently provide scientists with a greater insight regarding the enigmatic origin of the highest energy particles in nature – neutrinos.
Neutrinos: an introduction
Neutrinos are created by violent astrophysical events (i.e., exploding stars [supernovas] or gamma ray bursts) and are critical to the make-up of the universe. Although very similar to electrons, neutrinos are not electrically charged, and so remain unaffected by electromagnetic forces; instead, they are mildly affected by gravity and by subatomic weak forces. Neutrinos can therefore travel great distances through cosmic matter and for this reason are known as cosmic messengers – key elements in understanding the powerful cosmic accelerators in our Universe.
Owing to their very small mass and their lack of charge, neutrinos (unlike photons) are capable of escaping from very dense astronomical environments. So, in order to investigate cosmic ray acceleration, scientists rely solely on neutrinos to act as tracers of such environments. Imagine the broken clay pieces that are revealed by archaeologists to provide distinct and unique information regarding a past era – neutrinos act in this way for the era that formed the origins of sub-atomic particles.
Essentially, there are three types, or ‘flavours’, of neutrinos that have no net electric charge, where each one of them is connected to a corresponding charged particle. These comprise the electron neutrinos (ve), which were discovered in 1956, the muon neutrinos (vΜ), which were discovered in 1962, and the tau neutrinos (vτ), which were discovered in 2000. Neutrinos of all flavours form the core of the so-called IceCube project – a project that Dr Kiryluk has been an integral member of since 2007.
The IceCube project
The IceCube project is a particle detector located below the surface of the South Pole. It’s target is to glean information from neutrinos, particularly due to the ability of these particles to travel long distances without being disturbed by prevailing cosmic interactions.
IceCube is the first gigaton neutrino detector ever built, with its primary function aimed at searching for neutrinos in the most violent astrophysical sources, including cataclysmic phenomena, such as black holes and neutron stars. Currently, there is no direct method of observing such particles. Instead, they can be indirectly observed as a result of their interactions with the ice and the subsequent production of secondary electrically charged particles. These secondary charged particles can travel much faster than light in a dielectric medium such as ice, and hence they emit the so-called Cherenkov light. The IceCube, with its sensors, collects this generated light and subsequently digitises it, allowing scientists to gain a greater insight into its initial direction, energy and origins. Since 2012, this project has announced new observations regarding high-energy neutrinos that originate beyond our solar system.
Dr Kiryluk’s work at the IceCube
There are currently two principal aims for Dr Kiryluk’s research. The first one involves determining the flux of electron and tau neutrinos. This will, in turn, allow scientists to gain a greater insight into the diffuse astrophysical electron and tau neutrino energy spectrum, and their characteristics including their flavour composition down to single (individual) neutrino flavours.
In other words, Dr Kiryluk’s work intends to unravel the neutrinos’ production mechanisms and acceleration mechanisms and the properties of the neutrino sources. This, in turn, will allow unique observations and suggestions to be made in relation to the origin of cosmic neutrinos and of ultra-high-energy cosmic rays. Her second research aim involves the search for anisotropy, which will provide solid evidence for a galactic and an extragalactic origin of the recently discovered astrophysical neutrinos. In fact, according to the current research performed in the IceCube, if the magnitude of anisotropy is found to be small, this will suggest that practically all cosmic neutrino radiation has an extragalactic origin.
Conclusively, Dr Kiryluk’s work at the IceCube has every potential to provide scientists with a greater insight that will provoke a critical impact on neutrino astrophysics by, simultaneously, offering ample potential for continued discovery. As Dr Kiryluk puts it herself: “With this study, we are able to present the first comprehensive characterisation of the astrophysical electron and tau neutrino flux at IceCube.”
Data collected with the IceCube experiment consist of a mixture of astrophysical neutrinos, signal, as well as cosmic ray-induced atmospherical backgrounds. In the analysis, we develop methods to reject the dominant atmospheric background contributions from our data, while keeping the neutrino signals of interest. Following this selection, we sort the remaining data in intervals of angle and energy. For each interval we evaluate the expected background using Monte Carlo techniques. An excess observed in the data over the expected background is our measurement of the flux of astrophysical neutrinos.
We can describe the energy dependence of the flux of astrophysical neutrinos with a single power law form. We use maximum likelihood fitting techniques to extract the overall size, or normalisation, of the astrophysical neutrino flux and the energy slope, or spectral index, from the data. Future data will make it possible to investigate more intricate energy dependences, beyond a single power law.
Why is IceCube located where it is and not, for example, in space – much like the Hubble telescope?
Neutrinos are elementary particles that interact weakly. Their detection requires large and massive instruments. To detect high-energy astrophysical neutrinos, instruments with a volume of one cubic kilometre or larger are needed. This is the reason why neutrino telescopes use ice (Antarctica) or water (Mediterranean Sea, Lake Baikal) as part of the instrument. It is currently not practical to construct such detectors in space.
The determination and verification of anisotropy would mean that an important part of the cosmic neutrinos originates outside our galaxy. What is the significance of this discovery?
The astrophysical neutrinos that have been observed by IceCube so far are isotropically distributed in the sky. This means that the data strongly favour extragalactic origins for these neutrinos. Any indication of a neutrino flux anisotropy between the Northern and Southern skies would point to the existence of a galactic component, and would mean that the energy spectrum will have a rich structure. The characterisation of the galactic flux of neutrinos, if or when it is observed, will enable new studies of various models of possible galactic sources, such as the Galactic Halo, Sagittarius A* or Fermi Bubbles as well as of the diffuse emission from galactic cosmic rays.
Why have you selected this project, a project that you have been working with over the last ten years?
I am fascinated by high-energy phenomena, nuclear physics, and astro particle physics. IceCube is an experiment that speaks to the imagination, requiring the combined expertise from many disciplines to succeed. Neutrino astronomy offers tremendous potential for discovery, despite the experimental challenges that have required development over generations. I feel fortunate to be able to pursue science, do so with IceCube, and contribute to its science goals.
What are your objectives in terms of time scale, in order to accomplish the targets of your research?
Over the next several years, we aim to observe a sufficiently large number of astrophysical neutrinos to be able to characterise their energy spectrum and learn about the physical mechanisms that are at the origin of the most energetic processes in the universe.
Dr Kiryluk’s research focuses on studying subatomic particles called neutrinos. Her latest work looks to target the energy and flavour characteristics of a diffuse flux of highly energetic neutrinos.
National Science Foundation (NSF)
- IceCube (www.icecube.wisc.edu/collaboration/icecube)
- National Science Foundation (www.nsf.gov)
- Department of Physics and Astronomy, Stony Brook University (www.physics.sunysb.edu/Physics/)
- Women in Science and Engineering programme at Stony Brook University (www.stonybrook.edu/wise/)
Dr Kiryluk is an assistant professor in the Department of Physics and Astronomy at Stony Brook University. She obtained her PhD in Physics from Warsaw University, Poland, and has worked at the University of California Los Angeles, the Massachusetts Institute of Technology, and Lawrence Berkeley National Laboratory.
Dr Joanna Kiryluk
Assistant Professor of Physics
Department of Physics and Astronomy
Stony Brook University
Stony Brook, NY 11794-3800
T: +1 (631) 632 7734