34,276 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
On Thermal Gravitational Contribution to Particle Production and Dark Matter
We investigate the particle production from thermal gravitational
annihilation in the very early universe, which is an important contribution for
particles that might not be in thermal equilibrium or/and only have
gravitational interaction, such as dark matter (DM). For particles with spin 0,
1/2 and 1 we calculate the relevant cross sections through gravitational
annihilation and give the analytic formulas with full mass-dependent terms. We
find that DM with mass between TeV and GeV could have the relic
abundance that fits the observation, with small dependence on its spin. We also
discuss the effects of gravitational annihilation from inflatons.
Interestingly, contributions from inflatons could be dominant and have the same
power dependence on Hubble parameter of inflation as that from vacuum
fluctuation. Also, fermion production from inflatons, in comparison to boson,
is suppressed by its mass due to helicity selection.Comment: 10 pages, 3 figures and 2 tables, published versio
Pure Gravitational Dark Matter, Its Mass and Signatures
In this study, we investigate a scenario that dark matter (DM) has only
gravitational interaction. In the framework of effective field theory of
gravity, we find that DM is still stable at tree level even if there is no
symmetry to protect its longevity, but could decay into standard model
particles due to gravitational loop corrections. The radiative corrections can
lead to both higher- and lower-dimensional effective operators. We also first
explore how DM can be produced in the early universe. Through gravitational
interaction at high temperature, DM is then found to have mass around TeV
GeV to get the right relic abundance. When DM
decays, it mostly decays into gravitons, which could be tested by current and
future CMB experiments. We also estimate the resulting fluxes for cosmic rays,
gamma-ray and neutrino.Comment: 6 pages, 3 figure
Spin content of Lambda and its longitudinal polarization in annihilation at high energies
Longitudinal polarization of Lambda produced in annihilation at LEP
energies is calculated in a picture for the spin content of Lambda which is
consistent with the polarized deep inelastic lepton-nucleon scattering data and
SU(3) flavor symmetry for hyperon decay so that the spin of Lambda is not
completely carried by its -valence quark. A comparison with the recent ALEPH
data and the results of earlier calculations based on the static quark model in
which the spin of Lambda is completely determined by the -quark is given.
The result shows that further measurements of such polarization should provide
useful information to the question of which picture is more suitable in
describing the spin effects in the fragmentation processes.Comment: 12 pages, 2 figure
Single spin asymmetries in inclusive high energy hadron-hadron collision processes
It has been realized for quite a long time that single-spin experiments, in
which one of the colliding objects is transversely polarized, can be helpful in
studying the properties of strong interaction in general and in testing Quantum
Chromodynamics (QCD) in particular.
Striking effects have been observed in the past few years which deviate
drastically from the expectation of the perturbative QCD parton model.
These effects have received much attention.
New experiments of the similar type are underway and/or planned.
Different theoretical attempts have been made to understand these effects.
In this review, the special role played by singly polarized high-energy
hadron-hadron collisions in High Energy Spin Physics is emphasized.
Characteristics of the available data for inclusive hadron productions are
briefly summarized.
Different theoretical approaches for such processes are reviewed with special
attention to a non-perturbative model which explicitly takes the orbital motion
of the valence quarks and hadronic surface effects into account.
The connection between such asymmetries and hyperon polarization in
unpolarized reactions is discussed.
An example of the possible application of such experimental results in other
processes is given.Comment: 62 pages, 17 ps-figures (Review article to appear in Inter. J. Mod.
Phys. A
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