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TOPIC: PAMELA experiment
Blobrana


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RE: PAMELA experiment
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Title: PAMELA through a Magnetic Lense
Authors: J. P. Roberts

The PAMELA satellite has observed an excess of positrons over electrons in the energy range 1-100 GeV that increases with energy. We propose that the excess is not due to a change in the local interstellar spectrum, but is due to heliospheric modulation. We motivate this from the known form of the heliospheric magnetic field and predict that the excess will disappear when we enter a period of solar maximum activity.

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Blobrana


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Galactic electrons are thought to originate in the explosion of supernovae, and conventional models predict that they lose energy as they pass through the Milky Way's magnetic field. The annihilation of proposed dark-matter particles would also create electrons, and some theorists had interpreted the recent experimental detections of surplus high-energy electrons as evidence for this process.
But starlight also scatters the electrons. Petrosian says that starlight suppresses the energy of most electrons in a way that makes it seem as if there is an excess of certain high-energy electrons. The Stanford group's models show an excess that is similar to that reported by NASA's Fermi Gamma-ray Space Telescope; the High Energy Stereoscopic System (HESS), a ground-based detector in Namibia; and the Advanced Thin Ionisation Calorimeter (ATIC), a balloon-borne detector that flew over Antarctica.
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Blobrana


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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 v3)

We show that the anomaly of the positron fraction observed by the PAMELA experiment can be attributed to recent supernova explosion(s) in a dense gas cloud (DC) near the Earth. Protons are accelerated around the supernova remnant (SNR). Electrons and positrons are created through hadronic interactions inside the DC. Their spectrum is harder than that of the background because the SNR spends much time in a radiative phase. Our scenario predicts that the anti-proton flux dominates that of the background for >~100 GeV. We compare the results with observations (Fermi, HESS, PPB-BETS, and ATIC).

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Blobrana


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Positron Excess
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Particles as tracers for the most massive explosions in the Milky Way
Astronomers recently observed a mysterious flux of particles in the universe, and the hope was born that this may be the first observation of the remnants of "dark matter".
But scientists from the University of Gothenburg have shown that there is another explanation of the flux.
Several independent studies recently discovered a mysterious flux of electrons and positrons in the universe. Several theories were presented that suggested that these particles arise from the decay of "dark matter" - the hypothetical material that is believed to influence the rotation of galaxies. Dark matter is one of the most challenging questions in astrophysics. An international research group with members from the University of Gothenburg has now published new results showing that the mysterious flux actually arises from exploding stars.

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Blobrana


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Excess positrons
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Excess positrons are linked to Geminga pulsar
Are recently-detected excesses of cosmic electrons and positrons the first direct evidence for the existence of dark matter particles? That has been the hope of many physicists, while others have suggested a more mundane origin in a nearby pulsar. Now researchers in the US claim that the excesses can be linked to high-energy gamma rays emitted by the Geminga pulsar.
Cold dark matter is the most accepted explanation as to why the universe appears to have at least 80% more gravitating mass than is directly visible. Dark-matter particles are expected to collide with one another and annihilate - producing high energy particles such as electrons and positrons. If these particles could be observed, they would represent the most direct evidence yet for the existence of dark matter.

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Blobrana


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Posts: 131433
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RE: PAMELA experiment
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Title: TeV Gamma Rays from Geminga and the Origin of the GeV Positron Excess
Authors: Yüksel, Hasan; Kistler, Matthew D.; Stanev, Todor

The Geminga pulsar has long been one of the most intriguing MeV-GeV -ray point sources. We examine the implications of the recent Milagro -ray Observatory detection of extended, multi-TeV -ray emission from Geminga, finding that this reveals the existence of an ancient, powerful cosmic-ray accelerator that can plausibly account for the multi-GeV positron excess that has evaded explanation. We explore a number of testable predictions for -ray and electron or positron experiments (up to ~100TeV) that can confirm the first "direct" detection of a cosmic-ray source.

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Blobrana


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PAMELA and WMAP data
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Title: Explaining PAMELA and WMAP data through Coannihilations in Extended SUGRA with Collider Implications
Authors: Daniel Feldman, Zuowei Liu, Pran Nath, Brent D. Nelson

The PAMELA positron excess is analysed within the framework of nonuniversal SUGRA models with an extended U(1)^n gauge symmetry in the hidden sector leading to neutralino dark matter with either a mixed Higgsino-wino LSP or an essentially pure wino dominated LSP. The Higgsino-wino LSP can produce the observed PAMELA positron excess and satisfy relic density constraints in the extended class of models due to a near degeneracy of the mass spectrum of the extended neutralino sector with the LSP mass. The simultaneous satisfaction of the WMAP relic density data and the PAMELA data is accomplished through a co-annihilation mechanism (B_{Co}-mechanism), and leads to predictions of a neutralino and a chargino in the mass range (180-200) GeV as well as low lying sparticles accessible at colliders. We show that the models are consistent with the antiproton constraints from PAMELA as well as photon flux data from EGRET and FERMI-LAT. Predictions for the scalar neutralino proton cross section relevant for the direct detection of dark matter are also discussed and signatures at the LHC for these PAMELA inspired models are analysed. It is shown that the mixed Higgsino-wino LSP model will be discoverable with as little as 1 fb^{-1} of data and is thus a prime candidate for discovery in the low luminosity runs at the LHC.

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Blobrana


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PAMELA Positron Excess
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Title: Extended MSSM Neutralinos as the Source of the PAMELA Positron Excess
Authors: Dan Hooper, Tim M.P. Tait

We consider a scenario within the Minimal Supersymmetric Standard Model extended by a singlet chiral superfield, in which neutralino dark matter annihilates to light singlet-like Higgs bosons, which proceed to decay to either electron-positron or muon-antimuon pairs. Unlike neutralino annihilations in the MSSM, this model can provide a good fit to the PAMELA cosmic ray positron fraction excess. Furthermore, the singlet-like scalar Higgs can induce a large Sommerfeld enhancement and provide an annihilation rate sufficient to accommodate the observed positron excess.

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Blobrana


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Title: Extragalactic Inverse Compton Light from Dark Matter Annihilation and the Pamela Positron Excess
Authors: Stefano Profumo, Tesla E. Jeltema

We calculate the extragalactic diffuse emission originating from the up-scattering of cosmic microwave photons by energetic electrons and positrons produced in particle dark matter annihilation events at all redshifts and in all halos. We outline the observational constraints on this emission and we study its dependence on both the particle dark matter model (including the particle mass and its dominant annihilation final state) and on assumptions on structure formation and on the density profile of halos. We find that for low-mass dark matter models, data in the X-ray band provide the most stringent constraints, while the gamma-ray energy range probes models featuring large masses and pair-annihilation rates, and a hard spectrum for the injected electrons and positrons. Specifically, we point out that the all-redshift, all-halo inverse Compton emission from many dark matter models that might provide an explanation to the anomalous positron fraction measured by the Pamela payload severely overproduces the observed extragalactic gamma-ray background.

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Blobrana


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Title: Testing astrophysical models for the PAMELA positron excess with cosmic ray nuclei
Authors: Philipp Mertsch, Subir Sarkar (Oxford)

The excess in the positron fraction reported by the PAMELA collaboration has been interpreted as due to annihilation or decay of dark matter in the Galaxy. More prosaically, it has been ascribed to direct production of positrons by nearby pulsars, or due to pion production during stochastic acceleration of hadronic cosmic rays in nearby sources. We point out that measurements of secondary nuclei produced by cosmic ray spallation can discriminate between these possibilities. New data on the titanium-to-iron ratio from the ATIC-2 experiment support the hadronic source model above and enable a prediction to be made for the boron-to-carbon ratio at energies above 100 GeV. Presently, all cosmic ray data are consistent with the positron excess being astrophysical in origin.

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