Tag Archives: IceCube

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.

Farewell to Per Olof Hulth, pioneer of Swedish particle astrophysics

Professor emeritus Per Olof Hulth
Professor emeritus Per Olof Hulth
Our colleague and friend Per Olof has passed away after a very brief period of illness. Peo, as we all called him, played a key role in the field of neutrino physics, both nationally and in the international arena. He started his career with six years at CERN (1976-1982), and was coordinator and spokesperson for several neutrino experiments using bubble chambers. In the early 1990’s, he started the experimental particle astrophysics activities in Sweden, first hoping that a high-energy neutrino telescope could be built in the Torneträsk lake of northern Sweden. After realizing that the transparency of the lake water was not adequate for a large Cerenkov telescope, Peo led the Swedish effort in AMANDA, the neutrino telescope buried in the Antartic ice and the predecessor of IceCube. Peo was the first spokesperson of the IceCube collaboration in 2001-2005, at the crucial stage which included the first year of deployment at the Pole. IceCube quickly grew to be a world leading observatory, now using 5160 light sensors deep in the ice to look for the rare signals from astrophysical neutrinos. These were indeed discovered in 2013, a spectacular achievement and a great satisfaction for Peo, who remained a figure in the experiment, also after retirement. Peo’s commitment to Physics and enthusiasm for his work left no room for hesitation. He went to the remote experimental site at South Pole 11 times, and would have gladly gone there again and again. He was a constant source of inspiration for younger colleagues. We are proud to have had him as a colleague and friend. We will miss him dearly.

– Lars Bergström, Chad Finley, Ariel Goobar, Klas Hultqvist and Christian Walck on behalf of the colleagues at OKC.

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

Interview with Christopher Savage

Christopher SavageChristopher Savage is a Oskar Klein Fellow since the summer of 2009. He is working on direct detection of Dark Matter and seems to be very happy about it! I asked him to tell us more.

Why did you choose the OKC for doing a postdoc?
The broad focus on cosmology, with an emphasis on interaction between different areas, was very appealing. In addition, I had started working on capture of dark matter in stars and there is a lot of expertise in that area here in Stockholm (particularly Joakim Edsjö).

When I was given the offer, I was at a conference in Michigan with four OKC people in attendance (Marcus Berg and Joakim Edsjö plus PhD students Erik Lundström and Sara Rydbeck). That gave me a chance to see what the group was like and it made the decision an easy one.

What is your field of research?
My field of research is in the phenomenology of detecting dark matter, both directly and indirectly. The former case (direct detection) involves looking for interactions (scattering) of dark matter particles inside a detector, while the latter case (indirect detection) involves looking for products of dark matter particles that annihilate elsewhere.

Direct detection has long been a focus of my research and I continue to work in that area here at the OKC. I look at how various issues affect the signals seen in direct detection experiments, which might explain the apparent incompatibility between experiments that observe signals consistent with dark matter (CoGeNT, CRESST, and DAMA) and those that do not (CDMS and XENON, to name a few). The issues include how dark matter couples to ordinary matter, how the dark matter is distributed in the galaxy, and potential systematic issues in the experiments themselves (often involving energy calibrations). In addition, several of us (Yashar Akrami, Pat Scott, Jan Conrad, Joakim Edsjö and I) are looking at how direct detection results constrain supersymmetric models, which provide a natural candidate for a dark matter particle (the neutralino).
Continue reading Interview with Christopher Savage

Time for PhD thesis defenses at the Oskar Klein Center

This spring there have been several PhD theses defenses here at the Oskar Klein Center, and as much as we hate saying good bye to some of our best students, we are proud to have been part of their professional lives.
The first to go was Jakob Nordin who defended his thesis with the title “Spectral Properties of Type Ia Supernovae and Implications for Cosmology” the 27th May 2011. During his PhD under the guidance of Professor Ariel Goobar, Jakob collected spectroscopic data of Type Ia supernovae with the purpose to study if SNIa are indeed good “standard candles” over a wide redshift range, a necessary condition to use these explosions to study the properties of dark energy. He has also investigated the nature of one of the color-brightness relation, one of the largest astrophysical corrections in the use of SNIa to measure distances. Finally, he has written a Monte-Carlo simulation package to investigate how systematic uncertainties in the use of Type Ia SNe as distance indicators propagate into our cosmological fits of dark energy parameters. Jakob is now ready to move on with a postdoc at the Berkeley, and we really wish him good luck with his life in California!

Another of Ariel Goobar students, Teresa Riehm also student at the Astronomy department, defended her thesis “Investigating the Dark Universe through Gravitational Lensing”. Continue reading Time for PhD thesis defenses at the Oskar Klein Center

New instruments for Dark Matter

Things have been happening lately with experiments which could eventually shed some light on dark matter.

The IceCube Neutrino Observatory, which was completed last December, defiantly started data taking in its final configuration on Friday the 13:th of this month. Data from 5397 optical modules are recorded at a rate of 2370 events per second, and about 50 million events per day are sent North for analysis via satellite (unfortunately,
almost all are due to atmospheric muons, not neutrinos). We are expecting to take data with the completed IceCube for fifteen years at least, and analyses have to be designed to include ever more data without introducing biases. Neutrinos from dark matter annihilation are being searched for from different sources, including the Sun which is the prime target of the Stockholm group. Continue reading New instruments for Dark Matter