New gamma/hadron separation parameters for a neural network for HAWC

The High-Altitude Water Cherenkov experiment (HAWC) observatory is located 4100 meters above sea level. HAWC is able to detect secondary particles from extensive air showers (EAS) initiated in the interaction of a primary particle (either a gamma or a charged cosmic ray) with the upper atmosphere. Because an overwhelming majority of EAS events are triggered by cosmic rays, background noise suppression plays an important role in the data analysis process of the HAWC observatory. Currently, HAWC uses cuts on two parameters (whose values depend on the spatial distribution and luminosity of an event) to separate gamma-ray events from background hadronic showers. In this work, a search for additional gamma-hadron separation parameters was conducted to improve the efficiency of the HAWC background suppression technique. The best-performing parameters were integrated to a feed-foward Multilayer Perceptron Neural Network (MLP-NN), along with the traditional parameters. Various iterations of MLP-NN's were trained on Monte Carlo data, and tested on Crab data. Preliminary results show that the addition of new parameters can improve the significance of the point source at high-energies (~ TeV), at the expense of slightly worse performance in conventional low-energy bins (~ GeV). Further work is underway to improve the efficiency of the neural network at low energies.Comment: Presented at the 35th International Cosmic Ray Conference (ICRC2017), Bexco, Busan, Korea. See arXiv:1708.02572 for all HAWC contribution

Coupling of state-resolved rovibrational coarse-grain model for nitrogen to stochastic particle method for simulating internal energy excitation and dissociation

We propose to couple a state-resolved rovibrational coarse-grain model to a stochastic particle method for simulating internal energy excitation and dissociation of a molecular gas. An existing coarse-grain model based on the NASA Ames ab initio database for the N2-N system is modified using variably-spaced energy bins. Thermodynamic properties of the new coarse-grained model closely match those of the full set of rovibrational levels over a wide temperature range, using a number of bins much smaller than the complete mechanism. The chemical-kinetic behavior of the original equally -- and new variably -- spaced bin formulations is compared by simulating excitation and dissociation of N2 in an adiabatic, isochoric reactor. The variably-spaced formulation is better suited for reproducing the dynamics of the full database at conditions of interest in Earth reentry. Furthermore, we discuss details of our Direct Simulation Monte Carlo (DSMC) implementation for the coarse-grain model and describe changes to the collision algorithm necessary to accommodate our state-resolved reaction mechanism. The DSMC code is then verified against equivalent master equation (ME) calculations. In these simulations, state-resolved cross sections are used in analytical form. They verify micro-reversibility for the bins and allow for faster execution of the code. In our verification, we obtain very close agreement for the N and N2 concentrations, as well as the translational and rovibrational mode temperatures obtained independently using both methods. In addition to macroscopic moments, we compare internal energy populations predicted at selected time steps via DSMC and ME. We observe good agreement between both methods within the statistical scatter limits imposed by DSMC. In future work, the rovibrational coarse-grain model coupled to the particle method will allow us to study 3D reentry flow configurations.Comment: 40 pages, 19 figure

The white dwarf population within 40 pc of the Sun

The white dwarf luminosity function is an important tool to understand the properties of the Solar neighborhood, like its star formation history, and its age. Here we present a population synthesis study of the white dwarf population within 40~pc from the Sun, and compare the results of this study with the properties of the observed sample. We use a state-of-the-art population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable white dwarf cooling sequences, an accurate description of the Galactic neighborhood, and a realistic treatment of all the known observational biases and selection procedures. We find a good agreement between our theoretical models and the observed data. In particular, our simulations reproduce a previously unexplained feature of the bright branch of the white dwarf luminosity function, which we argue is due to a recent episode of star formation. We also derive the age of the Solar neighborhood employing the position of the observed cut-off of the white dwarf luminosity function, obtaining ~8.9+-0.2 Gyr. We conclude that a detailed description of the ensemble properties of the population of white dwarfs within 40pc of the Sun allows us to obtain interesting constraints on the history of the Solar neighborhood.Comment: 8 pages, 7 figures, accepted for publication in A&

Study of the γd→K+K−np\gamma d\to K^{+}K^{-}np reaction and an alternative explanation for the "Θ+(1540)\Theta^{+}(1540) pentaquark" peak

We present a calculation of the $\gamma d \to K^+ K^- n p$ reaction with the aim of seeing if the experimental peak observed in the $K^+ n$ invariant mass around 1526 MeV, from where evidence for the existence of the $\Theta^+$ has been claimed, can be obtained without this resonance as a consequence of the particular dynamics of the process and the cuts applied in the experimental set up. We find that a combination of facts leads indeed to a peak around 1530 MeV for the invariant mass of $K^+ n$ without the need to invoke any new resonance around this energy. This, together with statistical fluctuations that we prove to be large with the statistics of the experiment, is likely to produce the narrower peak observed there.Comment: published versio

On the time variability of gamma-ray sources: A numerical analysis of variability indices

We present a Monte Carlo analysis of the recently introduced variability indices $\tau$ (Tompkins 1999) and $I$ (Zhang et al. 2000 & Torres et al. 2001) for $\gamma$-ray sources. We explore different variability criteria and prove that these two indices, despite the very different approaches used to compute them, are statistically correlated (5 to 7$\sigma$). This conclusion is maintained also for the subset of AGNs and high latitude ($|b|>10$ deg) sources, whereas the correlation is lowered for the low latitude ones, where the influence of the diffuse galactic emission background is strong.Comment: Small changes to match published version in Astronomische Nachrichten (2001). Paper accepted in July 200

Non-paraxial Talbot effect in one-dimensional gratings with period comparable to the wavelength

The Talbot effect is a diffractive phenomenon which was found since its inception in the Gaussian optics, but when the period of the object is comparable with the wavelength is considered entering its non-paraxial regime. There, it has attracted interest for its applications related to the design of objects and their restorative effects posing in damaged gratings on this scale. In this contribution a study of this phenomenon at level non-paraxial (metaxial) is presented by using scalar diffraction theory in plane waves representation for periodic transversal fields. To this end, a one-dimensional amplitude grating of period p is used which is illuminated by a monochromatic plane wave of wavelength λ where p ranges between 1,5 λ and 4,2 λ.Fil: Arrieta, E.. Universidad Popular del Cesar; ColombiaFil: Bolognini, Nestor Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Ópticas. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones Ópticas. Universidad Nacional de La Plata. Centro de Investigaciones Ópticas; ArgentinaFil: Torres, C.O.. Universidad Popular del Cesar; Colombi