16,265 research outputs found
The surprising influence of late charged current weak interactions on Big Bang Nucleosynthesis
The weak interaction charged current processes (, , ) interconvert neutrons and protons in the early universe and
have significant influence on Big Bang Nucleosynthesis (BBN) light-element
abundance yields, particulary that for . We demonstrate that the
influence of these processes is still significant even when they operate well
below temperatures usually invoked for "weak freeze-out,"
and in fact down nearly into the alpha-particle formation epoch (). This physics is correctly captured in commonly used BBN
codes, though this late-time, low-temperature persistent effect of the
isospin-changing weak processes, and the sensitivity of the associated rates to
lepton energy distribution functions and blocking factors are not widely
appreciated. We quantify this late-time influence by analyzing weak interaction
rate dependence on the neutron lifetime, lepton energy distribution functions,
entropy, the proton-neutron mass difference, and Hubble expansion rate. The
effects we point out here render BBN a keen probe of any beyond-standard-model
physics that alters lepton number/energy distributions, even subtly, in epochs
of the early universe all the way down to near .Comment: 27 pages, 8 figure
Baryon Number Transport in a Cosmic QCD-Phase Transition
We investigate the transport of baryon number across phase boundaries in a
putative first order QCD-phase transition. Two independent phenomenological
models are employed to estimate the baryon penetrability at the phase boundary:
chromoelectric flux tube models; and an analogy to baryon-baryon coalescence in
nuclear physics. Our analysis indicates that baryon transport across phase
boundaries may be order of magnitude more efficient than other work has
suggested. We discuss the substantial uncertainties involved in estimating
baryon penetrability at phase boundaries.Comment: 25 pages, 4 figures (available upon request by mail or fax), plain
tex, UCRL-JC-00000
Neutrinos in Cosmology and Astrophysics
We briefly review the recent developments in neutrino physics and
astrophysics which have import for frontline research in nuclear physics. These
developments, we argue, tie nuclear physics to exciting developments in
observational cosmology and astrophysics in new ways. Moreover, the behavior of
neutrinos in dense matter is itself a fundamental problem in many-body quantum
mechanics, in some ways akin to well-known issues in nuclear matter and nuclei,
and in some ways radically different, especially because of nonlinearity and
quantum de-coherence. The self-interacting neutrino gas is the only many body
system driven by the weak interactions.Comment: 7 pages, 1 figur
Nuclear neutrino energy spectra in high temperature astrophysical environments
Astrophysical environments that reach temperatures greater than 100
keV can have significant neutrino energy loss via both plasma processes and
nuclear weak interactions. We find that nuclear processes likely produce the
highest-energy neutrinos. Among the important weak nuclear interactions are
both charged current channels (electron capture/emission and positron
capture/emission) and neutral current channels (de-excitation of nuclei via
neutrino pair emission). We show that in order to make a realistic prediction
of the nuclear neutrino spectrum, one must take nuclear structure into account;
in some cases, the most important transitions may involve excited states,
possibly in both parent and daughter nuclei. We find that the standard
technique of producing a neutrino energy spectrum by using a single transition
with a Q-value and matrix element chosen to fit published neutrino production
rates and energy losses will not accurately capture important spectral
features.Comment: 11 pages, 17 figure
Absence of a Lower Limit on Omega_b in Inhomogeneous Primordial Nucleosynthesis
We show that a class of inhomogeneous big bang nucleosynthesis models exist
which yield light-element abundances in agreement with observational
constraints for baryon-to-photon ratios significantly smaller than those
inferred from standard homogeneous big bang nucleosynthesis (HBBN). These
inhomogeneous nucleosynthesis models are characterized by a bimodal
distribution of baryons in which some regions have a local baryon-to-photon
ratio eta=3*10e-10, while the remaining regions are baryon-depleted. HBBN
scenarios with primordial (2H+3He)/H<9*10e-5 necessarily require that most
baryons be in a dark or non-luminous form, although new observations of a
possible high deuterium abundance in Lyman-alpha clouds may relax this
requirement somewhat. The models described here present another way to relax
this requirement and can even eliminate any lower bound on the baryon-to-photon
ratio.Comment: 13 pages, 2 figures (available upon request by email), plain te
The art of being human : a project for general philosophy of science
Throughout the medieval and modern periods, in various sacred and secular guises, the unification of all forms of knowledge under the rubric of âscienceâ has been taken as the prerogative of humanity as a species. However, as our sense of species privilege has been called increasingly into question, so too has the very salience of âhumanityâ and âscienceâ as general categories, let alone ones that might bear some essential relationship to each other. After showing how the ascendant Stanford School in the philosophy of science has contributed to this joint demystification of âhumanityâ and âscienceâ, I proceed on a more positive note to a conceptual framework for making sense of science as the art of being human. My understanding of âscienceâ is indebted to the red thread that runs from Christian theology through the Scientific Revolution and Enlightenment to the Humboldtian revival of the university as the site for the synthesis of knowledge as the culmination of self-development. Especially salient to this idea is scienceâs epistemic capacity to manage modality (i.e. to determine the conditions under which possibilities can be actualised) and its political capacity to organize humanity into projects of universal concern. However, the challenge facing such an ideal in the twentyfirst century is that the predicate âhumanâ may be projected in three quite distinct ways, governed by what I call âecologicalâ, âbiomedicalâ and âcyberneticâ interests. Which one of these future humanities would claim todayâs humans as proper ancestors and could these futures co-habit the same world thus become two important questions that general philosophy of science will need to address in the coming years
Prospects for Detecting Supernova Neutrino Flavor Oscillations
The neutrinos from a Type II supernova provide perhaps our best opportunity
to probe cosmologically interesting muon and/or tauon neutrino masses. This is
because matter enhanced neutrino oscillations can lead to an anomalously hot
nu_e spectrum, and thus to enhanced charged current cross sections in
terrestrial detectors. Two recently proposed supernova neutrino observatories,
OMNIS and LAND, will detect neutrons spalled from target nuclei by neutral and
charged current neutrino interactions. As this signal is not flavor specific,
it is not immediately clear whether a convincing neutrino oscillation signal
can be extracted from such experiments. To address this issue we examine the
responses of a series of possible light and heavy mass targets, 9Be, 23Na,
35Cl, and 208Pb. We find that strategies for detecting oscillations which use
only neutron count rates are problematic at best, even if cross sections are
determined by ancillary experiments. Plausible uncertainties in supernova
neutrino spectra tend to obscure rate enhancements due to oscillations.
However, in the case of 208Pb, a signal emerges that is largely flavor specific
and extraordinarily sensitive to the nu_e temperature, the emission of two
neutrons. This signal and its flavor specificity are associated with the
strength and location of the first-forbidden responses for neutral and charge
current reactions, aspects of the 208Pb neutrino cross section that have not
been discussed previously. Hadronic spin transfer experiments might be helpful
in confirming some of the nuclear structure physics underlying our conclusions.Comment: 27 pages, RevTeX, 2 figure
The initial conditions of stellar protocluster formation. II. A catalogue of starless and protostellar clumps embedded in IRDCs in the Galactic longitude range 15<l<55
We present a catalogue of starless and protostellar clumps associated with
infrared dark clouds (IRDCs) in a 40 degrees wide region of the inner Galactic
Plane (b<1). We have extracted the far-infrared (FIR) counterparts of 3493
IRDCs with known distance in the Galactic longitude range 15<l<55 and searched
for the young clumps using Hi-GAL, the survey of the Galactic Plane carried out
with the Herschel satellite. Each clump is identified as a compact source
detected at 160, 250 and 350 mum. The clumps have been classified as
protostellar or starless, based on their emission (or lack of emission) at 70
mum. We identify 1723 clumps, 1056 (61%) of which are protostellar and 667
(39%) starless. These clumps are found within 764 different IRDCs, 375 (49%) of
which are only associated with protostellar clumps, 178 (23%) only with
starless clumps, and 211 (28%) with both categories of clumps. The clumps have
a median mass of 250 M_sun and range up to >10^4$ M_sun in mass and up to 10^5
L_sun in luminosity. The mass-radius distribution shows that almost 30% of the
starless clumps identified in this survey could form high-mass stars, however
these massive clumps are confined in only ~4% of the IRDCs. Assuming a minimum
mass surface density threshold for the formation of high-mass stars, the
comparison of the numbers of massive starless clumps and those already
containing embedded sources suggests an upper limit lifetime for the starless
phase of 10^5 years for clumps with a mass M>500 M_sun.Comment: accepted for publication in MNRAS. Online catalogues available soon,
please contact the authors if intereste
The Influence of Nuclear Composition on the Electron Fraction in the Post-Core-Bounce Supernova Environment
We study the early evolution of the electron fraction (or, alternatively, the
neutron-to-proton ratio) in the region above the hot proto-neutron star formed
after a supernova explosion. We study the way in which the electron fraction in
this environment is set by a competition between lepton (electron, positron,
neutrino, and antineutrino) capture processes on free neutrons and protons and
nuclei. Our calculations take explicit account of the effect of nuclear
composition changes, such as formation of alpha particles (the alpha effect)
and the shifting of nuclear abundances in nuclear statistical equilibrium
associated with cooling in near-adiabatic outflow. We take detailed account of
the process of weak interaction freeze-out in conjunction with these nuclear
composition changes. Our detailed treatment shows that the alpha effect can
cause significant increases in the electron fraction, while neutrino and
antineutrino capture on heavy nuclei tends to have a buffering effect on this
quantity. We also examine the effect on weak rates and the electron fraction of
fluctuations in time in the neutrino and antineutrino energy spectra arising
from hydrodynamic waves. Our analysis is guided by the Mayle & Wilson supernova
code numerical results for the neutrino energy spectra and density and velocity
profiles.Comment: 38 pages, AAS LaTeX, 8 figure
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