532 research outputs found
Tungsten Behavior at High Temperature and High Stress
Recently reported results on the tungsten lifetime/fatigue tests under conditions expected in the Neutrino Factory target have strengthened the case of solid target option for a Neutrino Factory. This paper gives description of the detailed measurements of the tungsten properties at high temperature and high stress. We have performed extensive set of measurements of the surface displacement and velocity of the tungsten wires that were stressed by passing a fast, high current pulse through a thin sample. Radial and longitudinal oscillations of the wire were measured by a Laser Doppler Vibrometer. The wire was operated at temperatures of 300-2500 K by adjusting the pulse repetition rate. In doing so we have tried to simulate the conditions (high stress and temperature) expected at the Neutrino Factory. Most important result of this study is an experimental confirmation that strength of tungsten remains high at high temperature and high stress. The experimental results have been found to agree very well with LS-DYNA modelling results
Parton rescattering and screening in Au+Au collisions at RHIC
We study the microscopic dynamics of quarks and gluons in relativistic heavy
ion collisions in the framework of the Parton Cascade Model. We use lowest
order perturbative QCD cross sections with fixed lower momentum cutoff p_0. We
calculate the time-evolution of the Debye-screening mass for Au+Au collisions
at sqrt(s)=200 GeV per nucleon pair. The screening mass is used to determine a
lower limit for the allowed range of p_0. We also determine the energy density
reached through hard and semi-hard processes at RHIC, obtain a lower bound for
the rapidity density of charged hadrons produced by semihard interactions, and
analyze the extent of perturbative rescattering among partons.Comment: 6 pages, 4 figures, uses RevTeX 4.0; revised version with minor
corrections and one updated figur
Boost Invariance and Multiplicity Dependence of the Charge Balance Functionin and Collisions at GeV/c
Boost invariance and multiplicity dependence of the charge balance function
are studied in \pi^{+}\rp and \rK^{+}\rp collisions at 250 GeV/ incident
beam momentum. Charge balance, as well as charge fluctuations, are found to be
boost invariant over the whole rapidity region, but both depend on the size of
the rapidity window. It is also found that the balance function becomes
narrower with increasing multiplicity, consistent with the narrowing of the
balance function when centrality and/or system size increase, as observed in
current relativistic heavy ion experiments.Comment: 4 pages, 5 figures, Revte
Transverse flow and hadro-chemistry in Au+Au collisions at \sqrt{s_{NN}}=200 GeV
We present a hydrodynamic assessment of preliminary particle spectra observed
in Au+Au collisions at \sqrt{s_{NN}}=200 GeV. The hadronic part of the
underlying equation of state is based on explicit conservation of (measured)
particle ratios throughout the resonance gas stage after chemical freezeout by
employing chemical potentials for stable mesons, nucleons and anti-nucleons. We
find that under these conditions the data (in particular the proton spectra)
favor a low freeze-out temperature of around 100 MeV. Furthermore we show that
through inclusion of a moderate pre-hydrodynamic transverse flow field the
shape of the spectra improves with respect to the data. The effect of the
initial transverse boost on elliptic flow and the freeze-out geometry of the
system is also elucidated.Comment: as published: more data included in Fig. 1, discussions throughout
the text improved, 6 pages, 4 figure
Anisotropic Flow from RHIC to the LHC
Anisotropic flow is recognized as one of the main observables providing
information on the early stage of a heavy-ion collision. At RHIC the large
observed anisotropic flow and its successful description by ideal hydrodynamics
is considered evidence for an early onset of thermalization and almost ideal
fluid properties of the produced strongly coupled Quark Gluon Plasma. This
write-up discusses some key RHIC anisotropic flow measurements and for
anisotropic flow at the LHC some predictions.Comment: 4 pages, 6 figures, hotquarks 200
Hydrodynamics and Flow
In this lecture note, we present several topics on relativistic hydrodynamics
and its application to relativistic heavy ion collisions. In the first part we
give a brief introduction to relativistic hydrodynamics in the context of heavy
ion collisions. In the second part we present the formalism and some
fundamental aspects of relativistic ideal and viscous hydrodynamics. In the
third part, we start with some basic checks of the fundamental observables
followed by discussion of collective flow, in particular elliptic flow, which
is one of the most exciting phenomenon in heavy ion collisions at relativistic
energies. Next we discuss how to formulate the hydrodynamic model to describe
dynamics of heavy ion collisions. Finally, we conclude the third part of the
lecture note by showing some results from ideal hydrodynamic calculations and
by comparing them with the experimental data.Comment: 40 pages, 35 figures; lecture given at the QGP Winter School, Jaipur,
India, Feb.1-3, 2008; to appear in Springer Lecture Notes in Physic
Synthesis of Strategies and the Hoare Logic of Angelic Nondeterminism
Abstract. We study a propositional variant of Hoare logic that can be used for reasoning about programs that exhibit both angelic and demonic nondeterminism. We work in an uninterpreted setting, where the mean-ing of the atomic actions is specified axiomatically using hypotheses of a certain form. Our logical formalism is entirely compositional and it sub-sumes the non-compositional formalism of safety games on finite graphs. We present sound and complete Hoare-style (partial-correctness) calculi that are useful for establishing Hoare assertions, as well as for synthesiz-ing implementations. The computational complexity of the Hoare theory of dual nondeterminism is investigated using operational models, and it is shown that the theory is complete for exponential time.
A model for net-baryon rapidity distribution
In nuclear collisions, a sizable fraction of the available energy is carried
away by baryons. As the baryon number is conserved, the net-baryon
retains information on the energy-momentum carried by the incoming nuclei. A
simple and consistent model for net-baryon production in high energy
proton-proton and nucleus-nucleus collisions is presented. The basic
ingredients of the model are valence string formation based on standard PDFs
with QCD evolution and string fragmentation via the Schwinger mechanism. The
results of the model are presented and compared with data at different
centre-of-mass energies and centralities, as well as with existing models.
These results show that a good description of the main features of net-baryon
data is possible in the framework of a simplistic model, with the advantage of
making the fundamental production mechanisms manifest.Comment: 9 pages, 12 figures; in fig. 11 a) the vertical scale was correcte
Chemical freeze-out temperature in hydrodynamical description of Au+Au collisions at sqrt(s_NN) = 200 GeV
We study the effect of separate chemical and kinetic freeze-outs to the ideal
hydrodynamical flow in Au+Au collisions at RHIC (sqrt(s_NN) = 200 GeV energy).
Unlike in earlier studies we explore how these effects can be counteracted by
changes in the initial state of the hydrodynamical evolution. We conclude that
the reproduction of pion, proton and antiproton yields necessitates a chemical
freeze-out temperature of T = 150 MeV instead of T = 160 - 170 MeV motivated by
thermal models. Unlike previously reported, this lower temperature makes it
possible to reproduce the p_T-spectra of hadrons if one assumes very small
initial time, tau_0 = 0.2 fm/c. However, the p_T-differential elliptic flow,
v_2(p_T) remains badly reproduced. This points to the need to include
dissipative effects (viscosity) or some other refinement to the model.Comment: 8 pages, 7 figures; Accepted for publication in European Physical
Journal A; Added discussion about the effect of weak decays to chemical
freeze-out temperature and a figure showing isentropic curves in T-mu plan
Collective flow and two-pion correlations from a relativistic hydrodynamic model with early chemical freeze out
We investigate the effect of early chemical freeze-out on radial flow,
elliptic flow and HBT radii by using a fully three dimensional hydrodynamic
model. When we take account of the early chemical freeze-out, the space-time
evolution of temperature in the hadron phase is considerably different from the
conventional model in which chemical equilibrium is always assumed. As a
result, we find that radial and elliptic flows are suppressed and that the
lifetime and the spatial size of the fluid are reduced. We analyze the p_t
spectrum, the differential elliptic flow, and the HBT radii at the RHIC energy
by using hydrodynamics with chemically non-equilibrium equation of state.Comment: One subsection and two figures adde
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