Tag Archives: neutrino physics

Interview with Erin O’Sullivan

Dr. Erin O’Sullivan

I am from Toronto, Canada. I did my undergraduate degree at the University of Guelph and my PhD at Queen’s University. I chose to become a physicist because I liked how collaborative the work was, even at the high school level. I was fortunate to have a strong science department in high school, including a physics teacher that had a talent for piquing our interest in current physics events. During one class, he discussed the Sudbury Neutrino Observatory and I was really interested in the idea of studying this elusive particle deep in a Northern Ontario mine. I carried this idea with me into my undergraduate career and it set the course of the eventual research I would choose to do. I still like how collaborative the work is and that I get to learn from and share ideas with my collaborators. I also like the freedom that comes with being a researcher, especially that I get to pursue the ideas that are interesting to me. The biggest challenge of being a scientist is having to get through the monotony (getting code to run, making small adjustments to figures, etc) in order to extract the science.

What is your field of research and/or what project are you involved in at the OKC?
I am a neutrino astrophysicist. I am a collaborator on IceCube, as well as the proposed Hyper-Kamiokande experiment.

What are your research plans for your time in Sweden?
I am a software convener for Hyper-Kamiokande, so I spend some time developing the Monte Carlo software package, WCSim. I also have plans to get involved with multi messenger astronomy involving IceCube neutrinos. We would like to expand the number of public alert channels that we send out from IceCube, as well as get involved with the search for coincidences between IceCube’s neutrinos and gravitational events from LIGO-Virgo.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I have been fortunate to work on a few different neutrino experiments throughout my career. I did my PhD focusing on solar neutrinos in SNO+, which are low energy (MeV-scale). Before coming to the OKC I was a postdoc with the Super-Kamiokande experiment where I looked for astrophysical neutrinos that were in the mid-energy range (GeV-scale). Now at the OKC, I am working with IceCube where neutrinos with energies above PeV can be measured. So, I guess the skill that I am most proud of is my general knowledge of the whole picture of neutrino detection. I am still learning a lot about detecting neutrinos at the highest energies, so I would like to continue to learn more about that in the next year.

What advances or new results are you excited about or looking forward to?
I would love to measure a galactic supernova using neutrinos. The last time there was a nearby supernova was in 1987 and there were only a few neutrino experiments online and they weren’t as powerful as today’s detectors. If there was a supernova today, detectors would measure a detailed picture of the rate and energy evolution of neutrinos coming from a supernova. This would allow us to see inside a supernova and learn about the progenitor properties, and it would allow us to study how neutrinos behave in dense matter environments. Too bad there are only a few galactic supernovae per century!

Why did you choose the OKC?
I was working in the US when we visited Stockholm for vacation, and I was surprised that there was a similarity between Sweden and Canada. I knew that there was a good IceCube group here, and just after our vacation a faculty position became available in the astronomy department. My husband is an astrophysicist and and getting jobs together can be a challenge, so this seemed like it could be a good opportunity, and indeed it worked out for us.

How do you relax after a hard day of work?
On the weekends I like to explore Stockholm and maybe cook something nice for dinner. Weeknights are pretty busy, but if there’s time I like to catch up on my favorite tv shows (we just finished Stranger Things 2) or play board games (currently Pandemic Legacy 2).

What do you hope to see accomplished scientifically in the next 50 years?
Neutrino astronomy is interesting because neutrinos allow us to see into the interior of violent astrophysical events that are difficult to probe with electromagnetic messengers. I would love for neutrino astronomy to become similar to how we do electromagnetic astronomy now where we could see many neutrinos coming from the same astrophysical object. This would allow us to really start using neutrinos to probe the behavior of astrophysical phenomena. I would also like to see neutrinos enter into the multi-messenger picture where they could be detected in coincidence with gravitational waves or EM detections (or both!). I’m actually hopeful that doing multi messenger physics with neutrinos won’t be 50 years away and could soon be a reality. We have had some recent hints that this could already be happening!

Erin is a postdoc in the SU IceCube group. She joined the OKC in July 2017.
Thanks Erin, and while you’re exploring Stockholm check out Millesgården (one of my favorite places)!

The Nobel Prize in Physics 2015 for Neutrino Oscillations

The announcement of the Nobel Prize in Physics awarded to Takaaki Kajita (Univ. of Tokyo) and Arthur McDonald (Queen’s University, Canada) for the discovery of neutrino oscillations, and thus the revelation that neutrinos have mass, is an exciting occasion for its recognition of fundamental scientific research of the kind done by all of us at the Oskar Klein Centre.

While 2015 is the UN’s International Year of Light, the prize reminds us that the neutrino, the second most abundant particle in the Universe after the photon and devilishly difficult to study, continues to surprise us and deepen our understanding of the laws of nature.

It’s an especially exciting occasion for members of the OKC who work with the IceCube Neutrino Observatory. As is generally known, IceCube is built to use neutrinos for a wide variety of goals, from the study of astrophysical particle accelerators and supernova explosions to the search for dark matter. In the past few years, the study of neutrino oscillations—and therefore of the neutrino itself—has also come to the forefront of IceCube activity.

IceCube is able today to measure neutrino oscillations thanks to the DeepCore detector, an initiative that began at the universities of Stockholm and Uppsala with funding from the K&A Wallenberg foundation. DeepCore consists of optical modules buried deep in the South Pole ice much like the rest of the IceCube detector that surrounds it. But in DeepCore the modules are spaced much closer together, lowering the energy threshold from 100 GeV down to 10 GeV. The initial aim of the DeepCore detector was to search for dark matter, via neutrinos emitted when dark matter particles annihilate in places like the sun or the galactic center. In 2012, Stockholm University PhD student Matthias Danninger led the first dark matter analysis to use DeepCore, setting tight constraints on the spin-dependent cross-section of dark matter particles. Atmospheric neutrinos are the background for this search, and DeepCore records a lot of them. It was soon realized that the atmospheric neutrinos in the energy range of tens of GeV could provide a strong signal for neutrino oscillation measurements.

At energies around 25 GeV, a sizable fraction of the muon neutrinos created on the opposite side of the Earth will “disappear” by oscillating to another neutrino flavor before they arrive at the detector. The rate of muon neutrinos as a function of energy and distance traveled through Earth can be measured, and compared to the expected rate if no oscillations had occurred. The measurable effect is not small: in an analysis published this year, 5174 events were observed compared to an expectation of 6830 events assuming no oscillations. More importantly, from a full analysis of the event distributions one can extract the relevant oscillation parameters: a mass difference between types of neutrinos, and a so-called mixing angle, related to how strong the oscillation effect is. The most recent DeepCore oscillation parameter measurements have reached a precision comparable to that of dedicated oscillation experiments.

Planning is underway now for a next-generation IceCube, and a major component is the PINGU sub-detector array. It can be thought of as a DeepCore inside DeepCore: an even denser instrumentation in the center of the IceCube detector allowing an energy threshold down to a few GeV. With this threshold, PINGU would be able to study a new range of oscillation phenomena. Among the top priorities is a determination of the neutrino mass ordering, a major unanswered question in fundamental physics. Until now, oscillation measurements have been able to tell us the mass differences between the types of neutrinos, but not their absolute ordering.

Credit: Sven Lidstrom, IceCube/NSF The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica, hosts the computers collecting raw data. Due to satellite bandwidth allocations, the first level of reconstruction and event filtering happens in near real-time in this lab. Only events selected as interesting for physics studies are sent to UW-Madison, where they are prepared for use by any member of the IceCube Collaboration.
Credit: Sven Lidstrom, IceCube/NSF
The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica. The IceCube detector modules are deployed between 1.5 and 2.5 kilometres below the surface and monitor a cubic kilometre of ice for neutrino interactions.

Meanwhile at the other end of the spectrum, neutrino oscillations are central to our understanding the high energy neutrinos from space that IceCube discovered two years ago. If the physics of oscillations holds for neutrinos at PeV energies, as we expect, then the large distances traveled by astrophysical neutrinos imply that oscillations en route will fully mix the neutrino flavors by the time they reach us. The sample we measure is thus expected to be a nearly 1:1:1 ratio of electron, muon, and tau neutrinos (with only small deviations allowed, depending on the exact ratio at the source.) So far, this is consistent with what IceCube sees, based on the fraction of the astrophysical flux measured with muon neutrino interactions only, compared with all neutrino interactions. A significant deviation, if it survived experimental scrutiny, could be an indication of new physics at high energies.

Exclusion regions for astrophysical neutrino flavor ratios ( f_e : f_mu : f_tau ) measured at Earth.  Due to neutrino oscillations, any initial flavor ratio at the source is expected to oscillate to a final ratio measured at Earth that sits inside the long blue triangle. (Note that the blue triangle is so narrow, it looks almost like a line.)  While the best-fit composition at Earth is (0:0.2:0.8), the one-sigma contour includes all of the expected final flavor ratios.  The three symbols on the triangle depict the flavor ratios that we would expect to measure on Earth if the flavor ratio at the source were 1:0:0 (green square), 0:1:0 (red triangle), or 1:2:0 (blue circle).  IceCube Collaboration, Phys. Rev. Lett. 114, 171102 (2015)
Exclusion regions for the astrophysical neutrino flavor ratio ( f_e : f_mu : f_tau ) measured at Earth. Due to neutrino oscillations, any initial flavor ratio at the source is expected to oscillate to a final ratio measured at Earth that sits inside the long blue triangle. (Note that the blue triangle is so narrow that it looks almost like a line.) The best-fit composition at Earth is (0: 0.2 : 0.8), but the one-sigma contour includes all of the expected final flavor ratios. The three symbols on the triangle indicate the flavor ratios that we would expect to measure on Earth if the flavor ratio at the source were 1:0:0 (green square), 0:1:0 (red triangle), or 1:2:0 (blue circle). IceCube Collaboration, Phys. Rev. Lett. 114, 171102 (2015)

Neutrino oscillations are of great interest for elementary particle physics and for cosmology, both of which are tackled by cosmologists and theorists at the Oskar Klein Centre. Some of the questions studied are the hierarchy of neutrino masses mentioned above, whether CP violation is also present in the neutrino sector, and the origin of the neutrino masses. Are the neutrinos their own antiparticles or not, and does the lepton sector—to which neutrinos belong—take part in explaining the puzzle of the particle-antiparticle asymmetry we observe in the Universe?

As with all great discoveries in fundamental research, the discovery of neutrino oscillations celebrated with the Nobel Prize did much more than answer one question definitively; it also set the stage for advancing knowledge on many new fronts. The list of experiments worldwide pursuing neutrino oscillations in particular and neutrino research in general continues to grow. We at the Oskar Klein Centre can be almost certain that the neutrino has more revelations about the universe ahead.

*Listen to Fysikerpodden, a Swedish podcast by OKC PhD student Jessica Elevant, including 2 episodes about neutrino oscillations.

Breakthrough from IceCube: High-Energy Neutrinos from Space

Credit: IceCube Collaboration. Reprinted with permission from AAAS

Today’s issue of Science highlights a breakthrough in astroparticle physics many decades in the making. After tantalizing hints in the past year, the IceCube Collaboration now reports on a follow-up analysis, leading to a larger event sample and compelling evidence that the first high-energy neutrinos from outer space are starting to be seen.


The IceCube Neutrino Observatory is a strange kind of telescope, because it searches for neutrinos—rather than photons—from space. It’s situated at the South Pole, and it consists of thousands of detector modules buried up to 2.5 km deep in the glacial ice there. At that depth, the ice is very clear and very dark. What the modules see is the passage of subatomic particles zipping through the ice, emitting a well-known, faint blue light called Cherenkov light. When enough modules detect this light from a passing particle, the information is sent to the surface where a computer cluster starts crunching the `event’ to make a rough estimate of the direction and energy of the particle.

In the analysis reported today, 28 events are in the final sample, sifted out of over 10ˆ11 (one hundred thousand million) events in total, collected over two years. The overwhelming majority of these events are background particles from cosmic rays striking the atmosphere. Finding the handful of neutrinos that potentially originated in space is extraordinarily challenging. One of the innovations of the new analysis is to use the outermost detector modules as a `veto’, to tag any particle observed coming into the detector through the outer layer as a background event. All that remains are events which first appear, mysteriously, in the middle of the detector. This is the tell-tale signature of a neutrino event. In fact, famous for being “ghost” particles, almost all the neutrinos which pass through the detector never leave a trace at all. But fortunately a tiny fraction do happen to bump into an atom in the ice. When that happens, many other particles (like electrons, and their heavier cousins, muons) are created, and these particles create the Cherenkov light that IceCube detects.

It turns out that this veto design, selecting only those events where the light emission begins inside the detector, is extremely efficient for separating high energy neutrinos from background. Applying this technique and selecting only very energetic events, the two years of data would be expected to boil down to only about ten surviving background events (including some background neutrinos called atmospheric neutrinos). The fact that 28 events survive is thus strongly inconsistent with an explanation purely in terms of background. There are, moreover, many additional details such as their energy distribution and their distribution over the sky that are consistent with an origin in space but not with any known backgrounds.

The two highest energy events are remarkable in their own right. Their energy weighs in at about 1 PeV each (that’s roughly 100 times more energetic than the particles in the world’s most powerful accelerator, the LHC at CERN). They are the highest energy neutrinos ever observed, (or rather, were, as the news has already leaked that another, even higher energy neutrino, has been found in a newer analysis not yet published.)

With the results reported in Science today, the picture that is starting to emerge is that the universe is filled with a flux of high-energy neutrinos extending up to a few PeV in energy. The origin of this flux is not yet known. One clue is that the energy distribution seems to drop off not far above the highest energy events. IceCube has sensitivity for similar events at much higher energy, but none have been detected so far.

Credit: Sven Lidstrom. IceCube/NSF

Another clue about the neutrinos’ origins is that they appear to be coming from many different directions. With the current data there is not yet significant evidence of any preferred directions in the sky. Such evidence would of course greatly narrow down the possible sources. Because of the way the veto technique works, however, it sacrifices many potential neutrino events that would have much more precise directional information. New analyses using other techniques will try to identify more of these neutrinos with good directional reconstruction, and hopefully a sharper picture will start to emerge. It is widely believed that finding the sources of high-energy neutrinos will also reveal the origins of cosmic rays, a question that is now over a century old.

We thus arrive at a wonderful moment where a goal has been reached, yet upon reaching it we see new and more exciting challenges ahead. In Sweden, this dream to open the field of neutrino astronomy has a long and distinguished history. Swedish research groups were involved from the beginning twenty years ago with the first neutrino detector at the South Pole, AMANDA. Over the next decade that transitioned to the much larger IceCube detector, of which Per Olof Hulth from Stockholm University was the first spokesperson. Today the IceCube collaboration consists of about 250 scientists in a dozen countries, with Francis Halzen of University of Wisconsin as Principal Investigator, and Olga Botner of Uppsala University as spokesperson.

– Chad Finley (researcher at the Oskar Klein Centre) – cfinley@fysik.su.se

Link: Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector

News from the Venice Neutrino Workshop

Here is just a very brief summary of results that have been presented so far. (For an extensive blog coverage about the event, see this link.)
Unfortunately, Elena Aprile did not present the new results from Xenon100, but she said that they will be presented at a press conference in Gran Sasso in April. It seems that they have new accurate measurements of the efficiency L_eff over a substantial energy range, that of course will be crucial when interpreting the data.
In neutrino physics, the present buzz concerns the possibility of sterile neutrinos, as seems to be mildly preferred by cosmological data. Continue reading News from the Venice Neutrino Workshop

Welcome to the Oskar Klein Centre blog

As a way to communicate more quickly and efficiently between ourselves in the Oskar Klein Centre (OKC) and with the outside world, we have with the advice of Serena Nobili, responsible for OKC information and outreach, started this blog. Here you will now and then get updates on what is happening in the Centre Continue reading Welcome to the Oskar Klein Centre blog