1,324 research outputs found
Aspects of the Screening Length and Drag Force in Two Alternative Gravity Duals of the Quark-gluon Plasma
We compute the screening length of mesons with different angular momentum
from two gravity dual theories. Both the asymptotically and
Sakai-Sugimoto metrics are considered in the calculations. Using the dual
description of the quark as a classical string ending on the probe brane, we
obtain the interacting potential between the heavy quark and antiquark after
rotating the background metric. The result shows that the screening length of
mesons with different is well fit to . The constant is
determined for by taking advantage of numerical techniques. Finally,
we calculate the drag force and relaxation times from the Sakai-Sugimoto metric
and compare with the ones obtained in the . The application of our
result to charmonium and bottomonium at RHIC and LHC is briefly discussed.Comment: 18 pages, 6 figures, 2 tables, version to appear in JHEP,
acknowledgment adde
Drag force in a string model dual to large-N QCD
We compute the drag force exerted on a quark and a di-quark systems in a
background dual to large-N QCD at finite temperature. We find that appears a
drag force in the former setup with flow of energy proportional to the mass of
the quark while in the latter there is no dragging as in other studies. We also
review the screening length.Comment: 15 pages, typos removed, error corrected, refs adde
A Tiling Approach to Counting Inherent Structures in Hard Potential Systems
The number of distinguishable inherent structures of a liquid is the key
component to understanding the thermodynamics of glass formers. In the case of
hard potential systems such as hard discs, spheres and ellipsoids, an inherent
structure corresponds to a collectively jammed configuration. This work
develops a tiling based approach to counting inherent structures that
constructs packings by combining sets of elementary locally jammed structures
but eliminates those final packings that either, do not tile space, or are not
collectively jammed, through the use of tile incompatibility rules. The
resulting theory contains a number of geometric quantities, such as the number
of growth sites on a tile and the number of tile compatibilities that provide
insight into the number of inherent structures in certain limits. We also show
that these geometric quantities become quite simple in a system of highly
confined hard discs.Comment: 12 pages, 2 figures. Accepted for Publication : Journal of
Non-Crystalline Solid
Investigation of Ultrasonic Assisted Friction Stir Spot Welding of Magnesium Alloy to Aluminum Alloy
A new welding method: ultrasonic assisted friction stir spot welding (UAFSSW) was put forward in the present study. UAFSSW was successfully applied to weld dissimilar AZ31 Mg alloy and 6061 Al alloy. Results show that for either conventional FSSW or UAFSSW, sound joints are obtained in the configuration of upper Mg alloy and lower Al alloy. Ultrasonic vibration is beneficial to the upward flow of lower aluminum alloy, the increase of the stir zone (SZ) width and the refinement of the grains in the SZ. All cross sections of the Al–Mg joints exhibit the formation of intermetallic compounds (IMC) in the SZ. The crack of the conventional FSSW joint propagates exactly along the interface between the dissimilar materials and presents an inverted “V” morphology. After reaching the highest point of the hook defect, crack of the UAFSSW joint extends to the keyhole, leaving a portion of Mg alloy on the lower sheet. Conventional FSSW joint and UAFSSW joint show different IMC compositions at the faying interface
Search for Invisible Decays of and in and
Using a data sample of decays collected with the BES
II detector at the BEPC, searches for invisible decays of and
in to and are performed.
The signals, which are reconstructed in final states, are used
to tag the and decays. No signals are found for the
invisible decays of either or , and upper limits at the 90%
confidence level are determined to be for the ratio
and for . These are the first
searches for and decays into invisible final states.Comment: 5 pages, 4 figures; Added references, Corrected typo
Observation of Two New N* Peaks in J/psi -> and Decays
The system in decays of is limited to be
isospin 1/2 by isospin conservation. This provides a big advantage in studying
compared with and experiments which mix
isospin 1/2 and 3/2 for the system. Using 58 million decays
collected with the Beijing Electron Positron Collider, more than 100 thousand
events are obtained. Besides two well known
peaks at 1500 MeV and 1670 MeV, there are two new, clear peaks in
the invariant mass spectrum around 1360 MeV and 2030 MeV. They are the
first direct observation of the peak and a long-sought "missing"
peak above 2 GeV in the invariant mass spectrum. A simple
Breit-Wigner fit gives the mass and width for the peak as MeV and MeV, and for the new peak above 2 GeV
as MeV and MeV, respectively
Static Observers in Curved Spaces and Non-inertial Frames in Minkowski Spacetime
Static observers in curved spacetimes may interpret their proper acceleration
as the opposite of a local gravitational field (in the Newtonian sense). Based
on this interpretation and motivated by the equivalence principle, we are led
to investigate congruences of timelike curves in Minkowski spacetime whose
acceleration field coincides with the acceleration field of static observers of
curved spaces. The congruences give rise to non-inertial frames that are
examined. Specifically we find, based on the locality principle, the embedding
of simultaneity hypersurfaces adapted to the non-inertial frame in an explicit
form for arbitrary acceleration fields. We also determine, from the Einstein
equations, a covariant field equation that regulates the behavior of the proper
acceleration of static observers in curved spacetimes. It corresponds to an
exact relativistic version of the Newtonian gravitational field equation. In
the specific case in which the level surfaces of the norm of the acceleration
field of the static observers are maximally symmetric two-dimensional spaces,
the energy-momentum tensor of the source is analyzed.Comment: 28 pages, 4 figures
Star cluster formation and star formation: the role of environment and star-formation efficiencies
“The original publication is available at www.springerlink.com”. Copyright Springer. DOI: 10.1007/s10509-009-0088-5By analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to determine whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star-formation rate, burst strength, star-formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few billion years later.Peer reviewe
Drag and jet quenching of heavy quarks in a strongly coupled N=2* plasma
The drag of a heavy quark and the jet quenching parameter are studied in the
strongly coupled N=2* plasma using the AdS/CFT correspondence. Both increase in
units of the spatial string tension as the theory departs from conformal
invariance. The description of heavy quark dynamics using a Langevin equation
is also considered. It is found that the difference between the velocity
dependent factors of the transverse and longitudinal momentum broadening of the
quark admit an interpretation in terms of relativistic effects, so the
distribution is spherical in the quark rest frame. When conformal invariance is
broken there is a broadening of the longitudinal momentum distribution. This
effect may be useful in understanding the jet distribution observed in
experiments.Comment: 30 pages, 5 figures, references added, minor corrections. To be
published in JHE
Drag force in SYM plasma with B field from AdS/CFT
We investigate drag force in a thermal plasma of N=4 super Yang-Mills theory
via both fundamental and Dirichlet strings under the influence of non-zero NSNS
-field background. In the description of AdS/CFT correspondence the endpoint
of these strings correspondes to an external monopole or quark moving with a
constant electromagnetic field. We demonstrate how the configuration of string
tail as well as the drag force obtains corrections in this background.Comment: 13 pages, 2 figures, more discussion and reference adde
- …