357 research outputs found
Photodissociation of H2 in Protogalaxies: Modeling Self-Shielding in 3D Simulations
The ability of primordial gas to cool in proto-galactic haloes exposed to
Lyman-Werner (LW) radiation is critically dependent on the self-shielding of
H_2. We perform radiative transfer calculations of LW line photons,
post-processing outputs from three-dimensional adaptive mesh refinement (AMR)
simulations of haloes with T_vir > 10^4 K at redshifts around z=10. We
calculate the optically thick photodissociation rate numerically, including the
effects of density, temperature, and velocity gradients in the gas, as well as
line overlap and shielding of H_2 by HI, over a large number of sight-lines. In
low-density regions (n<10^4 cm^-3) the dissociation rates exceed those obtained
using most previous approximations by more than an order of magnitude; the
correction is smaller at higher densities. We trace the origin of the
deviations primarily to inaccuracies of (i) the most common fitting formula
(Draine & Bertoldi 1996) for the suppression of the dissociation rate and (ii)
estimates for the effective shielding column density from local properties of
the gas. The combined effects of gas temperature and velocity gradients are
comparatively less important, typically altering the spherically averaged rate
only by a factor of less than two. We present a simple modification to the DB96
fitting formula for the optically thick rate which improves agreement with our
numerical results to within approx. 15 per cent, and can be adopted in future
simulations. We find that estimates for the effective shielding column can be
improved by using the local Sobolev length. Our correction to the H_2
self-shielding reduces the critical LW flux to suppress H_2-cooling in
T_vir>10^4 K haloes by an order of magnitude; this increases the number of such
haloes in which supermassive (approx. M=10^5 M_sun) black holes may have
formed.Comment: 17 pages, 11 figures. Submitted to MNRA
Direct Measurement of Nuclear Dependence of Charged Current Quasielastic-like Neutrino Interactions using MINERvA
Charged-current interactions on carbon, iron, and lead with a
final state hadronic system of one or more protons with zero mesons are used to
investigate the influence of the nuclear environment on quasielastic-like
interactions. The transfered four-momentum squared to the target nucleus,
, is reconstructed based on the kinematics of the leading proton, and
differential cross sections versus and the cross-section ratios of iron,
lead and carbon to scintillator are measured for the first time in a single
experiment. The measurements show a dependence on atomic number. While the
quasielastic-like scattering on carbon is compatible with predictions, the
trends exhibited by scattering on iron and lead favor a prediction with
intranuclear rescattering of hadrons accounted for by a conventional particle
cascade treatment. These measurements help discriminate between different
models of both initial state nucleons and final state interactions used in the
neutrino oscillation experiments
First evidence of coherent meson production in neutrino-nucleus scattering
Neutrino-induced charged-current coherent kaon production,
, is a rare, inelastic electroweak process
that brings a on shell and leaves the target nucleus intact in its ground
state. This process is significantly lower in rate than neutrino-induced
charged-current coherent pion production, because of Cabibbo suppression and a
kinematic suppression due to the larger kaon mass. We search for such events in
the scintillator tracker of MINERvA by observing the final state ,
and no other detector activity, and by using the kinematics of the final state
particles to reconstruct the small momentum transfer to the nucleus, which is a
model-independent characteristic of coherent scattering. We find the first
experimental evidence for the process at significance.Comment: added ancillary file with information about the six kaon candidate
Single neutral pion production by charged-current interactions on hydrocarbon at 3.6 GeV
Single neutral pion production via muon antineutrino charged-current
interactions in plastic scintillator (CH) is studied using the \minerva
detector exposed to the NuMI low-energy, wideband antineutrino beam at
Fermilab. Measurement of this process constrains models of neutral pion
production in nuclei, which is important because the neutral-current analog is
a background for appearance oscillation experiments. The
differential cross sections for momentum and production angle, for
events with a single observed and no charged pions, are presented and
compared to model predictions. These results comprise the first measurement of
the kinematics for this process.Comment: 6 pages, 5 figures, submitted to Physics Letters
Measurement of the muon anti-neutrino double-differential cross section for quasi-elastic scattering on hydrocarbon at~ GeV
We present double-differential measurements of anti-neutrino quasi-elastic
scattering in the MINERvA detector. This study improves on a previous single
differential measurement by using updated reconstruction algorithms and
interaction models, and provides a complete description of observed muon
kinematics in the form of a double-differential cross section with respect to
muon transverse and longitudinal momentum. We include in our signal definition
zero-meson final states arising from multi-nucleon interactions and from
resonant pion production followed by pion absorption in the primary nucleus. We
find that model agreement is considerably improved by a model tuned to MINERvA
inclusive neutrino scattering data that incorporates nuclear effects such as
weak nuclear screening and two-particle, two-hole enhancements.Comment: 47 pages, 31 figure
Measurement of Partonic Nuclear Effects in Deep-Inelastic Neutrino Scattering using MINERvA
The MINERvA collaboration reports a novel study of neutrino-nucleus
charged-current deep inelastic scattering (DIS) using the same neutrino beam
incident on targets of polystyrene, graphite, iron, and lead. Results are
presented as ratios of C, Fe, and Pb to CH. The ratios of total DIS cross
sections as a function of neutrino energy and flux-integrated differential
cross sections as a function of the Bjorken scaling variable x are presented in
the neutrino-energy range of 5 - 50 GeV. Good agreement is found between the
data and predicted ratios, based on charged-lepton nucleus scattering, at
medium x and low neutrino energies. However, the data rate appears depleted in
the vicinity of the nuclear shadowing region, x < 0.1. This apparent deficit,
reflected in the DIS cross-section ratio at high neutrino energy , is
consistent with previous MINERvA observations and with the predicted onset of
nuclear shadowing with the the axial-vector current in neutrino scattering
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