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  • richardmitnick 9:23 pm on September 21, 2017 Permalink | Reply
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    From Penn State: “Mystery solved: Super-energetic space particles crash to Earth from far away” 

    Penn State Bloc

    Pennsylvania State University

    September 21, 2017

    An image of the Earth showing the continent of South America, with faint white streaks representing cosmic rays streaming toward the Pierre Auger Observatory in Argentina. Image: Pierre Auger Observatory

    Super-energetic space particles, which were thought to have been blasted toward Earth from somewhere outside our solar system, now have been discovered to be from very far away indeed — from far outside our Milky Way galaxy. The discovery was made by an international team that includes Penn State scientists and the Pierre Auger Collaboration, using the largest cosmic-ray instrument ever built, the Pierre Auger Observatory in Argentina. A paper describing the discovery will be published in the journal Science on Sept. 22.

    This animation illustrates the long journey of high-energy cosmic waves from the time they are shot into space from powerful events in galaxies far away from our Milky Way Galaxy until they eventually crash on Earth, leaving clues among the large array of cosmic-ray detectors in western Argentina, the Pierre Auger Observatory. Penn State scientists are members of the Pierre Auger Consortium.
    Pierre Auger Collaboration

    “After more than a century since cosmic rays were first detected, this is the first truly significant result from our analysis of the detections, which now have revealed the distant origin of these ultra-high-energy cosmic rays,” said Miguel Mostafá at Penn State. He and Stephane Coutu — both professors of physics and of astronomy and astrophysics and Fellows of the American Physical Society — lead teams of students and post-doctoral scientists in research at Penn State’s Pierre Auger Collaboration group.

    Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes

    “Now we know that the highest-energy particles in the universe came from other galaxies in our cosmological neighborhood,” Mostafá said.

    Mostafá and Coutu have been working on the project since 1996 and 1997, respectively, with support from the U.S. National Science Foundation. Mostafá has been a coordinator of the Auger team in charge of this analysis of cosmic-ray arrival directions, and is one of the corresponding authors on the Science article.

    Although the Pierre Auger Collaboration’s discovery clearly shows an origin outside our Milky Way galaxy, the specific sources that are producing the particles have not yet been discovered. “We are now considerably closer to solving the mystery of where and how these extraordinary particles are produced, a question of great interest to astrophysicists,” said Karl-Heinz Kampert, professor of physics at the University of Wuppertal in Germany and spokesperson for the Pierre Auger Collaboration.

    See the full article here .

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  • richardmitnick 11:09 am on November 8, 2016 Permalink | Reply
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    From Pierre Auger Observatory: “Evidence for a mixed mass composition at the ‘ankle’ in the cosmic ray spectrum” 


    Pierre Auger Observatory

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    Pierre Auger Observatory Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes
    Pierre Auger Observatory Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes

    The highest energy cosmic rays remain elusive and mysterious, and their study requires extraordinary efforts. At the Pierre Auger Observatory in Argentina, the giant air showers of particles created by these cosmic rays are detected when they slam into the ground by a large array of water tanks equipped with electronic detectors. But on dark nights they are simultaneously detected by telescopes sensitive to the faint sky glow left by the air showers. The new report by the Pierre Auger Collaboration correlates in detail the signals in the water tanks with those from the telescopes. The correlation is uniquely sensitive to the presence in the primary beam of nuclei with different masses, and is used in particular to help resolve how many types of atomic nuclei contribute to the cosmic ray flux.

    Evidence for a mixed mass composition at the ‘ankle’ in the cosmic ray spectrum

    Cosmic rays are energetic particles (atomic nuclei) impinging upon the Earth from the vast reaches of the cosmos. They can have tremendous energies and thus must originate in remarkable but still mysterious astrophysical sources. If we understand the nature of these particles, we may be able to find the extragalactic sources of the highest energy cosmic rays.

    The distribution of particle numbers with energy, or ‘spectrum’, shows a striking and very rapid reduction in numbers with increasing energy. At an energy of E ≈ 5 × 1018 eV a flattening in the spectrum (slightly slower rate of reduction with energy) is observed. This feature is called the ‘ankle’ in the cosmic ray spectrum.

    The transition from cosmic rays from a Galactic origin to an extragalactic component may cause such a flattening. Alternatively, the ‘dip’ model in which highly energetic extragalactic protons interact with photons from the cosmic microwave background also creates a flattening of the spectrum. The different models can be distinguished by the predicted cosmic ray composition in the ankle region.

    In this paper the important characteristics of the mass composition of the cosmic rays — the spread of their masses — is measured using a method relatively insensitive to either experimental uncertainties or uncertainties in the particle interaction models. The method takes advantage of the hybrid design of the Pierre Auger Observatory, which measures both the cascade of particles when it reaches the ground and the development of the shower through the atmosphere using specialized fluorescence telescopes on dark clear nights.

    The method uses the independent information on the depth of shower maximum (Xmax) from the fluorescence telescopes and the signal at 1000 m from the shower axis, S(1000), from the surface detector. The original idea of the method is that a direct and robust estimation of the spread of masses in the primary beam can be obtained via a measurement of the correlation between Xmax and S(1000). For any given type of cosmic ray nucleus this correlation is close to or larger than zero as shown in the first figure (left) for proton and iron air showers generated with the particle interaction model EPOS-LHC (the correlation coefficient is denoted as rG). For mixed compositions a negative correlation emerges due to a very general characteristic of air showers: showers from heavier nuclei have smaller Xmax and larger S(1000) (due to a larger number of muons). Thus in a mixed composition shallower showers have on average larger signals. The correlation becomes more negative the larger the spread of masses.

    For events successfully reconstructed with both Auger fluorescence and surface detectors in the energy range around the ankle E = 1018.5 – 1019.0 eV a significant negative correlation was found rG = -0.125 ± 0.024 (stat), as illustrated in the first figure (right). This value is at least 5 standard deviations away from predictions for pure compositions or any composition made up of protons and helium only.

    Correlation between X*max (Xmax scaled to 1019 eV) and S*38 (S(1000) scaled to 1019 eV,
    38° of zenith angle). Left: simulations for protons and iron nuclei with EPOS-LHC. Right: Auger data.

    To estimate the spread of the primary mass numbers σ(ln A) the value of the correlation found in the data is compared to values from simulations for compositions with varying fractions of protons, helium, oxygen and iron. As illustrated in the second figure, for different particle interaction models (EPOS-LHC or QGSJetII-04) the data can be described with compositions having a spread in atomic mass numbers within the range 1.0 ≲ σ(ln A)≲ 1.7. The results are practically unaffected by systematic uncertainties on Xmax and S(1000), or by modifications of the key parameters of the particle interaction models.

    Correlation coefficient as a function of spread of cosmic ray mass number, in simulated (with two separate particle interaction models, EPOS-LHC and QGSJetII-04) mixtures with different fractions of protons, helium, oxygen and iron nuclei (points) compared to the correlation value found in Auger data (shaded area). The ranges of cosmic ray mass spread compatible with the data are marked by vertical lines.

    The conclusion that the mass composition around the cosmic ray ankle energy is not pure but mixed has important consequences for theoretical source models. Proposals of almost pure compositions, such as the dip scenario, are disfavored as a sole explanation of the ultra-high energy cosmic rays. These findings, together with other observations made at the Pierre Auger Observatory, indicate that nuclei with mass number A > 4 are accelerated to ultra-high energies E ≥ 1018.5 eV and are able to escape the source environment. The search for the final resolution of the ankle puzzle is continuing and new astrophysical models are already emerging, e.g., including modifications of the nuclear composition in the environment of the acceleration sites.

    Related paper:
    Evidence for a mixed mass composition at the ‘ankle’ in the cosmic-ray spectrum
    A. Aab et al. (Pierre Auger Collaboration), Phys.Lett. B762 (2016) 288-295

    See the full article here .

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    Pierre Augur Observatory

    The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling nearly at the speed of light and each with energies beyond 1018 eV. In Earth’s atmosphere such particles interact with air nuclei and produce various other particles. These effect particles (called an “air shower”) can be detected and measured. But since these high energy particles have an estimated arrival rate of just 1 per km2 per century, the Auger Observatory has created a detection area of 3,000 km2 (1,200 sq mi) — the size of Rhode Island, or Luxembourg — in order to record a large number of these events. It is located in the western Mendoza Province, Argentina, near the Andes.

    Construction began in 2000,[1] the observatory has been taking production-grade data since 2005 and was officially completed in 2008.

    The observatory was named after the French physicist Pierre Victor Auger. The project was proposed by Jim Cronin and Alan Watson in 1992. Today, more than 500 physicists from nearly 100 institutions around the world[2] are collaborating to maintain and upgrade the site in Argentina and collect and analyse the measured data. The 15 participating countries shared the $50 million construction budget, each providing a small portion of the total cost.

  • richardmitnick 2:35 pm on November 1, 2016 Permalink | Reply
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    From APS Physics: “Viewpoint: Cosmic-Ray Showers Reveal Muon Mystery” 

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    APS Physics

    October 31, 2016
    Thomas Gaisser, Department of Physics and Astronomy, University of Delaware

    The Pierre Auger Observatory has detected more muons from cosmic-ray showers than predicted by the most up-to-date particle-physics models.

    Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes
    Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes

    Figure 1:This illustration shows the detection of a hybrid event from a cosmic-ray shower in the Pierre Auger Observatory. The pixels in the camera of the fluorescence telescope (light blue semicircle) trace the shower profile—specifically, the energy loss of the shower as a function of its penetration into the atmosphere. Particles from the same shower are detected on the ground by an array of water tanks (white dots). The red line shows the trajectory of the shower.

    The Large Hadron Collider at CERN produces proton collisions with center-of-mass energies that are 13 thousand times greater than the proton’s rest mass. At such extreme energies these collisions create many secondary particles, whose distribution in momentum and energy reveals how the particles interact with one another. A key question is whether the interactions determined at the LHC are the same at higher energies. Luckily, nature already provides such high-energy collisions—albeit at a much lower rate—in the form of cosmic rays entering our atmosphere. Using its giant array of particle detectors, the Pierre Auger Observatory in Argentina has found that more muons arrive on the ground from cosmic-ray showers than expected from models using LHC data as input [1]. The showers that the Auger collaboration analyzed come from atmospheric cosmic-ray collisions that are 10 times higher in energy than the collisions produced at the LHC. This result may therefore suggest that our understanding of hadronic interactions (that is, interactions between protons, neutrons, and mesons) from accelerator measurements is incomplete.

    Cosmic rays are relativistic particles (mostly protons and light nuclei) that are produced by supernovae and other powerful sources in and beyond our galaxy. When a cosmic-ray particle collides with a molecule in Earth’s atmosphere, it generates a cascade of secondary particles. An incident proton, for example, will typically expend 40% of its energy producing a secondary proton or neutron, together with a large number of other hadrons, mostly pions. Neutral pions decay immediately to two photons that generate an electromagnetic “cascade” comprising electron-positron pairs and gamma rays. Charged pions with high energies interact again in the atmosphere. The neutral pions they produce contribute further to the electromagnetic component of the shower, while other particles carry energy forward to subsequent interactions. Lower-energy charged pions decay before interacting again and produce muons, which largely survive to the ground.

    Unlike detectors at accelerators, experiments like Auger do not directly detect the initial collision but only the secondary cascade that it generates. This is simply because the rate of events is too low: At an energy equivalent to 10 times the center-of-mass energy at the LHC, the cosmic-ray flux is only about one particle per square kilometer per year. This is far too low to observe the collision directly with a detector in space or a balloon-borne detector above the atmosphere. Auger, with a detector array that spans 3000 square kilometers, may collect only a few thousand such events per year. In comparison, the LHC can produce a billion proton collisions per second.

    Auger observes the first interaction indirectly by analyzing the shower of particles it generates [2]. To detect shower particles that reach the ground, the observatory uses 1660 water-filled tanks separated from each other by more than a kilometer. When struck by a high-speed particle, the water emits a flash of light (Cherenkov radiation). Auger complements the detection of particles on the ground by tracking the path of a cascade in the atmosphere with four telescopes, placed at the perimeter of the array, that are sensitive to the fluorescent light generated by the cascade (Fig. 1).

    Events seen by both the fluorescence telescopes and the water tanks are called hybrid events. They constitute only a small fraction of all of the ground events because the fluorescence can only be observed on clear, moonless nights. However, they are a particularly valuable subsample because the fluorescence from the shower as a function of its penetration into the atmosphere—the shower profile—is sensitive to the mixture of nuclei in the primary cosmic radiation [3]. Also, because most of the muons arriving at the ground are from interactions involving charged pions, the ground signal is primarily sensitive to the properties of hadronic interactions. On the other hand, the atmospheric cascade probed by the telescope consists mostly of electrons and positrons descended from the first few hadronic interactions. It therefore reflects the primary particles’ energies. In their new analysis, the Auger collaboration uses a sample of 411 hybrid events, collected over nine years, in a narrow energy range of around 1019 eV.

    For each hybrid event, the Auger researchers compare two quantities: the signal measured at the ground and the signal expected at the ground, which they compute with models that use parameters determined by the latest LHC measurements. A complication for these computations is that they depend on the identity of the nucleus involved in the first collision and on where in the atmosphere the shower starts and how it develops, all of which vary from shower to shower. To solve this problem, the Auger team simulates each event 25,000 times, on average, thereby sampling all the possibilities for how the different particle interactions are distributed in energy and in the atmosphere. They then pick several simulations that fit the telescope measurements well. From these “best fit” telescope measurements, they predict the signal on the ground using two models based on LHC data.

    But there is an additional wrinkle. The signal at the ground comes both from muons and from the electrons and positrons produced by the electromagnetic cascade. Since these two components cannot be distinguished, the researchers must predict them separately. Fortunately, the electromagnetic component dominates for cascades arriving from straight above the observatory, while the muon component dominates for angles of arrival exceeding 37 degrees. (The data correspond to events with arrival angles from 0 to 60 degrees.) Taking account of this difference, the researchers scale the two components predicted by the models separately to obtain the best fit to the data. The scaling factor they get for the electromagnetic component is near unity, but it is between 1.3 and 1.6 for the hadronic component. In other words, Auger has detected about 30–60% more muons than expected.

    This discrepancy has been seen before. In 2000, the HiRes-MIA hybrid array in Utah found a higher density of muons at 600 m from the shower’s trajectory than expected from (then current) models of hadronic interactions [4]. Last year, the problem showed up in the analysis of nearly horizontal showers at Auger [5]. The new results from Auger put the muon excess on a firmer basis by making a tight connection between the telescope measurements and the signal on the ground. This finding suggests that the best models of hadronic interactions are missing something. One possibility is that they do not account for a process that keeps more energy in the hadronic component; for example, a higher production of baryon-antibaryon pairs [6]. Another option is that the physics of strong interactions changes at energies beyond those tested at the LHC [7, 8].

    What’s next? The Auger collaboration can extend its analysis outside the narrow energy range to look for an energy dependence of the discrepancy, which would provide a clue to its origin. For a complementary test, they could also analyze other observables that are sensitive to hadronic interactions, such as the height at which muons are produced. Finally, a significant upgrade called “Auger Prime” is underway [9]. This will allow the team to measure the muon and electromagnetic contributions to the ground signal separately, removing a significant source of uncertainty in their current analysis.

    This research is published in Physical Review Letters.


    1. A. Aab et al. (Pierre Auger Collaboration), “Testing Hadronic Interactions at Ultrahigh Energies with Air Showers Measured by the Pierre Auger Observatory,” Phys. Rev. Lett. 117, 192001 (2016).
    2. A. Aab et al. (Pierre Auger Collaboration), “The Pierre Auger Cosmic Ray Observatory,” Nucl. Instrum. Methods Phys. Res., Sect. A 798, 172 (2015).
    3.A. Aab et al. (Pierre Auger Collaboration), “Depth of Maximum of Air-Shower Profiles at the Pierre Auger Observatory. II. Composition Implications,” Phys. Rev. D 90, 122006 (2014).
    4.T. Abu-Zayyad et al. (HiRes-MIA Collaboration), “Evidence for Changing of Cosmic Ray Composition between 1017 and 1018 eV from Multicomponent Measurements,” Phys. Rev. Lett. 84, 4276 (2000).
    5 A. Aab et al. (Pierre Auger Collaboration), “Muons in Air Showers at the Pierre Auger Observatory: Mean Number in Highly Inclined Events,” Phys. Rev. D 91, 032003 (2015).
    6 T. Pierog, “LHC Data and Extensive Air Showers,” EPJ Web Conf. 52, 03001 (2013).
    7 G. R. Farrar and J. D. Allen, “A New Physical Phenomenon in Ultra-High Energy Collisions,” EPJ Web Conf. 53, 07007 (2013).
    8 J. Alvarez-Muñiz, L. Cazon, R. Conceição, J. Dias de Deus, C. Pajares, and M. Pimenta, “Muon Production and String Percolation Effects in Cosmic Rays at the Highest Energies,” arXiv:1209.6474.
    9 A. Aab et al. (Pierre Auger Collaboration), “The Pierre Auger Observatory Upgrade – Preliminary Design Report,” arXiv:1604.03637.

    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).

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