3,204 research outputs found
Absorbing systematic effects to obtain a better background model in a search for new physics
This paper presents a novel approach to estimate the Standard Model
backgrounds based on modifying Monte Carlo predictions within their systematic
uncertainties. The improved background model is obtained by altering the
original predictions with successively more complex correction functions in
signal-free control selections. Statistical tests indicate when sufficient
compatibility with data is reached. In this way, systematic effects are
absorbed into the new background model. The same correction is then applied on
the Monte Carlo prediction in the signal region. Comparing this method to other
background estimation techniques shows improvements with respect to statistical
and systematical uncertainties. The proposed method can also be applied in
other fields beyond high energy physics
A Proposal to Localize Fermi GBM GRBs Through Coordinated Scanning of the GBM Error Circle via Optical Telescopes
We investigate the feasibility of implementing a system that will coordinate
ground-based optical telescopes to cover the Fermi GBM Error Circle (EC). The
aim of the system is to localize GBM detected GRBs and facilitate
multi-wavelength follow-up from space and ground. This system will optimize the
observing locations in the GBM EC based on individual telescope location, Field
of View (FoV) and sensitivity. The proposed system will coordinate GBM EC
scanning by professional as well as amateur astronomers around the world. The
results of a Monte Carlo simulation to investigate the feasibility of the
project are presented.Comment: 2011 Fermi Symposium proceedings - eConf C11050
The D0 Run IIb Luminosity Measurement
An assessment of the recorded integrated luminosity is presented for data
collected with the D0 detector at the Fermilab Tevatron Collider from June 2006
to September 2011 (Run IIb). In addition, a measurement of the effective cross
section for inelastic interactions, also referred to as the luminosity
constant, is reported. This measurement incorporates new features that lead to
a substantial improvement in the precision of the result. A luminosity constant
of \sigma_{LM} = 48.3\pm1.9\pm0.6 mb is obtained, where the first uncertainty
is due to the accuracy of the inelastic cross section used by both CDF and D0,
and the second uncertainty is due to D0 sources. The recorded luminosity for
the highest E_T jet trigger is L_rec = 9.2 \pm 0.4 fb^{-1}, with a relative
uncertainty of 4.3%.Comment: 20 pages, 23 figure
Identification of mixed-symmetry states in an odd-mass nearly-spherical nucleus
The low-spin structure of 93Nb has been studied using the (n,n' gamma)
reaction at neutron energies ranging from 1.5 to 3.0 MeV and the 94Zr(p,2n
gamma)93Nb reaction at bombarding energies from 11.5 to 19 MeV. States at
1779.7 and 1840.6 keV, respectively, are proposed as mixed-symmetry states
associated with the coupling of a proton hole in the p_1/2 orbit to the 2+_1,ms
state in 94Mo. These assignments are derived from the observed M1 and E2
transition strengths to the symmetric one-phonon states, energy systematics,
spins and parities, and comparison with shell model calculations.Comment: 5 pages, 3 figure
Milagro Constraints on Very High Energy Emission from Short Duration Gamma-Ray Bursts
Recent rapid localizations of short, hard gamma-ray bursts (GRBs) by the
Swift and HETE satellites have led to the observation of the first afterglows
and the measurement of the first redshifts from this type of burst. Detection
of >100 GeV counterparts would place powerful constraints on GRB mechanisms.
Seventeen short duration (< 5 s) GRBs detected by satellites occurred within
the field of view of the Milagro gamma-ray observatory between 2000 January and
2006 December. We have searched the Milagro data for >100 GeV counterparts to
these GRBs and find no significant emission correlated with these bursts. Due
to the absorption of high-energy gamma rays by the extragalactic background
light (EBL), detections are only expected for redshifts less than ~0.5. While
most long duration GRBs occur at redshifts higher than 0.5, the opposite is
thought to be true of short GRBs. Lack of a detected VHE signal thus allows
setting meaningful fluence limits. One GRB in the sample (050509b) has a likely
association with a galaxy at a redshift of 0.225, while another (051103) has
been tentatively linked to the nearby galaxy M81. Fluence limits are corrected
for EBL absorption, either using the known measured redshift, or computing the
corresponding absorption for a redshift of 0.1 and 0.5, as well as for the case
of z=0.Comment: Accepted for publication in the Astrophysical Journa
The Study of TeV Variability and Duty Cycle of Mrk 421 from 3 Years of Observations with the Milagro Observatory
TeV flaring activity with time scales as short as tens of minutes and an
orphan TeV flare have been observed from the blazar Markarian 421 (Mrk 421).
The TeV emission from Mrk 421 is believed to be produced by leptonic
synchrotron self-Compton (SSC) emission. In this scenario, correlations between
the X-ray and the TeV fluxes are expected, TeV orphan flares are hardly
explained and the activity (measured as duty cycle) of the source at TeV
energies is expected to be equal or less than that observed in X-rays if only
SSC is considered. To estimate the TeV duty cycle of Mrk 421 and to establish
limits on its variability at different time scales, we continuously observed
Mrk 421 with the Milagro observatory. Mrk 421 was detected by Milagro with a
statistical significance of 7.1 standard deviations between 2005 September 21
and 2008 March 15. The observed spectrum is consistent with previous
observations by VERITAS. We estimate the duty cycle of Mrk 421 for energies
above 1 TeV for different hypothesis of the baseline flux and for different
flare selections and we compare our results with the X-ray duty cycle estimated
by Resconi et al. 2009. The robustness of the results is discussed.Comment: 27 pages, 6 figures, ApJ accepte
Discovery of Localized Regions of Excess 10-TeV Cosmic Rays
An analysis of 7 years of Milagro data performed on a 10-degree angular scale
has found two localized regions of excess of unknown origin with greater than
12 sigma significance. Both regions are inconsistent with gamma-ray emission
with high confidence. One of the regions has a different energy spectrum than
the isotropic cosmic-ray flux at a level of 4.6 sigma, and it is consistent
with hard spectrum protons with an exponential cutoff, with the most
significant excess at ~10 TeV. Potential causes of these excesses are explored,
but no compelling explanations are found.Comment: Submitted to PhysRevLet
Search for very high energy gamma-rays from WIMP annihilations near the Sun with the Milagro Detector
The neutralino, the lightest stable supersymmetric particle, is a strong
theoretical candidate for the missing astronomical ``dark matter''. A profusion
of such neutralinos can accumulate near the Sun when they lose energy upon
scattering and are gravitationally captured. Pair-annihilations of those
neutralinos may produce very high energy (VHE, above ) gamma-rays.
Milagro is an air shower array which uses the water Cherenkov technique to
detect extensive air showers and is capable of observing VHE gamma-rays from
the direction of the Sun with an angular resolution of . Analysis
of Milagro data with an exposure to the Sun of 1165 hours presents the first
attempt to detect TeV gamma-rays produced by annihilating neutralinos captured
by the Solar system and shows no statistically significant signal. Resulting
limits that can be set on gamma-ray flux due to near-Solar neutralino
annihilations and on neutralino cross-section are presented
Observation and Spectral Measurements of the Crab Nebula with Milagro
The Crab Nebula was detected with the Milagro experiment at a statistical
significance of 17 standard deviations over the lifetime of the experiment. The
experiment was sensitive to approximately 100 GeV - 100 TeV gamma ray air
showers by observing the particle footprint reaching the ground. The fraction
of detectors recording signals from photons at the ground is a suitable proxy
for the energy of the primary particle and has been used to measure the photon
energy spectrum of the Crab Nebula between ~1 and ~100 TeV. The TeV emission is
believed to be caused by inverse-Compton up-scattering scattering of ambient
photons by an energetic electron population. The location of a TeV steepening
or cutoff in the energy spectrum reveals important details about the underlying
electron population. We describe the experiment and the technique for
distinguishing gamma-ray events from the much more-abundant hadronic events. We
describe the calculation of the significance of the excess from the Crab and
how the energy spectrum is fit. The fit is consistent with values measured by
IACTs between 1 and 20 TeV. Fixing the spectral index to values that have been
measured below 1 TeV by IACT experiments (2.4 to 2.6), the fit to the Milagro
data suggests that Crab exhibits a spectral steepening or cutoff between about
20 to 40 TeV.Comment: Submitted to Astrophysical Journa
TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro
A survey of Galactic gamma-ray sources at a median energy of ~20 TeV has been
performed using the Milagro Gamma Ray Observatory. Eight candidate sources of
TeV emission are detected with pre-trials significance in the
region of Galactic longitude and latitude
. Four of these sources, including the Crab nebula
and the recently published MGRO J2019+37, are observed with significances
after accounting for the trials involved in searching the 3800
square degree region. All four of these sources are also coincident with EGRET
sources. Two of the lower significance sources are coincident with EGRET
sources and one of these sources is Geminga. The other two candidates are in
the Cygnus region of the Galaxy. Several of the sources appear to be spatially
extended. The fluxes of the sources at 20 TeV range from ~25% of the Crab flux
to nearly as bright as the Crab.Comment: Submitted to Ap
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