142 research outputs found
Time-variability in the Interstellar Boundary Conditions of the Heliosphere: Effect of the Solar Journey on the Galactic Cosmic Ray Flux at Earth
During the solar journey through galactic space, variations in the physical
properties of the surrounding interstellar medium (ISM) modify the heliosphere
and modulate the flux of galactic cosmic rays (GCR) at the surface of the
Earth, with consequences for the terrestrial record of cosmogenic
radionuclides. One phenomenon that needs studying is the effect on cosmogenic
isotope production of changing anomalous cosmic ray fluxes at Earth due to
variable interstellar ionizations. The possible range of interstellar ram
pressures and ionization levels in the low density solar environment generate
dramatically different possible heliosphere configurations, with a wide range
of particle fluxes of interstellar neutrals, their secondary products, and GCRs
arriving at Earth. Simple models of the distribution and densities of ISM in
the downwind direction give cloud transition timescales that can be directly
compared with cosmogenic radionuclide geologic records. Both the interstellar
data and cosmogenic radionuclide data are consistent with cloud transitions
during the Holocene, with large and assumption-dependent uncertainties. The
geomagnetic timeline derived from cosmic ray fluxes at Earth may require
adjustment to account for the disappearance of anomalous cosmic rays when the
Sun is immersed in ionized gas.Comment: Submitted to Space Sciences Review
Exclusive electroproduction of K+ Lambda and K+ Sigma^0 final states at Q^2 = 0.030-0.055 (GeV/c)^2
Cross section measurements of the exclusive p(e,e'K+)Lambda,Sigma^0
electroproduction reactions have been performed at the Mainz Microtron MAMI in
the A1 spectrometer facility using for the first time the Kaos spectrometer for
kaon detection. These processes were studied in a kinematical region not
covered by any previous experiment. The nucleon was probed in its third
resonance region with virtual photons of low four-momenta, Q^2= 0.030-0.055
(GeV/c)^2. The MAMI data indicate a smooth transition in Q^2 from
photoproduction to electroproduction cross sections. Comparison with
predictions of effective Lagrangian models based on the isobar approach reveal
that strong longitudinal couplings of the virtual photon to the N* resonances
can be excluded from these models.Comment: 16 pages, 7 figure
P-wave excited baryons from pion- and photo-induced hyperon production
We report evidence for , , ,
, , and , and find
indications that might have a companion state at 1970\,MeV. The
controversial is not seen. The evidence is derived from a
study of data on pion- and photo-induced hyperon production, but other data are
included as well. Most of the resonances reported here were found in the
Karlsruhe-Helsinki (KH84) and the Carnegie-Mellon (CM) analyses but were
challenged recently by the Data Analysis Center at GWU. Our analysis is
constrained by the energy independent scattering amplitudes from either
KH84 or GWU. The two amplitudes from KH84 or GWU, respectively, lead to
slightly different branching ratios of contributing resonances but the
debated resonances are required in both series of fits.Comment: 22 pages, 28 figures. Some additional sets of data are adde
Differential cross section and recoil polarization measurements for the gamma p to K+ Lambda reaction using CLAS at Jefferson Lab
We present measurements of the differential cross section and Lambda recoil
polarization for the gamma p to K+ Lambda reaction made using the CLAS detector
at Jefferson Lab. These measurements cover the center-of-mass energy range from
1.62 to 2.84 GeV and a wide range of center-of-mass K+ production angles.
Independent analyses were performed using the K+ p pi- and K+ p (missing pi -)
final-state topologies; results from these analyses were found to exhibit good
agreement. These differential cross section measurements show excellent
agreement with previous CLAS and LEPS results and offer increased precision and
a 300 MeV increase in energy coverage. The recoil polarization data agree well
with previous results and offer a large increase in precision and a 500 MeV
extension in energy range. The increased center-of-mass energy range that these
data represent will allow for independent study of non-resonant K+ Lambda
photoproduction mechanisms at all production angles.Comment: 22 pages, 16 figure
Improved constraints on the expansion rate of the Universe up to z~1.1 from the spectroscopic evolution of cosmic chronometers
We present new improved constraints on the Hubble parameter H(z) in the
redshift range 0.15 < z < 1.1, obtained from the differential spectroscopic
evolution of early-type galaxies as a function of redshift. We extract a large
sample of early-type galaxies (\sim11000) from several spectroscopic surveys,
spanning almost 8 billion years of cosmic lookback time (0.15 < z < 1.42). We
select the most massive, red elliptical galaxies, passively evolving and
without signature of ongoing star formation. Those galaxies can be used as
standard cosmic chronometers, as firstly proposed by Jimenez & Loeb (2002),
whose differential age evolution as a function of cosmic time directly probes
H(z). We analyze the 4000 {\AA} break (D4000) as a function of redshift, use
stellar population synthesis models to theoretically calibrate the dependence
of the differential age evolution on the differential D4000, and estimate the
Hubble parameter taking into account both statistical and systematical errors.
We provide 8 new measurements of H(z) (see Tab. 4), and determine its change in
H(z) to a precision of 5-12% mapping homogeneously the redshift range up to z
\sim 1.1; for the first time, we place a constraint on H(z) at z \neq 0 with a
precision comparable with the one achieved for the Hubble constant (about 5-6%
at z \sim 0.2), and covered a redshift range (0.5 < z < 0.8) which is crucial
to distinguish many different quintessence cosmologies. These measurements have
been tested to best match a \Lambda CDM model, clearly providing a
statistically robust indication that the Universe is undergoing an accelerated
expansion. This method shows the potentiality to open a new avenue in constrain
a variety of alternative cosmologies, especially when future surveys (e.g.
Euclid) will open the possibility to extend it up to z \sim 2.Comment: 34 pages, 15 figures, 6 tables, published in JCAP. It is a companion
to Moresco et al. (2012b, http://arxiv.org/abs/1201.6658) and Jimenez et al.
(2012, http://arxiv.org/abs/1201.3608). The H(z) data can be downloaded at
http://www.physics-astronomy.unibo.it/en/research/areas/astrophysics/cosmology-with-cosmic-chronometer
Measurement of the Nucleon Structure Function F2 in the Nuclear Medium and Evaluation of its Moments
We report on the measurement of inclusive electron scattering off a carbon
target performed with CLAS at Jefferson Laboratory. A combination of three
different beam energies 1.161, 2.261 and 4.461 GeV allowed us to reach an
invariant mass of the final-state hadronic system W~2.4 GeV with four-momentum
transfers Q2 ranging from 0.2 to 5 GeV2. These data, together with previous
measurements of the inclusive electron scattering off proton and deuteron,
which cover a similar continuous two-dimensional region of Q2 and Bjorken
variable x, permit the study of nuclear modifications of the nucleon structure.
By using these, as well as other world data, we evaluated the F2 structure
function and its moments. Using an OPE-based twist expansion, we studied the
Q2-evolution of the moments, obtaining a separation of the leading-twist and
the total higher-twist terms. The carbon-to-deuteron ratio of the leading-twist
contributions to the F2 moments exhibits the well known EMC effect, compatible
with that discovered previously in x-space. The total higher-twist term in the
carbon nucleus appears, although with large systematic uncertainites, to be
smaller with respect to the deuteron case for n<7, suggesting partial parton
deconfinement in nuclear matter. We speculate that the spatial extension of the
nucleon is changed when it is immersed in the nuclear medium.Comment: 37 pages, 15 figure
First measurement of direct photoproduction on the proton
We report on the results of the first measurement of exclusive
meson photoproduction on protons for GeV and GeV. Data were collected with the CLAS detector at the Thomas
Jefferson National Accelerator Facility. The resonance was detected via its
decay in the channel by performing a partial wave analysis of the
reaction . Clear evidence of the meson
was found in the interference between and waves at GeV. The -wave differential cross section integrated in the mass range of
the was found to be a factor of 50 smaller than the cross section
for the meson. This is the first time the meson has been
measured in a photoproduction experiment
Integral potential method for a transmission problem with Lipschitz interface in R^3 for the Stokes and Darcy–Forchheimer–Brinkman PDE systems
The purpose of this paper is to obtain existence and uniqueness results in weighted Sobolev spaces for transmission problems for the non-linear Darcy-Forchheimer-Brinkman system and the linear Stokes system in two complementary Lipschitz domains in R3, one of them is a bounded Lipschitz domain with connected boundary, and the other one is the exterior Lipschitz domain R3 n. We exploit a layer potential method for the Stokes and Brinkman systems combined with a fixed point theorem in order to show the desired existence and uniqueness results, whenever the given data are suitably small in some weighted Sobolev spaces and boundary Sobolev spaces
Nucleon resonances in the fourth resonance region
Nucleon and resonances in the fourth resonance region are studied in
a multichannel partial-wave analysis which includes nearly all available data
on pion- and photo-induced reactions off protons. In the high-mass range, above
1850\,MeV, several alternative solutions yield a good description of the data.
For these solutions, masses, widths, pole residues and photo-couplings are
given. In particular, we find evidence for nucleon resonances with
spin-parities . For one set of solutions, there are four
resonances forming naturally a spin-quartet of resonances with orbital angular
momentum L=2 and spin S=3/2 coupling to . Just below 1.9\,GeV we
find a spin doublet of resonances with and . Since a spin
partner with is missing at this mass, the two resonances form a
spin doublet which must have a symmetric orbital-angular-momentum wave function
with L=1. For another set of solutions, the four positive-parity resonances are
accompanied by mass-degenerate negative-parity partners -- as suggested by the
conjecture of chiral symmetry restoration. The possibility of a spin doublet at 1900\,MeV belonging to a 20-plet is discussed.Comment: 16 page
Partial wave analysis of the reaction gamma p -> p omega$ and the search for nucleon resonances
An event-based partial wave analysis (PWA) of the reaction gamma p -> p omega
has been performed on a high-statistics dataset obtained using the CLAS at
Jefferson Lab for center-of-mass energies from threshold up to 2.4 GeV. This
analysis benefits from access to the world's first high precision spin density
matrix element measurements, available to the event-based PWA through the decay
distribution of omega-> pi+ pi - pi0. The data confirm the dominance of the
t-channel pi0 exchange amplitude in the forward direction. The dominant
resonance contributions are consistent with the previously identified states
F[15](1680) and D[13](1700) near threshold, as well as the G[17](2190) at
higher energies. Suggestive evidence for the presence of a J(P)=5/2(+) state
around 2 GeV, a "missing" state, has also been found. Evidence for other states
is inconclusive
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