2,014 research outputs found
Nuclear dependence of azimuthal asymmetry in semi-inclusive deep inelastic scattering
Within the framework of a generalized factorization, semi-inclusive deeply
inelastic scattering (SIDIS) cross sections can be expressed as a series of
products of collinear hard parts and transverse-momentum-dependent (TMD) parton
distributions and correlations. The azimuthal asymmetry $ is studied. It is shown that the azimuthal
asymmetry is suppressed by multiple parton scattering and the transverse
momentum dependence of the suppression depends on the relative shape of the
twist-2 and 3 quark distributions in the nucleon. A Gaussian ansatz for TMD
twist-2 and 3 quark distributions in nucleon is used to demonstrate the nuclear
dependence of the azimuthal asymmetry and to estimate the smearing effect due
to fragmentation.Comment: 9 pages in RevTex with 2 figure
Twist-4 contributions to the azimuthal asymmetry in SIDIS
We calculate the differential cross section for the unpolarized
semi-inclusive deeply inelastic scattering (SIDIS) process
in leading order (LO) of perturbative QCD and up to twist-4 in power
corrections and study in particular the azimuthal asymmetry . The
final results are expressed in terms of transverse momentum dependent (TMD)
parton matrix elements of the target nucleon up to twist-4. %Under the maximal
two-gluon correlation approximation, these TMD parton matrix elements in a
nucleus %can be expressed terms of a Gaussian convolution of that in a nucleon
with the width given by the jet transport %parameter inside cold nuclei. We
also apply it to $e^-+A \to e^-+q+X$ and illustrate numerically the nuclear
dependence of the azimuthal asymmetry by using a Gaussian ansatz
for the TMD parton matrix elements.Comment: 9 pages, afigur
Global quark polarization in non-central collisions
Partons produced in the early stage of non-central heavy-ion collisions can
develop a longitudinal fluid shear because of unequal local number densities of
participant target and projectile nucleons. Under such fluid shear, local
parton pairs with non-vanishing impact parameter have finite local relative
orbital angular momentum along the direction opposite to the reaction plane.
Such finite relative orbital angular momentum among locally interacting quark
pairs can lead to global quark polarization along the same direction due to
spin-orbital coupling. Local longitudinal fluid shear is estimated within both
Landau fireball and Bjorken scaling model of initial parton production. Quark
polarization through quark-quark scatterings with the exchange of a thermal
gluon is calculated beyond small-angle scattering approximation in a
quark-gluon plasma. The polarization is shown to have a non-monotonic
dependence on the local relative orbital angular momentum dictated by the
interplay between electric and magnetic interaction. It peaks at a value of
relative orbital angular momentum which scales with the magnetic mass of the
exchanged gluons. With the estimated small longitudinal fluid shear in
semi-peripheral collisions at the RHIC energy, the final quark
polarization is found to be small in the weak coupling limit.
Possible behavior of the quark polarization in the strong coupling limit and
implications on the experimental detection of such global quark polarization at
RHIC and LHC are also discussed.Comment: 28 pages,11 figure
Disentangling covariant Wigner functions for chiral fermions
We develop a general formalism for the quantum kinetics of chiral fermions in
a background electromagnetic field based on a semiclassical expansion of
covariant Wigner functions in the Planck constant . We demonstrate to
any order of that only the time-component of the Wigner function is
independent while other components are explicit derivative. We further
demonstrate to any order of that a system of quantum kinetic equations
for multiple-components of Wigner functions can be reduced to one chiral
kinetic equation involving only the single-component distribution function.
These are remarkable properties of the quantum kinetics of chiral fermions and
will significantly simplify the description and simulation of chiral effects in
heavy ion collisions and Dirac/Weyl semimetals. We present the unintegrated
chiral kinetic equations in four-momenta up to and the integrated
ones in three-momenta up to . We find that some singular terms emerge
in the integration over the time component of the four-momentum, which result
in a new source term contributing to the chiral anomaly, in contrast to the
well-known scenario of the Berry phase term. Finally we rewrite our results in
any Lorentz frame with a reference four-velocity and show how the non-trivial
transformation of the distribution function in different frames emerges in a
natural way.Comment: RevTex 4, 14 pages, no figure. Section II and III have been
re-organized and expended to three sections (II-IV) to include more details
of calculations. Section V has been expanded to include more discussions. A
new section (VI) is added about Wigner functions in a general Lorentz frame.
Some references are adde
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