20,847 research outputs found
Gluon Transport Equation with Effective Mass and Dynamical Onset of Bose-Einstein Condensation
We study the transport equation describing a dense system of gluons, in the
small scattering angle approximation, taking into account medium-generated
effective masses of the gluons. We focus on the case of overpopulated systems
that are driven to Bose-Einstein condensation on their way to thermalization.
The presence of a mass modifies the dispersion relation of the gluon, as
compared to the massless case, but it is shown that this does not change
qualitatively the scaling behavior in the vicinity of the onset.Comment: 25 pages, 8 figure
Anomalous Chiral Transport in Heavy Ion Collisions from Anomalous-Viscous Fluid Dynamics
Chiral anomaly is a fundamental aspect of quantum theories with chiral
fermions. How such microscopic anomaly manifests itself in a macroscopic
many-body system with chiral fermions, is a highly nontrivial question that has
recently attracted significant interest. As it turns out, unusual transport
currents can be induced by chiral anomaly under suitable conditions in such
systems, with the notable example of the Chiral Magnetic Effect (CME) where a
vector current (e.g. electric current) is generated along an external magnetic
field. A lot of efforts have been made to search for CME in heavy ion
collisions, by measuring the charge separation effect induced by the CME
transport. A crucial challenge in such effort, is the quantitative prediction
for the CME signal. In this paper, we develop the Anomalous-Viscous Fluid
Dynamics (AVFD) framework, which implements the anomalous fluid dynamics to
describe the evolution of fermion currents in QGP, on top of the neutral bulk
background described by the VISH2+1 hydrodynamic simulations for heavy ion
collisions. With this new tool, we quantitatively and systematically
investigate the dependence of the CME signal to a series of theoretical inputs
and associated uncertainties. With realistic estimates of initial conditions
and magnetic field lifetime, the predicted CME signal is quantitatively
consistent with measured change separation data in 200GeV Au-Au collisions.
Based on analysis of Au-Au collisions, we further make predictions for the CME
observable to be measured in the planned isobaric (Ru-Ru v.s. Zr-Zr ) collision
experiment, which could provide a most decisive test of the CME in heavy ion
collisions.Comment: 28 pages, 13 figures; published versio
Quantification of Chiral Magnetic Effect from Event-by-Event Anomalous-Viscous Fluid Mechanics
Chiral Magnetic Effect (CME) is the macroscopic manifestation of the
fundamental chiral anomaly in a many-body system of chiral fermions, and
emerges as anomalous transport current in hydrodynamic framework. Experimental
observation of CME is of great interest and significant efforts have been made
to look for its signals in heavy ion collisions. Encouraging evidence of
CME-induced charge separation has been reported from both RHIC and LHC, albeit
with ambiguity due to potential background contributions. Crucial for
addressing such issue, is the need of quantitative predictions for both CME
signal and the non-CME background consistently, with sophisticated modeling
tool. In this contribution we report a recently developed Anomalous Viscous
Fluid Dynamics (AVFD) framework, which simulates the evolution of fermion
currents in QGP on top of the data-validated VISHNU bulk hydro evolution. In
particular, this framework has been extended to event-by-event simulations with
proper implementation of known flow-driven background contributions. We report
quantitative results from such simulations and evaluate the implications for
interpretations of current experimental measurements. Finally we give our
prediction for the CME signal in upcoming isobaric collisions.Comment: 5 pages, 7 figures; plenary talk at CPOD 2017 conference, Stony Brook
University, Stony Brook, NY. arXiv admin note: substantial text overlap with
arXiv:1704.05531; text overlap with arXiv:1611.0458
Off-Policy Evaluation of Probabilistic Identity Data in Lookalike Modeling
We evaluate the impact of probabilistically-constructed digital identity data
collected from Sep. to Dec. 2017 (approx.), in the context of
Lookalike-targeted campaigns. The backbone of this study is a large set of
probabilistically-constructed "identities", represented as small bags of
cookies and mobile ad identifiers with associated metadata, that are likely all
owned by the same underlying user. The identity data allows to generate
"identity-based", rather than "identifier-based", user models, giving a fuller
picture of the interests of the users underlying the identifiers. We employ
off-policy techniques to evaluate the potential of identity-powered lookalike
models without incurring the risk of allowing untested models to direct large
amounts of ad spend or the large cost of performing A/B tests. We add to
historical work on off-policy evaluation by noting a significant type of
"finite-sample bias" that occurs for studies combining modestly-sized datasets
and evaluation metrics involving rare events (e.g., conversions). We illustrate
this bias using a simulation study that later informs the handling of inverse
propensity weights in our analyses on real data. We demonstrate significant
lift in identity-powered lookalikes versus an identity-ignorant baseline: on
average ~70% lift in conversion rate. This rises to factors of ~(4-32)x for
identifiers having little data themselves, but that can be inferred to belong
to users with substantial data to aggregate across identifiers. This implies
that identity-powered user modeling is especially important in the context of
identifiers having very short lifespans (i.e., frequently churned cookies). Our
work motivates and informs the use of probabilistically-constructed identities
in marketing. It also deepens the canon of examples in which off-policy
learning has been employed to evaluate the complex systems of the internet
economy.Comment: Accepted by WSDM 201
Quantifying the Chiral Magnetic Effect from Anomalous-Viscous Fluid Dynamics
In this contribution we report a recently developed Anomalous-Viscous Fluid
Dynamics (AVFD) framework, which simulates the evolution of fermion currents in
QGP on top of the bulk expansion from data-validated VISHNU hydrodynamics. With
reasonable estimates of initial conditions and magnetic field lifetime, the
predicted CME signal is quantitatively consistent with change separation
measurements in 200GeV Au-Au collisions at RHIC. We further develop the
event-by-event AVFD simulations that allow direct evaluation of two-particle
correlations arising from CME signal as well as the non-CME backgrounds.
Finally we report predictions from AVFD simulations for the upcoming isobaric
(Ru-Ru v.s. Zr-Zr ) collisions that could provide the critical test of the CME
in heavy ion collisions.Comment: Contribution to the Proceedings of the XXVIth International
Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter
2017), Feb 5-11, Chicago, U.S.A. 4 pages, 6 figure
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