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The American Association of Physics Teachers have collated a set of educational resources on “Neutrinos: Teaching the science behind the 2015 Nobel Prize in Physics” (see The Cosmos, Section 12.7, pp. 322-325).

Find out more on their website.

 

 

The 2015 Nobel Prize in Physics was given on October 6 to Takaaki Kajita of the Superkamiokande experiment and to Arthur McDonald of the Sudbury Neutrino Observatory.  The work at both those sites is thoroughly discussed in The Cosmos (see Section 12.7, p. 322-325) about the solar-neutrino experiment.

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By showing definitively that a mix of the three types of neutrinos reaches the Earth, combining the knowledge that only electron-neutrinos leave the Sun shows that neutrinos change in type en route.  Only if neutrinos have mass can such changes take place, so the discovery is a major challenge to the Standard Model of particle physics.

This is the third Nobel Prize for neutrino research.  Half the 1995 Nobel Prize went Fred Reines for the discovery of neutrinos in an atomic-reactor beam (his co-discoverer, Clyde Cowan, having died before the prize was given, making him ineligible).  Half the 2002 Nobel Prize went to Ray Davis, who ran the chlorine version of the neutrino experiment at the Homestake Mine, and Masatoshi Koshiba, who was in charge of Kamiokande (the Neutrino Detection Experiment [NDE] in the Kamioka mine in Japan). John Bahcall from the Institute for Advanced Study, who had done the bulk of the theoretical work involved, was omitted from the prize, unfortunately (again, as the prize is not awarded posthumously).

Links: 2015 Nobel Prize in Physics at the Royal Swedish Academy of Sciences; Dennis Overbye’s analysis for the NY Times (including a discussion of the next investigations via The Deep Underground Neutrino Experiment, DUNE).

From a Fremilab press release, April 22, 2015:

The Italian ICARUS neutrino experiment – the world’s largest of its type – will move to Fermilab and become an integral part of the future of neutrino research in the United States (see Section 12.7c, p. 324-325).

Scientists will transport the liquid-argon neutrino detector across the Atlantic Ocean to its new home at the U.S. Department of Energy’s Fermi National Accelerator Laboratory. The 760-ton, 65-foot-long detector took data for the ICARUS experiment at the Italian Institute for Nuclear Physics’ (INFN) Gran Sasso National Laboratory in Italy from 2010 to 2014, using a beam of neutrinos sent through the Earth from CERN. The detector is now being refurbished at CERN, where it is the first beneficiary of a new test facility for neutrino detectors.

Icarus

Credit: INFN

When it arrives at Fermilab, the detector will become part of an on-site suite of three experiments dedicated to studying neutrinos, ghostly particles that are all around us but have given up few of their secrets. All three detectors will be filled with liquid argon, which enables the use of state-of-the-art time projection technology, drawing charged particles created in neutrino interactions toward planes of fine wires that can capture a 3-D image of the tracks those particles leave. Each detector will contribute different yet complementary results to the hunt for a fourth type of neutrino.

Many theories in particle physics predict the existence of a so-called “sterile” neutrino, which would behave differently from the three known types and, if it exists, could provide a route to understanding the mysterious dark matter that makes up 25 percent of the Universe. Discovering this fourth type of neutrino would revolutionize physics, changing scientists’ picture of the Universe and how it works.

Links: Fermilab press release, CERN press release, ICARUS home.

An article in ScienceNews reports that data from the IceCube experiment under Antarctic ice have shown that the highest energy neutrinos they detect come from all directions, indicating that they are probably at cosmological distances (see Section 12.7c, pp. 324-325). The results were first announced at the American Physical Society’s meeting in April 2014.

Neutrinos open a window into the very distant and high-energy Universe that is extremely difficult to access by any other means. This is because neutrinos, unlike every other subatomic particle, are electrically neutral and rarely interact with matter. By detecting these particles and charting the directions they come from, scientists aim to identify the sources of neutrinos: star-forming galaxies, supermassive black holes or perhaps some as-yet unknown violent objects. These sources can accelerate neutrinos and other subatomic particles to energies far greater than any human-made machine could achieve.

Credit: Sven Lidstrom, IceCube/NSF

IceCube was specifically built to aid in this quest. For three years, strings of sensors stretching as deep as 2.5 kilometers below the surface of an Antarctic glacier have detected subtle flashes of light created when neutrinos and other particles collide with atoms. Last year, IceCube researchers identified 28 high-energy neutrinos from all directions that are almost certainly from outside the Solar System. The researchers have since found nine more, including the highest energy neutrino ever detected.

To complement this painstaking search for the highest energy neutrinos, Christopher Weaver, an IceCube physicist at the University of Wisconsin-Madison, decided to cast a wider net for the larger population of slightly lower-energy astronomical neutrinos. His approach relied on selecting particles that fell from the skies of the Northern Hemisphere, whizzed through Earth’s interior and arrived at IceCube from below. Only neutrinos, and not other particles that often trigger IceCube’s sensors from above, can make it through Earth’s dense crust and core. He also limited his search to detections at a specific energy – about 100 trillion electron volts – so that the number of neutrinos from space wouldn’t be dwarfed by the amount of neutrinos produced in the atmosphere. (IceCube’s sensors can’t distinguish between the two.)

That left Weaver with about 35,000 neutrinos, at least some of which began their journeys beyond the Solar System. He tracked the directions they came from and found no evidence of clustering in any particular parts of the sky – a finding that confirmed previous analyses and suggests that no local source is primarily responsible for the population of neutrinos whizzing by Earth. As IceCube continues to collect more data, scientists hope these two independent neutrino search methods will converge on trends in the neutrinos’ direction of arrival. It’s an exciting time in neutrino astrophysics.

Links: ScienceNews article

A massive telescope buried in the Antarctic ice has detected 28 record-breaking, extremely high-energy neutrinos – elementary particles that likely originate far beyond our Solar System. (See Sections 12.7c and 13.2g)

The achievement, which comes nearly 25 years after the pioneering idea of detecting neutrinos in ice, provides the first solid evidence for astrophysical neutrinos from cosmic accelerators and has been hailed as the dawn of a new age of astronomy. The team of researchers that detected the neutrinos with the IceCube Neutrino Observatory in Antarctica published a paper describing the detections on November 22, 2013, in the journal Science.

Credit: IceCube Collaboration

The neutrinos had energies greater than 1,000,000,000,000,000 electron volts, or 1 peta-electron volt (PeV). Two of these neutrinos had energies many thousands of times higher than the highest-energy neutrino that any man-made particle accelerator has ever produced. (1 joule of energy = 6.2419 × 1018 eV.)

While not telling scientists what the cosmic accelerators are or where they’re located, the IceCube results do provide scientists with a compass that can help guide them to the answers. Unlike other cosmic particles, neutrinos are electrically neutral and nearly massless, so that they travel through space in a straight line from their point of origin, passing through virtually everything in their path without being deflected by interstellar masses and magnetic fields.

Credit: Jamie Yang, IceCube Collaboration

The IceCube observatory consists of over 5,000 basketball-sized light detectors called Digital Optical Modules (DOMs). These are suspended along 86 strings that are embedded in a cubic kilometer of clear ice starting 1.5 kilometers beneath the Antarctic surface. Out of the trillions of neutrinos that pass through the ice each day, a couple of hundred will collide with oxygen nuclei, yielding the blue light of Cherenkov radiation that IceCube’s DOMs detect.

Links: LBNL press release; U. Wisconsin press release (with image gallery); Penn State press release (with movie).