A robust upper limit on N_eff from BBN, circa 2011

We derive here a robust bound on the effective number of neutrinos from constraints on primordial nucleosynthesis yields of deuterium and helium. In particular, our results are based on very weak assumptions on the astrophysical determination of the helium abundance, namely that the minimum effect of stellar processing is to keep constant (rather than increase, as expected) the helium content of a low-metallicity gas. Using the results of a recent analysis of extragalactic HII regions as upper limit, we find that Delta Neff<= 1 at 95 % C.L., quite independently of measurements on the baryon density from cosmic microwave background anisotropy data and of the neutron lifetime input. In our approach, we also find that primordial nucleosynthesis alone has no significant preference for an effective number of neutrinos larger than the standard value. The ~2 sigma hint sometimes reported in the literature is thus driven by CMB data alone and/or is the result of a questionable regression protocol to infer a measurement of primordial helium abundance.Comment: 5 pages, 1 table, 1 figure. Minor improvements and extensions in the analysis, clarifications and reference added, conclusions slightly strengthened. Matches version published in Phys. Lett.

Effects of non-standard neutrino-electron interactions on relic neutrino decoupling

We consider the decoupling of neutrinos in the early Universe in presence of non-standard neutral current neutrino-electron interactions (NSI). We first discuss a semi-analytical approach to solve the relevant kinetic equations and then present the results of fully numerical and momentum-dependent calculations, including flavor neutrino oscillations. We present our results in terms of both the effective number of neutrino species (N_eff) and the impact on the abundance of He-4 produced during Big Bang Nucleosynthesis. We find that, for NSI parameters within the ranges allowed by present laboratory data, non-standard neutrino-electron interactions do not essentially modify the density of relic neutrinos nor the bounds on neutrino properties from cosmological observables, such as their mass. Nonetheless, the presence of neutrino-electron NSI may enhance the entropy transfer from electron-positron pairs into neutrinos instead of photons, up to a value of N_eff=3.12. This is almost three times the correction to N_eff=3 that appears for standard weak interactions.Comment: 23 pages, 5 figures. To be published in NP

Generalized Gluon Currents and Applications in QCD

We consider the process containing two quark lines and an arbitrary number of gluons in a spinor helicity framework. A current with two off-shell gluons appears in the amplitude. We first study this modified gluon current using recursion relations. The recursion relation for the modified gluon current is solved for the case of like-helicity gluons. We apply the modified gluon current to compute the amplitude for $q \bar q \rightarrow q \bar q gg \cdots g$ in the like-helicity gluon case.Comment: 80 pages, 2 figures (appended in pictex), CLNS 91/112

Minimal Gauge Invariant Classes of Tree Diagrams in Gauge Theories

We describe the explicit construction of groves, the smallest gauge invariant classes of tree Feynman diagrams in gauge theories. The construction is valid for gauge theories with any number of group factors which may be mixed. It requires no summation over a complete gauge group multiplet of external matter fields. The method is therefore suitable for defining gauge invariant classes of Feynman diagrams for processes with many observed final state particles in the standard model and its extensions.Comment: 13 pages, RevTeX (EPS figures

LOMO sapiens: de la photographie

Une des questions les plus fr\ue9quentes, dans les recherches sur la photographie, est li\ue9e \ue0 la valeur de m\ue9diation de l\u2019image et donc au rapport qu\u2019elle entretient avec la soi-disant r\ue9alit\ue9. D\u2019un c\uf4t\ue9, la certitude de la pr\ue9sence devant l\u2019objectif de ce que nous voyons repr\ue9sent\ue9, une copie du monde excluant la n\ue9cessit\ue9 d\u2019un code pour \ueatre comprise, de l\u2019autre, l\u2019imposition d\u2019un point de vue situ\ue9 dans le temps et l\u2019espace qui, par ces choix oblig\ue9s, porte en soi le germe d\u2019une strat\ue9gie de la communication. La s\ue9miotique devient ainsi l\u2019instrument pour analyser ces strat\ue9gies, pour rechercher des signifi\ue9s plus ou moins cach\ue9s, en d\ue9montant l\u2019\ue9vidence suppos\ue9e de l\u2019image photographique. De tr\ue8s nombreuses \ue9tudes et analyses ont eu pour objectif d\u2019enqu\ueater sur ces questions, en se concentrant sur le produit de l\u2019acte photographique : le texte-image. La perspective inverse, comme le rappelait Barthes1, a \ue9galement \ue9t\ue9 poursuivie, surtout par les historiens, qui se sont concentr\ue9s sur la figure du producteur, c\u2019est-\ue0-dire du photographe. On a peu \ue9crit en revanche sur la production, et en particulier sur l\u2019objet qui la rend possible, l\u2019appareil \ue0 photo. Les processus s\ue9mantiques explicitement impliqu\ue9s dans l\u2019utilisation de l\u2019appareil semblent n\u2019\ueatre en rapport qu\u2019avec son fonctionnement, la fa\ue7on dont l\u2019appareil communique avec l\u2019utilisateur et dont, par son biais, il doit \ueatre utilis\ue9. La s\ue9miotique comme mode d\u2019emploi, en quelque sorte : objectif noble, certes, mais non pas unique \u2014 et cela pour une bonne raison : les instruments que nous utilisons pour effectuer nos activit\ue9s les plus vari\ue9es ne sont jamais des prolongements fournissant des capacit\ue9s \ue0 un sujet sans intervenir en retour sur lui-m\ueame. Si nous sommes oblig\ue9s d\u2019agir sur un objet pour produire une image, il est logique de penser qu\u2019\ue0 cette action correspondra une r\ue9action, que la machine en quelque sorte agira sur nous. Dans le cas de la photographie, le sujet agissant n\u2019est jamais seulement humain. Une photo est le r\ue9sultat d\u2019une relation entre un homme et une machine, produisant une nouvelle subjectivit\ue9, le photographe justement, qui ne formule pas seulement des programmes d\u2019action sp\ue9cifiques, mais qui, surtout, per\ue7oit le monde de fa\ue7on diff\ue9rente

Disentangling neutrino-nucleon cross section and high energy neutrino flux with a km^3 neutrino telescope

The energy--zenith angular event distribution in a neutrino telescope provides a unique tool to determine at the same time the neutrino-nucleon cross section at extreme kinematical regions, and the high energy neutrino flux. By using a simple parametrization for fluxes and cross sections, we present a sensitivity analysis for the case of a km^3 neutrino telescope. In particular, we consider the specific case of an under-water Mediterranean telescope placed at the NEMO site, although most of our results also apply to an under-ice detector such as IceCube. We determine the sensitivity to departures from standard values of the cross sections above 1 PeV which can be probed independently from an a-priori knowledge of the normalization and energy dependence of the flux. We also stress that the capability to tag downgoing neutrino showers in the PeV range against the cosmic ray induced background of penetrating muons appears to be a crucial requirement to derive meaningful constraints on the cross section.Comment: 10 pages, 28 figure

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.Comment: 4 pages, 3 figures, Proceedings of the 30th ICRC 200

Monte Carlo studies of the jet activity in Higgs + 2 jet events

Tree-level studies have shown in the past that kinematical correlations between the two jets in Higgs+2-jet events are direct probes of the Higgs couplings, e.g. of their CP nature. In this paper we explore the impact of higher-order corrections on the azimuthal angle correlation of the two leading jets and on the rapidity distribution of extra jets. Our study includes matrix-element and shower MC effects, for the two leading sources of Higgs plus two jet events at the CERN LHC, namely vector-boson and gluon fusion. We show that the discriminating features present in the previous leading-order matrix element studies survive.Comment: 12 pages, 10 figures. Version to appear on JHEP. Figs. 5-8 replaced with colour version

Multiphoton Production at High Energies in the Standard Model I

We examine multiphoton production in the electroweak sector of the Standard Model in the high energy limit using the equivalence theorem in combination with spinor helicity techniques. We obtain recursion relations for currents consisting of a charged scalar, spinor, or vector line that radiates $n$ photons. Closed form solutions to these recursion relations for arbitrary $n$ are presented for the cases of like-helicity and one unlike-helicity photon production. We apply the currents singly and in pairs to obtain amplitudes for processes involving the production of $n$ photons with up to two unlike helicities from a pair of charged particles. The replacement of one or more photons by transversely polarized $$Z$-bosons is also discussed.Comment: 75 pages, CLNS 91/111