Search Talks

The positron excess measured by PAMELA may be the long waited hint of the presence of dark matter particles in the Milky Way halo. But before we rejoice, we need to examine the other Possible astrophysical explanations. Whatever the sources -- DM or conventional -- a crucial ingredient is the transport of cosmic rays in the magnetic fields of the Galaxy to which I will pay particular attention in this presentation.

The cosmic-ray excess observed by PAMELA in the positron fraction and by FERMI and HESS in the electron + positron flux can be interpreted in terms of DM annihilations or decays into leptonic final states. Final states into tau's or 4mu give the best fit to the excess. However, in the annihilation scenario, they are incompatible with photon and neutrino constraints, unless DM has a quasi-constant density profile. Final states involving electrons are less constrained but poorly fit the excess, unless hidden sector radiation makes their energy spectrum smoother, allowing a fit to all the data with a combination of leptonic modes. In general, DM lighter than about a TeV cannot fit the excesses, so PAMELA should find a greater positron fraction at higher energies. The DM interpretation can be tested by FERMI gamma observations above 10 GeV: if the electronic excess is everywhere in the DM halo, inverse Compton scattering on ambient light produces a well-predicted gamma excess that FERMI should soon detect.

Recent data from the PAMELA satellite and a number of balloon experiment have reported unexpected excesses in the measured fluxes of cosmic rays. Are these the first direct evidences for Dark Matter? If yes, which DM models and candidates can explain these anomalies and what do they imply for future searches?

Successfully launched on June 11, 2008, Fermi is the reference high-energy gamma-ray space observatory of the current decade. The Fermi Large Area Telescope (LAT) has been collecting data continuously in nominal operations since August 2008, providing exciting results that are contributing to changing our understanding of the extreme Universe.Being a high sensitivity gamma-ray detector, the LAT is by its nature also a powerful electron detector and has in fact delivered the first high precision measurement of the primary cosmic-ray electron spectrum between 20 GeV and 1 TeV, based on six months of data. I will present this result and discuss the implications for dark matter scenarios; possible signatures detectable by Fermi (in both electrons and gammas) that might be helpful to disentangle different models and preliminary results on selected DM searches based on the first 3 months of data will be briefly discussed

The balloon-borne Advanced Thin Ionization Calorimeter (ATIC) experiment has measured the cosmic-ray electron spectrum over the energy range from 20 GeV to 3 TeV. The totally active Bismuth Germanate (BGO) calorimeter provides energy measurements with resolution of ~2%. The finely segmented Silicon matrix provides charge measurements with an excellent resolution of ~0.2 e. Below 100 GeV, the ATIC spectrum agrees with previous data and with a calculated spectrum based on a conventional galactic propagation model. Above ~100 GeV the results depart from the calculated spectrum and show an excess electron flux up to about 650 GeV, above which the spectrum drops rapidly. The source of this electron surplus would need to be a previously unidentified and relatively nearby cosmic object within ~1 kilo parsec of the Sun. It could be an astrophysical source, but it might also result from annihilation of dark matter particles. The measurement technique, and the implication of the results will be discussed.

New results on the antiproton-to-proton and positron-to-all electron ratios over a wide energy range (1 – 100 GeV) have been obtained by the PAMELA mission. These data are mainly interpreted in terms of dark matter annihilation or pulsar contribution. The instrument PAMELA, in orbit since June 15th, 2006 on board the Russian satellite Resurs DK1, is daily delivering to ground 16 Gigabytes of data. The apparatus is designed to study charged particles in the cosmic radiation, with a particular focus on antiparticles for searching antimatter and signals of dark matter annihilation. A combination of a magnetic spectrometer and different detectors allows antiparticles to be reliably identified from a large background of other charged particles. This talk reviews the design of the apparatus and illustrates the most recent scientific results obtained by PAMELA, together to some of the recent theoretical interpretations. In particular new data on antiprotons, protons, positrons, electrons absolute fluxes will be presented.

I will discuss the contribution to black hole thermodynamics from a variation in the cosmological constant. The description of black hole with a cosmological constant is facilitated by introducing a two-form potential for the static Killing field. The resulting Smarr formula then includes a term proportional to the cosmological constant times an effective volume, which arises as the difference between the Killing potential on the horizon and the boundary at infinity. This volume is shown to be equal to the difference between the (infinite) volume of AdS and the (infinite) volume outside the black hole horizon of AdS containing a black hole--and so can be interpreted as the volume occupied by the black hole. I will outline the derivation for the first law for AdS black holes including a variation in the cosmological constant. This yields a new work term, the change in the cosmological constant times the effective volume. Hence this is analogous to a "volume times change in pressure" work term in classical thermodynamics. This suggests that the usual change in mass term is better interpreted as a change in the enthalpy, the mass plus the pressure times the volume. In the AdS/CFT correspondence a change in the cosmological constant corresponds to a change in the t'Hooft coupling, for example, a change in the number of degrees of freedom. The effective volume multiplier then looks like a chemical potential. Members of the audience will be asked to contribute their own interpretations at this point.

We suggest here a mechanism for the seeding of the primordial density fluctuations. We point out that a process like reheating at the end of inflation will inevitably generate perturbations, even on superhorizon scales, by the local diffusion of energy. Provided that the final temperature is of order the GUT scale, the density contrast $\delta_R$ for spheres of radius $R$ will be of order $10^{-5}$ at horizon entry, consistent with the values measured by \texttt{WMAP}. If this were a purely classical process, $\delta_R^2$ would fall as $1/R^4$ beyond the horizon, and the resulting primordial density power spectrum would be $P(k) \propto k^n$ with $n=4$. However, as shown by Gabrielli et al, a quantum diffusion process can generate a power spectrum with any index in the range $0<N\LEQ $n="1$" for R^4$ $1 and $n<1$ R^{3+n}$ 1 $\propto be then will ($\delta_R^2$ observed the to close values including 4$,>1$). Thus, the two characteristic parameters that determine the appearance of present day structures could be natural consequences of this mechanism. These are in any case the minimum density variations that must have formed if the universe was rapidly heated to GUT temperatures by the decay of a `false vacuum'. There is then no \emph{a priori} necessity to postulate additional (and fine tuned) quantum fluctuations in the `false vacuum', nor a pre-inflationary period. Given also the very stringent pre-conditions required to trigger a satisfactory period of inflation, altogether it seems at least as natural to assume that the universe began in a flat and homogeneously expanding phase.

The massless fields of closed string theory on toroidal backgrounds naturally depend on coordinates dual to momentum and coordinates dual to winding. Their dynamical theory, which contains gravitation, must include diffeomorphism and dual diffeomorphism invariance. We begin a serious attempt to construct this generalized form of field theory.

The black body nature of the first acoustic peak of the cosmic microwave background (CMB) was tested using foreground reduced WMAP 5-year data, by producing subtraction maps between pairs of cosmological bands, viz. the Q, V, and W bands, for masked sky areas that avoid the Galactic disk. The resulting maps revealed a non black body signal that has three main properties. (a) It fluctuates on the degree scale preferentially in one half of the sky, producing an extra {\it random} noise there of amplitude $\approx$ 3.5 $\mu$K, which is $\gtrsim$ 10 $\sigma$ above the pixel noise even after beam size differences between bands are taken into account. (b) The signal exhibits large scale asymmetry in the form of a dipole ($\approx$ 3 $\mu$K) in the Q-V and Q-W maps; and (c) a quadrupole ($\approx$ 1.5 $\mu$K) in the Q-V, Q-W, and V-W maps. While (b) is due most probably to cross-band calibration residuals of the CMB COBE dipole, the amplitude of (c) is well beyond systematics of the kind, and in any case no {\it a priori} quadrupole in the CMB exists to leave behind such a residual. The axes of symmetry of (a), (b), and (c) are tilted towards the same general direction of the ecliptic plane. This strongly suggests that foreground emission contaminates the CMB signatures at the 4 -- 5 % level even on the angular scale of the first acoustic peak.