44 research outputs found
Determination of nuclear parton distributions
Parametrization of nuclear parton distributions is investigated in the
leading order of alpha_s. The parton distributions are provided at Q^2=1 GeV^2
with a number of parameters, which are determined by a chi^2 analysis of the
data on nuclear structure functions. Quadratic or cubic functional form is
assumed for the initial distributions. Although valence quark distributions in
the medium x region are relatively well determined, the small x distributions
depend slightly on the assumed functional form. It is difficult to determine
the antiquark distributions at medium x and gluon distributions. From the
analysis, we propose parton distributions at Q^2=1 GeV^2 for nuclei from
deuteron to heavy ones with the mass number A~208. They are provided either
analytical expressions or computer subroutines for practical usage. Our studies
should be important for understanding the physics mechanism of the nuclear
modification and also for applications to heavy-ion reactions. This kind of
nuclear parametrization should also affect existing parametrization studies in
the nucleon because "nuclear" data are partially used for obtaining the optimum
distributions in the "nucleon".Comment: 16 pages, REVTeX4b5, revtex4.cls, url.sty, natbib.sty, 10pt.rtx,
aps.rtx, revsymb.sty, 21 eps figures. Submitted for publication. Computer
codes for the nuclear parton distributions could be obtained from
http://www-hs.phys.saga-u.ac.jp Email: [email protected]
Relativistic nuclear structure effects in quasielastic neutrino scattering
Charged-current cross sections are calculated for quasielastic neutrino and
antineutrino scattering using a relativistic meson-nucleon model. We examine
how nuclear-structure effects, such as relativistic random-phase-approximation
(RPA) corrections and momentum-dependent nucleon self-energies, influence the
extraction of the axial form factor of the nucleon. RPA corrections are
important only at low-momentum transfers. In contrast, the momentum dependence
of the relativistic self-energies changes appreciably the value of the
axial-mass parameter, , extracted from dipole fits to the axial form
factor. Using Brookhaven's experimental neutrino spectrum we estimate the
sensitivity of M to various relativistic nuclear-structure effects.Comment: 26 pages, revtex, 6 postscript figures (available upon request
Energy Loss versus Shadowing in the Drell-Yan Reaction on Nuclei
We present a new analysis of the E772 and E866 experiments on the nuclear
dependence of Drell-Yan (DY) lepton pair production resulting from the
bombardment of , Be, C, Ca, Fe, and W targets by 800 GeV/c protons at
Fermilab. We employ a light-cone formulation of the DY reaction in the rest
frame of the nucleus, where the dimuons detected at small values of Bjorken x_2
<< 1 may be considered to originate from the decay of a heavy photon radiated
from an incident quark in a bremsstrahlung process. We infer the energy loss of
the quark by examining the suppression of the nuclear-dependent DY ratios seen
as a function of projectile momentum fraction x_1 and dimuon mass M. Shadowing,
which also leads to nuclear suppression of dimuons, is calculated within the
same approach employing the results of phenomenological fits to deep inelastic
scattering data from HERA. The analysis yields -dE/dz =2.73 +/- 0.37 +/- 0.5
GeV/fm for the rate of quark energy loss per unit path length, a value
consistent with theoretical expectations including the effects of the inelastic
interaction of the incident proton at the surface of the nucleus. This is the
first observation of a nonzero energy loss effect in such experiments.Comment: 43 pages including 17 figure
Evidence for SU(3) symmetry breaking from hyperon production
We examine the SU(3) symmetry breaking in hyperon semileptonic decays (HSD)
by considering two typical sets of quark contributions to the spin content of
the octet baryons: Set-1 with SU(3) flavor symmetry and Set-2 with SU(3) flavor
symmetry breaking in HSD. The quark distributions of the octet baryons are
calculated with a successful statistical model. Using an approximate relation
between the quark fragmentation functions and the quark distributions, we
predict polarizations of the octet baryons produced in annihilation
and semi-inclusive deeply lepton-nucleon scattering in order to reveal the
SU(3) symmetry breaking effect on the spin structure of the octet baryons. We
find that the SU(3) symmetry breaking significantly affects the hyperon
polarization. The available experimental data on the polarization
seem to favor the theoretical predictions with SU(3) symmetry breaking. We
conclude that there is a possibility to get a collateral evidence for SU(3)
symmetry breaking from hyperon production. The theoretical errors for our
predictions are discussed.Comment: 3 tables, 14 figure
Nonperturbative and perturbative aspects of photo- and electroproduction of vector mesons
We discuss various aspects of vector meson production, first analysing the
interplay between perturbative and nonperturbative aspects of the QCD
calculation. Using a general method adapted to incorporate both perturbative
and nonpertubative aspects, we show that nonperturbative effects are important
for all experimentally available values of the photon virtuality Q2. We compare
the huge amount of experimental information now available with our theoretical
results obtained using a specific nonperturbative model without free
parameters, showing that quite simple features are able to explain the data.Comment: 19 page
Measurement of the - and -Dependence of the Asymmetry on the Nucleon
We report results for the virtual photon asymmetry on the nucleon from
new Jefferson Lab measurements. The experiment, which used the CEBAF Large
Acceptance Spectrometer and longitudinally polarized proton (NH) and
deuteron (ND) targets, collected data with a longitudinally
polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present
paper, we concentrate on our results for and the related ratio
in the resonance and the deep inelastic regions for our lowest
and highest beam energies, covering a range in momentum transfer from
0.05 to 5.0 GeV and in final-state invariant mass up to about 3 GeV.
Our data show detailed structure in the resonance region, which leads to a
strong --dependence of for below 2 GeV. At higher , a
smooth approach to the scaling limit, established by earlier experiments, can
be seen, but is not strictly --independent. We add
significantly to the world data set at high , up to . Our data
exceed the SU(6)-symmetric quark model expectation for both the proton and the
deuteron while being consistent with a negative -quark polarization up to
our highest . This data setshould improve next-to-leading order (NLO) pQCD
fits of the parton polarization distributions.Comment: 7 pages LaTeX, 5 figure
Star clusters near and far; tracing star formation across cosmic time
© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00690-x.Star clusters are fundamental units of stellar feedback and unique tracers of their host galactic properties. In this review, we will first focus on their constituents, i.e.\ detailed insight into their stellar populations and their surrounding ionised, warm, neutral, and molecular gas. We, then, move beyond the Local Group to review star cluster populations at various evolutionary stages, and in diverse galactic environmental conditions accessible in the local Universe. At high redshift, where conditions for cluster formation and evolution are more extreme, we are only able to observe the integrated light of a handful of objects that we believe will become globular clusters. We therefore discuss how numerical and analytical methods, informed by the observed properties of cluster populations in the local Universe, are used to develop sophisticated simulations potentially capable of disentangling the genetic map of galaxy formation and assembly that is carried by globular cluster populations.Peer reviewedFinal Accepted Versio
TRY plant trait database â enhanced coverage and open access
Plant traitsâthe morphological, anatomical, physiological, biochemical and phenological characteristics of plantsâdetermine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of traitâbased plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traitsâalmost complete coverage for âplant growth formâ. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and traitâenvironmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives