42424969
Cosmological Parameter Uncertainties from SALT-II Type Ia Supernova Light Curve Models
We use simulated type Ia supernova (SN Ia) samples, including both photometry and spectra, to perform the first direct validation of cosmology analysis using the SALT-II light curve model. This validation includes residuals from the light curve training process, systematic biases in SN Ia distance measurements, and a bias on the dark energy equation of state parameter w. Using the SN-analysis package SNANA, we simulate and analyze realistic samples corresponding to the data samples used in the SNLS3 analysis: ∼120 low-redshift (z < 0.1) SNe Ia, ∼255 Sloan Digital Sky Survey SNe Ia (z < 0.4), and ∼290 SNLS SNe Ia (z ≤ 1). To probe systematic uncertainties in detail, we vary the input spectral model, the model of intrinsic scatter, and the smoothing (i.e., regularization) parameters used during the SALT-II model training. Using realistic intrinsic scatter models results in a slight bias in the ultraviolet portion of the trained SALT-II model, and w biases (w {sub input} – w {sub recovered}) ranging from –0.005 ± 0.012 to –0.024 ± 0.010. These biases are indistinguishable from each other within the uncertainty; the average bias on w is –0.014 ± 0.007.
18389855
Can luminosity distance measurements probe the equation of state of dark energy
Abstract Distance measurements to type Ia supernovae (SNe Ia) at cosmological distances indicate that the Universe is accelerating and that a large fraction of the critical energy density exists in a component with negative pressure. Various hypotheses on the nature of this “dark energy” can be tested via their prediction for the equation of state of this component. If the dark energy is due to a scalar field, its equation of state will in general vary with time and is related to the potential of the field. We review the intrinsic degeneracies of luminosity distance measurements and compute the expected accuracies that can be obtained for the equation of state parameter from a realistic high statistic SNe Ia experiment.
119301091
Observational evidence of the accelerated expansion of the universe
Abstract The discovery of cosmic acceleration is one of the most important developments in modern cosmology. The observation, thirteen years ago, that type Ia supernovae appear dimmer that they would have been in a decelerating universe followed by a series of independent observations involving galaxies and cluster of galaxies as well as the cosmic microwave background, all point in the same direction: we seem to be living in a flat universe whose expansion is currently undergoing an acceleration phase. In this article, we review the various observational evidences, most of them gathered in the last decade, and the improvements expected from projects currently collecting data or in preparation.
53466158
Experimental constraints on the uncoupled Galileon model from SNLS3 data and other cosmological probes
Aims. The Galileon model is a modified gravity theory that may provide
 an explanation for the accelerated expansion of the Universe. This model does not suffer
 from instabilities or ghost problems (normally associated with higher-order derivative
 theories), restores local General Relativity – thanks to the Vainshtein screening effect –
 and predicts late-time acceleration of the expansion.Methods. We derive a new definition of the Galileon parameters that
 allows us to avoid having to choose initial conditions for the Galileon field. We tested
 this model against precise measurements of the cosmological distances and the rate of
 growth of cosmic structures.Results. We observe a weak tension between the constraints set by growth
 data and those from distances. However, we find that the Galileon model remains consistent
 with current observations and is still competitive with the ΛCDM model, contrary to what
 was concluded in recent publications.
121654518
Cosmology and dark energy
Global cosmological parameters are now known to an accuracy totally unexpected ten years ago, and considerable improvements are expected in the next decade both from space missions and ground-based facilities. Although there were some early indications, convincing evidences of the existence of dark energy appeared in the late 90's. This definitely makes the measurement of cosmological parameters more complicated, because the nature of this component of the universe is totally unknown, and present data only constraints the simplest scenarios. We will review here how dark energy existence was established and confirmed, and the current observational constraints we can place. We will also describe the observational approaches expected to provide new cosmological constrains over approximately the next decade.
119269586
Improved photometric calibration of the SNLS and the SDSS supernova surveys
Context. We present a combined photometric calibration of the Supernova Legacy Survey (SNLS) and the SDSS supernova survey, which results from a joint effort of the SDSS and the SNLS collaborations. Aims: Our primary motivation is to eventually sharpen cosmological constraints derived from type Ia supernova measurements by improving the accuracy of the photometric calibration. We deliver fluxes calibrated to the HST spectrophotometric star network for large sets of tertiary stars that cover the science fields of both surveys in all photometric bands. We also cross-calibrate directly the two surveys and demonstrate their consistency. Methods: For each survey the flat-fielding is revised based on the analysis of dithered star observations. The calibration transfer from the HST spectrophotometric standard stars to the multi-epoch tertiary standard star catalogs in the science fields follows three different paths: observations of primary standard stars with the SDSS PT telescope; observations of Landolt secondary standard stars with SNLS MegaCam instrument at CFHT; and direct observation of faint HST standard stars with MegaCam. In addition, the tertiary stars for the two surveys are cross-calibrated using dedicated MegaCam observations of stripe 82. This overlap enables the comparison of these three calibration paths and justifies using their combination to improve the calibration accuracy. Results: Flat-field corrections have improved the uniformity of each survey as demonstrated by the comparison of photometry in overlapping fields: the rms of the difference between the two surveys is 3 mmag in gri, 4 mmag in z and 8 mmag in u. We also find a remarkable agreement (better than 1%) between the SDSS and the SNLS calibration in griz. The cross-calibration and the introduction of direct calibration observations bring redundancy and strengthen the confidence in the resulting calibration. We conclude that the surveys are calibrated to the HST with a precision of about 0.4% in griz. This precision is comparable to the external uncertainty affecting the color of the HST primary standard stars. Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products available at the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.Photometric catalogs for stars in the CFTHLS deep fields and the SDSS stripe 82, and measurements of the MegaPrime transmission curves are available in electronic form at the CDS via anonymous ftp to ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/cgi-bin/qcat?J/A+A/552/A124
2764261
The Drift Chambers Of The Nomad Experiment
Abstract We present a detailed description of the drift chambers used as an active target and a tracking device in the NOMAD experiment at CERN. The main characteristics of these chambers are a large area (3×3 m 2 ) , a self-supporting structure made of light composite materials and a low cost. A spatial resolution of 150 μm has been achieved with a single hit efficiency of 97%. 29.40.Cs; 29.40.Gx; 07.05.Kf; 13.15.+g Drift chambers; Spatial resolution; Tracking; Neutrino oscillations
6507259
Kalman filter track fits and track breakpoint analysis
We give an overview of track fitting using the Kalman filter method in the NOMAD detector at CERN, and emphasize how the wealth of by-product information can be used to analyze track breakpoints (discontinuities in track parameters caused by scattering, decay, etc.). After reviewing how this information has been previously exploited by others, we describe extensions which add power to breakpoint detection and characterization. We show how complete fits to the entire track, with breakpoint parameters added, can be easily obtained from the information from unbroken fits. Tests inspired by the Fisher F-test can then be used to judge breakpoints. Signed quantities (such as change in momentum at the breakpoint) can supplement unsigned quantities such as the various chisquares. We illustrate the method with electrons from real data, and with Monte Carlo simulations of pion decays.