1,811 research outputs found

    Experimental Investigation of the Nature of the Knee in the Primary Cosmic Ray Energy Spectrum with the GAMMA experiment

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    We present preliminary results obtained by a novel difference method for the study of the nature of the knee in the energy spectrum of the primary cosmic radiation. We have applied this method to data from the GAMMA experiment in Armenia. The analysis provides evidence for the possible existence of a nearby source of primary cosmic rays in the Southern hemisphere.Comment: 17 pages, 5 figure

    Tkachenko modes as sources of quasiperiodic pulsar spin variations

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    We study the long wavelength shear modes (Tkachenko waves) of triangular lattices of singly quantized vortices in neutron star interiors taking into account the mutual friction between the superfluid and the normal fluid and the shear viscosity of the normal fluid. The set of Tkachenko modes that propagate in the plane orthogonal to the spin vector are weakly damped if the coupling between the superfluid and normal fluid is small. In strong coupling, their oscillation frequencies are lower and are undamped for small and moderate shear viscosities. The periods of these modes are consistent with the observed ~100-1000 day variations in spin of PSR 1828-11.Comment: 7 pages, 3 figures, uses RevTex, v2: added discussion/references, matches published versio

    Should we doubt the cosmological constant?

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    While Bayesian model selection is a useful tool to discriminate between competing cosmological models, it only gives a relative rather than an absolute measure of how good a model is. Bayesian doubt introduces an unknown benchmark model against which the known models are compared, thereby obtaining an absolute measure of model performance in a Bayesian framework. We apply this new methodology to the problem of the dark energy equation of state, comparing an absolute upper bound on the Bayesian evidence for a presently unknown dark energy model against a collection of known models including a flat LambdaCDM scenario. We find a strong absolute upper bound to the Bayes factor B between the unknown model and LambdaCDM, giving B < 3. The posterior probability for doubt is found to be less than 6% (with a 1% prior doubt) while the probability for LambdaCDM rises from an initial 25% to just over 50% in light of the data. We conclude that LambdaCDM remains a sufficient phenomenological description of currently available observations and that there is little statistical room for model improvement.Comment: 10 pages, 2 figure

    A measure on the set of compact Friedmann-Lemaitre-Robertson-Walker models

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    Compact, flat Friedmann-Lemaitre-Robertson-Walker (FLRW) models have recently regained interest as a good fit to the observed cosmic microwave background temperature fluctuations. However, it is generally thought that a globally, exactly-flat FLRW model is theoretically improbable. Here, in order to obtain a probability space on the set F of compact, comoving, 3-spatial sections of FLRW models, a physically motivated hypothesis is proposed, using the density parameter Omega as a derived rather than fundamental parameter. We assume that the processes that select the 3-manifold also select a global mass-energy and a Hubble parameter. The inferred range in Omega consists of a single real value for any 3-manifold. Thus, the obvious measure over F is the discrete measure. Hence, if the global mass-energy and Hubble parameter are a function of 3-manifold choice among compact FLRW models, then probability spaces parametrised by Omega do not, in general, give a zero probability of a flat model. Alternatively, parametrisation by the injectivity radius r_inj ("size") suggests the Lebesgue measure. In this case, the probability space over the injectivity radius implies that flat models occur almost surely (a.s.), in the sense of probability theory, and non-flat models a.s. do not occur.Comment: 19 pages, 4 figures; v2: minor language improvements; v3: generalisation: m, H functions of

    Radio-Frequency Measurements of Coherent Transition and Cherenkov Radiation: Implications for High-Energy Neutrino Detection

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    We report on measurements of 11-18 cm wavelength radio emission from interactions of 15.2 MeV pulsed electron bunches at the Argonne Wakefield Accelerator. The electrons were observed both in a configuration where they produced primarily transition radiation from an aluminum foil, and in a configuration designed for the electrons to produce Cherenkov radiation in a silica sand target. Our aim was to emulate the large electron excess expected to develop during an electromagnetic cascade initiated by an ultra high-energy particle. Such charge asymmetries are predicted to produce strong coherent radio pulses, which are the basis for several experiments to detect high-energy neutrinos from the showers they induce in Antarctic ice and in the lunar regolith. We detected coherent emission which we attribute both to transition and possibly Cherenkov radiation at different levels depending on the experimental conditions. We discuss implications for experiments relying on radio emission for detection of electromagnetic cascades produced by ultra high-energy neutrinos.Comment: updated figure 10; fixed typo in equation 2.2; accepted by PR

    Photon Physics in Heavy Ion Collisions at the LHC

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    Various pion and photon production mechanisms in high-energy nuclear collisions at RHIC and LHC are discussed. Comparison with RHIC data is done whenever possible. The prospect of using electromagnetic probes to characterize quark-gluon plasma formation is assessed.Comment: Writeup of the working group "Photon Physics" for the CERN Yellow Report on "Hard Probes in Heavy Ion Collisions at the LHC", 134 pages. One figure added in chapter 5 (comparison with PHENIX data). Some figures and correponding text corrected in chapter 6 (off-chemical equilibrium thermal photon rates). Some figures modified in chapter 7 (off-chemical equilibrium photon rates) and comparison with PHENIX data adde

    Large scale cosmic-ray anisotropy with KASCADE

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    The results of an analysis of the large scale anisotropy of cosmic rays in the PeV range are presented. The Rayleigh formalism is applied to the right ascension distribution of extensive air showers measured by the KASCADE experiment.The data set contains about 10^8 extensive air showers in the energy range from 0.7 to 6 PeV. No hints for anisotropy are visible in the right ascension distributions in this energy range. This accounts for all showers as well as for subsets containing showers induced by predominantly light respectively heavy primary particles. Upper flux limits for Rayleigh amplitudes are determined to be between 10^-3 at 0.7 PeV and 10^-2 at 6 PeV primary energy.Comment: accepted by The Astrophysical Journa

    Observation of exotic meson production in the reaction πpηπp \pi^{-} p \to \eta^{\prime} \pi^- p at 18 GeV/c

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    An amplitude analysis of an exclusive sample of 5765 events from the reaction πpηπp\pi^{-} p \to \eta^{\prime} \pi^- p at 18 GeV/c is described. The ηπ\eta^{\prime} \pi^- production is dominated by natural parity exchange and by three partial waves: those with JPC=1+,2++,J^{PC} = 1^{-+}, 2^{++}, and 4++4^{++}. A mass-dependent analysis of the partial-wave amplitudes indicates the production of the a2(1320)a_2(1320) meson as well as the a4(2040)a_4(2040) meson, observed for the first time decaying to ηπ\eta^{\prime}\pi^-. The dominant, exotic (non-qqˉ)q\bar{q}) 1+1^{-+} partial wave is shown to be resonant with a mass of 1.597±0.0100.010+0.0451.597 \pm 0.010^{+0.045}_{-0.010} GeV/c^2 and a width of 0.340±0.040±0.0500.340 \pm 0.040 \pm 0.050 GeV/c^2 . This exotic state, the π1(1600)\pi_1(1600), is produced with a tt dependence which is different from that of the a2(1320)a_2(1320) meson, indicating differences between the production mechanisms for the two states.Comment: 5 pages with 4 figure

    A partial wave analysis of the π0π0\pi ^0\pi ^0 system produced in πp\pi ^-p charge exchange collisions

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    A partial wave analysis of the of the π0π0\pi ^0\pi ^0 system produced in the charge exchange reaction: πpπ0π0n\pi ^-p\to \pi ^0\pi ^0n at an incident momentum of 18.3GeV/c18.3 GeV/c is presented as a function of π0π0{\pi ^0\pi ^0} invariant mass, mπ0π0m_{\pi^0\pi^0}, and momentum transfer squared, t| {t} |, from the incident π\pi^- to the outgoing π0π0{\pi ^0\pi ^0} system.Comment: 24 pages total,8 pages text, 14 figures, 1 table. Submitted to Phys Rev
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