Optimization of multivariate analysis for IACT stereoscopic systems

Multivariate methods have been recently introduced and successfully applied for the discrimination of signal from background in the selection of genuine very-high energy gamma-ray events with the H.E.S.S. Imaging Atmospheric Cerenkov Telescope. The complementary performance of three independent reconstruction methods developed for the H.E.S.S. data analysis, namely Hillas, model and 3D-model suggests the optimization of their combination through the application of a resulting efficient multivariate estimator. In this work the boosted decision tree method is proposed leading to a significant increase in the signal over background ratio compared to the standard approaches. The improved sensitivity is also demonstrated through a comparative analysis of a set of benchmark astrophysical sources.Comment: 10 pages, 8 figures, 3 tables, accepted for publication in Astroparticle Physic

El modelado de los valles longitudinales de San Juan

Fil: Barrera, Rosier Omar. Universidad Nacional de Cuyo. Facultad de Filosofía y LetrasFil: Panocchia, Sergio A.. Universidad Nacional de Cuyo. Facultad de Filosofía y Letra

Improved sensitivity of H.E.S.S.-II through the fifth telescope focus system

The Imaging Atmospheric Cherenkov Telescope (IACT) works by imaging the very short flash of Cherenkov radiation generated by the cascade of relativistic charged particles produced when a TeV gamma ray strikes the atmosphere. This energetic air shower is initiated at an altitude of 10-30 km depending on the energy and the arrival direction of the primary gamma ray. Whether the best image of the shower is obtained by focusing the telescope at infinity and measuring the Cherenkov photon angles or focusing on the central region of the shower is a not obvious question. This is particularly true for large size IACT for which the depth of the field is much smaller. We address this issue in particular with the fifth telescope (CT5) of the High Energy Stereoscopic System (H.E.S.S.); a 28 m dish large size telescope recently entered in operation and sensitive to an energy threshold of tens of GeVs. CT5 is equipped with a focus system, its working principle and the expected effect of focusing depth on the telescope sensitivity at low energies (50-200 GeV) is discussed.Comment: In Proceedings of the 33rd International Cosmic Ray Conference (ICRC2013), Rio de Janeiro (Brazil

A new look at the cosmic ray positron fraction

The positron fraction in cosmic rays was found to be a steadily increasing in function of energy, above $\sim$ 10 GeV. This behaviour contradicts standard astrophysical mechanisms, in which positrons are secondary particles, produced in the interactions of primary cosmic rays during the propagation in the interstellar medium. The observed anomaly in the positron fraction triggered a lot of excitement, as it could be interpreted as an indirect signature of the presence of dark matter species in the Galaxy. Alternatively, it could be produced by nearby astrophysical sources, such as pulsars. Both hypotheses are probed in this work in light of the latest AMS-02 positron fraction measurements. The transport of the primary and secondary positrons in the Galaxy is described using a semi-analytic two-zone model. MicrOMEGAs is used to model the positron flux generated by dark matter species. The description of the positron fraction from astrophysical sources is based on the pulsar observations included in the ATNF catalogue. We find that the mass of the favoured dark matter candidates is always larger than 500 GeV. The only dark matter species that fulfils the numerous gamma ray and cosmic microwave background bounds is a particle annihilating into four leptons through a light scalar or vector mediator, with a mixture of tau (75%) and electron (25%) channels, and a mass between 0.5 and 1 TeV. The positron anomaly can also be explained by a single astrophysical source and a list of five pulsars from the ATNF catalogue is given. Those results are obtained with the cosmic ray transport parameters that best fit the B/C ratio. Uncertainties in the propagation parameters turn out to be very significant. In the WIMP annihilation cross section to mass plane for instance, they overshadow the error contours derived from the positron data.Comment: 20 pages, 16 figures, accepted for publication in A&A, corresponds to published versio

Continuous Uniform Finite Time Stabilization of Planar Controllable Systems

Continuous homogeneous controllers are utilized in a full state feedback setting for the uniform finite time stabilization of a perturbed double integrator in the presence of uniformly decaying piecewise continuous disturbances. Semiglobal strong $\mathcal{C}^1$ Lyapunov functions are identified to establish uniform asymptotic stability of the closed-loop planar system. Uniform finite time stability is then proved by extending the homogeneity principle of discontinuous systems to the continuous case with uniformly decaying piecewise continuous nonhomogeneous disturbances. A finite upper bound on the settling time is also computed. The results extend the existing literature on homogeneity and finite time stability by both presenting uniform finite time stabilization and dealing with a broader class of nonhomogeneous disturbances for planar controllable systems while also proposing a new class of homogeneous continuous controllers

A Fixed-Target ExpeRiment at the LHC (AFTER@LHC) : luminosities, target polarisation and a selection of physics studies

We report on a future multi-purpose fixed-target experiment with the proton or lead ion LHC beams extracted by a bent crystal. The multi-TeV LHC beams allow for the most energetic fixed-target experiments ever performed. Such an experiment, tentatively named AFTER for "A Fixed-Target ExperRiment", gives access to new domains of particle and nuclear physics complementing that of collider experiments, in particular at RHIC and at the EIC projects. The instantaneous luminosity at AFTER using typical targets surpasses that of RHIC by more than 3 orders of magnitude. Beam extraction by a bent crystal offers an ideal way to obtain a clean and very collimated high-energy beam, without decreasing the performance of the LHC. The fixed-target mode also has the advantage of allowing for spin measurements with a polarised target and for an access over the full backward rapidity domain up to xF ~ - 1. Here, we elaborate on the reachable luminosities, the target polarisation and a selection of measurements with hydrogen and deuterium targets.Comment: 6 pages. Proceedings of the Sixth International Conference on Quarks and Nuclear Physics QNP2012 (16-20 April 2012, Ecole Polytechnique, Palaiseau,France

Modeling Antarctic tides in response to ice shelf thinning and retreat

Tides play an important role in ice sheet dynamics by modulating ice stream velocity, fracturing, and moving ice shelves and mixing water beneath them. Any changes in ice shelf extent or thickness will alter the tidal dynamics through modification of water column thickness and coastal topography but these will in turn feed back onto the overall ice shelf stability. Here, we show that removal or reduction in extent and/or thickness of the Ross and Ronne-Filchner ice shelves would have a significant impact on the tides around Antarctica. The Ronne-Filchner appears particularly vulnerable, with an increase in M2 amplitude of over 0.5 m beneath much of the ice shelf potentially leading to tidally induced feedbacks on ice shelf/sheet dynamics. These results highlight the importance of understanding tidal feedbacks on ice shelves/streams due to their influence on ice sheet dynamics