13,304 research outputs found
Constituent quark scaling violation due to baryon number transport
In ultra-relativistic heavy ion collisions at \roots\approx200 GeV, the
azimuthal emission anisotropy of hadrons with low and intermediate transverse
momentum ( GeV/c) displays an intriguing scaling. In particular,
the baryon (meson) emission patterns are consistent with a scenario in which a
bulk medium of flowing quarks coalesces into three-quark (two-quark) "bags."
While a full understanding of this number of constituent quark (NCQ) scaling
remains elusive, it is suggestive of a thermalized bulk system characterized by
colored dynamical degrees of freedom-- a quark-gluon plasma (QGP). In this
scenario, one expects the scaling to break down as the central energy density
is reduced below the QGP formation threshold; for this reason, NCQ-scaling
violation searches are of interest in the energy scan program at the
Relativistic Heavy Ion Collider (RHIC). However, as \roots is reduced, it is
not only the initial energy density that changes; there is also an increase in
the net baryon number at midrapidity, as stopping transports entrance-channel
partons to midrapidity. This phenomenon can result in violations of simple NCQ
scaling. Still in the context of the quark coalescence model, we describe a
specific pattern for the break-down of the scaling that includes different flow
strengths for particles and their anti-partners. Related complications in the
search for recently suggested exotic phenomena are also discussed.Comment: 7 pages, 2 tables, 2 figures. Wording sharpened. Two tables added, to
quantify the estimate of stopped quark fraction
Nuclear recoil energy scale in liquid xenon with application to the direct detection of dark matter
We show for the first time that the quenching of electronic excitation from
nuclear recoils in liquid xenon is well-described by Lindhard theory, if the
nuclear recoil energy is reconstructed using the combined (scintillation and
ionization) energy scale proposed by Shutt {\it et al.}. We argue for the
adoption of this perspective in favor of the existing preference for
reconstructing nuclear recoil energy solely from primary scintillation. We show
that signal partitioning into scintillation and ionization is well-described by
the Thomas-Imel box model. We discuss the implications for liquid xenon
detectors aimed at the direct detection of dark matter
First Direct Detection Limits on sub-GeV Dark Matter from XENON10
The first direct detection limits on dark matter in the MeV to GeV mass range
are presented, using XENON10 data. Such light dark matter can scatter with
electrons, causing ionization of atoms in a detector target material and
leading to single- or few-electron events. We use 15 kg-days of data acquired
in 2006 to set limits on the dark-matter-electron scattering cross section. The
strongest bound is obtained at 100 MeV where sigma_e < 3 x 10^{-38} cm^2 at 90%
CL, while dark matter masses between 20 MeV and 1 GeV are bounded by sigma_e <
10^{-37} cm^2 at 90% CL. This analysis provides a first proof-of-principle that
direct detection experiments can be sensitive to dark matter candidates with
masses well below the GeV scale.Comment: Submitted to PR
Statistical mechanics of a Feshbach coupled Bose-Fermi gas in an optical lattice
We consider an atomic Fermi gas confined in a uniform optical lattice
potential, where the atoms can pair into molecules via a magnetic field
controlled narrow Feshbach resonance. The phase diagram of the resulting
atom-molecule mixture in chemical and thermal equilibrium is determined
numerically in the absence of interactions under the constraint of particle
conservation. In the limiting cases of vanishing or large lattice depth we
derive simple analytical results for important thermodynamic quantities. One
such quantity is the dissociation energy, defined as the detuning of the
molecular energy spectrum with respect to the atomic one for which half of the
atoms have been converted into dimers. Importantly we find that the
dissociation energy has a non-monotonic dependence on lattice depth.Comment: 9 pages, 5 figure
Suppression of Non-photonic Electrons from Enhancement of Charm Baryons in Heavy Ion Collisions
At intermediate transverse momentum (2 < p_T < 6 GeV/c), baryon production in
Au+Au collisions is enhanced compared to p+p collisions. Since charm baryon
decays produce electrons less frequently than charm meson decays, the
non-photonic electron spectrum is sensitive to the Lambda_c/D ratio. In this
report we study the dependence of the non-photonic electron spectrum on the
baryon-to-meson ratio for charm hadrons. As an example, we take the Lambda_c/D
ratio to have the same form as the Lambda/K^0_S ratio. In this case, even if
the total charm quark yield in Au+Au collisions scales with the number of
binary nucleon-nucleon collisions (N_bin), the electron spectrum at 2 < p_T < 5
GeV/c is suppressed relative to N_bin scaled p+p collisions by as much as 20%.Comment: Added STAR data to figure 1 and made slight text modifications (fixed
figure replacement
Comparative genetic resistance to Ascaridia galli infections of 4 different commercial layer-lines
1. The objective of the study was to compare the establishment and effect of Ascaridia galli infections in 4 different layer-lines.
2. A total of 160 birds comprising 4 different commercial layer-lines, ISA Brown, New Hampshire, Skalborg and a cross of New Hampshire(NH) and Skalborg (Sk), were infected with A. galli eggs. The birds were examined for the presence of parasite eggs and parasites at weeks 3, 6 and 9 post infection (pi).
3. At week 6 pi the chickens of the NH line harboured more larvae compared with the three other lines. The Sk line chickens excreted more A. galli eggs throughout the study compared with the other lines. Female worms in the Sk line were more fecund than the worms in the other lines. Male and female worms recovered from the Sk line at week 9 pi were longer. Male worms recovered from the NH line 6 weeks pi were shorter than male worms from the other lines. Female worms recovered from the NH line were shorter than the female worms from the ISA line and the Sk line. No differences were seen
in weight gain among the 4 lines.
4. The results suggest that genetic factors are involved in the establishment and survival of A. galli in the intestine of layers. Further studies are needed to elucidate the genetic mechanisms behind the observed parasitological findings
Different quantization mechanisms in single-electron pumps driven by surface acoustic waves
We have studied the acoustoelectric current in single-electron pumps driven
by surface acoustic waves. We have found that in certain parameter ranges two
different sets of quantized steps dominate the acoustoelectric current versus
gate-voltage characteristics. In some cases, both types of quantized steps
appear simultaneously though at different current values, as if they were
superposed on each other. This could indicate two independent quantization
mechanisms for the acoustoelectric current.Comment: 6 pages, 3 figure
The Asymmetric Effect of the Business Cycle on the Realtion between Stock Market Returns and their Volatility
We examine the relation between US stock market returns and the US business cycle for the period 1960 - 2003 using a new methodology that allows us to estimate a time-varying equity premium. We identify two channels in the transmission mechanism. One is through the mean of stock returns via the equity risk premium, and the other is through the volatility of returns. We provide support for previous findings based on simple correlation analysis that the relation is asymmetric with downturns in the business cycle having a greater negative impact on stock returns than the positive effect of upturns. We also obtain a new result, that demand and supply shocks affect stock returns differently. Our model of the relation between returns and their volatility encompasses CAPM, consumption CAPM and Merton's (1973) inter-temporal CAPM. It is implemented using a multi-variate GARCH-in-mean model with an asymmetric time-varying conditional heteroskedasticity and correlation structure.Equity returns, risk premium, asymmetry
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