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  • richardmitnick 10:09 am on August 6, 2018 Permalink | Reply
    Tags: , , Las Cruces Sun News, , Sterile neutrinos   

    From Fermilab via Las Cruces Sun News: “NMSU physicist may have evidence of the elusive sterile neutrino” 

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    From Fermilab , an enduring source of strength for the US contribution to scientific research world wide.

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    Las Cruces Sun News

    July 21, 2018 [Just now in social media]
    Billy Huntsman

    A New Mexico State University professor is part of an international research team that recently released the results of an experiment that may support a theory for a fourth fundamental particle – the mysterious sterile neutrino that passes through matter without interacting with it….

    Robert Cooper, assistant professor in NMSU’s Department of Physics, is one of a group of scientists from 17 universities and labs, which conducted their experiments collectively known as MiniBooNE at the Fermi National Accelerator Laboratory near Chicago.

    FNAL/MiniBooNE

    3
    MiniBooNE’s oil tank is 12 meters in diameter and lined with 1,520 photodetectors. Ryan Patterson / Princeton / Fermilab.

    “We are presenting very compelling evidence that could support the sterile neutrino hypothesis,” Cooper said. “It will take other types of experiments that look at this phenomenon from a slightly different ‘angle’ to claim discovery.”

    But they are not jumping to conclusions. Cooper emphasized the research team is not claiming the discovery of a fourth neutrino.

    3
    Robert Cooper, in the Department of Physics at New Mexico State University, is part of a research team whose research may support evidence for a fourth fundamental particle: the sterile neutrino. (Photo: NMSU photo by Billy Huntsman)

    “Besides the sterile neutrino theory, another explanation could be that there is a new or underestimated source of background events,” Cooper said. “Clearly, sterile neutrinos could rewrite our understanding of neutrino and particle physics, but either way, something new is occurring.”

    MiniBooNE has been conducting experiments for over 20 years. The anomaly that resulted in the sterile-neutrino hypothesis was first detected in the 1990s at Los Alamos National Laboratory.

    “Not much was done differently this time around,” Cooper said. “The detector was run twice as long, and as a result, MiniBooNE doubled the number of neutrino events. Some have argued that the excess of electron events could have been due to an ‘unlucky’ statistical fluctuation. The increased number of events has improved our ‘statistical sensitivity’ and shows that this result is significant and very unlikely to be due to a statistical accident.”

    MiniBooNE specifically researches neutrino oscillations. Currently there are three kinds, or flavors, of neutrinos known: an electron neutrino, a muon neutrino, and a tau neutrino. Neutrino oscillation is the phenomenon in which neutrinos change into different flavors.

    “MiniBooNE was designed to search for electron neutrinos,” Cooper said. “It identifies electron neutrinos by searching for interactions that produce electrons in the final state. We use the Booster Neutrino Beamline at Fermilab to produce a pure beam of muon neutrinos to search for these.”

    “An electron neutrino can produce an electron when it interacts with matter, and the muon neutrino can produce a muon when it interacts with matter,” Cooper said. “At the neutrino energies we produce in our experiments, more than 800 million electron volts, and the distance scales of our experiment – half a kilometer – we expect to see almost no electrons in the final state.”

    But at the conclusion of their experiments, the researchers saw an excess of final-state electrons beyond what they expected.

    “We have measurements and models that constrain this number to a small amount,” Cooper said. “Another source of ‘false’ electrons is gamma rays. Our detector does an extraordinary job of distinguishing background gamma ray events from electrons, but some may slip through the cracks of our analysis. Just like contamination, we have multiple measurements of these backgrounds and extensive simulation and modeling. Therefore, our excess of electron events seems significant, validated, and real.”

    The existence of a sterile fourth neutrino would mean a total rethinking of particle physics would be necessary.

    “Our understanding of particle physics, cosmology, and possibly dark matter would be affected,” Cooper said. “Discovering new particles is a very big deal. Supposing sterile neutrinos exist, this would have deep consequences about the types of future experiments we perform to explore the boundaries of our particle physics knowledge.”

    Sterile neutrinos could answer a number of questions about dark matter, which physicists theorize makes up most of the universe and about which very little is known.

    Cooper said there could even be two or more types of sterile neutrinos.

    A formal discovery will require more work and the MiniBooNE team is continuing its research.

    “I’m working on multiple new efforts to validate this possible discovery,” Cooper said. “All that being said, we have very significant evidence, but it is not absolute proof yet.”

    The preprint article has been submitted to a peer-reviewed journal and is available at: https://arxiv.org/abs/1805.12028 Observation of a Significant Excess of Electron-Like Events in the MiniBooNE Short-Baseline Neutrino Experiment.

    See the full article here .


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  • richardmitnick 3:57 pm on June 4, 2018 Permalink | Reply
    Tags: , , , , , Sterile neutrinos   

    From physicsworld.com: “Evidence for sterile neutrinos claimed by Fermilab experiment” 

    physicsworld
    From physicsworld.com

    04 Jun 2018
    Edwin Cartlidge

    Physicists working on the Mini Booster Neutrino Experiment (MiniBooNE) at Fermilab in the US have released new results that they argue provide strong evidence for the existence of a new type of particle known as a sterile neutrino. The researchers say that their data are fully consistent with a previous hint of sterile neutrinos that emerged more than 20 years ago from the Liquid Scintillator Neutrino Detector (LSND) at the Los Alamos National Laboratory in New Mexico, although other groups have failed to reproduce the findings.

    FNAL/MiniBooNE

    Sterile neutrinos, if they exist, would be even more elusive than standard neutrinos, which themselves are chargeless and nearly massless. The Standard Model of particle physics tells us that neutrinos come in three flavours – electron, muon and tau – and that they “oscillate” from one flavour to another as they travel through space. But some extensions of the Standard Model predict that these known neutrinos can also oscillate into and out of sterile neutrinos, which might not interact at all with any other ordinary matter.

    MiniBooNE monitors the tiny flashes of light that are produced occasionally when electron neutrinos interact with atomic nuclei in about 800 tonnes of pure mineral oil contained in a spherical tank located underground on the Fermilab site near Chicago.

    MiniBooNE receptors

    Those neutrinos are generated after protons from the lab’s Booster accelerator are fired into a beryllium target to create muon neutrinos, which then oscillate to electron neutrinos as they travel several hundred metres through the Earth to the detector.

    Statistically speaking

    In a preprint uploaded recently to the arXiv server, the MiniBooNE collaboration reports having detected far more electron neutrinos than would be expected from purely Standard Model oscillations after collecting data for 15 years. According to collaboration member William Louis of Los Alamos, the measurement suggests that some of the muon neutrinos oscillate into sterile neutrinos that in turn transform into electron neutrinos. That interpretation, he says, is bolstered by the fact that the variation of electron-neutrino excess with neutrino energy – a parameter of neutrino oscillations – seen in MiniBooNE matches that recorded at Liquid Scintillator Neutrino Detector. He and his colleagues conclude that the combined excess from the two experiments has a statistical significance of 6.1σ, which is well above the 5σ that is normally considered a discovery in particle physics.

    Although MiniBooNE is quite similar to LSND – in using mineral oil to observe neutrinos from an accelerator-based source – and indeed has inherited personnel from the earlier project, its researchers are nevertheless confident that there are no common sources of error. “We think that is very unlikely,” says Louis. “The two experiments have very different energies and backgrounds, and therefore very different possible systematic errors”.

    Not a done deal

    However, the existence of sterile neutrinos is not yet a done deal. While some groups operating experiments that exploit neutrinos from nuclear reactors or radioactive sources have also gathered (somewhat weaker) evidence for the hypothetical particles, other groups have looked and found nothing. These include the IceCube collaboration, which operates a detector at the South Pole, and the now completed Main Injector Neutrino Oscillation Search (MINOS) at Fermilab.

    U Wisconsin ICECUBE neutrino detector at the South Pole

    U Wisconsin ICECUBE neutrino detector at the South Pole

    Lunar Icecube

    IceCube DeepCore annotated

    IceCube PINGU annotated


    DM-Ice II at IceCube annotated

    FNAL MINOS experiment

    FNAL Minos map

    FNAL MINOS near detector

    FNAL MINOS Far Detector in the Soudan Mine in northern Minnesota

    Indeed, some physicists outside the MiniBooNE collaboration have told Physics World that they remain unconvinced by the latest results.

    To try and settle the issue, numerous other dedicated experiments are either operating or under development. For their part, Louis and colleagues are currently setting up another three detectors at Fermilab – one of which is now running – to monitor the neutrino flux closer to the beryllium target than MiniBooNE and also further away. The idea, he says, is to show beyond doubt that the electron neutrino excess really is due to oscillations, given that the measured oscillation rate should vary with distance from the source as well as with energy. Results are expected “over the next three to five years,” he says.

    See the full article here .


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  • richardmitnick 11:29 am on September 25, 2017 Permalink | Reply
    Tags: , , , , , Sterile neutrinos,   

    From U Wisconsin IceCube: “Looking for new physics in the neutrino sector” 

    icecube
    U Wisconsin IceCube South Pole Neutrino Observatory

    25 Sep 2017
    Sílvia Bravo

    ICECUBE neutrino detector

    Neutrinos are intriguing in more ways than one. And although the fact that they have such tiny mass explains their quirky behavior, their allure remains intact. The issue is that neutrino masses are not predicted by the Standard Model; thus, on its own, the existence of a neutrino with mass is an indication of new physics. And that’s what scientists around the world, including at IceCube, want to learn: what type of new physics are neutrinos pointing to?

    New physics could appear in the form of a new type of neutrino or it could help us understand the nature of dark matter. The possibilities are endless. In a new search for nonstandard neutrino interactions, the IceCube Collaboration has tested theories that introduce heavy bosons, such as some Grand Unified Theories. These heavy bosons would explain, for example, why neutrinos have masses much smaller than their lepton partners. The study resulted in new constraints on these models, which are among the world’s best limits for nonstandard interactions in the muon-tau neutrino sector. These results have just been submitted to Physical Review D.

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    Confidence limits from this analysis are shown as solid vertical red lines. The light blue and light green vertical lines show previous limits by Super-Kamiokande and another study using IceCube data at higher energy. Credit: IceCube Collaboration.

    Super-Kamiokande experiment. located under Mount Ikeno near the city of Hida, Gifu Prefecture, Japan

    The flavor of neutrinos oscillates as they travel through matter or empty space, a quantum effect on macroscopic scales that proves that they have mass. When atmospheric neutrinos reach IceCube after crossing the Earth, they have often morphed from muon into tau neutrinos. If TeV-scale bosons predicted by nonstandard theories exist, they will modify the probability that a given type of neutrino oscillates into other types. The result is that the disappearance pattern of muon neutrinos in IceCube will change, with effects that span a large range of energies.

    In IceCube, for studies using atmospheric neutrinos that sail through the Earth, these nonstandard interactions (NSIs) can be parametrized in terms of the strength of muon neutrino to tau neutrino morphing due to an NSI, a parameter called .

    IceCube researchers have analyzed three years of data, using the same neutrino sample used for a recent measurement of the neutrino oscillation parameters, but with an additional selection criterion to improve the signal purity. The remaining 4,625 candidate neutrino events were used to fit the oscillation parameters, including the NSI contribution.

    The best fit of muon to tau NSI oscillations was consistent with no nonstandard interactions. “Even though no new physics was shown by this study, it narrows in on the possible existence of new neutrino interactions with regular matter” says Carlos Argüelles, an IceCube researcher from MIT. “It also showcases the advantages of having a very broad energy range, so experiments like IceCube can look for new oscillation physics with neutrinos, which are 10 to 1000 times more energetic than the average proton.”

    The 90% confidence level upper limit on the NSI parameter is consistent with previous measurements by Super-Kamiokande, which at that time had set the world’s best limits. The new IceCube measurement slightly improves Super-Kamiokande’s measurements, also extending the energy range. A more recent study using published IceCube data at even higher energies has also set limits on the parameter, which in turn were slightly more stringent than the ones of the present study.

    Albrecht Karle, a professor of physics at UW–Madison, comments that “the results shown here are based on only a relatively small set of muon neutrinos available.” IceCube is collecting more than 100,000 muon neutrinos per year, which are yet to be mined for physics beyond the Standard Model. “With almost a million atmospheric neutrinos, IceCube has an incredible data set for investigating even small deviations from Standard Model physics.”

    And keeping in mind that it’s not all about the detector, Melanie Day, another IceCube researcher and co-author on this paper, adds, “Not enough is said about the value of teamwork and collaboration over individual contributions to scientific results. But without that, this result would not have been possible.”

    See the full article here .

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    IceCube is a particle detector at the South Pole that records the interactions of a nearly massless sub-atomic particle called the neutrino. IceCube searches for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The IceCube telescope is a powerful tool to search for dark matter, and could reveal the new physical processes associated with the enigmatic origin of the highest energy particles in nature. In addition, exploring the background of neutrinos produced in the atmosphere, IceCube studies the neutrinos themselves; their energies far exceed those produced by accelerator beams. IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice.

     
  • richardmitnick 1:39 pm on May 4, 2017 Permalink | Reply
    Tags: , , Sterile neutrinos,   

    From Symmetry: “Sterile neutrino search hits roadblock at reactors” 

    Symmetry Mag

    Symmetry

    05/04/17
    Kathryn Jepsen

    1
    LBNL

    A new result from the Daya Bay collaboration reveals both limitations and strengths of experiments studying antineutrinos at nuclear reactors.

    As nuclear reactors burn through fuel, they produce a steady flow of particles called neutrinos. Neutrinos interact so rarely with other matter that they can flow past the steel and concrete of a power plant’s containment structures and keep on moving through anything else that gets in their way.

    Physicists interested in studying these wandering particles have taken advantage of this fact by installing neutrino detectors nearby. A recent result using some of these detectors demonstrated both their limitations and strengths.

    The reactor antineutrino anomaly

    In 2011, a group of theorists noticed that several reactor-based neutrino experiments had been publishing the same, surprising result: They weren’t detecting as many neutrinos as they thought they would.

    Or rather, to be technically correct, they weren’t seeing as many antineutrinos as they thought they would; nuclear reactors actually produce the antimatter partners of the elusive particles. About 6 percent of the expected antineutrinos just weren’t showing up. They called it “the reactor antineutrino anomaly.”

    The case of the missing neutrinos was a familiar one. In the 1960s, the Davis experiment located in Homestake Mine in South Dakota reported a shortage of neutrinos coming from processes in the sun.

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    Construction of the Homestake Mine tank. BNL.

    Other experiments confirmed the finding. In 2001, the Sudbury Neutrino Observatory in Ontario demonstrated that the missing neutrinos weren’t missing at all; they had only undergone a bit of a costume change.

    SNOLAB, Sudbury, Ontario, Canada.

    Neutrinos come in three types. Scientists discovered that neutrinos could transform from one type to another. The missing neutrinos had changed into a different type of neutrino that the Davis experiment couldn’t detect.

    Since 2011, scientists have wondered whether the reactor antineutrino anomaly was a sign of an undiscovered type of neutrino, one that was even harder to detect, called a sterile neutrino.

    A new result from the Daya Bay experiment in China not only casts doubt on that theory, it also casts doubt on the idea that scientists understand their model of reactor processes well enough at this time to use it to search for sterile neutrinos.

    The word from Daya Bay

    The Daya Bay experiment studies antineutrinos coming from six nuclear reactors on the southern coast of China, about 35 miles northeast of Hong Kong. The reactors are powered by the fission of uranium. Over time, the amount of uranium inside the reactor decreases while the amount of plutonium increases. The fuel is changed—or cycled—about every 18 months.

    The main goal of the Daya Bay experiment was to look for the rarest of the known neutrino oscillations. It did that, making a groundbreaking discovery after just nine weeks of data-taking.

    But that wasn’t the only goal of the experiment. “We realized right from the beginning that it is important for Daya Bay to address as many interesting physics problems as possible,” says Daya Bay co-spokesperson Kam-Biu Luk of the University of California, Berkeley and the US Department of Energy’s Lawrence Berkeley National Laboratory.

    For this result, Daya Bay scientists took advantage of their enormous collection of antineutrino data to expand their investigation to the reactor antineutrino anomaly.

    Using data from more than 2 million antineutrino interactions and information about when the power plants refreshed the uranium in each reactor, Daya Bay physicists compared the measurements of antineutrinos coming from different parts of the fuel cycle: early ones dominated by uranium through later ones dominated by both uranium and plutonium.

    n theory, the type of fuel producing the antineutrinos should not affect the rate at which they transform into sterile neutrinos. According to Bob Svoboda, chair of the Department of Physics at the University of California, Davis, “a neutrino wouldn’t care how it got made.” But Daya Bay scientists found that the shortage of antineutrinos existed only in processes dominated by uranium.

    Their conclusion is that, once again, the missing neutrinos aren’t actually missing. This time, the problem of the missing antineutrinos seems to stem from our understanding of how uranium burns in nuclear power plants. The predictions for how many antineutrinos the scientists should detect may have been overestimated.

    “Most of the problem appears to come from the uranium-235 model (uranium-235 is a fissile isotope of uranium), not from the neutrinos themselves,” Svoboda says. “We don’t fully understand uranium, so we have to take any anomaly we measured with a grain of salt.”

    This knock against the reactor antineutrino anomaly does not disprove the existence of sterile neutrinos. Other, non-reactor experiments have seen different possible signs of their influence. But it does put a damper on the only evidence of sterile neutrinos to have come from reactor experiments so far.

    Other reactor neutrino experiments, such as NEOS in South Korea and PROSPECT in the United States will fill in some missing details.

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    NEOS

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    Prospect. BNL

    NEOS scientists directly measured antineutrinos coming from reactors in the Hanbit nuclear power complex using a detector placed about 80 feet away, a distance some scientists believe is optimal for detecting sterile neutrinos should they exist. PROSPECT scientists will make the first precision measurement of antineutrinos coming from a highly enriched uranium core, one that does not produce plutonium as it burns.

    A silver lining

    The Daya Bay result offers the most detailed demonstration yet of scientists’ ability to use neutrino detectors to peer inside running nuclear reactors.

    “As a study of reactors, this is a tour de force,” says theorist Alexander Friedland of SLAC National Accelerator Laboratory. “This is an explicit demonstration that the composition of the reactor fuel has an impact on the neutrinos.”

    Some scientists are interested in monitoring nuclear power plants to find out if nuclear fuel is being diverted to build nuclear weapons.

    “Suppose I declare my reactor produces 100 kilograms of plutonium per year,” says Adam Bernstein of the University of Hawaii and Lawrence Livermore National Laboratory. “Then I operate it in a slightly different way, and at the end of the year I have 120 kilograms.” That 20-kilogram surplus, left unmeasured, could potentially be moved into a weapons program.

    Current monitoring techniques involve checking what goes into a nuclear power plant before the fuel cycle begins and then checking what comes out after it ends. In the meantime, what happens inside is a mystery.

    Neutrino detectors allow scientists to understand what’s going on in a nuclear reactor in real time.

    Scientists have known for decades that neutrino detectors could be useful for nuclear nonproliferation purposes. Scientists studying neutrinos at the Rovno Nuclear Power Plant in Ukraine first demonstrated that neutrino detectors could differentiate between uranium and plutonium fuel.

    Most of the experiments have done this by looking at changes in the aggregate number of antineutrinos coming from a detector. Daya Bay showed that neutrino detectors could track the plutonium inventory in nuclear fuel by studying the energy spectrum of antineutrinos produced.

    “The most likely use of neutrino detectors in the near future is in so-called ‘cooperative agreements,’ where a $20-million-scale neutrino detector is installed in the vicinity of a reactor site as part of a treaty,” Svoboda says. “The site can be monitored very reliably without having to make intrusive inspections that bring up issues of national sovereignty.”

    Luk says he is dubious that the idea will take off, but he agrees that Daya Bay has shown that neutrino detectors can give an incredibly precise report. “This result is the best demonstration so far of using a neutrino detector to probe the heartbeat of a nuclear reactor.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 9:27 pm on December 9, 2016 Permalink | Reply
    Tags: , , Janet Conrad, , , , Sterile neutrinos,   

    From Quanta: Women in STEM – “On a Hunt for a Ghost of a Particle” Janet Conrad 

    Quanta Magazine
    Quanta Magazine

    Janet Conrad has a plan to catch the sterile neutrino — an elusive particle, possibly glimpsed by a number of experiments, that would upend what we know about the subatomic world.

    December 8, 2016
    Maggie McKee

    1
    Kayana Szymczak for Quanta Magazine

    Even for a particle physicist, Janet Conrad thinks small. Early in her career, when her peers were fanning out in search of the top quark, now known to be the heaviest elementary particle, she broke ranks to seek out the neutrino, the lightest.

    In part, she did this to avoid working as part of a large collaboration, demonstrating an independent streak shared by the particles she studies. Neutrinos eschew the strong and electromagnetic forces, maintaining only the most tenuous of ties to the rest of the universe through the weak force and gravity. This aloofness makes neutrinos hard to study, but it also allows them to serve as potential indicators of forces or particles entirely new to physics, according to Conrad, a professor at the Massachusetts Institute of Technology. “If there’s a force out there we haven’t seen, it must be because it is very, very weak — very quiet. So looking at a place where things are only whispering is a good idea.”

    In fact, neutrinos have already hinted at the existence of a new type of whispery particle. Neutrinos come in three flavors, morphing from one flavor to another by means of some quantum jujitsu. In 1995, the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory suggested that these oscillations involve more than the three flavors “we knew and loved,” Conrad said. Could there be another, more elusive type of “sterile” neutrino that can’t feel even the weak force? Conrad has been trying to find out ever since, and she expects to get the latest result from a long-running follow-up experiment called MiniBooNE within a year.

    FNAL/MiniBooNE
    FNAL/MiniBooNE

    Still, even MiniBooNE is unlikely to settle the question, especially since a number of other experiments have found no signs of sterile neutrinos. So Conrad is designing what she hopes will be a decisive test using — naturally — a small particle accelerator called a cyclotron rather than a behemoth like the Large Hadron Collider in Europe. “I feel like my field just keeps deciding to get at our problems by growing, and I think that there’s going to be a point at which that’s not sustainable,” Conrad said. “When the great meteor hits, I want to be a small, fuzzy mammal. That’s my plan: small, fuzzy mammal.”

    Quanta Magazine spoke with Conrad about her hunt for sterile neutrinos, her penchant for anthropomorphizing particles, and her work on the latest Ghostbusters reboot. An edited and condensed version of the interview follows.

    QUANTA MAGAZINE: What would it mean for physics if sterile neutrinos exist?

    JANET CONRAD: The Standard Model of particle physics has done very well in predicting what’s going on, but there’s a great deal it can’t explain — for example, dark matter.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Right now we’re desperately looking for clues as to what the larger theory would be. We have been working on ideas, and in many of these “grand unified theories,” you actually get sterile neutrinos falling out of the theory. If we were to discover that there were these extra neutrinos, it would be huge. It would really be a major clue to what the larger theory would be.

    You’ve been looking for neutrinos your entire career. Was that always the plan?

    I started out thinking I was going to be an astronomer. I went to Swarthmore College and discovered that astronomy is cold and dark. I was lucky enough to get hired to work in a particle physics lab. I worked for the Harvard Cyclotron, which was at that time treating eye cancers. But in the evenings physicists would bring their detectors down and calibrate them using the same accelerator. I was really interested in what they were doing and got a position the next summer at Fermilab [FNAL]. It was such a good fit for me. I just think the idea of creating these tiny little universes is so wondrous. Every collision is a little world. And the detectors are really big and fun to work on — I like to climb around stuff. I liked the juxtaposition of the scales; this incredibly tiny little world you create and this enormous detector you see it in.

    And how did you get into neutrino research in particular?

    When I was in grad school, the big question was: What is the mass of the top quark? Everybody expected me to join one of the collider experiments to find the top quark and measure its mass, and instead I was looking around and was quite interested in what was going on in the neutrino world. I actually had some senior people tell me it would be the end of my career.

    Why did you take that risk?

    I was very interested in the questions that were coming out of the neutrino experiments, and also I didn’t really want to join an enormously large collaboration. I was more interested in the funny little anomalies that were already showing up in the neutrino world than I was in a particle which had to exist — the top quark — and the question of what was its precise mass. I am really, I suppose, an anomaly chaser. I admit it. Some people might call it an epithet. I wear it with pride.

    One of those anomalies was the hint of an extra type of neutrino beyond the three known flavors in the Standard Model. That result from LSND was such an outlier that some physicists suggested dismissing it. Instead, you helped lead an experiment at Fermilab, called MiniBooNE, to follow up on it. Why?

    You’re not allowed to throw out data, I’m sorry. That is exactly how to miss important new physics. We can’t be so in love with our Standard Model that we aren’t willing to question it. Even if the question doesn’t align with our prejudices, we have to ask the question anyway. When I started out, nobody was really interested in sterile neutrinos. It was a lonely land out there.

    MiniBooNE’s results have added to the mystery. In one set of experiments using antineutrinos, it found LSND-like hints of sterile neutrinos, and in another, using neutrinos, it did not.

    The antineutrino result matched up with LSND very well, but the neutrino result, which is the one we produced first, is the one that doesn’t match up. The whole world would be a very different place if we had started with antineutrino running and gotten a result that matched LSND. I think there would have been a lot more interest immediately in the sterile-neutrino question. We would have been where we are now at least 10 years earlier.

    Where are we now?

    There are eight experiments total that have anomalies suggesting the presence of more than the three known flavors of neutrino. There are also seven experiments that don’t. Recently, some of the experiments that have not seen an effect have gotten a lot of press, including IceCube, which is a result that my group worked on. A lot of press came out about how IceCube didn’t see a sterile-neutrino signal. But while the data rules out some of the possible sterile-neutrino masses, it doesn’t rule out all of them, a result we point out in an article that has just been published in Physical Review Letters.

    Why are neutrino studies so hard?

    Most neutrino experiments need very large detectors that need to be underground, almost always under mountains, to be protected from cosmic rays that themselves produce neutrinos. And all of the accelerator systems we build tend to be in plains — like Fermilab is in Illinois. So once you decide you’re going to build a beam and shoot it for such a long distance, the costs are enormous, and the beams are very difficult to design and produce.

    FNAL LBNF/DUNE from FNAL to SURF
    FNAL LBNF/DUNE from FNAL to SURF

    FNAL/NOvA experiment
    FNAL/NOvA

    Is there any way around these problems?

    What I would really like to see is a future series of experiments that are really decisive. One possibility for this is IsoDAR, which is part of a larger experiment called DAEδALUS.

    3
    DAEδALUS

    IsoDAR will take a small cyclotron and use it as a driver to produce lithium-8 that decays, resulting in a very pure source of antielectron neutrinos. If we paired that with the KamLAND detector in Japan, then you would be able to see the whole neutrino oscillation.

    KamLAND at the Kamioka Observatory in Japan
    KamLAND at the Kamioka Observatory in Japan

    You don’t just measure an effect at a few points, you can trace the entire oscillation wave. The National Science Foundation has given us a little over $1 million to demonstrate the system can work. We’re excited about that.

    Why would IsoDAR be a more decisive sterile-neutrino hunter?

    This is a case where you don’t produce a beam in the normal way, by smashing protons into a target and using a series of magnetic fields to herd the resulting charged particles into a wide beam where they decay into several kinds of neutrinos, among other particles. Instead you allow the particle you produce, which has a short lifetime, to decay. And it decays uniformly into one kind of neutrino in all directions. All of the aspects of this neutrino beam — the flavor, the intensity, the energies — are driven by the interaction that’s involved in the decay, not by anything that human beings do. Human beings cannot screw up this beam! It’s really a new way of thinking and a new kind of source for the neutrino community that I think can become very widely used once we prove the first one.

    So the resulting neutrino interactions are easier to interpret?

    We’re talking about a signal-to-background ratio of 10 to one. By contrast, most of the reactor experiments looking for antineutrinos are running with a signal-to-background ratio of one to one if they do well, since the neutrons that come out of the reactor core can actually produce a signal that looks a lot like the antineutrino signal you are looking for.

    Speaking of spectral signals, tell me about your connection with the recent Ghostbusters movie remake.

    It’s the first movie I’ve consulted for. It happened because of Lindley Winslow. She was at the University of California, Los Angeles, before she came to MIT. At UCLA, she had made a certain amount of connection with the film industry, and so they had gotten in touch with her. She showed them my office, and they really liked my books. My books are stars — you do get to see them in the movie and some of the other things from my office here and there. When they brought the books back, they put them all back exactly the way they were. What’s really funny about that was that they were not in any order.

    What did you think of the movie itself? Did you relate to the way Kristen Wiig played a physicist?

    I was really happy to see a whole new rendering of it. To watch the characters interact; I think there was a lot of impromptu work. It really came through that these women resonated with each other. In the movie, Kristen Wiig goes into an empty auditorium and she rehearses for her lecture. I felt for that character. When I started out as a faculty member, I had very little experience as somebody who actually taught — I had done all this research. It’s kind of ridiculous to think about now, but I went through those first lectures and really rehearsed them.

    In a way, your career has come full circle, since you started out working at a cyclotron in college and now you want to use another one to hunt for sterile neutrinos. Can you really do cutting-edge research with cyclotrons that accelerate particles to energies just a thousandth of a percent of those reached at the Large Hadron Collider?

    Cyclotrons were invented back at the beginning of the last century.

    5
    The prototype cyclotrons built by E.O. Lawrence. On display at the Lawrence Hall of Science. Picture by Deb McCaffrey.

    They were limited in energy, and as a result, they went out of fashion as particle physicists decided that they needed larger and larger accelerators going up to higher and higher energies. But in the meantime, the research that was done for the nuclear physics community and also for medical isotopes and for treating people with cancer took cyclotrons in a whole different direction. They’ve turned into these amazing machines, which now we can bring back to particle physics. There are questions that can perhaps be better answered if you are working at lower energies but with much purer beams, with more intense beams, and with much better-understood beams. And they’re really nice because they’re small. You can bring your cyclotron to your ultra-large detector, whereas it’s very hard to move Fermilab to your ultra-large detector.

    A single type of sterile neutrino is hard to reconcile with existing experiments, right?

    I think the little beast looks different from what we thought. The very simplistic model introduces only one sterile neutrino. That would be a little weird if you were guided by patterns. If you look at the patterns of all the other particles, they’re appearing in sets of three. If you introduce three, and you do all the dynamics between them properly, does that fix the problem? People have taken a few steps toward answering that, but we’re still doing approximations.

    You just called the sterile neutrino a “little beast.” Do you anthropomorphize particles?

    There’s no question about that. They all have these great little personalities. The quarks are the mean girls. They’re stuck in their little cliques and they won’t come out. The electron is the girl next door. She’s the one you can always depend on to be your friend — you plug in and there she is, right? And she’s much more interesting than people would think. What I like about the neutrinos is they’re very independent. With that said, with neutrinos as friends, you will never be lonely, because there are a billion neutrinos in every cubic meter of space. I have opinions about all of them.

    When did you start creating these characterizations?

    I’ve always thought about them that way. I have in fact been criticized for thinking about them that way and I don’t care. I don’t know how you think about things that are disconnected from your own experience. You have to be really careful not to go down a route that you shouldn’t go down, but it’s a way of thinking about things that’s completely legitimate and gives you some context. I still remember once describing some of the work I was doing as fun. I had one physicist say to me, “This is not fun; this is serious research.” I was, like, you know, serious research can be a lot of fun. Being fun doesn’t make it less important — those are not mutually exclusive.

    See the full article here .

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 1:08 pm on October 7, 2016 Permalink | Reply
    Tags: , , Sterile neutrinos,   

    From Symmetry: “Hunting the nearly un-huntable” 

    Symmetry Mag
    Symmetry

    10/07/16
    Andre Salles

    1
    Artwork by Sandbox Studio, Chicago with Corinne Mucha

    The MINOS and Daya Bay experiments weigh in on the search for sterile neutrinos.

    In the 1990s, the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos National Laboratory saw intriguing hints of an undiscovered type of particle, one that (as of yet) cannot be detected. In 2007, the MiniBooNE experiment at the US Department of Energy’s Fermi National Accelerator Laboratory followed up and found a similar anomaly.

    LSND Experiment University of California
    LSND Experiment University of California

    FNAL/MiniBooNE
    FNAL/MiniBooNE

    Today scientists on two more neutrino experiments—the MINOS experiment at Fermilab and the Daya Bay experiment in China—entered the discussion, presenting results that limit the places where these particles, called sterile neutrinos, might be hiding.

    FNAL/MINOS
    FNAL/MINOS

    Daya Bay, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China
    “Daya Bay, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China

    “This combined result was a two-year effort between our collaborations,” says MINOS scientist Justin Evans of the University of Manchester. “Together we’ve set what we believe is a very strong limit on a very intriguing possibility.”

    In three separate papers—two published individually by MINOS and Daya Bay and one jointly, all in Physical Review Letters—scientists on the two experiments detail the results of their hunt for sterile neutrinos.

    Both experiments are designed to see evidence of neutrinos changing, or oscillating, from one type to another. Scientists have so far observed three types of neutrinos, and have detected them changing between those three types, a discovery that was awarded the 2015 Nobel Prize in physics.

    What the LSND and MiniBooNE experiments saw—an excess of electron neutrino-like signals—could be explained by a two-step change: muon neutrinos morphing into sterile neutrinos, then into electron neutrinos. MINOS and Daya Bay measured the rate of these steps using different techniques.

    MINOS, which is fed by Fermilab’s neutrino beam—the most powerful in the world—looks for the disappearance of muon neutrinos. MINOS can also calculate how often muon neutrinos should transform into the other two known types and can infer from that how often they could be changing into a fourth type that can’t be observed by the MINOS detector.

    Daya Bay performed a similar observation with electron anti-neutrinos (assumed, for the purposes of this study, to behave in the same way as electron neutrinos).

    The combination of the two experiments’ data (and calculations based thereon) cannot account for the apparent excess of neutrino-like signals observed by LSND. That along with a reanalysis of results from Bugey, an older experiment in France, leaves only a very small region where sterile neutrinos related to the LSND anomaly could be hiding, according to scientists on both projects.

    “There’s a very small parameter space left that the LSND signal could correspond to,” says Alex Sousa of the University of Cincinnati, one of the MINOS scientists who worked on this result. “We can’t say that these light sterile neutrinos don’t exist, but the space where we might find them oscillating into the neutrinos we know is getting narrower.”

    Both Daya Bay and MINOS’ successor experiment, MINOS+, have already taken more data than was used in the analysis here. MINOS+ has completely analyzed only half of its collected data to date, and Daya Bay plans to quadruple its current data set. The potential reach of the final joint effort, says Kam-Biu Luk, co-spokesperson of the Daya Bay experiment, “could be pretty definitive.”

    The IceCube collaboration, which measures atmospheric neutrinos with a detector deep under the Antarctic ice, recently conducted a similar search for sterile neutrinos and also came up empty.

    U Wisconsin ICECUBE neutrino detector at the South Pole
    IceCube neutrino detector interior
    U Wisconsin ICECUBE neutrino detector at the South Pole

    All of this might seem like bad news for fans of sterile neutrinos, but according to theorist André de Gouvea of Northwestern University, the hypothesis is still alive.

    Sterile neutrinos are “still the best new physics explanation for the LSND anomaly that we can probe, even though that explanation doesn’t work very well,” de Gouvea says. “The important thing to remember is that these results from MINOS, Daya Bay, Ice Cube and others don’t rule out the concept of sterile neutrinos, as they may be found elsewhere.”

    Theorists have predicted the existence of sterile neutrinos based on anomalous results from several different experiments. The results from MINOS and Daya Bay address the sterile neutrinos predicted based on the LSND and MiniBooNE anomalies. Theorists predict other types of sterile neutrinos to explain anomalies in reactor experiments and in experiments using the chemical gallium. Much more massive types of sterile neutrinos would help explain why the neutrinos we know are so very light and how the universe came to be filled with more matter than antimatter.

    Searches for sterile neutrinos have focused on the LSND neutrino excess, de Gouvea says, because it provides a place to look. If that particular anomaly is ruled out as a key to finding these nigh-undetectable particles, then they could be hiding almost anywhere, leaving no clues. “Even if sterile neutrinos do not explain the LSND anomaly, their existence is still a logical possibility, and looking for them is always interesting,” de Gouvea says.

    Scientists around the world are preparing to search for sterile neutrinos in different ways.

    Fermilab is preparing a three-detector suite of short-baseline experiments dedicated to nailing down the cause of both the LSND anomaly and an excess of electrons seen in the MiniBooNE experiment. These liquid-argon detectors will search for the appearance of electron neutrinos, a method de Gouvea says is a more direct way of addressing the LSND anomaly. One of those detetors, MicroBooNE, is specifically chasing down the MiniBooNE excess.

    FNAL/MicrobooNE
    FNAL/MicrobooNE

    Scientists at Oak Ridge National Laboratory are preparing the Precision Oscillation and Spectrum Experiment (PROSPECT), which will search for sterile neutrinos generated by a nuclear reactor.

    Yale PROSPECT Neutrino experiment
    Yale PROSPECT Neutrino experiment

    CERN’s SHiP experiment, which stands for Search for Hidden Particles, is expected to look for sterile neutrinos with much higher predicted masses.

    CERN SHiP Experiment
    CERN SHiP Experiment

    Obtaining a definitive answer to the sterile neutrino question is important, Evans says, because the existence (or non-existence) of these particles might impact how scientists interpret the data collected in other neutrino experiments, including Japan’s T2K, the United States’ NOvA, the forthcoming DUNE, and other future projects.

    T2K map
    T2K map

    FNAL/NOvA experiment
    FNAL/NOvA experiment

    FNAL LBNF/DUNE from FNAL to SURF
    FNAL LBNF/DUNE from FNAL to SURF

    DUNE in particular will be able to look for sterile neutrinos across a broad spectrum, and evidence of a fourth kind of neutrino would enhance its already rich scientific program.

    “It’s absolutely vital that we get this question resolved,” Evans says. “Whichever way it goes, it will be a crucial part of neutrino experiments in the future.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:43 am on August 8, 2016 Permalink | Reply
    Tags: , , Sterile neutrinos,   

    From Physics- “Viewpoint: Hunting the Sterile Neutrino” 

    Physics LogoAbout Physics

    Physics Logo 2

    Physics

    August 8, 2016
    David W. Schmitz
    Enrico Fermi Institute and Department of Physics, University of Chicago

    A search for sterile neutrinos with the IceCube detector has found no evidence for the hypothetical particles, significantly narrowing the range of masses that a new kind of neutrino could possibly have.

    U Wisconsin ICECUBE neutrino detector at the South Pole
    IceCube neutrino detector interior
    U Wisconsin ICECUBE neutrino detector at the South Pole

    1
    Figure 1: To search for sterile neutrinos, the IceCube experiment looks for the disappearance of atmospheric muon neutrinos (νμ) that have traveled to its detector (black dots) through the Earth. If sterile neutrinos exist, then the matter in Earth’s core should enhance the oscillation of muon neutrinos into sterile neutrinos (νS), creating a larger disappearance of muon neutrinos than would be expected with only the three standard neutrino flavors.

    Neutrinos only interact with matter through the feeblest of forces, the weak nuclear force and gravity, yet they play critical roles in an incredible range of phenomena. They influenced the formation of the early Universe and may be the reason why matter came to dominate over antimatter shortly after the big bang. They are also integral to the inner workings of stars, including during their explosive demise as a supernova. Moreover, neutrinos are practically everywhere: even a single banana emits a million neutrinos a day from the unstable potassium isotopes it contains.

    Although only three types of neutrino are known to exist, hints of a new kind of neutrino that solely interacts with matter through gravity have appeared in several experiments. If such a “sterile” neutrino does indeed exist, it might also play an important role in the evolution of the Universe. The hunt for sterile neutrinos has gone on for decades and has been full of twists and turns, with tantalizing positive signals that were later found to be in tension with null results in follow-up experiments. Now the world’s largest neutrino detector, the IceCube experiment at the South Pole, has released an analysis that eliminates a large portion of the parameter space in which sterile neutrinos could exist [1].

    Standard neutrinos come in three flavors, each of which is associated with a charged partner: the electron, muon, or tau particle. The discovery that neutrinos oscillate, meaning one type of neutrino can transform into another, led to the realization that each flavor state is a linear superposition of three mass states with masses m1, m2, and m3—a beautiful example of basic quantum mechanics at work (see 7 October 2015 Focus story.) Based on precision oscillation measurements, we know that the mixing between neutrinos is quite large compared to similar effects among the quarks. Also, the distance needed for one neutrino type to turn into another, the neutrino oscillation wavelength, is determined by the difference between the squared masses of the participating mass states. These differences, m22−m21 and m23−m22, are known with good precision for the standard neutrinos.

    However, experiments have found possible evidence for neutrinos oscillating with a wavelength that doesn’t match any combination of the known neutrino masses. The most significant results are from the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos National Lab, which observed electron antineutrinos appearing in a beam of muon antineutrinos [2], and from the Mini Booster Neutrino Experiment (MiniBooNE) at Fermilab, which found excesses of both electron neutrinos and antineutrinos [3].

    LSND Experiment University of California
    LSND Experiment University of California

    FNAL/MiniBooNE
    FNAL/MiniBooNE

    Other hints come from the anomalous disappearance of electron neutrinos and antineutrinos produced in nuclear power reactors [4] or by powerful radioactive sources [5, 6].

    Neutrino oscillations involving sterile neutrinos can be understood if there is a fourth mass state with mass m4. This fourth state must be mostly sterile, containing only a small mixture of the standard neutrino flavors. If it exists, then it should be possible to observe small-amplitude neutrino oscillations with a wavelength set by the difference between m24 and the square of the mass of one of the standard neutrino mass states. (Limits on the neutrino masses from cosmological measurements suggest that the hypothetical fourth mass state would have to be heavier than the standard neutrino mass states.) So far, the positive experimental hints for sterile neutrinos point to a squared-mass difference somewhere in the range 0.1–10 eV2.

    Unlike the “traditional” particle physics experiments that have undertaken searches for sterile neutrinos, IceCube is primarily designed to detect high-energy neutrinos from some of the most powerful astrophysical events in the Universe. The detector is spread over a cubic kilometer and consists of thousands of optical sensors buried in the Antarctic ice. When a high-energy neutrino interacts with the ice, it creates charged particles. These in turn produce large amounts of light. From the amplitude and timing of these light signals, the IceCube researchers can reconstruct the properties of the parent neutrino that induced the interaction.

    The key to IceCube’s sensitivity to sterile neutrinos is its ability to determine, with high accuracy, the energy and arrival direction of muon neutrinos and antineutrinos that are produced in Earth’s atmosphere with energies around 1 TeV. Normally, the oscillation of muon neutrinos caused by an additional neutrino mass state should be small. But if this oscillation occurs as the neutrinos pass through dense matter, it may be greatly enhanced by a so-called matter-induced resonance effect [7], creating a sizable disappearance of the muon neutrinos at certain energies. (The precise energy depends on the mass of the hypothetical fourth mass state.) In a unique experiment, the IceCube researchers have tapped into this matter effect by looking for the disappearance of atmospheric muon neutrinos and antineutrinos that have arrived from the North Pole and have therefore passed through Earth’s dense core (see Fig. 1). They looked for this disappearance for neutrinos and antineutrinos with energies between 320 GeV and 20 TeV, a range in which the matter effect has not been explored before. Assuming the additional neutrino mass state is heavier than the known neutrinos, a nearly 100% disappearance of muon antineutrinos is expected at the resonant energy. However, no such disappearance is observed in the energy range explored by IceCube.

    IceCube’s finding places strong limits on the possible existence of a sterile neutrino. In fact, a new analysis incorporating IceCube’s result with data from other experiments indicates that the value of the possible sterile-neutrino mass splitting is now limited to a small region around 1 to 2 eV2 [8]. Several new experiments are being constructed to explore exactly this region. Researchers are, for example, planning next-generation experiments to search for the disappearance of electron antineutrinos from nuclear reactors and radioactive sources. At Fermilab, we are building the Short-Baseline Neutrino (SBN) program using an accelerator neutrino beam and three precision detectors [9]. SBN will investigate both muon-neutrino disappearance and electron-neutrino appearance with maximum sensitivity in exactly the 1–2 eV2 region. With the first SBN detector already running and the remaining two scheduled to begin operation in 2018, we are poised to settle the question of the sterile neutrino’s existence in the coming years. Whether we will soon rule out the possibility of sterile neutrinos in this region or are narrowing in on a thrilling discovery is still to be determined. But thanks to IceCube’s new result, we have a much better idea of where to look.

    This research is published in Physical Review Letters.

    References

    M. G. Aartsen et al. (IceCube Collaboration), “Searches for Sterile Neutrinos with the IceCube Detector,” Phys. Rev. Lett. 117, 071801 (2016).

    A. Aguilar et al. (LSND Collaboration), “Evidence for Neutrino Oscillations from the Observation of ν̄ eAppearance in a ν̄ μ
    Beam,” Phys. Rev. D 64, 112007 (2001).

    A. A. Aguilar-Arevalo et al. (MiniBooNE Collaboration), “Improved Search for ν̄ μ→ν̄ e
    Oscillations in the MiniBooNE Experiment,” Phys. Rev. Lett. 110, 161801 (2013).

    G. Mention, M. Fechner, Th. Lasserre, Th. A. Mueller, D. Lhuillier, M. Cribier, and A. Letourneau, “Reactor Antineutrino Anomaly,” Phys. Rev. D 83, 073006 (2011).

    W Hampel et al. (GALLEX Collaboration), “Final Results of the 51Cr
    Neutrino Source Experiments in GALLEX,” Phys. Rev. B 420, 114 (1998).

    J. N. Abdurashitov et al. (SAGE Collaboration), “Measurement of the Response of a Gallium Metal Solar Neutrino Experiment to Neutrinos from a 51Cr Source,” Phys. Rev. C 59, 2246 (1999).

    H. Nunokawa, O. L. G. Peres, and R. Zukanovich Funchal, “Probing the LSND Mass Scale and Four Neutrino Scenarios with a Neutrino Telescope,” Phys. Lett. B 562, 279 (2003).

    G. H. Collin, C. A. Arguelles, J. M. Conrad, and M. H. Shaevitz, “First Constraints on the Complete Neutrino Mixing Matrix with a Sterile Neutrino,” arXiv:1607.00011.

    R. Acciarri et al. (ICARUS-WA104, LAr1-ND, MicroBooNE Collaborations), “A Proposal for a Three Detector Short-Baseline Neutrino Oscillation Program in the Fermilab Booster Neutrino Beam,” arXiv:1503.01520.

    See the full article here .

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    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

     
  • richardmitnick 1:37 pm on May 9, 2016 Permalink | Reply
    Tags: , , Sterile neutrinos   

    From IceCube: “A first search for sterile neutrinos in IceCube” 

    icecube
    IceCube South Pole Neutrino Observatory

    09 May 2016
    Sílvia Bravo

    IceCube studies of neutrino physics usually happen in the low-energy regime, where the inner and more dense DeepCore is the most relevant subdetector of the Antarctic observatory. However, searches for new physics beyond the Standard Model can also use the main IceCube array when the signature signal is expected at energies above 100 GeV.

    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    This is the case for searches for light sterile neutrinos with IceCube. Sterile neutrinos could be a fourth type of neutrino that only interacts gravitationally and is able to answer questions such as why neutrinos have mass or if neutrinos are important contributors to the dark matter pool in the universe. A typical signature of light sterile neutrinos, those with mass around 1 eV, is expected to produce a strong disappearance of atmospheric muon neutrinos crossing the Earth. This effect results in a depletion at energies of a few TeV due to matter effects in neutrino oscillations.

    The IceCube Collaboration has performed two independent searches, both with one year of data, searching for sterile neutrinos in the energy range between approximately 320 GeV and 20 TeV. IceCube has not found any anomalous disappearance of muon neutrinos and has placed new exclusion limits on the parameter space of the 3+1 model, a scenario with only one sterile neutrino. These results* have been submitted today to Physical Review Letters.

    Results from the IceCube search. The 90% (orange solid line) CL contour is shown with bands containing 68% (green) and 95% (yellow) of the 90% contours in simulated pseudo-experiments, respectively. The contours and bands are overlaid on 90% CL exclusions from previous experiments, and the MiniBooNE / LSND 90% CL allowed region. Image: IceCube Collaboration.
    Results from the IceCube search. The 90% (orange solid line) CL contour is shown with bands containing 68% (green) and 95% (yellow) of the 90% contours in simulated pseudo-experiments, respectively. The contours and bands are overlaid on 90% CL exclusions from previous experiments, and the MiniBooNE / LSND 90% CL allowed region. Image: IceCube Collaboration.

    IceCube has proven to be a great tool for the study of neutrino oscillations, i.e., their change from one flavor to another, in the energy regime from a few GeV up to 50 GeV. But the detailed properties of neutrinos have not yet been fully revealed, and the international scientific community is working on several experiments that will shed new light on what neutrinos can tell us about the universe.

    We know that neutrinos traveling through matter will change their oscillation pattern because of interactions with atomic electrons and nucleons. These interactions, which depend on the energy of the neutrinos and on the density of the medium, create matter enhanced oscillations that can result in a strong disappearance of antineutrinos at specific energies.

    Following the first observations of neutrino oscillations back in 1998, several experiments have measured oscillations patterns at different energies and for different neutrino types. And a few of them, including LSND and MiniBooNE, have found anomalies that cannot be explained with the current model of three neutrinos.

    LSND Experiment University of California
    LSND Experiment University of California

    FNAL/MiniBooNE
    FNAL/MiniBooNE

    However, theories that postulate the existence of sterile neutrinos could accommodate these results. Sterile neutrinos only interact gravitationally, and they would be harder to detect than the currently known neutrinos.

    During the last few years, several experiments have been searching for sterile neutrinos. These searches have not been successful so far and have every time further constrained the sterile neutrino parameter space, namely, the relative mass and mixing angle of sterile neutrinos with the other three active neutrinos.

    The sterile neutrino search announced by IceCube today used throughgoing neutrino-induced muon events, which are neutrinos that reach the detector after crossing the Earth, from the first year of data with the full detector, i.e., with 86 strings of sensors, and its results are confirmed by an independent study using the so-called IC59 data, or data taken when the detector was running with 59 strings. IceCube researchers selected atmospheric neutrinos with energies between 320 GeV and 20 TeV, which includes the energies where the existence of sterile neutrinos would introduce a new resonance effect in matter neutrino oscillations.

    Sterile neutrino models predict a strong disappearance of muon antineutrinos for energies around a few TeV. And, although IceCube cannot differentiate neutrino and antineutrino interactions, if sterile neutrinos exist, it should be able to measure a significant disappearance in the total number of atmospheric muon neutrinos and antineutrinos reaching the South Pole after sailing through the Earth.

    “IceCube’s search for sterile neutrinos is an example of something that experimental physicists strive for: taking a phenomenon that is weak and difficult to study and examining it using a method where its effects would be amplified,” says Ben Jones, who co-led this study while a graduate student at MIT. “By not seeing sterile neutrinos in this way, we have excluded much of the parameter space that has been inaccessible to previous sterile neutrino experiments,” adds Jones.

    In fact, IceCube’s null result also excludes the allowed parameter space region for several experiments that had observed anomalies in the oscillation patterns, which were interpreted as hints of sterile neutrinos, at approximately the 99% confidence level.

    “Not finding the characteristic depletion signal has allowed us to place constraints in some parameter regions that are an order of magnitude stronger than past experiments. This greatly increases the tension between experiments that claim observation and those that do not,” explains Carlos Argüelles, an IceCube researcher who received his PhD at UW–Madison working on this study. “IceCube results have changed the parameter space where sterile neutrinos may exist, calling their existence into question and impacting future search strategies,” adds Argüelles.

    To sum up, sterile neutrinos are not yet ruled out, but their existence is now more remote than ever.

    + Info Searches for Sterile Neutrinos with the IceCube Detector, The IceCube Collaboration: M.G.Aartsen et al, Submitted to Physical Review Letters, arxiv.org/abs/1605.01990

    *Science paper:
    Searches for Sterile Neutrinos with the IceCube Detector

    See the full article here .

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    ICECUBE neutrino detector
    IceCube neutrino detector interior

    IceCube is a particle detector at the South Pole that records the interactions of a nearly massless sub-atomic particle called the neutrino. IceCube searches for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The IceCube telescope is a powerful tool to search for dark matter, and could reveal the new physical processes associated with the enigmatic origin of the highest energy particles in nature. In addition, exploring the background of neutrinos produced in the atmosphere, IceCube studies the neutrinos themselves; their energies far exceed those produced by accelerator beams. IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice.

     
  • richardmitnick 2:02 pm on October 6, 2015 Permalink | Reply
    Tags: , , , , , Sterile neutrinos   

    From NOVA: “Sterile Neutrinos: The Ghost Particle’s Ghost” July 2014. Old, but Worth It for the Details 

    PBS NOVA

    NOVA

    11 Jul 2014

    FNAL Don Lincoln
    Don Lincoln, FNAL

    What do you call the ghost of a ghost?

    If you’re a particle physicist, you might call it a sterile neutrino. Neutrinos, known more colorfully as “ghost particles,” can pass through (almost) anything. If you surrounded the Sun with five light years’ worth of solid lead, a full half of the Sun’s neutrinos would slip right on through. Neutrinos have this amazing penetrating capability because they do not interact by the electromagnetic force, nor do they feel the strong nuclear force. The only forces they feel are the weak nuclear force and the even feebler tug of gravity.

    2
    The Perseus galaxy cluster, one of 73 clusters from which mysterious x-rays, possible produced by sterile neutrinos, were observed. Credit: Chandra: NASA/CXC/SAO/E.Bulbul, et al.; XMM-Newton: ESA

    NASA Chandra Telescope
    NASA/Chandra

    ESA XMM Newton
    ESA/XMM-Newton

    When Wolfgang Pauli first postulated neutrinos in 1930, he thought that his proposed particles could never be detected. In fact, it took more than 25 years for physicists to confirm that neutrinos—Italian for “little neutral ones”—were real. Now, physicists are hunting for something even harder to spot: a hypothetical ghostlier breed of neutrinos called sterile neutrinos.

    Today, we know of three different “flavors” of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos (and their antimatter equivalents). In the the late 1960s, studies of the electron-type neutrinos emitted by the Sun led scientists to suspect that they were somehow disappearing or morphing into other forms. Measurements made in 1998 by the Super Kamiokande experiment strongly supported this hypothesis, and in 2001, the Sudbury Neutrino Observatory clinched it.

    Super-Kamiokande Detector
    Super-Kamiokande Detector

    Sudbury Neutrino Observatory
    Sudbury Neutrino Observatory

    One of the limitations of studying neutrinos from the Sun and other cosmic sources is that experimenters don’t have control over them. However, scientists can make beams of neutrinos in particle accelerators and also study neutrinos emitted by man-made nuclear reactors. When physicists studied neutrinos from these sources, a mystery presented itself. It looked like there weren’t three kinds of neutrinos, but rather four or perhaps more.

    Ordinarily, this wouldn’t be cause for alarm, as the history of particle physics is full of the discovery of new particles. However, in 1990, researchers using the LEP accelerator demonstrated convincingly that there were exactly three kinds of ordinary neutrinos. Physicists were faced with a serious puzzle.

    CERN LEP
    LEP at CERN

    There were some caveats to the LEP measurement. It was only capable of finding neutrinos if they were low mass and interacted via the weak nuclear force. This led scientists to hypothesize that perhaps the fourth (and fifth and…) forms of neutrinos were sterile, a word coined by Russian physicist Bruno Pontecorvo to describe a form of neutrino that didn’t feel the weak nuclear force.

    Searching for sterile neutrinos is a vibrant experimental program and a confusing one. Researchers pursuing some experiments, such as the LSND and MiniBoone, have published measurements consistent with the existence of these hypothetical particles, while others, like the Fermilab MINOS team, have ruled out sterile neutrinos with the same properties. Inconsistencies abound in the experimental world, leading to great consternation among scientists.

    LSND Experiment
    LANL/LSND Experiment

    FNAL MiniBoone
    FNAL/MiniBoone

    FNAL Minos Far Detector
    FNAL/MINOS

    In addition, theoretical physicists have been busy. There are many different ways to imagine a particle that doesn’t experience the strong, weak, or electromagnetic forces (and is therefore very difficult to make and detect); proposals for a variety of different kinds of sterile neutrinos have proliferated wildly, and sterile neutrinos are even a credible candidate for dark matter.

    Perhaps the only general statement we can make about sterile neutrinos is that they are spin ½ fermions, just like neutrinos, but unlike “regular” neutrinos, they don’t experience the weak nuclear force. Beyond that, the various theoretical ideas diverge. Some predict that sterile neutrinos have right-handed spin, in contrast to ordinary neutrinos, which have only left-handed spin. Some theories predict that sterile neutrinos will be very light, while others have them quite massive. If they are massive, that could explain why ordinary neutrinos have such a small mass: perhaps the mathematical product of the masses of these two species of neutrinos equals a constant, say proponents of what scientists call the “see-saw mechanism”; as one mass goes up, the other must go down, resulting in low-mass ordinary neutrinos and high-mass sterile ones.

    Now, some astronomers have proposed sterile neutrinos could be the source of a mysterious excess of x-rays coming from certain clusters of galaxies. Both NASA’s Chandra satellite and the European Space Agency’s XMM-Newton have spotted an excess of x-ray emission at 3.5 keV. It is brighter than could immediately be accounted for by known x-ray sources, but it could be explained by sterile neutrinos decaying into photons and regular neutrinos. However, one should be cautious. There are tons of atomic emission lines in this part of the x-ray spectrum. One such line, an argon emission line, happens to be at 3.62 keV. In fact, if the authors allow a little more of this line than predicted, the possible sterile neutrino becomes far less convincing.

    Thus the signal is a bit sketchy and could easily disappear with a better understanding of more prosaic sources of x-ray emission. This is not a criticism of the teams who have made the announcement, but an acknowledgement of the difficulty of the measurement. Many familiar elements emit x-rays in the 3.5 keV energy range, and though the researchers attempted to remove those expected signals, they may find that a fuller accounting negates the “neutrino” signal. Still, the excess was seen by more than one facility and in more than one cluster of galaxies, and the people involved are smart and competent, so it must be regarded as a possible discovery.

    It is an incredible long shot that the excess of 3.5 keV x-ray from galaxy clusters is a sterile neutrino but, if it is, it will be a really big deal. The first order of business is a more detailed understanding of more ordinary emission lines. Unfortunately, only time will tell if we’ve truly seen a ghost.

    See the full article here .

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