4,190 research outputs found
Online-offline activities and game-playing behaviors of avatars in a massive multiplayer online role-playing game
Massive multiplayer online role-playing games (MMORPGs) are very popular in
China, which provides a potential platform for scientific research. We study
the online-offline activities of avatars in an MMORPG to understand their
game-playing behavior. The statistical analysis unveils that the active avatars
can be classified into three types. The avatars of the first type are owned by
game cheaters who go online and offline in preset time intervals with the
online duration distributions dominated by pulses. The second type of avatars
is characterized by a Weibull distribution in the online durations, which is
confirmed by statistical tests. The distributions of online durations of the
remaining individual avatars differ from the above two types and cannot be
described by a simple form. These findings have potential applications in the
game industry.Comment: 6 EPL pages including 10 eps figure
Shear Viscosity of a Gluon Plasma in Perturbative QCD
We calculate the shear viscosity to entropy density ratio of a gluon plasma in kinetic theory including the gg->gg and ggggg
processes. Due to the suppressed contribution to in the gg->gg forward
scattering, it is known that the gluon bremsstrahlung ggggg process also
contributes at the same order in perturbative QCD. Using
the Gunion-Bertsch formula for the ggggg matrix element which is valid for
the limit of soft bremsstrahlung, we find that the result is sensitive to
whether the same limit is taken for the phase space. Using the exact phase
space, the ggggg contribution becomes more important to than gg->gg
for . Therefore, at , , between 2.7 obtained by Arnold, Moore and Yaffe (AMY) and 0.5
obtained by Xu and Greiner. If the soft bremsstrahlung limit is imposed on the
phase space such that the recoil effect from the bremsstrahlung gluon is
neglected, then the correction from the ggggg process is about 10-30% of the
total which is close to AMY's prediction. This shows that the soft
bremsstrahlung approximation is not as good as previously expected.Comment: RevTex 4, 14 pages, 3 figures; The results for the soft
bremsstrahlung limit for the phase space are added. The difference between
AMY and XG approach is addressed with more clarificatio
How Perfect a Gluon Plasma Can Be in Perturbative QCD?
The shear viscosity to entropy density ratio, \eta /s, characterizes how
perfect a fluid is. We calculate the leading order \eta /s of a gluon plasma in
perturbation using the kinetic theory. The leading order contribution only
involves the elastic gg -> gg (22) process and the inelastic ggggg (23)
process. The Hard-Thermal-Loop (HTL) treatment is used for the 22 matrix
element, while the exact matrix element in vacuum is supplemented by the gluon
Debye mass insertion for the 23 process. Also, the asymptotic mass is used for
the external gluons in the kinetic theory. The errors from not implementing HTL
and the Landau-Pomeranchuk-Migdal effect in the 23 process, and from the
uncalculated higher order corrections, are estimated. Our result for \eta /s
lies between that of Arnold, Moore and Yaffe (AMY) and Xu and Greiner (XG). Our
result shows that although the finite angle contributions are important at
intermediate \alpha_s (\alpha_s \sim 0.01-0.1), the 22 process is still more
important than 23 when \alpha_s < 0.1. This is in qualitative agreement with
AMY's result. We find no indication that the proposed perfect fluid limit \eta
/s \simeq 1/(4\pi) can be achieved by perturbative QCD alone.Comment: ReVTex 4, 11 pages, 5 figures. A coding error in the exact matrix
element for the 23 process is corrected. Results in Fig. 2,3 and Table I are
re-calculated, and relevant discussions are adjusted. Part of the conclusion
is change
Unifying ultrafast demagnetization and intrinsic Gilbert damping in Co/Ni bilayers with electronic relaxation near the Fermi surface
The ability to controllably manipulate the laser-induced ultrafast magnetic
dynamics is a prerequisite for future high speed spintronic devices. The
optimization of devices requires the controllability of the ultrafast
demagnetization time, , and intrinsic Gilbert damping, . In previous attempts
to establish the relationship between and , the rare-earth doping of a
permalloy film with two different demagnetization mechanism is not a suitable
candidate. Here, we choose Co/Ni bilayers to investigate the relations between
and by means of time-resolved magneto-optical Kerr effect (TRMOKE) via
adjusting the thickness of the Ni layers, and obtain an approximately
proportional relation between these two parameters. The remarkable agreement
between TRMOKE experiment and the prediction of breathing Fermi-surface model
confirms that a large Elliott-Yafet spin-mixing parameter is relevant to the
strong spin-orbital coupling at the Co/Ni interface. More importantly, a
proportional relation between and in such metallic films or heterostructures
with electronic relaxation near Fermi surface suggests the local spin-flip
scattering domains the mechanism of ultrafast demagnetization, otherwise the
spin-current mechanism domains. It is an effective method to distinguish the
dominant contributions to ultrafast magnetic quenching in metallic
heterostructures by investigating both the ultrafast demagnetization time and
Gilbert damping simultaneously. Our work can open a novel avenue to manipulate
the magnitude and efficiency of Terahertz emission in metallic heterostructures
such as the perpendicular magnetic anisotropic Ta/Pt/Co/Ni/Pt/Ta multilayers,
and then it has an immediate implication of the design of high frequency
spintronic devices
- …