arXiv subjects

P. D. Serpico

Publications and source records attributed to P. D. Serpico.

At least 37 records · Page 2Linked to original sources

arXiv2007

Model-independent dark matter annihilation bound from the diffuse gamma ray flux

An upper limit on the total annihilation cross section of dark matter (DM) has recently been derived from the observed atmospheric neutrino background. We show that comparable bounds are obtained for DM masses around the TeV scale by observations of the diffuse gamma-ray flux by EGRET, because electroweak bremsstrahlung leads to non-negligible electromagnetic branching ratios, even if DM particles only couple to neutrinos at tree level. A better mapping and the partial resolution of the diffuse gamma-ray background into astrophysical sources by the GLAST satellite will improve this bound in the near future.

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arXiv2007

Revisiting cosmological bounds on radiative neutrino lifetime

Neutrino oscillation experiments and direct bounds on absolute masses constrain neutrino mass differences to fall into the microwave energy range, for most of the allowed parameter space. As a consequence of these recent phenomenological advances, older constraints on radiative neutrino decays based on diffuse background radiations and assuming strongly hierarchical masses in the eV range are now outdated. We thus derive new bounds on the radiative neutrino lifetime using the high precision cosmic microwave background spectral data collected by the Far Infrared Absolute Spectrophotometer instrument on board of Cosmic Background Explorer. The lower bound on the lifetime is between a few x 10^19 s and 5 x 10^20 s, depending on the neutrino mass ordering and on the absolute mass scale. However, due to phase space limitations, the upper bound in terms of the effective magnetic moment mediating the decay is not better than ~ 10^-8 Bohr magnetons. We also comment about possible improvements of these limits, by means of recent diffuse infrared photon background data. We compare these bounds with pre-existing limits coming from laboratory or astrophysical arguments. We emphasize the complementarity of our results with others available in the literature.

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arXiv2007

High Energy Neutrinos with a Mediterranean Neutrino Telescope

The high energy neutrino detection by a km^3 Neutrino Telescope placed in the Mediterranean sea provides a unique tool to both determine the diffuse astrophysical neutrino flux and the neutrino-nucleon cross section in the extreme kinematical region, which could unveil the presence of new physics. Here is performed a brief analysis of possible NEMO site performances.

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arXiv2007

The path to metallicity: synthesis of CNO elements in standard BBN

We perform a reanalysis of the production of CNO elements in a standard Big Bang Nucleosynthesis scenario. The CNO yields in BBN are suppressed by the low density of the plasma, Coulomb barrier effects and the short time scales involved. Yet, the inclusion of nuclides and reactions traditionally disregarded may lead to an increase relevant enough to affect the pristine Population III stars. After a critical reanalysis and upgrade of the nuclear network our results show no major discrepancies with the ones obtained using a smaller nuclear network. The robustness of the standard predictions-the early generation of star developed in a metal-free environment-is confirmed.

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arXiv2007

The Signature of Large Scale Structures on the Very High Energy Gamma-Ray Sky

If the diffuse extragalactic gamma ray emission traces the large scale structures of the universe, peculiar anisotropy patterns are expected in the gamma ray sky. In particular, because of the cutoff distance introduced by the absorption of 0.1-10 TeV photons on the infrared/optical background, prominent correlations with the local structures within a range of few hundreds Mpc should be present. We provide detailed predictions of the signal based on the PSCz map of the local universe. We also use mock N-body catalogues complemented with the halo model of structures to study some statistical features of the expected signatures. The results are largely independent from cosmological details, and depend mostly on the index of correlation (or bias) of the sources with respect to the large scale distribution of galaxies. For instance, the predicted signal in the case of a quadratic correlation (as it may happen for a dark matter annihilation contribution to the diffuse gamma flux) differs substantially from a linear correlation case, providing a complementary tool to unveil the nature of the sources of the diffuse gamma ray emission. The chances of the present and future space and ground based observatories to measure these features are discussed.

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arXiv2007

Ultra High Energy Neutrinos in the Mediterranean: detecting nu_tau and nu_mu with a km^3 Telescope

We perform a study of the ultra high energy neutrino detection performances of a km^3 Neutrino Telescope sitting at the three proposed sites for ANTARES, NEMO and NESTOR in the Mediterranean sea. We focus on the effect of the underwater surface profile on the total amount of yearly expected tau and mu crossing the fiducial volume in the limit of full detection efficiency and energy resolution. We also emphasize the possible enhancement of matter effect by a suitable choice of the geometry of the Telescope.

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arXiv2006

First hints of large scale structures in the ultra-high energy sky?

The result of the recent publication [M. Kachelriess and D. V. Semikoz Astropart. Phys. 26, 10 (2006)] of a broad maximum around 25 degrees in the two-point autocorrelation function of ultra-high energy cosmic ray arrival directions has been intriguingly interpreted as the first imprint of the large scale structures (LSS) of baryonic matter in the near universe. We analyze this suggestion in light of the clustering properties expected from the PSCz astronomical catalogue of LSS. The chance probability of the signal is consistent within 2 sigma with the predictions based on the catalogue. No evidence for a significant cross-correlation of the observed events with known overdensities in the LSS is found, which may be due to the role of the galactic and extragalactic magnetic fields, and is however consistent with the limited statistics. The larger statistics to be collected by the Pierre Auger Observatory is needed to answer definitely the question.

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arXiv2006

Oscillations of solar atmosphere neutrinos

The Sun is a source of high energy neutrinos (E > 10 GeV) produced by cosmic ray interactions in the solar atmosphere. We study the impact of three-flavor oscillations (in vacuum and in matter) on solar atmosphere neutrinos, and calculate their observable fluxes at Earth, as well as their event rates in a kilometer-scale detector in water or ice. We find that peculiar three-flavor oscillation effects in matter, which can occur in the energy range probed by solar atmosphere neutrinos, are significantly suppressed by averaging over the production region and over the neutrino and antineutrino components. In particular, we find that the relation between the neutrino fluxes at the Sun and at the Earth can be approximately expressed in terms of phase-averaged ``vacuum'' oscillations, dominated by a single mixing parameter (the angle theta_23).

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arXiv2006

The Compton-Getting effect on ultra-high energy cosmic rays of cosmological origin

Deviations from isotropy have been a key tool to identify the origin and the primary type of cosmic rays at low energies. We suggest that the Compton-Getting effect can play a similar role at ultra-high energies: If at these energies the cosmic ray flux is dominated by sources at cosmological distances, then the movement of the Sun relative to the cosmic microwave background frame induces a dipole anisotropy at the 0.6% level. The energy dependence and the orientation of this anisotropy provide important information about the transition between galactic and extragalactic cosmic rays, the charge of the cosmic ray primaries, the galactic magnetic field and, at the highest energies, the energy-loss horizon of cosmic rays. A 3-sigma detection of this effect requires around 10^6 events in the considered energy range and is thus challenging but not impossible with present detectors. As a corollary we note that the Compton-Getting effect allows one also to constrain the fraction of the diffuse gamma-ray background emitted by sources at cosmological distance, with promising detection possibilities for the GLAST satellite.

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arXiv2006

The Galactic magnetic field as spectrograph for ultra-high energy cosmic rays

We study the influence of the regular component of the Galactic magnetic field (GMF) on the arrival directions of ultra-high energy cosmic rays (UHECRs). We find that, if the angular resolution of current experiments has to be fully exploited, deflections in the GMF cannot be neglected even for E=10^20 eV protons, especially for trajectories along the Galactic plane or crossing the Galactic center region. On the other hand, the GMF could be used as a spectrograph to discriminate among different source models and/or primaries of UHECRs, if its structure would be known with sufficient precision. We compare several GMF models introduced in the literature and discuss for the example of the AGASA data set how the significance of small-scale clustering or correlations with given astrophysical sources are affected by the GMF. We point out that the non-uniform exposure to the extragalactic sky induced by the GMF should be taken into account estimating the significance of potential (auto-)correlation signals.

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arXiv2006

Standard and non-standard primordial neutrinos

The standard cosmological model predicts the existence of a cosmic neutrino background with a present density of about 110 cm^{-3} per flavour, which affects big-bang nucleosynthesis, cosmic microwave background anisotropies, and the evolution of large scale structures. We report on a precision calculation of the cosmic neutrino background properties including the modification introduced by neutrino oscillations. The role of a possible neutrino-antineutrino asymmetry and the impact of non-standard neutrino-electron interactions on the relic neutrinos are also briefly discussed.

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arXiv2006

Photon-axion conversion in intergalactic magnetic fields and cosmological consequences

Photon-axion conversion induced by intergalactic magnetic fields causes an apparent dimming of distant sources, notably of cosmic standard candles such as supernovae of type Ia (SNe Ia). We review the impact of this mechanism on the luminosity-redshift relation of SNe Ia, on the dispersion of quasar spectra, and on the spectrum of the cosmic microwave background. The original idea of explaining the apparent dimming of distant SNe Ia without cosmic acceleration is strongly constrained by these arguments. However, the cosmic equation of state extracted from the SN Ia luminosity-redshift relation remains sensitive to this mechanism. For example, it can mimic phantom energy.

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arXiv2006

Earth matter effects in supernova neutrinos: Optimal detector locations

A model-independent experimental signature for flavor oscillations in the neutrino signal from the next Galactic supernova (SN) would be the observation of Earth matter effects. We calculate the probability for observing a Galactic SN shadowed by the Earth as a function of the detector's geographic latitude. This probability depends only mildly on details of the Galactic SN distribution. A location at the North Pole would be optimal with a shadowing probability of about 60%, but a far-northern location such as Pyhasalmi in Finland, the proposed site for a large-volume scintillator detector, is almost equivalent (58%). We also consider several pairs of detector locations and calculate the probability that only one of them is shadowed, allowing a comparison between a shadowed and a direct signal. For the South Pole combined with Kamioka this probability is almost 75%, for the South Pole combined with Pyhasalmi it is almost 90%. One particular scenario consists of a large-volume scintillator detector located in Pyhasalmi to measure the geo-neutrino flux in a continental location and another such detector in Hawaii to measure it in an oceanic location. The probability that only one of them is shadowed exceeds 50% whereas the probability that at least one is shadowed is about 80%. We provide an online tool to calculate different shadowing probabilities for the one- and two-detector cases.

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arXiv2006

The footprint of large scale cosmic structure on the ultra-high energy cosmic ray distribution

Current experiments collecting high statistics in ultra-high energy cosmic rays (UHECRs) are opening a new window on the universe. In this work we discuss a large scale structure model for the UHECR origin which evaluates the expected anisotropy in the UHECR arrival distribution starting from a given astronomical catalogue of the local universe. The model takes into account the main selection effects in the catalogue and the UHECR propagation effects. By applying this method to the IRAS PSCz catalogue, we derive the minimum statistics needed to significatively reject the hypothesis that UHECRs trace the baryonic distribution in the universe, in particular providing a forecast for the Auger experiment.

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arXiv2005

Neutrinos and Cosmology: an update

We review the current cosmological status of neutrinos, with particular emphasis on their effects on Big Bang Nucleosynthesis, Large Scale Structure of the universe and Cosmic Microwave Background Radiation measurements.

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arXiv2005

Photon-axion conversion as a mechanism for supernova dimming: Limits from CMB spectral distortion

Axion-photon conversion induced by intergalactic magnetic fields has been proposed as an explanation for the dimming of distant supernovae of type Ia SNe Ia) without cosmic acceleration. The effect depends on the intergalactic electron density $n_e$ as well as the $B$-field strength and domain size. We show that for $n_e\alt 10^{-9} {\rm cm}^{-3}$ the same mechanism would cause excessive spectral distortion of the cosmic microwave background (CMB). This small-$n_e$ parameter region had been left open by the most restrictive previous constraints based on the dispersion of quasar (QSO) spectra. The combination of CMB and QSO limits suggests that the photon-axion conversion mechanism can only play a subleading role for SN Ia dimming. A combined analysis of all the observables affected by the photon-axion oscillations would be required to give a final verdict on the viability of this model.

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arXiv2005

Measuring the 13-mixing angle and the CP phase with neutrino telescopes

The observed excess of high-energy cosmic rays from the Galactic plane in the energy range \sim 10^18 eV may be naturally explained by neutron primaries generated in the photo-dissociation of heavy nuclei. In this scenario, neutrons with lower energy decay before reaching the Earth and produce a detectable flux in a 1 km^3 neutrino telescope. The initial flavor composition of these neutrinos, ϕ(\barν_e):ϕ(\barν_μ):ϕ(\barν_τ)=1:0:0, offers the opportunity to perform a combined \barν_μ/\barν_τappearance and \barν_e disappearance experiment. The observable ratio ϕ(\barν_μ)/ϕ(\barν_e+\barν_τ) of fluxes arriving on Earth depends appreciably on the 13-mixing angle θ_13 and the leptonic CP phase δ_CP, opening thus a new experimental avenue to measure these two quantities.

hep-ph↗

arXiv2004

Neutrinos and Primordial Nucleosynthesis

The importance of the Big Bang Nucleosynthesis (BBN) as a unique tool for studying neutrino properties is discussed, and the recent steps towards a self-consistent and robust handling of the weak reaction decoupling from the thermal bath as well as of the neutrino reheating following the e+e- annihilation are summarized. We also emphasize the important role of the Cosmic Microwave Background (CMB) anisotropy in providing an accurate and independent determination of the baryon density parameter omegab. The BBN is presently a powerful parameter-free theory that can test the standard scenario of the neutrino decoupling in the early Universe. Moreover it can constrain new physics in the neutrino sector. The perspectives for improvements in the next years are outlined.

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