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  • richardmitnick 11:43 am on August 8, 2016 Permalink | Reply
    Tags: , , , U Wisconsin IceCube   

    From Physics- “Viewpoint: Hunting the Sterile Neutrino” 

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    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 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.

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    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 .

    Please help promote STEM in your local schools.

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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.

     
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