1,965 research outputs found
Identified Hadrons and Jet Chemistry for p+p and Au+Au Collisions at RHIC
The study of hadron spectra at high in p+p collisions provides a good
test of perturbative quantum chromo-dynamic calculations (pQCD) and baseline
for measurements of nuclear modification factors in Au+Au collisions. Using
events triggered by the Barrel Electro-Magnetic Calorimeter, identified charged
hadron transverse momentum () spectra are measured up to 15 GeV/ at
mid-rapidity ( 0.5) and neutral kaon spectra up to 12
GeV/ in p + p collisions at = 200 GeV. The particle ratios
of , and / in p + p
collisions are shown and compared with next-to-leading order pQCD calculations.
In central Au+Au collisions, we report nuclear modification factors ()
for pion, kaon, proton and and discuss several model calculations:
color-charge dependence of jet quenching and jet conversion. Finally,
centrality dependence of at high ( 5.5 GeV/c) for kaon are
compared with that of pion in Au + Au collisions at 200 GeV.Comment: 6 pages, 4 figures, SQM 2009 contributio
Testing the Color Charge and Mass Dependence of Parton Energy Loss with Heavy-to-light Ratios at RHIC and LHC
The ratio of nuclear modification factors of high-pT heavy-flavored mesons to
light-flavored hadrons (``heavy-to-light ratio'') in nucleus-nucleus collisions
tests the partonic mechanism expected to underlie jet quenching. Heavy-to-light
ratios are mainly sensitive to the mass and color-charge dependences of
medium-induced parton energy loss. Here, we assess the potential for
identifying these two effects in D and B meson production at RHIC and at the
LHC. To this end, we supplement the perturbative QCD factorized formalism for
leading hadron production with radiative parton energy loss. For D meson
spectra at high but experimentally accessible transverse momentum (10 < pT < 20
GeV) in Pb-Pb collisions at the LHC, we find that charm quarks behave
essentially like light quarks. However, since light-flavored hadron yields are
dominated by gluon parents, the heavy-to-light ratio of D mesons is a sensitive
probe of the color charge dependence of parton energy loss. In contrast, due to
the larger b quark mass, the medium modification of B mesons in the same
kinematical regime provides a sensitive test of the mass dependence of parton
energy loss. At RHIC energies, the strategies for identifying and disentangling
the color charge and mass dependence of parton energy loss are more involved
because of the smaller kinematical range accessible. We argue that at RHIC, the
kinematical regime best suited for such an analysis of D mesons is 7 < pT < 12
GeV, whereas the study of lower transverse momenta is further complicated due
to the known dominant contribution of additional, particle species dependent,
non-perturbative effects.Comment: 21 pages RevTex, 9 Figure
Violation of Wiedemann-Franz law at the Kondo breakdown quantum critical point
We study both the electrical and thermal transport near the heavy-fermion
quantum critical point (QCP), identified with the breakdown of the Kondo effect
as an orbital selective Mott transition. We show that the contribution to the
electrical conductivity comes mainly from conduction electrons while the
thermal conductivity is given by both conduction electrons and localized
fermions (spinons), scattered with dynamical exponent . This scattering
mechanism gives rise to a quasi-linear temperature dependence of the electrical
and thermal resistivity. The characteristic feature of the Kondo breakdown
scenario turns out to be emergence of additional entropy carriers, that is,
spinon excitations. As a result, we find that the Wiedemann-Franz ratio should
be larger than the standard value, a fact which enables to differentiate the
Kondo breakdown scenario from the Hertz-Moriya-Millis framework
Substituted bisphosphanylamines as ligands in gold(I) chemistry – synthesis and structures
Dimethyl 5-aminoisophthalate, which is a building block of amino-substituted tetralactam macrocycles, was used as ligand in gold(I) chemistry to form model complexes for macrocyclic gold compounds. Reaction of dimethyl 5-aminoisophthalate with chlorodiphenylphosphine gave the diphosphine compound dimethyl 5-[N,N-bis(diphenylphosphanyl)amino]isophthalate (dmbpaip). This compound can further be reacted with [AuCl(tht)] (tht = tetrahydrothiophene) to give the dinuclear complex [Au(2),Cl(2)(dmbpaip)]. In contrast, treatment of dinbpaip with [Au(tht)(2)]ClO(4) resulted in the ionic compound [Au(2)(dmbpaip)(2)](ClO(4))(2) in which the cation forms an eight-membered Au(2)P(4)N(2) heterocycle. In both gold(I) compounds Au center dot center dot center dot Au interactions are observed. All new compounds were characterized by single-crystal X-ray diffraction
Reconstructing the Freeze-out State in Pb + Pb Collisions at 158 AGeV/c
For a class of analytical parametrizations of the freeze-out state of relativistic heavy ion collisions, we perform a simultaneous analysis of the single-particle m_t-spectra and two-particle Bose-Einstein correlations measured in central Pb+Pb collisions at the CERN SPS. The analysis includes a full model parameter scan with chi^2 confidence levels. A comparison of different transverse density profiles for the particle emission region allows for a quantitative discussion of possible model dependencies of the results. Our fit results suggest a low thermal freeze-out temperature T approximately 95 +- 15 MeV and a large average transverse flow velocity of about 0.55c +- 0.07c. Moreover, the fit favours a box-shaped transverse density profile over a Gaussian one. We discuss the origins and the consequences of these results in detail. In order to reproduce the measured pion multiplicity our model requires a positive pion chemical potential. A study of the pion phase-space density indicates \mu_\pi approximately 60 MeV for T = 100 MeV
Bond activation in iron(II) and nickel(II) complexes of polypodal phosphanes
A pyridine-derived tetraphosphane ligand (donor set: NP4) has been found to undergo remarkably specific C-P bond cleavage reactions, thereby producing a ligand with an NP3 donor set. The reaction may be reversed under suitable conditions, with regeneration of the original NP4 ligand. In order to investigate the mechanism of this reaction, the NP3 donor ligand C5H3N[CMe(CH2PMe2)2][CMe2(CH2PMe2)] (11) was prepd., and its iron(II) complex 4 generated from Fe(BF4)2·6 H2O, with Me diethylphosphinite (7) as an addnl. monodentate ligand. Ligand 11 has, in addn. to the NP3 donor set, one Me group in close contact with the iron center, reminiscent of an agostic M···H-C interaction. Depending on the stoichiometric amt. of iron(II) salt, a side product 15 is formed, which has a diethylphosphane ligand instead of the phosphinite 7 coordinated to iron(II). While attempts to deprotonate the metal-coordinated Me group in 4 were unsuccessful, the reaction was shown to occur in an alternative complex (18), which is similar to 4 but has a trimethylphosphane ligand instead of the phosphinite 7. The reaction of complex 15 with CO gave two different products, which were both characterized by single-crystal X-ray diffraction. One (19) is the dicarbonyl iron(II) complex of the triphosphane ligand 11, the other (3) is the carbonyl iron(II) complex of the tetraphosphane C5H3N[CMe(CH2PMe2)2]2 (1). This suggests an intermol. mechanism for the C-P bond formation in question. [on SciFinder(R)
Infrared Observations During the Secondary Eclipse of HD 209458b: I. 3.6-Micron Occultation Spectroscopy Using the VLT
We search for an infrared signature of the transiting extrasolar planet HD
209458b during secondary eclipse. Our method, which we call `occultation
spectroscopy,' searches for the disappearance and reappearance of weak spectral
features due to the exoplanet as it passes behind the star and later reappears.
We argue that at the longest infrared wavelengths, this technique becomes
preferable to conventional `transit spectroscopy'. We observed the system in
the wing of the strong nu-3 band of methane near 3.6 microns during two
secondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolution
of 3300. Our analysis, which utilizes a model template spectrum, achieves
sufficient precision to expect detection of the spectral structure predicted by
an irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibrium
model for the exoplanet atmosphere. However, our observations show no evidence
for the presence of this spectrum from the exoplanet, with the statistical
significance of the non-detection depending on the timing of the secondary
eclipse, which depends on the assumed value for the orbital eccentricity. Our
results reject certain specific models of the atmosphere of HD 209458b as
inconsistent with our observations at the 3-sigma level, given assumptions
about the stellar and planetary parameters.Comment: 26 pages, 8 figures Accepted to Astrophysical Journa
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