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.

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