6,815 research outputs found
Event-by-event shape and flow fluctuations of relativistic heavy-ion collision fireballs
Heavy-ion collisions create deformed quark-gluon plasma (QGP) fireballs which
explode anisotropically. The viscosity of the fireball matter determines its
ability to convert the initial spatial deformation into momentum anisotropies
that can be measured in the final hadron spectra. A quantitatively precise
empirical extraction of the QGP viscosity thus requires a good understanding of
the initial fireball deformation. This deformation fluctuates from event to
event, and so does the finally observed momentum anisotropy. We present a
harmonic decomposition of the initial fluctuations in shape and orientation of
the fireball and perform event-by-event ideal fluid dynamical simulations to
extract the resulting fluctuations in the magnitude and direction of the
corresponding harmonic components of the final anisotropic flow at midrapidity.
The final harmonic flow coefficients are found to depend non-linearly on the
initial harmonic eccentricity coefficients. We show that, on average, initial
density fluctuations suppress the buildup of elliptic flow relative to what one
obtains from a smooth initial profile of the same eccentricity, and discuss
implications for the phenomenological extraction of the QGP shear viscosity
from experimental elliptic flow data.Comment: 22 pages, 17 figures. Relative to [v2], minor changes in text. Fig. 9
redrawn. This version accepted by Phys. Rev.
Azimuth quadrupole component spectra on transverse rapidity for identified hadrons from Au-Au collisions at 200 GeV
I present the first isolation of azimuth quadrupole components from published
data (called elliptic flow) as spectra on transverse rapidity
for identified pions, kaons and lambdas/protons from minimum-bias Au-Au
collisions at 200 GeV. The form of the spectra on indicates that the
three hadron species are emitted from a common boosted source with boost
. The quadrupole spectra have a L\'evy form similar to
the soft component of the single-particle spectrum, but with significantly
reduced () slope parameters . Comparison of quadrupole
spectra with single-particle spectra suggests that the quadrupole component
comprises a small fraction (%) of the total hadron yield, contradicting
the hydrodynamic picture of a thermalized, flowing bulk medium. The form of
is, within a constant factor, the product of ( in the
boost frame) times the ratio of quadrupole spectrum to single-particle
spectrum. That ratio in turn implies that above 0.5 GeV/c the form of
is dominated by the hard component of the single-particle spectrum
(interpreted as due to minijets). It is therefore unlikely that so-called {\em
constituent-quark scaling} attributed to is relevant to soft hadron
production mechanisms (e.g., chemical freezeout).Comment: 22 pages, 17 figure
Initial temperature and EoS of quark matter from direct photons
The time evolution of the quark gluon plasma created in gold-gold collisions
of the Relativistic Heavy Ion Collider (RHIC) can be described by
hydrodynamical models. Distribution of hadrons reflects the freeze-out state of
the matter. To investigate the time evolution one needs to analyze penetrating
probes, such as direct photon spectra. Distributions of low energy photons was
published in 2010 by PHENIX. In this paper we analyze a 3+1 dimensional
solution of relativistic hydrodynamics and calculate momentum distribution of
direct photons. Using earlier fits of this model to hadronic spectra, we
compare photon calculations to measurements and find that the initial
temperature of the center of the fireball is at least 519+-12 MeV, while for
the equation of state we get c_s= 0.36+-0.02.Comment: Talk at the VI Workshop on Particle Correlations and Femtoscopy,
Kiev, September 14-18, 2010. 6 pages, 1 figure. This work was supported by
the OTKA grant NK73143 and M. Csanad's Bolyai scholarshi
Interplay of shear and bulk viscosity in generating flow in heavy-ion collisions
We perform viscous hydrodynamic calculations in 2+1 dimensions to investigate
the influence of bulk viscosity on the viscous suppression of elliptic flow in
non-central heavy-ion collisions at RHIC energies. Bulk and shear viscous
effects on the evolution of radial and elliptic flow are studied with different
model assumptions for the transport coefficients. We find that the temperature
dependence of the relaxation time for the bulk viscous pressure, especially its
critical slowing down near the quark-hadron phase transition at T_c, partially
offsets effects from the strong growth of the bulk viscosity itself near T_c,
and that even small values of the specific shear viscosity eta/s of the
fireball matter can be extracted without large uncertainties from poorly
controlled bulk viscous effects.Comment: 13 pages, 7 figures, 1 table. Submitted to Physical Review C. v2:
corrected typos in several entries in Table
Identification of the YfgF MASE1 domain as a modulator of bacterial responses to aspartate
Complex 3'-5'-cyclic diguanylic acid (c-di-GMP) responsive regulatory networks that are modulated by the action of multiple diguanylate cyclases (DGC; GGDEF domain proteins) and phosphodiesterases (PDE; EAL domain proteins) have evolved in many bacteria. YfgF proteins possess a membrane-anchoring domain (MASE1), a catalytically inactive GGDEF domain and a catalytically active EAL domain. Here, sustained expression of the Salmonella enterica spp. Enterica ser. Enteritidis YfgF protein is shown to mediate inhibition of the formation of the aspartate chemotactic ring on motility agar under aerobic conditions. This phenomenon was c-di-GMP-independent because it occurred in a Salmonella strain that lacked the ability to synthesize c-di-GMP and also when PDE activity was abolished by site-directed mutagenesis of the EAL domain. YfgF-mediated inhibition of aspartate chemotactic ring formation was impaired in the altered redox environment generated by exogenous p-benzoquinone. This ability of YfgF to inhibit the response to aspartate required a motif, (213)Lys-Lys-Glu(215), in the predicted cytoplasmic loop between trans-membrane regions 5 and 6 of the MASE1 domain. Thus, for the first time the function of a MASE1 domain as a redox-responsive regulator of bacterial responses to aspartate has been shown
Constraints on models for the initial collision geometry in ultra relativistic heavy ion collisions
Monte Carlo (MC) simulations are used to compute the centrality dependence of
the collision zone eccentricities (), for both spherical and
deformed ground state nuclei, for different model scenarios. Sizable model
dependent differences are observed. They indicate that measurements of the
and order Fourier flow coefficients ,
expressed as the ratio , can provide robust constraints
for distinguishing between different theoretical models for the initial-state
eccentricity. Such constraints could remove one of the largest impediments to a
more precise determination of the specific viscosity from precision
measurements at the Relativistic Heavy Ion Collider (RHIC).Comment: 4 pages, 3 figs - version accepted for publicatio
Mass of Clusters in Simulations
We show that dark matter haloes, in n--body simulations, have a boundary
layer (BL) with precise features. In particular, it encloses all dynamically
stable mass while, outside it, dynamical stability is lost soon. Particles can
pass through such BL, which however acts as a confinement barrier for dynamical
properties. BL is set by evaluating kinetic and potential energies (T(r) and
W(r)) and calculating R=-2T/W. Then, on BL, R has a minimum which closely
approaches a maximum of w= -dlog W/dlog r. Such ``requirement'' is
consistent with virial equilibrium, but implies further regularities. We test
the presence of a BL around haloes in spatially flat CDM simulations, with or
without cosmological constant. We find that the mass M_c, enclosed within the
radius r_c, where the requirement is fulfilled, closely approaches the
mass M_{dyn}, evaluated from the velocities of all particles within r_c,
according to the virial theorem. Using r_c we can then determine an individual
density contrast Delta_c for each virialized halo, which can be compared with
the "virial" density contrast (Omega_m: matter
density parameter) obtained assuming a spherically symmetric and unperturbed
fluctuation growth. The spread in Delta_c is wide, and cannot be neglected when
global physical quantities related to the clusters are calculated, while the
average Delta_c is ~25 % smaller than the corresponding Delta_v; moreover if
is defined from the radius linked to Delta_v, we have a much worse
fit with particle mass then starting from {\it Rw} requirement.Comment: 4 pages, 5 figures, contribution to the XXXVIIth Rencontres de
Moriond, The Cosmological Model, Les Arc March 16-23 2002, to appear in the
proceeding
Feasibility and benefits of laminar flow control on supersonic cruise airplanes
An evaluation was made of the applicability and benefits of laminar flow control (LFC) technology to supersonic cruise airplanes. Ancillary objectives were to identify the technical issues critical to supersonic LFC application, and to determine how those issues can be addressed through flight and wind-tunnel testing. Vehicle types studied include a Mach 2.2 supersonic transport configuration, a Mach 4.0 transport, and two Mach 2-class fighter concepts. Laminar flow control methodologies developed for subsonic and transonic wing laminarization were extended and applied. No intractible aerodynamic problems were found in applying LFC to airplanes of the Mach 2 class, even ones of large size. Improvements of 12 to 17 percent in lift-drag ratios were found. Several key technical issues, such as contamination avoidance and excresence criteria were identified. Recommendations are made for their resolution. A need for an inverse supersonic wing design methodology is indicated
Measuring the Cosmic Equation of State with Counts of Galaxies
The classical dN/dz test allows the determination of fundamental cosmological
parameters from the evolution of the cosmic volume element. This test is
applied by measuring the redshift distribution of a tracer whose evolution in
number density is known. In the past, ordinary galaxies have been used as such
a tracer; however, in the absence of a complete theory of galaxy formation,
that method is fraught with difficulties. In this paper, we propose studying
instead the evolution of the apparent abundance of dark matter halos as a
function of their circular velocity, observable via the linewidths or rotation
speeds of visible galaxies. Upcoming redshift surveys will allow the linewidth
distribution of galaxies to be determined at both z~1 and the present day. In
the course of studying this test, we have devised a rapid, improved
semi-analytic method for calculating the circular velocity distribution of dark
halos based upon the analytic mass function of Sheth et al. (1999) and the
formation time distribution of Lacey & Cole (1993). We find that if selection
effects are well-controlled and minimal external constraints are applied, the
planned DEEP Redshift Survey should allow the measurement of the cosmic
equation-of-state parameter w to 10% (as little as 3% if Omega_m has been
well-determined from other observations). This type of test has the potential
also to provide a constraint on any evolution of w such as that predicted by
``tracker'' models.Comment: 4 pages plus 3 embedded figures; version approved by Ap. J. Letters.
A greatly improved error analysis has been added, along with a figure showing
complementarity to other cosmological test
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