arXiv subjects

Julien Lavalle

Publications and source records attributed to Julien Lavalle.

44 records · Page 3Linked to original sources

arXiv2009

Cosmic positron excess: is the dark matter solution a good bet?

The recent observation by the PAMELA satellite of a rising positron fraction up to $\sim$ 100 GeV has triggered a considerable amount of putative interpretations in terms of dark matter (DM) annihilation or decay. Here, we make a critical reassessment of such a possibility, recalling the elementary conditions with respect to the standard astrophysical background that would make it likely, showing that they are not fulfilled. Likewise, we argue that, as now well accepted, DM would need somewhat contrived properties to contribute significantly to the observed positron signal, even when including e.g. clumpiness effects. This means that most of natural DM candidates arising in particle physics beyond the standard model are not expected to be observed in the cosmic antimatter spectrum, unfortunately. However, this does not prevent them from remaining excellent DM candidates, this only points towards the crucial need of developing much more complex detection strategies (multimessenger, multiwavelength, multiscale searches).

astro-ph.GA↗

arXiv2009

Intermediate Mass Black Holes and Nearby Dark Matter Point Sources: A Critical Reassessment

The proposal of a galactic population of intermediate mass black holes (IMBHs), forming dark matter (DM) ``mini-spikes'' around them, has received considerable attention in recent years. In fact, leading in some scenarios to large annihilation fluxes in gamma rays, neutrinos and charged cosmic rays, these objects are sometimes quoted as one of the most promising targets for indirect DM searches. In this letter, we apply a detailed statistical analysis to point out that the existing EGRET data already place very stringent limits on those scenarios, making it rather unlikely that any of these objects will be observed with, e.g., the Fermi/GLAST satellite or upcoming Air Cherenkov telescopes. We also demonstrate that prospects for observing signals in neutrinos or charged cosmic rays seem even worse. Finally, we address the question of whether the excess in the cosmic ray positron/electron flux recently reported by PAMELA/ATIC could be due to a nearby DM point source like a DM clump or mini-spike; gamma-ray bounds, as well as the recently released Fermi cosmic ray electron and positron data, again exclude such a possibility for conventional DM candidates, and strongly constrain it for DM purely annihilating into light leptons.

astro-ph.CO↗

arXiv2009

Anti-Matter in Cosmic Rays : Backgrounds and Signals

Recent PAMELA and ATIC data seem to indicate an excess in positron cosmic rays above approximately 10 GeV which might be due to galactic Dark Matter particle annihilation. However the background of this signal suffers many uncertainties that make our task difficult in constraining Dark Matter or any other astrophysical explanation for these recent surprising data.

hep-ph↗

arXiv2009

Clarifying the covariant formalism for the SZ effect due to relativistic non-thermal electrons

We derive the covariant formalism associated with the relativistic Sunyaev-Zel'dovich effect due to a non-thermal population of high energy electrons in clusters of galaxies. More precisely, we show that the formalism proposed by Wright in 1979, based on an empirical approach (and widely used in the literature) to compute the inverse Compton scattering of a population of relativistic electrons on CMB photons, can actually be re-interpreted as a Boltzmann-like equation, in the single scattering approximation.

astro-ph↗

arXiv2008

On the antimatter signatures of the cosmological dark matter subhalos

While the PAMELA collaboration has recently confirmed the cosmic ray positron excess, it is interesting to review the effects of dark matter (DM) subhalos on the predicted antimatter signals. We recall that, according to general subhalo properties as inferred from theoretical cosmology, and for DM with constant annihilation cross section, the enhancement cannot be $\gtrsim$ 20 for the antimatter yield. This bound is obviously different from that found for gamma-rays. We also recall some predictions for supersymmetric benchmark models observable at the LHC and derived in the cosmological N-body framework, illustrating in the meantime the existing discrepancy between the profiles derived from N-body experiments and the current observations of the Milky Way.

astro-ph↗

arXiv2007

Anti-proton and positron Cosmic Rays from Dark Matter annihilation around Intermediate Mass Black Holes

Intermediate Mass Black Holes (IMBHs) are candidates to seed the Supermassive Black Holes (SMBHs), and some could still wander in the Galaxy. In the context of annihilating dark matter (DM), they are expected to drive huge annihilation rates, and could therefore significantly enhance the primary cosmic rays (CRs) expected from annihilation of the DM of the Galactic halo. In this proceeding (the original paper is Brun et al. 2007), we briefly explain the method to derive estimates of such exotic contributions to the anti-proton and positron CR spectra, and the associated statistical uncertainties connected to the properties of IMBHs. We find boost factors of order $10^4$ to the exotic fluxes, but associated with very large statistical uncertainties.

astro-ph↗

arXiv2007

Indirect detection of Dark Matter with antimatter: Demystifying the clumpiness boost factors

The hierarchical scenario of structure formation, in the frame of the $Λ$-CDM cosmology, predicts the existence of dark matter (DM) sub-halos down to very small scales, of which the minimal size depends on the microscopic properties of the DM. In the context of annihilating DM, such substructures are expected to enhance the primary cosmic ray (CR) fluxes originating from DM annihilation in the Galaxy. This enhancement has long been invoked to allow predictions of imprints of DM annihilation on the antimatter CR spectra. Taking advantage of the method developed by Lavalle et al (2007b), we (Lavalle et al, 2007a) accurately compute the boost factors for positrons and anti-protons, as well as the associated theoretical and statistical errors. To this aim, we use a compilation of the latest results of cosmological N-body simulations and the theoretical insights found in the literature. We find that sub-halos are not likely to significantly boost the exotic production of antimatter CRs.

astro-ph↗

arXiv2006

Clumpiness of Dark Matter and Positron Annihilation Signal: Computing the odds of the Galactic Lottery

The small-scale distribution of dark matter in Galactic halos is poorly known. Several studies suggest that it could be very clumpy, which turns out to be of paramount importance when investigating the annihilation signal from exotic particles (e.g. supersymmetric or Kaluza-Klein). In this paper we focus on the annihilation signal in positrons. We estimate the associated uncertainty, due to the fact that we do not know exactly how the clumps are distributed in the Galactic halo. To this aim, we perform a statistical study based on analytical computations, as well as numerical simulations. In particular, we study the average and variance of the annihilation signal over many Galactic halos having the same statistical properties. We find that the so-called boost factor used by many authors should be handled with care, as i) it depends on energy and ii) it may be different for positrons, antiprotons and gamma rays, a fact which has not received any attention before. As an illustration, we use our results to discuss the positron spectrum measurements by the HEAT experiment.

astro-ph↗