arXiv subjects

S. Dixon

Publications and source records attributed to S. Dixon.

32 records · Page 2Linked to original sources

arXiv2018

Correcting for peculiar velocities of Type Ia Supernovae in clusters of galaxies

Type Ia Supernovae (SNe Ia) are widely used to measure the expansion of the Universe. To perform such measurements the luminosity and cosmological redshift ($z$) of the SNe Ia have to be determined. The uncertainty on $z$ includes an unknown peculiar velocity, which can be very large for SNe Ia in the virialized cores of massive clusters. We determine which SNe Ia exploded in galaxy clusters. We then study how the correction for peculiar velocities of host galaxies inside the clusters improves the Hubble residuals. Using 145 SNe Ia from the Nearby Supernova Factory we found 11 candidates for membership in clusters. To estimate the redshift of a cluster we applied the bi-weight technique. Then, we use the galaxy cluster redshift instead of the host galaxy redshift to construct the Hubble diagram. For SNe Ia inside galaxy clusters the dispersion around the Hubble diagram when peculiar velocities are taken into account is smaller in comparison with a case without peculiar velocity correction, with a $wRMS=0.130\pm0.038$ mag instead of $wRMS=0.137\pm0.036$ mag. The significance of this improvement is 3.58 $σ$. If we remove the very nearby Virgo cluster member SN2006X ($z<0.01$) from the analysis, the significance decreases to 1.34 $σ$. The peculiar velocity correction is found to be highest for the SNe Ia hosted by blue spiral galaxies, with high local specific star formation rate and smaller stellar mass, seemingly counter to what might be expected given the heavy concentration of old, massive elliptical galaxies in clusters. As expected, the Hubble residuals of SNe Ia associated with massive galaxy clusters improve when the cluster redshift is taken as the cosmological redshift of the SN. This fact has to be taken into account in future cosmological analyses in order to achieve higher accuracy for cosmological redshift measurements. Here we provide an approach to do so.

astro-ph.CO↗

arXiv2018

A Binary Offset Effect in CCD Readout and Its Impact on Astronomical Data

We have discovered an anomalous behavior of CCD readout electronics that affects their use in many astronomical applications. An offset in the digitization of the CCD output voltage that depends on the binary encoding of one pixel is added to pixels that are read out one, two and/or three pixels later. One result of this effect is the introduction of a differential offset in the background when comparing regions with and without flux from science targets. Conventional data reduction methods do not correct for this offset. We find this effect in 16 of 22 instruments investigated, covering a variety of telescopes and many different front-end electronics systems. The affected instruments include LRIS and DEIMOS on the Keck telescopes, WFC3-UVIS and STIS on HST, MegaCam on CFHT, SNIFS on the UH88 telescope, GMOS on the Gemini telescopes, HSC on Subaru, and FORS on VLT. The amplitude of the introduced offset is up to 4.5 ADU per pixel, and it is not directly proportional to the measured ADU level. We have developed a model that can be used to detect this "binary offset effect" in data and correct for it. Understanding how data are affected and applying a correction for the effect is essential for precise astronomical measurements.

astro-ph.IM↗

arXiv2018

Understanding Type Ia supernovae through their U-band spectra

Context. Observations of Type Ia supernovae (SNe Ia) can be used to derive accurate cosmological distances through empirical standardization techniques. Despite this success neither the progenitors of SNe Ia nor the explosion process are fully understood. The U-band region has been less well observed for nearby SNe, due to technical challenges, but is the most readily accessible band for high-redshift SNe. Aims. Using spectrophotometry from the Nearby Supernova Factory, we study the origin and extent of U-band spectroscopic variations in SNe Ia and explore consequences for their standardization and the potential for providing new insights into the explosion process. Methods. We divide the U-band spectrum into four wavelength regions λ(uNi), λ(uTi), λ(uSi) and λ(uCa). Two of these span the Ca H&K λλ 3934, 3969 complex. We employ spectral synthesis using SYNAPPS to associate the two bluer regions with Ni/Co and Ti. Results. (1) The flux of the uTi feature is an extremely sensitive temperature/luminosity indicator, standardizing the SN peak luminosity to 0.116 $\pm$ 0.011 mag RMS. A traditional SALT2.4 fit on the same sample yields a 0.135 mag RMS. Standardization using uTi also reduces the difference in corrected magnitude between SNe originating from different host galaxy environments. (2) Early U-band spectra can be used to probe the Ni+Co distribution in the ejecta, thus offering a rare window into the source of lightcurve power. (3) The uCa flux further improves standardization, yielding a 0.086 $\pm$ 0.010 mag RMS without the need to include an additional intrinsic dispersion to reach χ$^2$/dof $\sim$ 1. This reduction in RMS is partially driven by an improved standardization of Shallow Silicon and 91T-like SNe.

astro-ph.HE↗

arXiv2017

SCALA: In-situ calibration for Integral Field Spectrographs

The scientific yield of current and future optical surveys is increasingly limited by systematic uncertainties in the flux calibration. This is the case for Type Ia supernova (SN Ia) cosmology programs, where an improved calibration directly translates into improved cosmological constraints. Current methodology rests on models of stars. Here we aim to obtain flux calibration that is traceable to state-of-the-art detector-based calibration. We present the SNIFS Calibration Apparatus (SCALA), a color (relative) flux calibration system developed for the SuperNova Integral Field Spectrograph (SNIFS), operating at the University of Hawaii 2.2 m (UH 88) telescope. By comparing the color trend of the illumination generated by SCALA during two commissioning runs, and to previous laboratory measurements, we show that we can determine the light emitted by SCALA with a long-term repeatability better than 1%. We describe the calibration procedure necessary to control for system aging. We present measurements of the SNIFS throughput as estimated by SCALA observations. The SCALA calibration unit is now fully deployed at the UH\,88 telescope, and with it color-calibration between 4000 Å and 9000 Å is stable at the percent level over a one-year baseline.

astro-ph.IM↗

arXiv2016

HiPERCAM: A high-speed, quintuple-beam CCD camera for the study of rapid variability in the Universe

HiPERCAM is a high-speed camera for the study of rapid variability in the Universe. The project is funded by a 3.5MEuro European Research Council Advanced Grant. HiPERCAM builds on the success of our previous instrument, ULTRACAM, with very significant improvements in performance thanks to the use of the latest technologies. HiPERCAM will use 4 dichroic beamsplitters to image simultaneously in 5 optical channels covering the u'g'r'i'z' bands. Frame rates of over 1000 per second will be achievable using an ESO CCD controller (NGC), with every frame GPS timestamped. The detectors are custom-made, frame-transfer CCDs from e2v, with 4 low-noise (2.5e-) outputs, mounted in small thermoelectrically-cooled heads operated at 180 K, resulting in virtually no dark current. The two reddest CCDs will be deep-depletion devices with anti-etaloning, providing high quantum efficiencies across the red part of the spectrum with no fringing. The instrument will also incorporate scintillation noise correction via the conjugate-plane photometry technique. The opto-mechanical chassis will make use of additive manufacturing techniques in metal to make a light-weight, rigid and temperature-invariant structure. First light is expected on the 4.2m William Herschel Telescope on La Palma in 2017 (on which the field of view will be 10' with a 0.3"/pixel scale), with subsequent use planned on the 10.4m Gran Telescopio Canarias on La Palma (on which the field of view will be 4' with a 0.11"/pixel scale) and the 3.5m New Technology Telescope in Chile.

astro-ph.IM↗

arXiv2016

First measurement of electron neutrino appearance in NOvA

We report results from the first search for $ν_μ\toν_e$ transitions by the NOvA experiment. In an exposure equivalent to $2.74\times10^{20}$ protons-on-target in the upgraded NuMI beam at Fermilab, we observe 6 events in the Far Detector, compared to a background expectation of $0.99\pm0.11$ (syst.) events based on the Near Detector measurement. A secondary analysis observes 11 events with a background of $1.07\pm0.14$ (syst.). The $3.3σ$ excess of events observed in the primary analysis disfavors $0.1π< δ_{CP} < 0.5π$ in the inverted mass hierarchy at the 90% C.L.

hep-ex↗

arXiv2016

First measurement of muon-neutrino disappearance in NOvA

This paper reports the first measurement using the NOvA detectors of $ν_μ$ disappearance in a $ν_μ$ beam. The analysis uses a 14 kton-equivalent exposure of $2.74 \times 10^{20}$ protons-on-target from the Fermilab NuMI beam. Assuming the normal neutrino mass hierarchy, we measure $Δm^{2}_{32}=(2.52^{+0.20}_{-0.18})\times 10^{-3}$ eV$^{2}$ and $\sin^2θ_{23}$ in the range 0.38-0.65, both at the 68% confidence level, with two statistically-degenerate best fit points at $\sin^2θ_{23} = $ 0.43 and 0.60. Results for the inverted mass hierarchy are also presented.

hep-ex↗

arXiv2015

Status of the DAMIC direct dark matter search experiment

The DAMIC experiment uses fully depleted, high resistivity CCDs to search for dark matter particles. With an energy threshold $\sim$50 eV$_{ee}$, and excellent energy and spatial resolutions, the DAMIC CCDs are well-suited to identify and suppress radioactive backgrounds, having an unrivaled sensitivity to WIMPs with masses $<$6 GeV/$c^2$. Early results motivated the construction of a 100 g detector, DAMIC100, currently being installed at SNOLAB. This contribution discusses the installation progress, new calibration efforts near the threshold, a preliminary result with 2014 data, and the prospects for physics results after one year of data taking.

physics.ins-det↗

arXiv2015

The DAMIC dark matter experiment

The DAMIC (Dark Matter in CCDs) experiment uses high resistivity, scientific grade CCDs to search for dark matter. The CCD's low electronic noise allows an unprecedently low energy threshold of a few tens of eV that make it possible to detect silicon recoils resulting from interactions of low mass WIMPs. In addition the CCD's high spatial resolution and the excellent energy response results in very effective background identification techniques. The experiment has a unique sensitivity to dark matter particles with masses below 10 GeV/c$^2$. Previous results have demonstrated the potential of this technology, motivating the construction of DAMIC100, a 100 grams silicon target detector currently being installed at SNOLAB. In this contribution, the mode of operation and unique imaging capabilities of the CCDs, and how they may be exploited to characterize and suppress backgrounds will be discussed, as well as physics results after one year of data taking.

physics.ins-det↗

arXiv2015

Measurement of radioactive contamination in the high-resistivity silicon CCDs of the DAMIC experiment

We present measurements of radioactive contamination in the high-resistivity silicon charge-coupled devices (CCDs) used by the DAMIC experiment to search for dark matter particles. Novel analysis methods, which exploit the unique spatial resolution of CCDs, were developed to identify $α$ and $β$ particles. Uranium and thorium contamination in the CCD bulk was measured through $α$ spectroscopy, with an upper limit on the $^{238}$U ($^{232}$Th) decay rate of 5 (15) kg$^{-1}$ d$^{-1}$ at 95% CL. We also searched for pairs of spatially correlated electron tracks separated in time by up to tens of days, as expected from $^{32}$Si-$^{32}$P or $^{210}$Pb-$^{210}$Bi sequences of $β$ decays. The decay rate of $^{32}$Si was found to be $80^{+110}_{-65}$ kg$^{-1}$ d$^{-1}$ (95% CI). An upper limit of $\sim$35 kg$^{-1}$ d$^{-1}$ (95% CL) on the $^{210}$Pb decay rate was obtained independently by $α$ spectroscopy and the $β$ decay sequence search. These levels of radioactive contamination are sufficiently low for the successful operation of CCDs in the forthcoming 100 g DAMIC detector.

astro-ph.IM↗

arXiv2015

Mu2e Technical Design Report

The Mu2e experiment at Fermilab will search for charged lepton flavor violation via the coherent conversion process mu- N --> e- N with a sensitivity approximately four orders of magnitude better than the current world's best limits for this process. The experiment's sensitivity offers discovery potential over a wide array of new physics models and probes mass scales well beyond the reach of the LHC. We describe herein the preliminary design of the proposed Mu2e experiment. This document was created in partial fulfillment of the requirements necessary to obtain DOE CD-2 approval.

physics.ins-det↗

arXiv2015

A Proposal for a Three Detector Short-Baseline Neutrino Oscillation Program in the Fermilab Booster Neutrino Beam

A Short-Baseline Neutrino (SBN) physics program of three LAr-TPC detectors located along the Booster Neutrino Beam (BNB) at Fermilab is presented. This new SBN Program will deliver a rich and compelling physics opportunity, including the ability to resolve a class of experimental anomalies in neutrino physics and to perform the most sensitive search to date for sterile neutrinos at the eV mass-scale through both appearance and disappearance oscillation channels. Using data sets of 6.6e20 protons on target (P.O.T.) in the LAr1-ND and ICARUS T600 detectors plus 13.2e20 P.O.T. in the MicroBooNE detector, we estimate that a search for muon neutrino to electron neutrino appearance can be performed with ~5 sigma sensitivity for the LSND allowed (99% C.L.) parameter region. In this proposal for the SBN Program, we describe the physics analysis, the conceptual design of the LAr1-ND detector, the design and refurbishment of the T600 detector, the necessary infrastructure required to execute the program, and a possible reconfiguration of the BNB target and horn system to improve its performance for oscillation searches.

physics.ins-det↗

arXiv2013

nuSTORM Project Definition Report

The objective of this Project Definition Report (PDR) is to document a possible solution for the conventional facilities required to house the nuSTORM experiment on the Fermilab site. It is recognized that this effort is done in support of obtaining DOE approval for Critical Decision Zero.

physics.ins-det↗

arXiv2012

Mu2e Conceptual Design Report

Mu2e at Fermilab will search for charged lepton flavor violation via the coherent conversion process mu- N --> e- N with a sensitivity approximately four orders of magnitude better than the current world's best limits for this process. The experiment's sensitivity offers discovery potential over a wide array of new physics models and probes mass scales well beyond the reach of the LHC. We describe herein the conceptual design of the proposed Mu2e experiment. This document was created in partial fulfillment of the requirements necessary to obtain DOE CD-1 approval, which was granted July 11, 2012.

physics.ins-det↗