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TOPIC: PAMELA experiment
Blobrana


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Posts: 131433
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ATIC and Fermi data
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Title: On discrepancy between ATIC and Fermi data
Authors: Dmitry Malyshev

Either ATIC or Fermi-LAT data can be fitted together with the PAMELA data by three components: primary background ~ E^{-3.3}, secondary background ~ E^{-3.6}, and an additional source of electrons ~ E^{-g_a} Exp(-E/E_{cut}). We find that the best fits for ATIC + PAMELA and for Fermi + PAMELA are approximately the same, g_a ~ 2 and E_{cut} ~ 500 GeV. However, the ATIC data have a narrow bump between 300 GeV and 600 GeV which contradicts the smooth Fermi spectrum. An interpretation of the ATIC bump as well as the featureless Fermi spectrum in terms of dark matter models and pulsars is discussed.

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Blobrana


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Posts: 131433
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Pamela/ATIC excess
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Title: Secondary radiation from the Pamela/ATIC excess and relevance for Fermi
Authors: E.Borriello, A.Cuoco, G.Miele
(Version v2)

The excess of electrons/positrons observed by the Pamela and ATIC experiments gives rise to a noticeable amount of synchrotron and Inverse Compton Scattering (ICS) radiation when the e^+e^- interact with the Galactic Magnetic Field, and the InterStellar Radiation Field (ISRF). In particular, the ICS signal produced within the WIMP annihilation interpretation of the Pamela/ATIC excess shows already some tension with the EGRET data. On the other hand, 1 yr of Fermi data taking will be enough to rule out or confirm this scenario with a high confidence level. The ICS radiation produces a peculiar and clean "ICS Haze" feature, as well, which can be used to discriminate between the astrophysical and Dark Matter scenarios. This ICS signature is very prominent even several degrees away from the galactic center, and it is thus a very robust prediction with respect to the choice of the DM profile and the uncertainties in the ISRF.

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Blobrana


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Posts: 131433
Date:
PAMELA/ATIC Anomaly
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Title: PAMELA/ATIC Anomaly from Exotic Mediated Dark Matter Decay
Authors: Kyu Jung Bae, Bumseok Kyae
(Version v2)

We discuss dark matter decay mediated by exotically charged particles ("exotics") in a supersymmetric model with two dark matter (DM) components: One is the (bino-like) lightest supersymmetric particle (LSP) \chi, and the other is a newly introduced meta-stable neutral singlet N. N decays to \chi e^+e^- via a dimension 6 operator induced by a penguin-type one loop diagram with the life time of 10^{26} sec., explaining energetic cosmic e^± excess observed recently by PAMELA and ATIC/PPB-BETS. The superheavy masses of exotics (~ 10^{15-16} GeV) are responsible for the longevity of N. The superpartner of N develops the vacuum expectation value (VEV) of order TeV so that the DM N achieves the desired mass of 2 TeV. By the VEV, the U(1)_R symmetry is broken to the discrete Z_2 symmetry, which is identified with the matter parity in the minimal supersymmetric standard model (MSSM). Since we have the two DM components, even extremely small amount of N [O(10^{-10}) < (n_N/n_\chi)] could account for the observed positron flux with relatively light exotics' masses [10^{12} GeV < M_{exo.} < 10^{16} GeV].

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Blobrana


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Posts: 131433
Date:
Positron Excess
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Title: The origin of the positron excess in cosmic rays
Authors: Pasquale Blasi (INAF/Arcetri)
(Version v2)

We show that the positron excess measured by the PAMELA experiment in the region between 10 and 100 GeV may well be a natural consequence of the standard scenario for the origin of Galactic cosmic rays. The 'excess' arises because of positrons created as secondary products of hadronic interactions inside the sources, but the crucial physical ingredient which leads to a natural explanation of the positron flux is the fact that the secondary production takes place in the same region where cosmic rays are being accelerated. Therefore secondary positrons (and electrons) participate in the acceleration process and turn out to have a very flat spectrum, which is responsible, after propagation in the Galaxy, for the observed positron 'excess'. This effect cannot be avoided though its strength depends on the values of the environmental parameters during the late stages of evolution of supernova remnants.

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Blobrana


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RE: PAMELA experiment
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Too many electrons, too high energy. That's what turned out in new data gathered by the Fermi Gamma-ray Space Telescope. The electrons could be coming from nearby pulsars - or they could be a longed-for signal of dark matter, the elusive, invisible material thought to make up nearly a quarter of the universe.
FGST's Large Area Telescope, a collaboration between NASA, the U.S. Department of Energy, and multiple international partners (built with an important Italian contribution, coordinated by the Italian Space Agency with the National Institute of Nuclear Physics and the National Institute for Astrophysics), has been scanning the skies for gamma rays and particles since its launch last summer. The LAT measured a strikingly high number of electrons with energies between 100 billion and one trillion electronvolts. It is not known from the LAT data alone if these electrons are coming from the distant background, or are the signal of a nearby source of high-energy particles.

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Blobrana


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Title: The PAMELA Positron Excess from Annihilations into a Light Boson
Authors: Ilias Cholis, Douglas P. Finkbeiner, Lisa Goodenough, Neal Weiner
(Version v3)

Recently published results from the PAMELA experiment have shown conclusive evidence for an excess of positrons at high (~ 10 - 100 GeV) energies, confirming earlier indications from HEAT and AMS-01. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in anti-protons is highly constraining on any model with hadronic annihilation modes. We revisit an earlier proposal, whereby the dark matter annihilates into a new light (<~GeV) boson phi, which is kinematically constrained to go to hard leptonic states, without anti-protons or pi0's. We find this provides a very good fit to the data. The light boson naturally provides a mechanism by which large cross sections can be achieved through the Sommerfeld enhancement, as was recently proposed. Depending on the mass of the WIMP, the rise may continue above 300 GeV, the extent of PAMELA's ability to discriminate electrons and positrons.

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Blobrana


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Posts: 131433
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Title: The Pamela Cosmic Ray Space Observatory: Detector, Objectives and First Results
Authors: M. Casolino, The Pamela collaboration

PAMELA is a satellite borne experiment designed to study with great accuracy cosmic rays of galactic, solar, and trapped nature in a wide energy range (protons: 80 MeV-700 GeV, electrons 50 MeV-400 GeV). Main objective is the study of the antimatter component: antiprotons (80 MeV-190 GeV), positrons (50 MeV-270 GeV) and search for antimatter with a precision of the order of $10^{-8}$). The experiment, housed on board the Russian Resurs-DK1 satellite, was launched on June, 15 2006 in a 350 x 600 km orbit with an inclination of 70 degrees. The detector is composed of a series of scintillator counters arranged at the extremities of a permanent magnet spectrometer to provide charge, Time-of-Flight and rigidity information. Lepton/hadron identification is performed by a Silicon-Tungsten calorimeter and a Neutron detector placed at the bottom of the device. An Anticounter system is used offline to reject false triggers coming from the satellite. In self-trigger mode the Calorimeter, the neutron detector and a shower tail catcher are capable of an independent measure of the lepton component up to 2 TeV. In this work we describe the experiment, its scientific objectives and the performance in its first two years of operation. Data on protons of trapped, secondary and galactic nature - as well as measurements of the December 13 2006 Solar Particle Event - are provided.

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Blobrana


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Posts: 131433
Date:
PAMELA Excess
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Title: Testing the Dark Matter Interpretation of the PAMELA Excess through Measurements of the Galactic Diffuse Emission
Authors: Marco Regis, Piero Ullio

We propose to test the dark matter (DM) interpretation of the positron excess observed by the PAMELA cosmic-ray (CR) detector through the identification of a Galactic diffuse gamma-ray component associated to DM-induced prompt and radiative emission. The goal is to present an analysis based on minimal sets of assumptions and extrapolations with respect to locally testable or measurable quantities. We discuss the differences between the spatial and spectral features for the DM-induced components (with an extended, possibly spherical, source function) and those for the standard CR contribution (with sources confined within the stellar disc), and propose to focus on intermediate and large latitudes. We address the dependence of the signal to background ratio on the model adopted to describe the propagation of charged CRs in the Galaxy, and find that, in general, the DM-induced signal can be detected by the Fermi Gamma-ray Space Telescope at energies above 100 GeV. An observational result in agreement with the prediction from standard CR components only, would imply very strong constraints on the DM interpretation of the PAMELA excess. On the other hand, if an excess in the diffuse emission above 100 GeV is identified, the angular profile for such emission would allow for a clean disentanglement between the DM interpretation and astrophysical explanations proposed for the PAMELA excess. We also compare to the radiative diffuse emission at lower frequencies, sketching in particular the detection prospects at infrared frequencies with the Planck satellite.

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Blobrana


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Posts: 131433
Date:
PAMELA anomaly
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Title: Is the PAMELA anomaly caused by the supernova explosions near the Earth?
Authors: Yutaka Fujita (Osaka), Kazunori Kohri (Lancaster), Ryo Yamazaki (Hiroshima), Kunihito Ioka (KEK)
(Version v2)

We show that recent supernova explosion(s) in a dense gas cloud (DC) near the Earth can be attributed to the electron/positron excesses observed with PAMELA and ATIC. Protons are accelerated around the supernova remnant (SNR). Electrons and positrons are created through hadronic interactions insides the DC. Their spectrum is harder than that of the background because the SNR spends much time in a radiative phase. Our model predicts that the anti-proton flux dominates that of the background for ~>100 GeV, while the gamma-ray and neutrino signals could currently be absent because the SNR has destroyed the DC.

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Blobrana


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Posts: 131433
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RE: PAMELA experiment
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When dark matter is destroyed, it leaves behind a burst of exotic particles, according to theory. Now scientists have found a possible signature of these remains. The discovery could help prove the existence of dark matter and reveal what it's made of.

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