19,696 research outputs found
Heavy flavour energy loss from AdS/CFT: A novel diffusion coefficient
Two AdS/CFT based energy loss models are used to compute the suppression and
azimuthal correlations of heavy quarks in heavy ion collisions. The model with
a velocity independent diffusion coefficient is in good agreement with B and D
meson data up to high . The partonic azimuthal correlations we calculate
exhibit an order of magnitude difference in low momentum correlations to pQCD
calculations [arXiv:1305.3823]. We thus propose heavy flavour momentum
correlations as a distinguishing observable of weakly- and strongly-coupled
energy loss mechanisms.Comment: 4 pages, 2 figures. Proceedings for Strangeness in Quark Matter 2017.
arXiv admin note: text overlap with arXiv:1703.0584
A proof of the S-genus identities for ternary quadratic forms
In this paper we prove the main conjectures of Berkovich and Jagy about
weighted averages of representation numbers over an S-genus of ternary lattices
(defined below) for any odd squarefree S \in N. We do this by reformulating
them in terms of local quantities using the Siegel-Weil and Conway-Sloane
formulas, and then proving the necessary local identities. We conclude by
conjecturing generalized formulas valid over certain totally real number fields
as a direction for future work.Comment: 14 page
Discounting of reward sequences: a test of competing formal models of hyperbolic discounting
Humans are known to discount future rewards hyperbolically in time. Nevertheless, a formal recursive model of hyperbolic discounting has been elusive until recently, with the introduction of the hyperbolically discounted temporal difference (HDTD) model. Prior to that, models of learning (especially reinforcement learning) have relied on exponential discounting, which generally provides poorer fits to behavioral data. Recently, it has been shown that hyperbolic discounting can also be approximated by a summed distribution of exponentially discounted values, instantiated in the μAgents model. The HDTD model and the μAgents model differ in one key respect, namely how they treat sequences of rewards. The μAgents model is a particular implementation of a Parallel discounting model, which values sequences based on the summed value of the individual rewards whereas the HDTD model contains a non-linear interaction. To discriminate among these models, we observed how subjects discounted a sequence of three rewards, and then we tested how well each candidate model fit the subject data. The results show that the Parallel model generally provides a better fit to the human data
Fluidization of collisionless plasma turbulence
In a collisionless, magnetized plasma, particles may stream freely along
magnetic-field lines, leading to phase "mixing" of their distribution function
and consequently to smoothing out of any "compressive" fluctuations (of
density, pressure, etc.,). This rapid mixing underlies Landau damping of these
fluctuations in a quiescent plasma-one of the most fundamental physical
phenomena that make plasma different from a conventional fluid. Nevertheless,
broad power-law spectra of compressive fluctuations are observed in turbulent
astrophysical plasmas (most vividly, in the solar wind) under conditions
conducive to strong Landau damping. Elsewhere in nature, such spectra are
normally associated with fluid turbulence, where energy cannot be dissipated in
the inertial scale range and is therefore cascaded from large scales to small.
By direct numerical simulations and theoretical arguments, it is shown here
that turbulence of compressive fluctuations in collisionless plasmas strongly
resembles one in a collisional fluid and does have broad power-law spectra.
This "fluidization" of collisionless plasmas occurs because phase mixing is
strongly suppressed on average by "stochastic echoes", arising due to nonlinear
advection of the particle distribution by turbulent motions. Besides resolving
the long-standing puzzle of observed compressive fluctuations in the solar
wind, our results suggest a conceptual shift for understanding kinetic plasma
turbulence generally: rather than being a system where Landau damping plays the
role of dissipation, a collisionless plasma is effectively dissipationless
except at very small scales. The universality of "fluid" turbulence physics is
thus reaffirmed even for a kinetic, collisionless system
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