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  • richardmitnick 2:01 pm on February 7, 2017 Permalink | Reply
    Tags: Anisotropies in Our Galaxy, , , , Origin and acceleration mechanisms of cosmic rays?, Tibet Air Shower experiment, U Wisconsin IceCube   

    From astrobites: ” Anisotropies in Our Galaxy” 

    Astrobites bloc

    Astrobites
    2.7.17
    Kelly Malone

    Title: Northern sky Galactic Cosmic Ray anisotropy between 10-1000 TeV with the Tibet Air Shower Array
    Authors: The Tibet AS Collaboration
    First Author’s Institution: Yale University
    Yale University bloc
    Status: Accepted to ApJ, open access

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    The Tibet Air Shower Array, located at over 14,000 ft in elevation, searches for signs of cosmic rays. [ICRR]

    One of the big unsolved mysteries in particle astrophysics is the origin and acceleration mechanisms of cosmic rays, or charged particles that are constantly bombarding the Earth. It is impossible to tell which astronomical source is the origin of any one particular cosmic ray. Because cosmic rays are charged particles, they are deflected on their way to Earth by our galactic magnetic field. Due to this effect, one might expect cosmic rays to arrive isotropically (in equal numbers in every direction), with slight anisotropy because of diffusion effects. However, that is not what is observed. Instead, there is a large-scale anisotropy with an energy-dependent amplitude. This causes problems for traditional diffusion models. Studying this anisotropy is important in learning more about cosmic rays.

    Today’s bite uses roughly five years of data from the Tibet Air Shower experiment, located at 4300 meters above sea level in Tibet, to provide an update in the study of the cosmic-ray anisotropy. The area of the sky they looked at is slightly larger than in their previous papers, and when combined with data from the IceCube experiment in Antarctica, allows us to have a complete picture of the entire sky in the energy range of a few hundred TeV.

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

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    Left: The anisotropy, as seen in 5 different energy bins. From top to bottom, the median energy is 15, 50, 100, 300 and 1000 TeV. Right: The 1D projection of the plot on the left. The blue curve is the first harmonic fit to the data. Note how the phase changes with energy. [Amenomori et al. 2017]

    Maps were binned according to energy and analyzed. A summary of results can be found below.

    In the map corresponding to a median energy of ~300 TeV, there are two regions that can be seen by eye: one area where there is an excess of cosmic rays, and another area where there is a deficit. Only the excess is statistically significant after taking trials into account. (For a description of what “trials” are in a statistical context, check out this page about the “look-elsewhere effect“). This result is consistent with what IceCube sees in a similar energy range. The authors looked to see if this anisotropy is due to the Compton-Getting effect (i.e. if it was caused by the motion of our solar system around the center of our Galaxy). The Compton-Getting effect predicts that we would see more cosmic rays coming from the direction that the Earth is moving toward, with a deficit coming from the other direction. They concluded that this particular excess is not related to the Compton-Getting effect.

    The anisotropy appears to be energy dependent. The maps in the 15–50 TeV range have completely different features than the maps at a few hundred TeV/1 PeV. (See the figure above for an illustration)

    These effects are not related to seasonal variations in the performance of the detector (atmospheric effects can affect experiments such as the Tibet Air Shower array, but it was shown to be negligible in this case).

    So what are the implications of this? Well, for one, studies like these give us some hints as to the origins of cosmic rays. For example, at a few hundred TeV, the most significant excess is coming from the direction of the Galactic center. However, the observed energy dependence shows that there is still a lot to learn about how cosmic rays diffuse. Changes in the anisotropy with energy may imply that the cosmic ray propagation parameters are evolving.

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 3:45 pm on February 1, 2017 Permalink | Reply
    Tags: , , , U Wisconsin IceCube   

    From SA: “IceCube Closes in on Mysterious Nature of Neutrinos” 

    Scientific American

    Scientific American

    February 1, 2017
    Calla Cofield

    The Antarctica-based observatory has found hints of strange patterns in the ghostly particles’ masses

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    IceCube neutrino detector interior
    U Wisconsin IceCube Neutrino detector

    Buried under the Antarctic ice, the IceCube experiment was designed primarily to capture particles called neutrinos that are produced by powerful cosmic events, but it is also helping scientists learn about the fundamental nature of these ghostly particles.

    At a meeting of the American Physical Society (APS) in Washington, D.C., this week, scientists with the IceCube collaboration presented new results that contribute to an ongoing mystery about the nature of neutrinos. These particles pour down on Earth from the sun, but they mostly pass unimpeded, like ghosts, through regular matter.

    The new results support evidence of a strange symmetry in measurements of one neutrino mass. In particle physics, symmetries often indicate underlying physics that scientists haven’t yet unearthed. [Neutrinos from Beyond the Solar System Found (Images)]

    Mystery of the neutrino mass

    Neutrinos are fundamental particles of nature. They aren’t one of the particles that make up atoms. (Those are electrons, protons and neutrons.) Neutrinos very, very rarely interact with regular matter, so they don’t really influence human beings at all (unless, of course, you happen to be a particle physicist who studies them). The sun generates neutrinos in droves, but for the most part, those particles pour through the Earth, like phantoms.

    The [U Wisconsin] IceCube Neutrino Observatory is a neutrino detector buried under 0.9 miles (1.45 kilometers) of ice in Antarctica. The ice provides a shield from other types of radiation and particles that would otherwise overwhelm the rare instances when neutrinos do interact with the detector and create a signal for scientists to study.

    Neutrinos come in three “flavors”: the tau neutrino, the muon neutrino and the electron neutrino. For a long time, scientists debated whether neutrinos had mass or if they were similar to photons (particles of light), which are considered massless. Eventually, scientists showed that neutrinos do have mass, and the 2015 Nobel Prize was awarded for work on neutrinos, including investigations into neutrino masses.

    But saying that neutrinos have mass is not the same as saying that a rock or an apple has mass. Neutrinos are particles that exist in the quantum world, and the quantum world is weird—light can be both a wave and a particle; cats can be both alive and dead. So it’s not that each neutrino flavor has its own mass, but rather that the neutrino flavors combine into what are called “mass eigenstates,” and those are what scientists measure. (For the purpose of simplicity, a Michigan State University statement describing the new findings calls the mass eigenstates “neutrino species.”)

    “One of the outstanding questions is whether there is a pattern to the fractions that go into each neutrino species,” Tyce DeYoung, an associate professor of physics and astronomy at Michigan State University and one of the IceCube collaborators working on the new finding, told Space.com.

    One neutrino species appears to be made up of mostly electron neutrinos, with some muon and tau neutrinos; the second neutrino species seems to be an almost equal mix of all three; and the third is still a bit of a mystery, but one previous study suggested that it might be an even split between muon and tau, with just a few electron neutrinos thrown in.

    At the APS meeting, Joshua Hignight, a postdoctoral researcher at Michigan State University working with DeYoung, presented preliminary results from IceCube that support the equal split of muon and tau neutrinos in that third mass species.

    “This question of whether the third type is exactly equal parts muon and tau is called the maximal mixing question,” he said. “Since we don’t know any reason that this neutrino species should be exactly half and half, that would either be a really astonishing coincidence or possibly telling us about some physical principle that we haven’t discovered yet.”

    Generally speaking, any given feature of the universe can be explained either by a random process or by some rule that governs how things behave. If the number of muon and tau neutrinos in the third neutrino species were determined randomly, there would be much higher odds that those numbers would not be equal.

    “To me, this is very interesting, because it implies a fundamental symmetry,” DeYoung said.

    To better understand why the equal number of muon and tau neutrinos in the mass species implies nonrandomness, DeYoung gave the example of scientists discovering that protons and neutrons (the two particles that make up the nucleus of an atom) have very similar masses. The scientists who first discovered those masses might have wondered if that similarity was a mere coincidence or the product of some underlying similarity.

    It turns out, it’s the latter: Neutrons and protons are both made of three elementary particles called quarks (though a different combination of two quark varieties). In that case, a similarity on the surface indicated something hidden below, the scientists said.

    The new results from IceCube are “generally consistent” with recent results from the T2K neutrino experiment in Japan, which is dedicated to answering questions about the fundamental nature of neutrinos.

    T2K Experiment
    T2K map
    T2K Experiment

    But the Nova experiment, based at Fermi National Accelerator Laboratory [FNAL] outside Chicago, did not “prefer the exact symmetry” between the muon and tau neutrinos in the third mass species, according to DeYoung.

    FNAL/NOvA experiment
    FNAL/NOvA experiment map
    FNAL NOvA Near Detector
    FNAL NOvA Near Detector

    “That’s a tension; that’s not a direct contradiction at this point,” he said. “It’s the sort of not-quite-agreement that we’re going to be looking into over the next couple of years.”

    IceCube was designed to detect somewhat-high-energy neutrinos from distant cosmic sources, but most neutrino experiments on Earth detect lower-energy neutrinos from the sun or nuclear reactors on Earth. Both T2K and Nova detect neutrinos at about an order of magnitude lower energy than IceCube. The consistency between the measurements made by IceCube and T2K are a test of “the robustness of the measurement” and “a success for our standard theory” of neutrino physics, DeYoung said.

    Neutrinos don’t affect most people’s day-to-day lives, but physicists hope that by studying these particles, they can find clues about some of the biggest mysteries in the cosmos. One of those cosmic mysteries could include an explanation for dark matter, the mysterious stuff that is five times more common in the universe than the “regular” matter that makes up planets, stars and all of the visible objects in the cosmos. Dark matter has a gravitational pull on regular matter, and it has shaped the cosmic landscape throughout the history of the universe. Some theorists think dark matter could be a new type of neutrino.

    The IceCube results are still preliminary, according to DeYoung. The scientists plan to submit the final results for publication after they’ve finished running the complete statistical analysis of the data.

    See the full article here .

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  • richardmitnick 12:35 pm on January 8, 2017 Permalink | Reply
    Tags: , , , U Wisconsin IceCube   

    From U Wisconsin IceCube: “Week 52 at the Pole” 

    icecube
    U Wisconsin IceCube South Pole Neutrino Observatory

    06 Jan 2017
    Jean DeMerit

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    Gwenhael De Wasseige, IceCube/NSF

    The year’s end doesn’t mean an end to the work going on at the Pole. Last week, continued detector upgrades and some inventory tasks were on the work roster. There was also considerable progress made on a new IceTop snow-depth sensor project, documented in the image above. New Year’s Eve was celebrated with a festive party in the gym. And the traditional unveiling of the new geographic South Pole marker was held the next day. Tired or not from the previous night’s festivities, plenty of folks showed up for the event—and many hands made light work of moving the sign close to the new marker. A beautiful day for photos, and winterover James along with other Georgia Tech alumni seized on the opportunity.

    All images below, Martin Wolf, IceCube/NSF

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    See the full article here .

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    ICECUBE neutrino detector
    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 9:07 am on October 8, 2016 Permalink | Reply
    Tags: , , , , High energy neutrinos, , U Wisconsin IceCube,   

    From IceCube: “Neutrinos and gamma rays, a partnership to explore the extreme universe” 

    icecube
    IceCube South Pole Neutrino Observatory

    07 Oct 2016
    Sílvia Bravo

    Solving the mystery of the origin of cosmic rays will not happen with a “one-experiment show.” High-energy neutrinos might be produced by galactic supernova remnants or by active galactic nuclei as well as other potential sources that are being sought. And, if our models are right, gamma rays at lower energies could also help identify neutrino sources and, thus, cosmic-ray sources. It’s sort of a “catch one, get them all” opportunity.

    IceCube’s collaborative efforts with gamma-ray, X-ray, and optical telescopes started long ago. Now, the IceCube, MAGIC and VERITAS collaborations present updates to their follow-up programs that will allow the gamma-ray community to collect data from specific sources during periods when IceCube detects a higher number of neutrinos.

    MAGIC Cherenkov gamma ray telescope  on the Canary island of La Palma, Spain
    MAGIC Cherenkov gamma ray telescope on the Canary island of La Palma, Spain

    CfA/VERITAS, AZ, USA
    “CfA/VERITAS, AZ, USA

    Details of the very high energy gamma-ray follow-up program have been submitted to the Journal of Instrumentation.

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    Image: Juan Antonio Aguilar and Jamie Yang. IceCube/WIPAC

    From efforts begun by its predecessor AMANDA, IceCube initiated a gamma-ray follow-up program with MAGIC for sources of electromagnetic radiation emissions with large time variations. If we can identify periods of increased neutrino emission, then we can look for gamma-ray emission later on from the same direction.

    For short transient sources, such as gamma-ray bursts and core-collapse supernovas, X-ray and optical wavelength telescopes might also detect the associated electromagnetic radiation. In this case, follow-up observations are much more time sensitive, with electromagnetic radiation expected only a few hours after neutrino emission from a GRB or a few weeks after a core-collapse supernova.

    Updates to this transient follow-up system will use a multistep high-energy neutrino selection to send alerts to gamma-ray telescopes, such as MAGIC and VERITAS, if clusters of neutrinos are observed from a predefined list of potential sources. The combined observation of an increased neutrino and gamma-ray flux could point us to the first source of astrophysical neutrinos. Also, the information provided by both cosmic messengers will improve our understanding of the physical processes that power those sources.

    The initial selection used simple cuts on a number of variables to discriminate between neutrinos and the atmospheric muon background. IceCube, MAGIC, and VERITAS are currently testing a new event selection that uses learning machines and other sophisticated discrimination algorithms to take into account the geometry and time evolution of the hit pattern in IceCube events. Preliminary studies show that this advanced event selection has a sensitivity comparable to offline point-source samples, with a 30-40% sensitivity increase in the Northern Hemisphere with respect to the old selection. The new technique does not rely only on catalogues of sources and allows observing neutrino flares in the Southern Hemisphere. Thus, those alerts will also be forwarded to the H.E.S.S. collaboration, expanding the gamma-ray follow-up program to the entire sky.

    HESS Cherenko Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg
    HESS Cherenko Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg

    During the last few years, IceCube has sent several alerts to VERITAS and MAGIC that have not yet resulted in any significant correlation between neutrino and gamma-ray emission. For some of those, however, the source was not in the reach of the gamma-ray telescopes, either because it was out of the field of view or due to poor weather conditions. Follow-up studies have allowed setting new limits on high-energy gamma-ray emission.

    With the increased sensitivity in the Northern Hemisphere and new alerts to telescopes in the Southern Hemisphere, the discovery potential of these joint searches for neutrino and gamma-ray sources is greatly enhanced. Stay tuned for new results!

    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 5:02 pm on August 30, 2016 Permalink | Reply
    Tags: Extragalactic supernovae, Samantha Pedek, U Wisconsin IceCube,   

    From IceCube: Women in STEM – “Exploring the possibility of detecting extragalactic supernovae with IceCube-Gen2, summer research with IceCube” Samantha Pedek 

    icecube
    IceCube South Pole Neutrino Observatory

    30 Aug 2016
    Samantha Pedek, UW–River Falls

    International Research Experiences for Students (IRES) is a program funded by the National Science Foundation to support active participation of US undergraduates in international research projects. Vanessa Esaw, Nick Kulacz, Nick Jensen, Jack Nuckles, and Samantha Pedek participated in the IRES program through UW–River Falls to work on IceCube research for the summer at Stockholm University.

    Growing up on a small, secluded hobby farm in southwestern Wisconsin, the night sky played a major role in my upbringing. Since there is almost no light pollution, the night sky was always bright and clear. In the summer months, my bedtime was determined by the time a specific satellite went over the house. Every year, my family would gather up all the blankets in the house and lay outside to watch meteor showers for hours. From a young age, I loved the idea of learning more about the stars and planets, and as I got into high school, I fell in love with physics. My original plan was to become a high school physics teacher, and I found the University of Wisconsin–River Falls (UWRF) not only has a fantastic physics program but is also involved with IceCube. I had heard about IceCube in 2013, when it won Physics World’s Breakthrough of the Year, and working for IceCube became my new goal and dream.

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    Samantha visiting Haga Park in Stockholm, Sweden

    Dr. Surujhdeo Seunarine encouraged me to apply for an IceCube research position during my first year at UWRF. I also knew a few other IceCubers, Laura Lusardi and Kelsey Kolell, who strongly recommended I apply. I was terrified that I wouldn’t get accepted because of my lack of experience, but thankfully I did. This summer was my second summer working for IceCube, and I had the experience of a lifetime in Stockholm, Sweden, working with Dr. Chad Finley.

    I started my project in parallel with Nick Kulacz, another student from UWRF, with the main focus of testing whether optical fibers would be better at detecting extragalactic supernovae than the traditional IceCube sensors. Supernova neutrinos have relatively low energy, typically 1-20MeV, and when they interact within the ice a positron is created. These positrons produce Cherenkov radiation that can be detected by IceCube. Since supernova neutrinos have low energy but come in large numbers, IceCube will “see” the background noise of the sensors increase significantly for periods of time. The challenge with extragalactic supernovae is the neutrinos are spread so thin by the time they reach IceCube that there are too few to notice above the normal detector noise. Currently, it is impossible to detect extragalactic supernovae due to background noise.

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    A simulation of an 18 MeV positron interacting 100 meters away from the ideal optical fiber.

    In order to test whether optical fibers would be able to detect extragalactic supernovae, we simulated an ideal cylindrical “fiber,” positron interactions, and propagation of the corresponding Cherenkov radiation. This determined the probability of detecting the positron produced from an extragalactic supernova neutrino with respect to how far away it traveled from the ideal, 100 percent efficient, cylindrical fiber. Then we set out to find the geometric effective volume of the cylindrical fiber. The effective volume is a way to compare complicated detectors in a fairly simple way. It is the volume for which a similar detector is 100 percent efficient. This value can then be scaled to account for other aspects of detector efficiency (e.g., photomultiplier response), which helps determine the detector design. The effective volume can then be used to compare the fiber and the current IceCube sensors, providing a valid argument as to which detector would be better for potential use in a future extension of IceCube.

    Overall, this experience has taught me invaluable life skills and broadened my perspective. It has furthered my passion for physics while challenging me in unexpected ways. I am now much more confident in my problem-solving abilities. But I have learned more than science. I have learned about another culture and another way of life. Every person I met in Sweden was very hospitable and welcoming.

    This experience has been the opportunity of a lifetime, and I am very thankful to have taken part in it.

    See the full article here .

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    ICECUBE neutrino detector
    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 12:40 pm on August 30, 2016 Permalink | Reply
    Tags: , , U Wisconsin IceCube   

    From IceCube: “Week 32 at the Pole” 

    icecube
    IceCube South Pole Neutrino Observatory

    26 Aug 2016
    Jean DeMeri

    What ever happened to the igloo from Weeks 30 and 31 at the Pole? Find out in Week 32 at the pole:

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    Hamish Wright, NSF

    The igloo—the prime attraction at the South Pole for the last few weeks—is no more. But before “disappearing,” its existence was memorialized in some final photos. Above, you can see it with the names of its builders carved into the side, and it appears to almost glow from the soft white light from within. The next image shows IceCube’s winterovers on the left along with the station’s water plant tech relaxing inside. Some folks took the opportunity to sleep (or attempt to sleep) in the igloo while it was still available—a thrill in and of itself but high winds in excess of 30 knots made it extra-exciting. A massive snow drift under the station entrance’s staircase attests to the ultrahigh winds last week. The last two images show the igloo before and during its ultimate demise. A shame to see it go, but that looks pretty cool. Until the next one!

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    Christian Krueger, IceCube/NSF

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    Christian Krueger, IceCube/NSF

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    Hamish Wright, NSF

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    Hamish Wright, NSF

    See the full article here .

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    ICECUBE neutrino detector
    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 4:32 pm on August 18, 2016 Permalink | Reply
    Tags: , U Wisconsin IceCube, Week 31 at the Pole   

    From IceCube: “Week 31 at the Pole” 

    icecube
    IceCube South Pole Neutrino Observatory

    1
    Christian Krueger, IceCube/NSF

    The igloo from last week is finally finished. What began as an afternoon project ended up taking an entire week (well, high winds were partly to blame). In the image above, you can see the igloo lit from within, and perhaps even discern that there are only few blocks missing to complete the ceiling. They had some fine auroras to watch while building the igloo, and once finished, they gathered inside for a group photo and a warm treat—not hot cocoa, but a Thai curry. With people inside, and with its low-profile, hidden-beneath-blankets entrance, the igloo can maintain an interior temperature above 0 ºF.

    The auroras were bountiful and varied in color last week, giving a nice purple show across a large part of the sky on one night. That can’t be what IceCube winterover Christian is reacting to in the last image below, since not only is he indoors but there are no unblocked windows to the outdoors that he could be looking through. What is it then? He is reacting to an elephant hiding in the janitor’s room—or “acting,” we should say, as it’s all part of shooting a short film for the Winter International Film Festival. Quite convincing, and a great teaser for their film!

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    Christian Krueger, IceCube/NSF

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    Christian Krueger, IceCube/NSF

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    South Pole WIFF Team/NSF

    See the full article here .

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    ICECUBE neutrino detector
    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 11:43 am on August 8, 2016 Permalink | Reply
    Tags: , , , U Wisconsin IceCube   

    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

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    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 .

    Please help promote STEM in your local schools.

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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 10:35 am on July 22, 2016 Permalink | Reply
    Tags: , , , U Wisconsin IceCube   

    From IceCube: “IceCube search for cosmogenic neutrinos favors heavy nuclei cosmic-ray sources” 

    icecube
    IceCube South Pole Neutrino Observatory

    21 Jul 2016
    Silvia Bravo

    The highest energy cosmic rays are known to reach energies a trillion times larger than those of protons in the LHC at CERN. These ultra-high-energy cosmic rays (UHECR) can produce neutrinos with energies above 100 PeV either by the interaction with photons and matter at the source, which are just very high energy astrophysical neutrinos, or by the interaction with the cosmic microwave background (CMB), which are referred to as cosmogenic neutrinos.

    The IceCube Collaboration has made public today that a new search for cosmogenic neutrinos resulted in two very high energy neutrinos. These neutrinos, which are found to be of astrophysical origin with a 92.3% probability, include the highest energy neutrino detected to date. While of astrophysical origin, the energy of these neutrinos does not match the expectation for a cosmogenic neutrino flux. The lack of evidence for such events in a search of seven years of IceCube data places very strong constraints on the sources of UHECR. Proton-dominated sources are greatly disfavored, and testing mixed and heavy nuclei cosmic-ray sources will require much bigger instruments, such as an extension of IceCube or radio Askaryan neutrino detectors. These results have been submitted yesterday to Physical Review Letters.

    1
    All-flavor-sum neutrino flux quasi-differential 90%-CL upper limit on one energy decade E^−1 flux windows. Credit: IceCube Collaboration

    Cosmogenic neutrinos, with energies reaching up to 50 EeV or more, are expected to be the highest energy neutrinos in nature. Their flux is supposed to exceed that of astrophysical neutrinos at energies of at least 100 PeV and above. But no one has ever detected a cosmogenic neutrino, not even a neutrino with an energy above 10 PeV.

    The results presented today by the IceCube Collaboration, using data from 2008 to 2015, have once more indicated a fruitless search for cosmogenic neutrinos. And, although this is not a totally unexpected scenario, it does set very strong constraints on the sources of UHECRs.

    Previous measurements of the spectrum and chemical composition of UHECRs by HiRes and the Telescope Array suggested a chemical composition compatible with proton-dominated sources up to the highest energies. However, Auger’s results pointed to the need for heavier nuclei UHECR to explain its data. IceCube results now confirm Auger’s hints and reject UHECR sources such as proton-dominated models of active galactic nuclei (AGNs) and gamma-ray bursts (GRBs).

    “Many scientists thought that AGNs or GRBs would be the standard scenario of UHECR production,” says Aya Ishihara, an IceCube researcher at Chiba University in Japan and the corresponding author of this work. “But neutrinos are changing our view of the ultra-high-energy universe,” adds Ishihara.

    Continued searches should now concentrate on models with weak or no cosmological evolution proton-dominated sources and those with heavier nuclei composition or, as most scientists lean toward, a combination of both. But these scenarios push cosmogenic neutrinos far below the detection threshold of any running detector.

    The production of cosmogenic neutrinos in muon and pion decays produced in the interaction of primary cosmic rays with CMB photons is efficient only if UHECR are protons. Models with a strong cosmological evolution of proton-dominated sources predict a flux of cosmogenic neutrinos in IceCube’s sensitivity region above 100 PeV. But these are now rejected by IceCube results.

    In the case of heavier nuclei, these CR interactions are suppressed and the neutrino flux falls rapidly with energy. “Neutrinos become more important if UHECRs are heavy nuclei since, due to the unknown magnetic fields in galactic and extragalactic environments, the cosmic rays’ path is even more unpredictable than for protons. But, no matter what, neutrinos always point to their sources,” states Ishihara.

    The more UHECRs are heavy nuclei, the smaller the EeV component of the cosmogenic neutrino flux, and the larger the detector required to first detect them. IceCube results strongly support the need for a full deployment of experiments such as ARA and ARIANNA , which are currently either in partial deployment or running as a pilot experiment.

    4
    A partially contained cascade with a deposited energy of 0.77 PeV was detected in IceCube on November 16, 2012. Credit: IceCube Collaboration

    5
    This is the highest energy neutrino event, detected in IceCube on June 11, 2014. The event deposited 2.6 PeV in the detector. Credit: IceCube Collaboration

    The search for cosmogenic neutrinos did find two very high energy neutrinos. One is a track with a deposited energy of 2.6 PeV, which had already been found in a previous study and is the highest energy neutrino recorded to date. The second is a partially contained cascade with a deposited energy of 0.77 PeV.

    The hypothesis that these events are of cosmogenic origin is rejected by IceCube researchers at more than a 99% confidence level. These are most likely astrophysical neutrinos since the probability of being atmospheric in origin has been determined to be very small.

    The track was detected in June 2014 and deposited some energy outside the detector. IceCube scientists estimated that the neutrino that induced this event had an energy about three times greater than what was deposited in the detector, i.e., the energy of the initial neutrino was well above 5 PeV.

    + Info Constraints on ultra-high-energy cosmic ray sources from a search for neutrinos above 10 PeV with IceCube, The IceCube Collaboration: M.G.Aartsen et al, Submitted to Physical Review Letters, arxiv.org/abs/1607.05886

    See the full article here .

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    STEM Icon

    Stem Education Coalition

    ICECUBE neutrino detector
    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.

     
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