188 research outputs found
Resonant Photonic Quasicrystalline and Aperiodic Structures
We have theoretically studied propagation of exciton-polaritons in
deterministic aperiodic multiple-quantum-well structures, particularly, in the
Fibonacci and Thue-Morse chains. The attention is concentrated on the
structures tuned to the resonant Bragg condition with two-dimensional
quantum-well exciton. The superradiant or photonic-quasicrystal regimes are
realized in these structures depending on the number of the wells. The
developed theory based on the two-wave approximation allows one to describe
analytically the exact transfer-matrix computations for transmittance and
reflectance spectra in the whole frequency range except for a narrow region
near the exciton resonance. In this region the optical spectra and the
exciton-polariton dispersion demonstrate scaling invariance and self-similarity
which can be interpreted in terms of the ``band-edge'' cycle of the trace map,
in the case of Fibonacci structures, and in terms of zero reflection
frequencies, in the case of Thue-Morse structures.Comment: 13 pages, 9 figures, submitted to Phys. Rev.
Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV
We report a measurement of the longitudinal double-spin asymmetry A_LL and
the differential cross section for inclusive Pi0 production at midrapidity in
polarized proton collisions at sqrt(s) = 200 GeV. The cross section was
measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be
in good agreement with a next-to-leading order perturbative QCD calculation.
The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T <
11 GeV/c and excludes a maximal positive gluon polarization in the proton. The
mean transverse momentum fraction of Pi0's in their parent jets was found to be
around 0.7 for electromagnetically triggered events.Comment: 6 pages, 3 figures, submitted to Phys. Rev. D (RC
Measurement of the parity-violating longitudinal single-spin asymmetry for boson production in polarized proton-proton collisions at GeV
We report the first measurement of the parity violating single-spin
asymmetries for midrapidity decay positrons and electrons from and
boson production in longitudinally polarized proton-proton collisions
at GeV by the STAR experiment at RHIC. The measured asymmetries,
and , are consistent with theory
predictions, which are large and of opposite sign. These predictions are based
on polarized quark and antiquark distribution functions constrained by
polarized DIS measurements.Comment: 6 pages, 4 figures, submitted to Physics Review Letter
Evolution of the differential transverse momentum correlation function with centrality in Au+Au collisions at GeV
We present first measurements of the evolution of the differential transverse
momentum correlation function, {\it C}, with collision centrality in Au+Au
interactions at GeV. {\it C} exhibits a strong dependence
on collision centrality that is qualitatively similar to that of number
correlations previously reported. We use the observed longitudinal broadening
of the near-side peak of {\it C} with increasing centrality to estimate the
ratio of the shear viscosity to entropy density, , of the matter formed
in central Au+Au interactions. We obtain an upper limit estimate of
that suggests that the produced medium has a small viscosity per unit entropy.Comment: 7 pages, 4 figures, STAR paper published in Phys. Lett.
High non-photonic electron production in + collisions at = 200 GeV
We present the measurement of non-photonic electron production at high
transverse momentum ( 2.5 GeV/) in + collisions at
= 200 GeV using data recorded during 2005 and 2008 by the STAR
experiment at the Relativistic Heavy Ion Collider (RHIC). The measured
cross-sections from the two runs are consistent with each other despite a large
difference in photonic background levels due to different detector
configurations. We compare the measured non-photonic electron cross-sections
with previously published RHIC data and pQCD calculations. Using the relative
contributions of B and D mesons to non-photonic electrons, we determine the
integrated cross sections of electrons () at 3 GeV/10 GeV/ from bottom and charm meson decays to be = 4.0({\rm
stat.})({\rm syst.}) nb and =
6.2({\rm stat.})({\rm syst.}) nb, respectively.Comment: 17 pages, 17 figure
Measurement of the Bottom contribution to non-photonic electron production in collisions at =200 GeV
The contribution of meson decays to non-photonic electrons, which are
mainly produced by the semi-leptonic decays of heavy flavor mesons, in
collisions at 200 GeV has been measured using azimuthal
correlations between non-photonic electrons and hadrons. The extracted
decay contribution is approximately 50% at a transverse momentum of GeV/. These measurements constrain the nuclear modification factor for
electrons from and meson decays. The result indicates that meson
production in heavy ion collisions is also suppressed at high .Comment: 6 pages, 4 figures, accepted by PR
Longitudinal scaling property of the charge balance function in Au + Au collisions at 200 GeV
We present measurements of the charge balance function, from the charged
particles, for diverse pseudorapidity and transverse momentum ranges in Au + Au
collisions at 200 GeV using the STAR detector at RHIC. We observe that the
balance function is boost-invariant within the pseudorapidity coverage [-1.3,
1.3]. The balance function properly scaled by the width of the observed
pseudorapidity window does not depend on the position or size of the
pseudorapidity window. This scaling property also holds for particles in
different transverse momentum ranges. In addition, we find that the width of
the balance function decreases monotonically with increasing transverse
momentum for all centrality classes.Comment: 6 pages, 3 figure
Energy and system size dependence of \phi meson production in Cu+Cu and Au+Au collisions
We study the beam-energy and system-size dependence of \phi meson production
(using the hadronic decay mode \phi -- K+K-) by comparing the new results from
Cu+Cu collisions and previously reported Au+Au collisions at \sqrt{s_NN} = 62.4
and 200 GeV measured in the STAR experiment at RHIC. Data presented are from
mid-rapidity (|y|<0.5) for 0.4 < pT < 5 GeV/c. At a given beam energy, the
transverse momentum distributions for \phi mesons are observed to be similar in
yield and shape for Cu+Cu and Au+Au colliding systems with similar average
numbers of participating nucleons. The \phi meson yields in nucleus-nucleus
collisions, normalised by the average number of participating nucleons, are
found to be enhanced relative to those from p+p collisions with a different
trend compared to strange baryons. The enhancement for \phi mesons is observed
to be higher at \sqrt{s_NN} = 200 GeV compared to 62.4 GeV. These observations
for the produced \phi(s\bar{s}) mesons clearly suggest that, at these collision
energies, the source of enhancement of strange hadrons is related to the
formation of a dense partonic medium in high energy nucleus-nucleus collisions
and cannot be alone due to canonical suppression of their production in smaller
systems.Comment: 20 pages and 5 figure
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