1,335 research outputs found
Open heavy flavor production at RHIC
The study of heavy flavor production in relativistic heavy ion collisions is
an extreme experimental challenge but provides important information on the
properties of the Quark-Gluon Plasma (QGP) created in Au+Au collisions at RHIC.
Heavy-quarks are believed to be produced in the initial stages of the
collision, and are essential on the understanding of parton energy loss in the
dense medium created in such environment. Moreover, heavy-quarks can help to
investigate fundamental properties of QCD in elementary p+p collisions. In this
work we review recent results on heavy flavor production and their interaction
with the hot and dense medium at RHIC.Comment: Quark Matter 2006 proceedings, 8 pages, 5 figure
Anisotropy of Imbalanced Alfvenic Turbulence in Fast Solar Wind
We present the first measurement of the scale-dependent power anisotropy of
Elsasser variables in imbalanced fast solar wind turbulence. The dominant
Elsasser mode is isotropic at lower spacecraft frequencies but becomes
increasingly anisotropic at higher frequencies. The sub-dominant mode is
anisotropic throughout, but in a scale-independent way (at higher frequencies).
There are two distinct subranges exhibiting different scalings within what is
normally considered the inertial range. The low Alfven ratio and shallow
scaling of the sub-dominant Elsasser mode suggest an interpretation of the
observed discrepancy between the velocity and magnetic field scalings. The
total energy is dominated by the latter. These results do not appear to be
fully explained by any of the current theories of incompressible imbalanced MHD
turbulence.Comment: 5 pages, 2 figure
Scaling anisotropy of the power in parallel and perpendicular components of the solar wind magnetic field
Power spectra of the components of the magnetic field parallel (Pzz) and perpendicular (Pzz+Pyy) to the local mean magnetic field direction were determined by wavelet methods from Ulysses’ MAG instrument data during eighteen 10-day segments of its first North Polar pass at high latitude at solar minimum in 1995. The power depends on frequency f and the angle θ between the solar wind direction and the local mean field, and with distance from the Sun. This data includes the solar wind whose total power (Pxx + Pyy + Pzz) in magnetic fluctuations we previously reported depends on f and the angle θ nearly as predicted by the GS95 critical balance model of strong incompressible MHD turbulence. Results at much wider range of frequencies during six evenly-spaced 10-day periods are presented here to illustrate the variability and evolution with distance from the Sun. Here we investigate the aniso tropic scaling of Pzz(f,θ) in particular because it is a reduced form of the Poloidal (pseudo-Alfvenic) component of the (incompressible) fluctuations. We also report the much larger Pxx(f,θ)+Pyy(f,θ) which is (mostly) reduced from the Toroidal (Alfvenic, i.e., perpendicular to both B and k) fluctuations, and comprises most of the total power. These different components of the total power evolve and scale differently in the inertial range. We compare these elements of the magnetic power spectral tensor with “critical balance” model predictions
Anisotropy of Solar Wind Turbulence between Ion and Electron Scales
The anisotropy of turbulence in the fast solar wind, between the ion and
electron gyroscales, is directly observed using a multispacecraft analysis
technique. Second order structure functions are calculated at different angles
to the local magnetic field, for magnetic fluctuations both perpendicular and
parallel to the mean field. In both components, the structure function value at
large angles to the field S_perp is greater than at small angles S_par: in the
perpendicular component S_perp/S_par = 5 +- 1 and in the parallel component
S_perp/S_par > 3, implying spatially anisotropic fluctuations, k_perp > k_par.
The spectral index of the perpendicular component is -2.6 at large angles and
-3 at small angles, in broad agreement with critically balanced whistler and
kinetic Alfven wave predictions. For the parallel component, however, it is
shallower than -1.9, which is considerably less steep than predicted for a
kinetic Alfven wave cascade.Comment: 4 pages, 4 figures, replaced to match published versio
Synthesis and Reactions of Halogenated Polyethers and Polysulfides
Polyepichlorohydrin (PECH) is well known as a reactive elastomer. Displacement at the carbon-chlorine bond of PECH has been accomplished with a wide variety of nucleophilic reagents, for the purposes of
polymer modification, grafting and cross1inking. On the other hand, the
PECH structure is hardly optimal from the point of view of its reactivity
as a substrate for nucleophilic substitution: chloride is modest in its
leaving group ability, and the β-branch point (i.e. the chain backbone) would
be expected to depress reaction rates by a factor of 10 or so
Energy Loss of a Heavy Quark Produced in a Finite Size Medium
We study the medium-induced energy loss suffered by a
heavy quark produced at initial time in a quark-gluon plasma, and escaping the
plasma after travelling the distance . The heavy quark is treated
classically, and within the same framework consistently
includes: the loss from standard collisional processes, initial bremsstrahlung
due to the sudden acceleration of the quark, and transition radiation. The
radiative loss {\it induced by rescatterings} is not
included in our study. For a ultrarelativistic heavy quark with momentum p
\gsim 10 {\rm GeV}, and for a finite plasma with L_p \lsim 5 {\rm fm}, the
loss is strongly suppressed compared to the stationary
collisional contribution . Our results
support that is the dominant contribution to the heavy quark
energy loss (at least for L_p \lsim 5 {\rm fm}), as indeed assumed in most of
jet-quenching analyses. However they might raise some question concerning the
RHIC data on large electron spectra.Comment: 18 pages, 3 figures. New version clarified and simplified. A critical
discussion added in section 2, and previous sections 3 and 4 have been merged
together. Main results are unchange
An Experimental Overview of Results Presented at SQM 2006
I have been asked to give an critical overview on the experimental results
shown in the conference with a emphasis of what has been learned and the
challenges that are ahead in trying to understand the physics of the strongly
interacting quark-gluon plasma. I will not try to summarize all of the results
presented, rather I will concentrate primarily on RHIC data from this
conference. Throughout this summary, I will periodically review some of the
previous results for those not familiar with the present state of the field.Comment: 15 pages, 12 Figure
Angular hadron correlations probing the early medium evolution
Hard processes are a well calibrated probe to study heavy-ion collisions.
However, the information to be gained from the nuclear suppression factor R_AA
is limited, hene one has to study more differential observables to do medium
tomography. The angular correlations of hadrons associated with a hard trigger
appear suitable as they show a rich pattern when going from low p_T to high
p_T. Of prime interest is the fate of away side partons with an in-medium
pathlength O(several fm). At high p_T the correlations become dominated by the
punchtrough of the away side parton with subsequent fragmentation. We discuss
what information about the medium density can be gained from the data.Comment: Talk given at the 19th International Conference on Ultrarelativistic
Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China,
14-20 Nov 200
ERRATUM: Correlations at Large Scales and the Onset of Turbulence in the Fast Solar Wind
We show that the scaling of structure functions of magnetic and velocity fields in a mostly highly Alfvenic fast solar wind stream depends strongly on the joint distribution of the dimensionless measures of cross helicity and residual energy. Already at very low frequencies, fluctuations that are both more balanced (cross helicity approx. 0) and equipartitioned (residual energy approx.0) have steep structure functions reminiscent of "turbulent" scalings usually associated with the inertial range. Fluctuations that are magnetically dominated (residual energy approx. 1), and so have closely anti-aligned Elsasser-field vectors, or are imbalanced (cross helicity approx. 1), and so have closely aligned magnetic and velocity vectors, have wide "1/f" ranges typical of fast solar wind. We conclude that the strength of nonlinear interactions of individual fluctuations within a stream, diagnosed by the degree of correlation in direction and magnitude of magnetic and velocity fluctuations, determines the extent of the 1/f region observed, and thus the onset scale for the turbulent cascade
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