9,702 research outputs found
Spin Effects in High Energy Fragmentation Processes
Recent measurements, in particular those on polarization and spin
alignment of vector mesons in annihilation at LEP, and those on the
azimuthal asymmetry at HERA, have attracted much attention on the spin effects
in high energy fragmentation processes. In this talk, we make a brief
introduction to the different topics studied in this connection and a short
summary of the available data. After that, we present a short summary of the
main theoretical results that we obtained in studying these different topics.
The talk was mainly based on the publications [5-9] which have been finished in
collaboration with C.Boros, Liu Chun-xiu and Xu Qing-hua.Comment: Plenary talk given at the 3rd Circum-Pan-Pacifc Symposium on High
Energy Spin Physics, October 2001, 8 pages, 4 figure
Ionizing radiation fluctuations and large-scale structure in the Lyman-alpha forest
We investigate the large-scale inhomogeneities of the hydrogen ionizing
radiation field in the Universe at redshift z=3. Using a raytracing algorithm,
we simulate a model in which quasars are the dominant sources of radiation. We
make use of large scale N-body simulations of a LambdaCDM universe, and include
such effects as finite quasar lifetimes and output on the lightcone, which
affects the shape of quasar light echoes. We create Lya forest spectra that
would be generated in the presence of such a fluctuating radiation field,
finding that the power spectrum of the Lya forest can be suppressed by as much
as 15 % for modes with k=0.05-1 Mpc/h. This relatively small effect may have
consequences for high precision measurements of the Lya power spectrum on
larger scales than have yet been published. We also investigate another
radiation field probe, the cross-correlation of quasar positions and the Lya
forest. For both quasar lifetimes which we simulate (10^7 yr and 10^8 yr), we
expect to see a strong decrease in the Lya absorption close to other quasars
(the ``foreground'' proximity effect). We then use data from the Sloan Digital
Sky Survey First Data Release to make an observational determination of this
statistic. We find no sign of our predicted lack of absorption, but instead
increased absorption close to quasars. If the bursts of radiation from quasars
last on average < 10^6 yr, then we would not expect to be able to see the
foreground effect. However, the strength of the absorption itself seems to be
indicative of rare objects, and hence much longer total times of emission per
quasar. Variability of quasars in bursts with timescales > 10^4yr and < 10^6 yr
could reconcile these two facts.Comment: Submitted to ApJ, 21 pages, 17 postscript figures, emulateapj.st
Heavy-to-light transition form factors and their relations in light-cone QCD sum rules
The improved light-cone QCD sum rules by using chiral current correlator is
systematically reviewed and applied to the calculation of all the
heavy-to-light form factors, including all the semileptonic and penguin ones.
By choosing suitable chiral currents, the light-cone sum rules for all the form
factors are greatly simplified and depend mainly on one leading twist
distribution amplitude of the light meson. As a result, relations between these
form factors arise naturally. At the considered accuracy these relations
reproduce the results obtained in the literature. Moreover, since the explicit
dependence on the leading twist distribution amplitudes is preserved, these
relations may be more useful to simulate the experimental data and extract the
information on the distribution amplitude.Comment: 1+16 pages, no figure
Landau parameters of nuclear matter in the spin and spin-isospin channels
The equation of state of spin and isospin polarized nuclear matter is
determined in the framework of the Brueckner theory including three-body
forces. The Landau parameters in the spin and spin-isospin sectors are derived
as a function of the baryonic density. The results are compared with the
Gamow-Teller collective modes. The relevance of and for neutron
stars is shortly discussed, including the magnetic susceptibility and the
neutron star cooling.Comment: 2 pages, 2 figures, RevTex4 forma
Azimuthal and single spin asymmetry in deep-inelastic lepton-nucleon scattering
We derive a general framework for describing semi-inclusive deep-inelastic
lepton-nucleon scattering in terms of the unintegrated parton distributions and
other higher twist parton correlations. Such a framework provides a consistent
approach to the calculation of inclusive and semi-inclusive cross sections
including higher twist effects. As an example, we calculate the azimuthal
asymmetries to the order of 1/Q in semi-inclusive process with transversely
polarized target. A non-vanishing single-spin asymmetry in the ``triggered
inclusive process'' is predicted to be 1/Q suppressed with a part of the
coefficient related to a moment of the Sivers function.Comment: 9 pages, 1 figur
Correlation effects in the ground state charge density of Mott-insulating NiO: a comparison of ab-initio calculations and high-energy electron diffraction measurements
Accurate high-energy electron diffraction measurements of structure factors
of NiO have been carried out to investigate how strong correlations in the Ni
3d shell affect electron charge density in the interior area of nickel ions and
whether the new ab-initio approaches to the electronic structure of strongly
correlated metal oxides are in accord with experimental observations. The
generalized gradient approximation (GGA) and the local spin density
approximation corrected by the Hubbard U term (LSDA+U) are found to provide the
closest match to experimental measurements. The comparison of calculated and
observed electron charge densities shows that correlations in the Ni 3d shell
suppress covalent bonding between the oxygen and nickel sublattices.Comment: 6 pages, LaTeX and 5 figures in the postscript forma
Anti-Lambda polarization in high energy pp collisions with polarized beam
We study the polarization of the anti-Lambda particle in polarized high
energy pp collisions at large transverse momenta. The anti-Lambda polarization
is found to be sensitive to the polarization of the anti-strange sea of the
nucleon. We make predictions using different parameterizations of the polarized
quark distribution functions. The results show that the measurement of
longitudinal anti-Lambda polarization can distinguish different
parameterizations, and that similar measurements in the transversely polarized
case can give some insights into the transversity distribution of the
anti-strange sea of nucleon.Comment: 11 pages, 4 figure
On the Search for Quasar Light Echoes
The UV radiation from a quasar leaves a characteristic pattern in the
distribution of ionized hydrogen throughout the surrounding space. This pattern
or light echo propagates through the intergalactic medium at the speed of
light, and can be observed by its imprint on the Ly-alpha forest spectra of
background sources. As the echo persists after the quasar has switched off, it
offers the possibility of searching for dead quasars, and constraining their
luminosities and lifetimes. We outline a technique to search for and
characterize these light echoes. To test the method, we create artificial
Ly-alpha forest spectra from cosmological simulations at z=3, apply light
echoes and search for them. We show how the simulations can also be used to
quantify the significance level of any detection. We find that light echoes
from the brightest quasars could be found in observational data. With
absorption line spectra of 100 redshift z~3-3.5 quasars or galaxies in a 1
square degree area, we expect that ~10 echoes from quasars with B band
luminosities L_B=3x10^45 ergs/s exist that could be found at 95% confidence,
assuming a quasar lifetime of ~10^7 yr. Even a null result from such a search
would have interesting implications for our understanding of quasar
luminosities and lifetimes.Comment: 9 pages, 7 figures, ApJ in pres
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