1,091 research outputs found
Temperature dependent sound velocity in hydrodynamic equations for relativistic heavy-ion collisions
We analyze the effects of different forms of the sound-velocity function
cs(T) on the hydrodynamic evolution of matter formed in the central region of
relativistic heavy-ion collisions. At high temperatures (above the critical
temperature Tc) the sound velocity is calculated from the recent lattice
simulations of QCD, while in the low temperature region it is obtained from the
hadron gas model. In the intermediate region we use different interpolations
characterized by the values of the sound velocity at the local maximum (at T =
0.4 Tc) and local minimum (at T = Tc). In all considered cases the temperature
dependent sound velocity functions yield the entropy density, which is
consistent with the lattice QCD simulations at high temperature. Our
calculations show that the presence of a distinct minimum of the sound velocity
leads to a very long (about 20 fm/c) evolution time of the system, which is not
compatible with the recent estimates based on the HBT interferometry. Hence, we
conclude that the hydrodynamic description is favored in the case where the
cross-over phase transition renders the smooth sound velocity function with a
possible shallow minimum at Tc.Comment: 6 pages, 3 figures, talk given at SQM'07 Levoca, Slovaki
On the formation of Hubble flow in Little Bangs
A dynamical appearance of scaling solutions in the relativistic hydrodynamics
applied to describe ultra-relativistic heavy-ion collisions is studied. We
consider the boost-invariant cylindrically symmetric systems and the effects of
the phase transition are taken into account by using a temperature dependent
sound velocity inferred from the lattice simulations of QCD. We find that the
transverse flow acquires the scaling form r/t within the short evolution times,
10 - 15 fm, only if the initial transverse flow originating from the
pre-equilibrium collective behavior is present at the initial stage of the
hydrodynamic evolution. The amount of such pre-equilibrium flow is correlated
with the initial pressure gradient; larger gradients require smaller initial
flow. The results of the numerical calculations support the phenomenological
parameterizations used in the Blast-Wave, Buda-Lund, and Cracow models of the
freeze-out process.Comment: 11 page
Solution of the RHIC HBT puzzle with Gaussian initial conditions
It is argued that the consistent description of the transverse-momentum
spectra, elliptic flow, and the HBT radii in the relativistic heavy-ion
collisions studied at RHIC may be obtained within the hydrodynamic model if one
uses the Gaussian profile for the initial energy density in the transverse
plane. Moreover, we show that the results obtained in the scenario with an
early start of hydrodynamics (at the proper time tau0 = 0.25 fm) are
practically equivalent to the results obtained in the model where the
hydrodynamics is preceded by the free-streaming stage of partons (in the proper
time interval 0.25 fm < tau < 1 fm) which suddenly equilibrate and with the
help of the Landau matching conditions are transformed into the hydrodynamic
regime (at the proper time tau0 = 1 fm).Comment: talk presented by WF at SQM2008 Conferenc
Charge dependent azimuthal correlations in Pb--Pb collisions at TeV
Separation of charges along the extreme magnetic field created in non-central
relativistic heavy--ion collisions is predicted to be a signature of local
parity violation in strong interactions. We report on results for charge
dependent two particle azimuthal correlations with respect to the reaction
plane for Pb--Pb collisions at TeV recorded in 2010 with
ALICE at the LHC. The results are compared with measurements at RHIC energies
and against currently available model predictions for LHC. Systematic studies
of possible background effects including comparison with conventional
(parity-even) correlations simulated with Monte Carlo event generators of
heavy--ion collisions are also presented.Comment: Published in the proceedings of "Quark Matter 2011", Annecy-Franc
Conductance spectra of (Nb, Pb, In)/NbP -- superconductor/Weyl semimetal junctions
The possibility of inducing superconductivity in type-I Weyl semimetal
through coupling its surface to a superconductor was investigated. A single
crystal of NbP, grown by chemical vapor transport method, was carefully
characterized by XRD, EDX, SEM, ARPES techniques and by electron transport
measurements. The mobility spectrum of the carriers was determined. For the
studies of interface transmission, the (001) surface of the crystal was covered
by several hundred nm thick metallic layers of either Pb, or Nb, or In. DC
current-voltage characteristics and AC differential conductance through the
interfaces as a function of the DC bias were investigated. When the metals
become superconducting, all three types of junctions show conductance increase,
pointing out the Andreev reflection as a prevalent contribution to the subgap
conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is
satisfactorily described by modified Blonder-Tinkham-Klapwijk model. The
absolute value of the conductance is much smaller than that for the bulk
crystal, indicating that the transmission occurs through only a small part of
the contact area. An opposite situation occurs in In-NbP junction, where the
conductance at the peak reaches the bulk value indicating that almost whole
contact area is transmitting and, additionally, a superconducting proximity
phase is formed in the material. We interpret this as a result of indium
diffusion into NbP, where the metal atoms penetrate the surface barrier and
form very transparent superconductor-Weyl semimetal contact inside. However,
further diffusion occurring already at room temperature leads to degradation of
the effect, so it is observed only in the pristine structures. Despite of this,
our observation directly demonstrates possibility of inducing superconductivity
in a type-I Weyl semimetal.Comment: Accepted for Phys. Rev. B. 13 pages, 12 figures. Second version with
major revisions. The title was changed. One author R. Jakiela added. New
inset to Fig. 8(A). New fits in Fig. 8 (B) and Fig. 10 (B). Added figures 12
(C)-(E). Added Fig. 12 (F) with SIMS data. Rewritten chapters III-C-2 and
III-C-3. Reference no. 38 removed, 11 new references: 9, 21, 22, 40-44, 46-49
were adde
Wounded nucleon model with realistic nucleon-nucleon collision profile and observables in relativistic heavy-ion collisions
We investigate the influence of the nucleon-nucleon collision profile
(probability of interaction as a function of the nucleon-nucleon impact
parameter) in the wounded nucleon model and its extensions on several
observables measured in relativistic heavy-ion collisions. We find that the
participant eccentricity coefficient, , as well as the higher
harmonic coefficients, , are reduced by 10-20% for
mid-peripheral collisions when the realistic (Gaussian) profile is used, as
compared to the case with the commonly-used hard-sphere profile. Similarly, the
multiplicity fluctuations, treated as the function of the number of wounded
nucleons in one of the colliding nuclei, are reduced by 10-20%. This
demonstrates that the Glauber Monte Carlo codes should necessarily use the
realistic nucleon-nucleon collision profile in precision studies of these
observables. The Gaussian collision profile is built-in in {\tt GLISSANDO}.Comment: 8 pages, 7 figure
Measurement of pion, kaon and proton production in proton-proton collisions at TeV
The measurement of primary , K, p and
production at mid-rapidity ( 0.5) in proton-proton collisions at
TeV performed with ALICE (A Large Ion Collider Experiment) at
the Large Hadron Collider (LHC) is reported. Particle identification is
performed using the specific ionization energy loss and time-of-flight
information, the ring-imaging Cherenkov technique and the kink-topology
identification of weak decays of charged kaons. Transverse momentum spectra are
measured from 0.1 up to 3 GeV/ for pions, from 0.2 up to 6 GeV/ for kaons
and from 0.3 up to 6 GeV/ for protons. The measured spectra and particle
ratios are compared with QCD-inspired models, tuned to reproduce also the
earlier measurements performed at the LHC. Furthermore, the integrated particle
yields and ratios as well as the average transverse momenta are compared with
results at lower collision energies.Comment: 33 pages, 19 captioned figures, 3 tables, authors from page 28,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/156
Overcoming the Challenges Associated with Image-based Mapping of Small Bodies in Preparation for the OSIRIS-REx Mission to (101955) Bennu
The OSIRIS-REx Asteroid Sample Return Mission is the third mission in NASA's
New Frontiers Program and is the first U.S. mission to return samples from an
asteroid to Earth. The most important decision ahead of the OSIRIS-REx team is
the selection of a prime sample-site on the surface of asteroid (101955) Bennu.
Mission success hinges on identifying a site that is safe and has regolith that
can readily be ingested by the spacecraft's sampling mechanism. To inform this
mission-critical decision, the surface of Bennu is mapped using the OSIRIS-REx
Camera Suite and the images are used to develop several foundational data
products. Acquiring the necessary inputs to these data products requires
observational strategies that are defined specifically to overcome the
challenges associated with mapping a small irregular body. We present these
strategies in the context of assessing candidate sample-sites at Bennu
according to a framework of decisions regarding the relative safety,
sampleability, and scientific value across the asteroid's surface. To create
data products that aid these assessments, we describe the best practices
developed by the OSIRIS-REx team for image-based mapping of irregular small
bodies. We emphasize the importance of using 3D shape models and the ability to
work in body-fixed rectangular coordinates when dealing with planetary surfaces
that cannot be uniquely addressed by body-fixed latitude and longitude.Comment: 31 pages, 10 figures, 2 table
Charge separation relative to the reaction plane in Pb-Pb collisions at TeV
Measurements of charge dependent azimuthal correlations with the ALICE
detector at the LHC are reported for Pb-Pb collisions at TeV. Two- and three-particle charge-dependent azimuthal correlations in
the pseudo-rapidity range are presented as a function of the
collision centrality, particle separation in pseudo-rapidity, and transverse
momentum. A clear signal compatible with a charge-dependent separation relative
to the reaction plane is observed, which shows little or no collision energy
dependence when compared to measurements at RHIC energies. This provides a new
insight for understanding the nature of the charge dependent azimuthal
correlations observed at RHIC and LHC energies.Comment: 12 pages, 3 captioned figures, authors from page 2 to 6, published
version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/286
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