932 research outputs found
Enhanced antiproton production in Pb(160 AGeV)+Pb reactions: evidence for quark gluon matter?
The centrality dependence of the antiproton per participant ratio is studied
in Pb(160 AGeV)+Pb reactions. Antiproton production in collisions of heavy
nuclei at the CERN/SPS seems considerably enhanced as compared to conventional
hadronic physics, given by the antiproton production rates in and
antiproton annihilation in reactions. This enhancement is consistent
with the observation of strong in-medium effects in other hadronic observables
and may be an indication of partial restoration of chiral symmetry
Strangeness Enhancement in Heavy Ion Collisions - Evidence for Quark-Gluon-Matter ?
The centrality dependence of (multi-)strange hadron abundances is studied for
Pb(158 AGeV)Pb reactions and compared to p(158 GeV)Pb collisions. The
microscopic transport model UrQMD is used for this analysis. The predicted
Lambda/pi-, Xi-/pi- and Omega-/pi- ratios are enhanced due to rescattering in
central Pb-Pb collisions as compared to peripheral Pb-Pb or p-Pb collisions. A
reduction of the constituent quark masses to the current quark masses m_s \sim
230 MeV, m_q \sim 10 MeV, as motivated by chiral symmetry restoration, enhances
the hyperon yields to the experimentally observed high values. Similar results
are obtained by an ad hoc overall increase of the color electric field strength
(effective string tension of kappa=3 GeV/fm). The enhancement depends strongly
on the kinematical cuts. The maximum enhancement is predicted around
midrapidity. For Lambda's, strangeness suppression is predicted at
projectile/target rapidity. For Omega's, the predicted enhancement can be as
large as one order of magnitude. Comparisons of Pb-Pb data to proton induced
asymmetric (p-A) collisions are hampered due to the predicted strong asymmetry
in the various rapidity distributions of the different (strange) particle
species. In p-Pb collisions, strangeness is locally (in rapidity) not
conserved. The present comparison to the data of the WA97 and NA49
collaborations clearly supports the suggestion that conventional (free)
hadronic scenarios are unable to describe the observed high (anti-)hyperon
yields in central collisions. The doubling of the strangeness to nonstrange
suppression factor, gamma_s \approx 0.65, might be interpreted as a signal of a
phase of nearly massless particles.Comment: published version, discussion on strange mesons and new table added,
extended discussion on strange baryon yields. Latex, 20 pages, including 5
eps-figure
Exact results in planar N=1 superconformal Yang-Mills theory
In the \beta-deformed N=4 supersymmetric SU(N) Yang-Mills theory we study the
class of operators O_J = Tr(\Phi_i^J \Phi_k), i\neq k and compute their exact
anomalous dimensions for N,J\to\infty. This leads to a prediction for the
masses of the corresponding states in the dual string theory sector. We test
the exact formula perturbatively up to two loops. The consistency of the
perturbative calculation with the exact result indicates that in the planar
limit the one--loop condition g^2=h\bar{h} for superconformal invariance is
indeed sufficient to insure the {\em exact} superconformal invariance of the
theory. We present a direct proof of this point in perturbation theory. The O_J
sector of this theory shares many similarities with the BMN sector of the N=4
theory in the large R--charge limit.Comment: LaTex, 14 pages, 3 figures; v2: minor corrections and one reference
adde
Universal Scaling of Strong-Field Localization in an Integer Quantum Hall Liquid
We study the Landau level localization and scaling properties of a disordered
two-dimensional electron gas in the presence of a strong external magnetic
field. The impurities are treated as random distributed scattering centers with
parameterized potentials. Using a transfer matrix for a finite-width strip
geometry, we calculate the localization length as a function of system size and
electron energy. The finite-size localization length is determined by
calculating the Lyapunov exponents of the transfer matrix. A detailed
finite-size scaling analysis is used to study the critical behavior near the
center of the Landau bands. The influence of varying the impurity
concentration, the scattering potential range and its nature, and the Landau
level index on the scaling behavior and on the critical exponent is
systematically investigated. Particular emphasis is put on studying the effects
of finite range of the disorder potential and Landau level coupling on the
quantum localization behavior. Our numerical results, which are carried out on
systems much larger than those studied before, indicate that pure
-function disorder in the absence of any Landau level coupling gives
rise to non-universal localization properties with the critical exponents in
the lowest two Landau levels being substantially different. Inclusion of a
finite potential range and/or Landau level mixing may be essential in producing
universality in the localization.Comment: 28 pages, Latex, 17 figures (available upon request), #phd0
Geant4 simulation of the response of phosphor screens for X-ray imaging
poster Pistrui-Maximean, submitted to NIM AIn order to predict and optimize the response of phosphor screens, it is important to understand the role played by the different physical processes inside the scintillator layer. Monte Carlo simulations were carried out to determine the Modulation Transfer Function (MTF) of phosphor screens for energies used in X-ray medical imaging and nondestructive testing applications. The visualization of the dose distribution inside the phosphor layer gives an insight into how the MTF is progressively degraded by X-ray and electron transport. The simulation model allows to study the inïŹuence of physical and technological parameters on the detector performances, as well as to design and optimize new detector conïŹgurations. Preliminary MTF measurements have been carried out and agreement with experimental data has been found in the case of a commercial screen (Kodak Lanex Fine), at an X-ray tube potential voltage of 100 kV. Further validation with other screens (transparent or granular) at different energies is under way
Coherent photon bremsstrahlung and dynamics of heavy-ion collisions: comparison of different models
Differential spectra of coherent photon bremsstrahlung in relativistic heavy
ion collisions are calculated within various schematic models of the
projectile-target stopping. Two versions of the degradation length model, based
on a phenomenological deceleration law, are considered. The simple shock wave
model is studied analytically. The predictions of these models agree in the
soft photon limit, where the spectrum is determined only by the final velocity
distribution of charged particles. The results of these models in the case of
central Au+Au collisions at various bombarding energies are compared with the
predictions of the microscopic transport model UrQMD. It is shown that at the
AGS energy the coherent photon bremsstrahlung exceeds the photon yield from
-decays at photon energies \omega\loo 50 MeV.Comment: 23 pages RevTeX, 9 eps Figure
gamma_i Deformed Lax Pair for Rotating Strings in the Fast Motion Limit
A 3-parameter generalization of the Lunin-Maldacena background has recently
been constructed by Frolov. This gamma_i-deformed background is
non-supersymmetric. We consider strings in this gamma_i-deformed R \times S^5
background rotating in three orthogonal planes (the 3-spin sector) in a fast
motion limit, in which the total angular momentum J is assumed to be large. We
show that there exists a consistent transformation which takes the undeformed
equations of motion into the gamma_i-deformed equations of motion. This
transformation is used to construct a Lax pair for the bosonic part of the
gamma_i-deformed theory in the fast motion limit. This implies the
integrability of the bosonic part of the gamma_i-deformed string sigma model in
the fast motion limit.Comment: 21 page
Effect of Nyquist Noise on the Nyquist Dephasing Rate in 2d Electron Systems
We measure the effect of externally applied broadband Nyquist noise on the
intrinsic Nyquist dephasing rate of electrons in a two-dimensional electron gas
at low temperatures. Within the measurement error, the phase coherence time is
unaffected by the externally applied Nyquist noise, including applied noise
temperatures of up to 300 K. The amplitude of the applied Nyquist noise from
100 MHz to 10 GHz is quantitatively determined in the same experiment using a
microwave network analyzer.Comment: 5 pages, 4 figures. Author affiliation clarified; acknowledgements
modified. Replacement reason clarifie
Semiclassical Strings, Dipole Deformations of N=1 SYM and Decoupling of KK Modes
In this paper we investigate the recently found -deformed
Maldacena-Nunez background by studying the behavior of different semiclassical
string configurations. This background is conjectured to be dual to dipole
deformations of SYM. We compare our results to those in the pure
Maldacena-Nunez background and show that the energies of our string
configurations are higher than in the undeformed background. Thinking in the
lines of (hep-th/0505100) we argue that this is an evidence for better
decoupling of the Kaluza-Klein modes from the pure SYM theory excitations.
Moreover we are able to find a limit of the background in which the string
energy is independent of , these strings are interpreted as
corresponding to pure gauge theory effects.Comment: 31 pages, references added, new solutions in Section 7 presented, an
appendix added, to appear in JHE
Electron beam induced oxidation of surfaces of Ni3Al-base alloys
This letter concentrates on phenomena of the electron beam (e-beam) induced oxidation of surfaces of Ni3Al-base alloys. In particular it is shown that an e-beam may contribute substantially to the oxidation behavior of polycrystalline Ni3Al and Ni3AlâB surfaces during Auger analysis at room temperature. Upon e-beam exposure oxidation occurs rapidly, even though any regime of oxygen chemisorption is absent. Auger peak-to-peak oxygen curves for Ni3Al(âB) surfaces appear to support the model based on the premise that the e-beam creates additional nucleation sites for oxidation. In contrast, for pure Ni containing B in solid solution a chemisorption regime is indeed present which is then followed by a rapid formation of oxides that saturates quickly at a time scale similar to that of the Ni3Al-base systems. Although Al does not participate in the e-beam induced oxidation, the cross-sections for creation of oxide sites are found to be drastically higher than those of Ni/B and pure Ni.
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