arXiv subjects

Graciela B. Gelmini

Publications and source records attributed to Graciela B. Gelmini.

83 records · Page 5Linked to original sources

arXiv2008

Ultra-cold WIMPs: relics of non-standard pre-BBN cosmologies

Weakly interacting massive particles (WIMPs) are one of very few probes of cosmology before Big Bang nucleosynthesis (BBN). We point out that in scenarios in which the Universe evolves in a non-standard manner during and after WIMP kinetic decoupling, the horizon mass scale at decoupling can be smaller and the dark matter WIMPs can be colder than in standard cosmology. This would lead to much smaller first objects in hierarchical structure formation. In low reheating temperature scenarios the effect may be large enough as to noticeably enhance indirect detection signals in GLAST and other detectors, by up to two orders of magnitude.

astro-ph↗

arXiv2008

Experimental relevance of low reheating temperature cosmologies

Standard simple assumptions are usually made about the pre-Big Bang Nucleosynthesis epoch, from which we do not have observations. Modifying these assumptions, the predicted density of relic particles such as neutralinos and sterile neutrinos can be very different from that in the standard case. For example, neutralinos could have the dark matter density in (almost) any supersymmetric model, and sterile neutrinos with mixings large enough to be soon detected in neutrino experiments would become cosmologically acceptable. These possibilities are important in view of what the LHC, and neutrino experiments could soon find.

hep-ph↗

arXiv2007

Composition of UHECR and the Pierre Auger Observatory Spectrum

We fit the recently published Pierre Auger ultra-high energy cosmic ray spectrum assuming that either nucleons or nuclei are emitted at the sources. We consider the simplified cases of pure proton, or pure oxygen, or pure iron injection. We perform an exhaustive scan in the source evolution factor, the spectral index, the maximum energy of the source spectrum Z E_{max}, and the minimum distance to the sources. We show that the Pierre Auger spectrum agrees with any of the source compositions we assumed. For iron, in particular, there are two distinct solutions with high and low E_{max} (e.g. 6.4 10^{20} eV and 2 10^{19} eV) respectively which could be distinguished by either a large fraction or the near absence of proton primaries at the highest energies. We raise the possibility that an iron dominated injected flux may be in line with the latest composition measurement from the Pierre Auger Observatory where a hint of heavy element dominance is seen.

astro-ph↗

arXiv2007

GZK Photons as Ultra High Energy Cosmic Rays

We calculate the flux of "GZK-photons", namely the flux of Ultra High Energy Cosmic Rays (UHECR) consisting of photons produced by extragalactic nucleons through the resonant photoproduction of pions, the so called GZK effect. We We calculate the flux of "GZK-photons", namely the flux of Ultra High Energy Cosmic Rays (UHECR) consisting of photons produced by extragalactic nucleons through the resonant photoproduction of pions, the so called GZK effect. We show that, for primary nucleons, the GZK photon fraction of the total UHECR flux is between $10^{-4}$ and $10^{-2}$ above $10^{19}$ eV and up to the order of 0.1 above $10^{20}$ eV. The GZK photon flux depends on the assumed UHECR spectrum, slope of the nucleon flux at the source, distribution of sources and intervening backgrounds. Detection of this photon flux would open the way for UHECR gamma-ray astronomy. Detection of a larger photon flux would imply the emission of photons at the source or new physics. We compare the photon fractions expected for GZK photons and the minimal predicted by Top-Down models. We find that the photon fraction above $10^{19}$ eV is a crucial test for Top-Down models.

astro-ph↗

arXiv2006

Neutralino with the Right Cold Dark Matter Abundance in (Almost) Any Supersymmetric Model

We consider non-standard cosmological models in which the late decay of a scalar field $ϕ$ reheats the Universe to a low reheating temperature, between 5 MeV and the standard freeze-out temperature of neutralinos of mass $m_χ$. We point out that in these models all neutralinos with standard density $Ω_{\rm std} \gtrsim 10^{-5} (100 {\rm GeV}/m_χ)$ can have the density of cold dark matter, provided the right combination of the following two parameters can be achieved in the high energy theory: the reheating temperature, and the ratio of the number of neutralinos produced per $ϕ$ decay over the $ϕ$ field mass. We present the ranges of these parameters where a combination of thermal and non-thermal neutralino production leads to the desired density, as functions of $Ω_{\rm std}$ and $m_χ$.

hep-ph↗

arXiv2005

DAMA detection claim is still compatible with all other DM searches

We show that the annual modulation signal observed by DAMA can be reconciled with all other negative results from dark matter searches with a conventional halo model for particle masses around 5 to 9 GeV. We also show which particular dark matter stream could produce the DAMA signal.

hep-ph↗

arXiv2005

GZK photons as UHECR above 10$^{19}$ eV

"GZK photons" are produced by extragalactic nucleons through the resonant photoproduction of pions. We present the expected range of the GZK photon fraction of UHECR, assuming a particular UHECR spectrum and primary nucleons, and compare it with the minimal photon fraction predicted by Top-Down models.

hep-ph↗

arXiv2004

Prospect for relic neutrino searches

Unlike the relic photons, relic neutrinos have not so far been observed. The Cosmic Neutrino Background (C$ν$B) is the oldest relic from the Big Bang, produced a few seconds after the Bang itself. Due to their impact in cosmology, relic neutrinos may be revealed indireclty in the near future through cosmological observations. In this talk we concentrate on other proposals, made in the last 30 years, to try to detect the C$ν$B directly, either in laboratory searches (through tiny accelerations they produce on macroscopic targets) or through astrophysical observations (looking for absorption dips in the flux of Ultra-High Energy neutrinos, due to the annihilation of these neutrinos with relic neutrinos at the Z-resonance). We concentrate mainly on the first of these two possibilities.

hep-ph↗

arXiv2004

Cosmology of "Visible" Sterile Neutrinos

We point out that in scenarios with a low reheating temperature $T_R << 100$ MeV at the end of (the last episode of) inflation or entropy production, the abundance of sterile neutrinos becomes largely independent of their coupling to active neutrinos. Thus, cosmological bounds become less stringent than usually assumed, allowing sterile neutrinos to be ``visible'' in future experiments. For example, the sterile neutrino required by the LSND result does not have any cosmological problem within these scenarios.

hep-ph↗

arXiv2003

Dark matter searches: looking for the cake or its frosting? (Detectability of a subdominant component of the CDM)

The dark matter candidates we are searching for, e.g. neutralinos, may be one of many components of the cold dark matter (CDM). We point out here that very subdominant components, constituting even 1% of the CDM for indirect detection and much less for direct detection, remain observable in current and future searches. So if a CDM signal is confirmed in CDM search experiments (except for a signal from annihilations in the dark halo) we will need to find out the halo fraction accounted for the CDM component we detected.

hep-ph↗

arXiv2000

Super-Kamiokande 0.07 eV Neutrinos in Cosmology: Hot Dark Matter and the Highest Energy Cosmic Rays

Relic neutrinos with mass in the range indicated by Super-Kamiokande results if neutrino masses are hierarchial (about 0.07 eV) are many times deemed too light to be cosmologically relevant. Here we remark that these neutrinos may significantly contribute to the dark matter of the Universe (with a large lepton asymmetry $L$) and that their existence might be revealed by the spectrum of ultra high energy cosmic rays (maybe even in the absence of a large $L$).

hep-ph↗