878 research outputs found
Thermal photons from fluctuating initial conditions
Event-by-event fluctuations of initial QCD-matter density produced in
heavy-ion collisions at RHIC enhance the production of thermal photons
significantly in the region GeV/ compared to a smooth
initial-state averaged profile in the ideal hydrodynamic calculation. This
enhancement is a an early time effect due to the presence of hotspots or
over-dense regions in the fluctuating initial state. The effect of fluctuations
is found to be stronger in peripheral than in central collisions.Comment: 4 pages, 3 figures. Talk given at Quark Matter 2011, 22-28 May 2011,
Annecy, Franc
Collision centrality and dependence of the emission of thermal photons from fluctuating initial state in ideal hydrodynamic calculation
Fluctuations in the initial QCD matter density distribution are found to
enhance the production of thermal photons significantly in the range 2 \leq pT
\leq 4 GeV/c compared to a smooth initial state averaged profile in ideal
hydrodynamic calculation for 200 AGeV Au+Au collisions at the Relativistic
Heavy Ion Collider (RHIC) and 2.76 ATeV Pb+Pb collisions at the Large Hadron
Collider (LHC). The thermal emission of photons is strongly dependent on the
initial temperature of the system where the presence of 'hotspots' in the
initial state translates into enhanced production of photons compared to a
smooth profile. The effect of fluctuations in the initial state is found to be
stronger for peripheral collisions and for lower beam energies. The pT spectra
are found to be quite sensitive to the value of the initial formation time of
the plasma which is not known unambiguously and which may vary with collision
centralities at a particular beam energy. Increase in the value of the
formation time lowers the production of thermal photons compared to the results
from a shorter formation time. However, the relative enhancement from
fluctuating initial tates (compared to a smooth initial state) is found to be
stronger for the larger values of formation time. The pT spectra alone are
found to be insufficient to quantify the fluctuations in the initial density
distribution due to the uncertainties in the initial conditions. A suitably
normalized ratio of central-to-peripheral yield as a function of collision
centrality and pT can be a useful measure of the fluctuation size scale.Comment: 10 pages, 10 figure
Simple Electromagnetic Motor Model for Torsional Analysis of Variable Speed Drives with an Induction Motor
Torsional vibrations must be considered in the design of all high-power drive-trains including an induction motor. Electromagnetic (EM) field in the air gap of an induction motor generates additional magnetic stiffness and damping between the rotor and stator. The inclusion of these magnetic effects is limited by the availability of simple and portable motor models. The main aim of this paper is to introduce a motor model including the speed and torque variation. The presented model is based on the linearization of the common space-vector models of induction motors. The parameters of this model are identified for the rated operating condition. This motor model can be extended to include variable speed and torque operation. The numerical results demonstrate that this model describes accurately the magnetic effects over the large speed and torque range. In addition, the numerical results demonstrate the significance of magnetic stiffness and damping in variable speed motor-driven compressors with a soft coupling
Dynamical freeze-out in event-by-event hydrodynamics
In hydrodynamical modeling of the ultrarelativistic heavy-ion collisions the
freeze-out is typically performed at a constant temperature or density. In this
work we apply a dynamical freeze-out criterion, which compares the
hydrodynamical expansion rate with the pion scattering rate. Recently many
calculations have been done using event-by-event hydrodynamics where the
initial density profile fluctuates from event to event. In these event-by-event
calculations the expansion rate fluctuates strongly as well, and thus it is
interesting to check how the dynamical freeze-out changes hadron distributions
with respect to the constant temperature freeze-out. We present hadron spectra
and elliptic flow calculated using (2+1)-dimensional ideal hydrodynamics, and
show the differences between constant temperature and dynamical freeze-out
criteria. We find that the differences caused by different freeze-out criteria
are small in all studied cases.Comment: 8 pages, 11 figures, to be published in the proceedings of the 28th
Winter Workshop on Nuclear Dynamics, Puerto Rico, April 7-14, 201
Asymptotic Dirichlet Problem for A-Harmonic Functions on Manifolds with Pinched Curvature
We study the asymptotic Dirichlet problem for -harmonic functions on a Cartan-Hadamard manifold whose radial sectional curvatures outside a compact set satisfy an upper bound and a pointwise pinching condition for some constants epsilon > 0 and C (K) a 1, where P and are any 2-dimensional subspaces of T (x) M containing the (radial) vector acr(x) and r(x) = d(o, x) is the distance to a fixed point o a M. We solve the asymptotic Dirichlet problem with any continuous boundary data . The results apply also to the Laplacian and p-Laplacian, as special cases.Peer reviewe
Unravelling the functions of biogenic volatiles in boreal and temperate forest ecosystems
Living trees are the main source of biogenic volatile organic compounds (BVOCs) in forest ecosystems, but substantial emissions originate from leaf and wood litter, the rhizosphere and from microorganisms. This review focuses on temperate and boreal forest ecosystems and the roles of BVOCs in ecosystem function, from the leaf to the forest canopy and from the forest soil to the atmosphere level. Moreover, emphasis is given to the question of how BVOCs will help forests adapt to environmental stress, particularly biotic stress related to climate change. Trees use their vascular system and emissions of BVOCs in internal communication, but emitted BVOCs have extended the communication to tree population and whole community levels and beyond. Future forestry practices should consider the importance of BVOCs in attraction and repulsion of attacking bark beetles, but also take an advantage of herbivore-induced BVOCs to improve the efficiency of natural enemies of herbivores. BVOCs are extensively involved in ecosystem services provided by forests including the positive effects on human health. BVOCs have a key role in ozone formation but also in ozone quenching. Oxidation products form secondary organic aerosols that disperse sunlight deeper into the forest canopy, and affect cloud formation and ultimately the climate. We also discuss the technical side of reliable BVOC sampling of forest trees for future interdisciplinary studies that should bridge the gaps between the forest sciences, health sciences, chemical ecology, conservation biology, tree physiology and atmospheric science
System size dependence of nuclear modification and azimuthal anisotropy of jet quenching
We investigate the system size dependence of jet-quenching by analyzing
transverse momentum spectra of neutral pions in Au+Au and Cu+Cu collisions at
=200 GeV for different centralities. The fast partons
are assumed to lose energy by radiating gluons as they traverse the plasma and
undergo multiple collisions. The energy loss per collision, , is
taken as proportional to (where is the energy of the parton),
proportional to , or a constant depending on whether the formation
time of the gluon is less than the mean path, greater than the mean free path
but less than the path length, or greater than the path length of the partons,
respectively. NLO pQCD is used to evaluate pion production by modifying the
fragmentation function to account for the energy loss. We reproduce the nuclear
modification factor by treating as the only free
parameter, depending on the centrality and the mechanism of energy loss. These
values are seen to explain the nuclear modification of prompt photons, caused
by the energy lost by final state quarks before they fragment into photons.
These also reproduce the azimuthal asymmetry of transverse momentum
distribution for pions within a factor of two and for prompt photons in a fair
agreement with experimental data.Comment: 26 pages, 17 figures. One more figure added. Discussion expanded.
Typographical corrections done, several references added. To appear in
Journal of Physics
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