1,317 research outputs found
Molecular hydrogen in high-redshift Damped Lyman-alpha systems: The VLT/UVES database
We present the current status of ongoing searches for molecular hydrogen in
high-redshift (1.8 < zabs <= 4.2) Damped Lyman-alpha systems (DLAs)
capitalising on observations performed with the ESO Very Large Telescope (VLT)
Ultraviolet and Visual Echelle Spectrograph (UVES). We identify 77 DLAs/strong
sub-DLAs, with log N(HI) >= 20 and z_abs > 1.8, which have data that include
redshifted H2 Lyman and/or Werner-band absorption lines. This sample of HI, H2
and metal line measurements, performed in an homogeneous manner, is more than
twice as large as our previous sample (Ledoux et al. 2003) and considers every
system in which searches for H2 could be completed so far, including all
non-detections. H2 is detected in thirteen of the systems with molecular
fractions of values between f=5x10^-7 and f=0.1, where f=2N(H2)/(2N(H2)+N(HI)).
Upper limits are measured for the remaining 64 systems with detection limits of
typically log N(H2)=14.3, corresponding to log f<-5. We find that about 35% of
the DLAs with metallicities relative to solar [X/H]>=-1.3 (i.e., 1/20th solar),
with X = Zn, S or Si, have molecular fractions log f>-4.5, while H2 is detected
-- regardless of the molecular fraction -- in 50% of them. In contrast, only
about 4% of the [X/H]-4.5. We show that the presence of
H2 does not strongly depend on the total neutral hydrogen column density,
although the probability of finding log f>-4.5 is higher for log N(HI)>=20.8
than below this limit (19% and 7% respectively). The overall H2 detection rate
in log N(HI)>=20 DLAs is found to be about 16% (10% considering only log f>-4.5
detections) after correction for a slight bias towards large N(HI). [truncated]Comment: 11 pages, 1 table, 10 figures. Accepted for publication in A&
HD molecules at high redshift: The absorption system at z=2.3377 towards Q 1232+082
We present a detailed analysis of the H_2 and HD absorption lines detected in
the Damped Lyman-alpha (DLA) system at z_abs=2.3377 towards the quasar
Q1232+082. We show that this intervening cloud has a covering factor smaller
than unity and covers only part of the QSO broad emission line region. The zero
flux level has to be corrected at the position of the saturated H_2 and
optically thin HD lines by about 10%. We accurately determine the Doppler
parameter for HD and CI lines (b = 1.86+/-0.20 km/s). We find a ratio
N(HD)/N(H_2)=(7.1 +3.7 -2.2)x10^-5 that is significantly higher than what is
observed in molecular clouds of the Galaxy. Chemical models suggest that in the
physical conditions prevailing in the central part of molecular clouds,
deuterium and hydrogen are mostly in their molecular forms. Assuming this is
true, we derive D/H = (3.6 +1.9 -1.1)x10^-5. This implies that the
corresponding baryon density of the Universe is \Omega_b h^2 = (0.0182 +0.0047
-0.0042). This value coincides within 1\sigma with that derived from
observations of the CMBR as well as from observations of the D/H atomic ratio
in low-metallicity QSO absorption line systems. The observation of HD at high
redshift is therefore a promising independent method to constrain \Omega_b.
This observation indicates as well a low astration factor of deuterium. This
can be interpreted as the consequence of an intense infall of primordial gas
onto the associated galaxy.Comment: 8 pages, 7 figures, 3 tables. Accepted for publication in MNRA
Claims of Potential Expansion throughout the U.S. by Invasive Python Species Are Contradicted by Ecological Niche Models
BACKGROUND: Recent reports from the United States Geological Survey (USGS) suggested that invasive Burmese pythons in the Everglades may quickly spread into many parts of the U.S. due to putative climatic suitability. Additionally, projected trends of global warming were predicted to significantly increase suitable habitat and promote range expansion by these snakes. However, the ecological limitations of the Burmese python are not known and the possible effects of global warming on the potential expansion of the species are also unclear. METHODOLOGY/PRINCIPAL FINDINGS: Here we show that a predicted continental expansion is unlikely based on the ecology of the organism and the climate of the U.S. Our ecological niche models, which include variables representing climatic extremes as well as averages, indicate that the only suitable habitat in the U.S. for Burmese pythons presently occurs in southern Florida and in extreme southern Texas. Models based on the current distribution of the snake predict suitable habitat in essentially the only region in which the snakes are found in the U.S. Future climate models based on global warming forecasts actually indicate a significant contraction in suitable habitat for Burmese pythons in the U.S. as well as in their native range. CONCLUSIONS/SIGNIFICANCE: The Burmese python is strongly limited to the small area of suitable environmental conditions in the United States it currently inhabits due to the ecological niche preferences of the snake. The ability of the Burmese python to expand further into the U.S. is severely limited by ecological constraints. Global warming is predicted to significantly reduce the area of suitable habitat worldwide, underscoring the potential negative effects of climate change for many species
A mathematical and computational review of Hartree-Fock SCF methods in Quantum Chemistry
We present here a review of the fundamental topics of Hartree-Fock theory in
Quantum Chemistry. From the molecular Hamiltonian, using and discussing the
Born-Oppenheimer approximation, we arrive to the Hartree and Hartree-Fock
equations for the electronic problem. Special emphasis is placed in the most
relevant mathematical aspects of the theoretical derivation of the final
equations, as well as in the results regarding the existence and uniqueness of
their solutions. All Hartree-Fock versions with different spin restrictions are
systematically extracted from the general case, thus providing a unifying
framework. Then, the discretization of the one-electron orbitals space is
reviewed and the Roothaan-Hall formalism introduced. This leads to a exposition
of the basic underlying concepts related to the construction and selection of
Gaussian basis sets, focusing in algorithmic efficiency issues. Finally, we
close the review with a section in which the most relevant modern developments
(specially those related to the design of linear-scaling methods) are commented
and linked to the issues discussed. The whole work is intentionally
introductory and rather self-contained, so that it may be useful for non
experts that aim to use quantum chemical methods in interdisciplinary
applications. Moreover, much material that is found scattered in the literature
has been put together here to facilitate comprehension and to serve as a handy
reference.Comment: 64 pages, 3 figures, tMPH2e.cls style file, doublesp, mathbbol and
subeqn package
Limits on WWZ and WW\gamma couplings from p\bar{p}\to e\nu jj X events at \sqrt{s} = 1.8 TeV
We present limits on anomalous WWZ and WW-gamma couplings from a search for
WW and WZ production in p-bar p collisions at sqrt(s)=1.8 TeV. We use p-bar p
-> e-nu jjX events recorded with the D0 detector at the Fermilab Tevatron
Collider during the 1992-1995 run. The data sample corresponds to an integrated
luminosity of 96.0+-5.1 pb^(-1). Assuming identical WWZ and WW-gamma coupling
parameters, the 95% CL limits on the CP-conserving couplings are
-0.33<lambda<0.36 (Delta-kappa=0) and -0.43<Delta-kappa<0.59 (lambda=0), for a
form factor scale Lambda = 2.0 TeV. Limits based on other assumptions are also
presented.Comment: 11 pages, 2 figures, 2 table
Search For Heavy Pointlike Dirac Monopoles
We have searched for central production of a pair of photons with high
transverse energies in collisions at TeV using of data collected with the D\O detector at the Fermilab Tevatron in
1994--1996. If they exist, virtual heavy pointlike Dirac monopoles could
rescatter pairs of nearly real photons into this final state via a box diagram.
We observe no excess of events above background, and set lower 95% C.L. limits
of on the mass of a spin 0, 1/2, or 1 Dirac
monopole.Comment: 12 pages, 4 figure
The Dijet Mass Spectrum and a Search for Quark Compositeness in bar{p}p Collisions at sqrt{s} = 1.8 TeV
Using the DZero detector at the 1.8 TeV pbarp Fermilab Tevatron collider, we
have measured the inclusive dijet mass spectrum in the central pseudorapidity
region |eta_jet| < 1.0 for dijet masses greater than 200 Gev/c^2. We have also
measured the ratio of spectra sigma(|eta_jet| < 0.5)/sigma(0.5 < |eta_jet| <
1.0). The order alpha_s^3 QCD predictions are in good agreement with the data
and we rule out models of quark compositeness with a contact interaction scale
< 2.4 TeV at the 95% confidence level.Comment: 11 pages, 4 figures, 2 tables, submitted to Phys. Rev. Let
Search for High Mass Photon Pairs in p-pbar --> gamma-gamma-jet-jet Events at sqrt(s)=1.8 TeV
A search has been carried out for events in the channel p-barp --> gamma
gamma jet jet. Such a signature can characterize the production of a
non-standard Higgs boson together with a W or Z boson. We refer to this
non-standard Higgs, having standard model couplings to vector bosons but no
coupling to fermions, as a "bosonic Higgs." With the requirement of two high
transverse energy photons and two jets, the diphoton mass (m(gamma gamma))
distribution is consistent with expected background. A 90(95)% C.L. upper limit
on the cross section as a function of mass is calculated, ranging from
0.60(0.80) pb for m(gamma gamma) = 65 GeV/c^2 to 0.26(0.34) pb for m(gamma
gamma) = 150 GeV/c^2, corresponding to a 95% C.L. lower limit on the mass of a
bosonic Higgs of 78.5 GeV/c^2.Comment: 9 pages, 3 figures. Replacement has new H->gamma gamma branching
ratios and corresponding new mass limit
Zgamma Production in pbarp Collisions at sqrt(s)=1.8 TeV and Limits on Anomalous ZZgamma and Zgammagamma Couplings
We present a study of Z +gamma + X production in p-bar p collisions at
sqrt{S}=1.8 TeV from 97 (87) pb^{-1} of data collected in the eegamma
(mumugamma) decay channel with the D0 detector at Fermilab. The event yield and
kinematic characteristics are consistent with the Standard Model predictions.
We obtain limits on anomalous ZZgamma and Zgammagamma couplings for form factor
scales Lambda = 500 GeV and Lambda = 750 GeV. Combining this analysis with our
previous results yields 95% CL limits |h{Z}_{30}| < 0.36, |h{Z}_{40}| < 0.05,
|h{gamma}_{30}| < 0.37, and |h{gamma}_{40}| < 0.05 for a form factor scale
Lambda=750 GeV.Comment: 17 Pages including 2 Figures. Submitted to PR
A Measurement of the W Boson Mass
We report a measurement of the W boson mass based on an integrated luminosity
of 82 pb from \ppbar collisions at TeV recorded in
1994--1995 by the \Dzero detector at the Fermilab Tevatron. We identify W
bosons by their decays to and extract the mass by fitting the transverse
mass spectrum from 28,323 W boson candidates. A sample of 3,563 dielectron
events, mostly due to Z to ee decays, constrains models of W boson production
and the detector. We measure \mw=80.44\pm0.10(stat)\pm0.07(syst)~GeV. By
combining this measurement with our result from the 1992--1993 data set, we
obtain \mw=80.43\pm0.11 GeV.Comment: 11 pages, 5 figure
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