154 research outputs found
Soft end-point and mass corrections to the eta' g*g* vertex function
Power-suppressed corrections arising from end-point integration regions to
the space-like vertex function of the massive eta'-meson virtual gluon
transition eta' - g*g* are computed. Calculations are performed within the
standard hard-scattering approach (HSA) and the running coupling method
supplemented by the infrared renormalon calculus. Contributions to the vertex
function from the quark and gluon contents of the eta' -meson are taken into
account and the Borel resummed expressions for F_{eta' g*g*}(Q2,\omega ,\eta),
as well as for F_{eta' g g*}}(Q^{2},\omega =\pm 1,\eta) and F_{eta'
g*g*}(Q^{2},\omega =0,\eta) are obtained. It is demonstrated that the
power-suppressed corrections \sim (\Lambda ^{2}/Q^{2})^{n}, in the explored
range of the total gluon virtuality 1 <Q2 < 25 GeV2, considerably enhance the
vertex function relative to the results found in the framework of the standard
HSA with a fixed coupling. Modifications generated by the eta ' -meson mass
effects are discussed
Semi-inclusive B Decays and Direct CP Violation in QCD Factorization
We have systematically investigated the semi-inclusive B decays B->MX, which
are manifestations of the quark decay b->Mq, within the framework of
QCD-improved factorization. These decays are theoretically clean and have
distinctive experimental signatures. We focus on a class of these that do not
require any form factor information and therefore may be especially suitable
for extracting information on the angles and of the unitarity
triangle. The nonfactorizable effects, such as vertex-type and penguin-type
corrections to the two-body b decay and hard spectator corrections to the
3-body decay are calculable in the heavy quark limit. QCD factorization is
applicable when the emitted meson is a light meson or a charmonium. We discuss
the issue of the CPT constraint on partial rate asymmetries. The strong phase
coming from final-state rescattering due to hard gluon exchange between the
final states can induce large rate asymmetries for tree-dominated
color-suppressed modes . The nonfactorizable
hard spectator interactions in the 3-body decay, though phase-space suppressed,
are extremely important for the tree-dominated modes
, and the
penguin-dominated mode . In fact, they are dominated by the
hard spectator corrections. Our result for is in
agreement with experiment. The semi-inclusive decay modes: , ,
and are the most promising ones in searching for
direct CP violation. In fact, they have branching ratios of order
and CP rate asymmetries of order .Comment: 28 page
Nonlinear ion-acoustic (IA) waves driven in a cylindrically symmetric flow
By employing a self-similar, two-fluid MHD model in a cylindrical geometry,
we study the features of nonlinear ion-acoustic (IA) waves which propagate in
the direction of external magnetic field lines in space plasmas. Numerical
calculations not only expose the well-known three shapes of nonlinear
structures (sinusoidal, sawtooth, and spiky or bipolar) which are observed by
numerous satellites and simulated by models in a Cartesian geometry, but also
illustrate new results, such as, two reversely propagating nonlinear waves,
density dips and humps, diverging and converging electric shocks, etc. A case
study on Cluster satellite data is also introduced.Comment: accepted by AS
A multi-species synthesis of physiological mechanisms in drought-induced tree mortality
Widespread tree mortality associated with drought 92 has been observed on all forested continents, and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water, and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analyzed across species and biomes using a standardized physiological framework. Here we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function
A multi-species synthesis of physiological mechanisms in drought-induced tree mortality
Widespread tree mortality associated with drought 92 has been observed on all forested continents, and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water, and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analyzed across species and biomes using a standardized physiological framework. Here we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function
Proximity effect at superconducting Sn-Bi2Se3 interface
We have investigated the conductance spectra of Sn-Bi2Se3 interface junctions
down to 250 mK and in different magnetic fields. A number of conductance
anomalies were observed below the superconducting transition temperature of Sn,
including a small gap different from that of Sn, and a zero-bias conductance
peak growing up at lower temperatures. We discussed the possible origins of the
smaller gap and the zero-bias conductance peak. These phenomena support that a
proximity-effect-induced chiral superconducting phase is formed at the
interface between the superconducting Sn and the strong spin-orbit coupling
material Bi2Se3.Comment: 7 pages, 8 figure
Heavy Quarks and Heavy Quarkonia as Tests of Thermalization
We present here a brief summary of new results on heavy quarks and heavy
quarkonia from the PHENIX experiment as presented at the "Quark Gluon Plasma
Thermalization" Workshop in Vienna, Austria in August 2005, directly following
the International Quark Matter Conference in Hungary.Comment: 8 pages, 5 figures, Quark Gluon Plasma Thermalization Workshop
(Vienna August 2005) Proceeding
Centrality Dependence of the High p_T Charged Hadron Suppression in Au+Au collisions at sqrt(s_NN) = 130 GeV
PHENIX has measured the centrality dependence of charged hadron p_T spectra
from central Au+Au collisions at sqrt(s_NN)=130 GeV. The truncated mean p_T
decreases with centrality for p_T > 2 GeV/c, indicating an apparent reduction
of the contribution from hard scattering to high p_T hadron production. For
central collisions the yield at high p_T is shown to be suppressed compared to
binary nucleon-nucleon collision scaling of p+p data. This suppression is
monotonically increasing with centrality, but most of the change occurs below
30% centrality, i.e. for collisions with less than about 140 participating
nucleons. The observed p_T and centrality dependence is consistent with the
particle production predicted by models including hard scattering and
subsequent energy loss of the scattered partons in the dense matter created in
the collisions.Comment: 7 pages text, LaTeX, 6 figures, 2 tables, 307 authors, resubmitted to
Phys. Lett. B. Revised to address referee concerns. Plain text data tables
for the points plotted in figures for this and previous PHENIX publications
are publicly available at
http://www.phenix.bnl.gov/phenix/WWW/run/phenix/papers.htm
Single Electrons from Heavy Flavor Decays in p+p Collisions at sqrt(s) = 200 GeV
The invariant differential cross section for inclusive electron production in
p+p collisions at sqrt(s) = 200 GeV has been measured by the PHENIX experiment
at the Relativistic Heavy Ion Collider over the transverse momentum range $0.4
<= p_T <= 5.0 GeV/c at midrapidity (eta <= 0.35). The contribution to the
inclusive electron spectrum from semileptonic decays of hadrons carrying heavy
flavor, i.e. charm quarks or, at high p_T, bottom quarks, is determined via
three independent methods. The resulting electron spectrum from heavy flavor
decays is compared to recent leading and next-to-leading order perturbative QCD
calculations. The total cross section of charm quark-antiquark pair production
is determined as sigma_(c c^bar) = 0.92 +/- 0.15 (stat.) +- 0.54 (sys.) mb.Comment: 329 authors, 6 pages text, 3 figures. Submitted to Phys. Rev. Lett.
Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
Nuclear Modification of Electron Spectra and Implications for Heavy Quark Energy Loss in Au+Au Collisions at sqrt(s_NN)=200 GeV
The PHENIX experiment has measured mid-rapidity transverse momentum spectra
(0.4 < p_T < 5.0 GeV/c) of electrons as a function of centrality in Au+Au
collisions at sqrt(s_NN)=200 GeV. Contributions from photon conversions and
from light hadron decays, mainly Dalitz decays of pi^0 and eta mesons, were
removed. The resulting non-photonic electron spectra are primarily due to the
semi-leptonic decays of hadrons carrying heavy quarks. Nuclear modification
factors were determined by comparison to non-photonic electrons in p+p
collisions. A significant suppression of electrons at high p_T is observed in
central Au+Au collisions, indicating substantial energy loss of heavy quarks.Comment: 330 authors, 6 pages text, 3 figures. Submitted to Phys. Rev. Lett.
Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
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