31,788 research outputs found
Energy dependence of the freeze out eccentricity from the azimuthal dependence of HBT at STAR
Non-central heavy ion collisions create an out-of-plane-extended participant
zone that expands toward a more round state as the system evolves. The recent
RHIC Beam Energy Scan at sqrt{s_{NN}} of 7.7, 11.5, and 39 GeV provide an
opportunity to explore the energy dependence of the freeze out eccentricity.
The new low energy data from STAR complements high statistics data sets at
sqrt{s_{NN}} of 62.4 and 200 GeV. Hanbury-Brown-Twiss (HBT) interferometry
allows to determine the size of pion emitting source regions. The dependence of
the HBT radius parameters on azimuthal angle relative to the reaction plane
have been extracted. These dependencies can be related to the freeze out
eccentricity. The new results from STAR are consistent with a monotonically
decreasing freeze out eccentricity and constrain any minimum, suggested by
previously available data, to lie in the range between 11.5 and 39 GeV. Of
several models UrQMD appears to best predict the STAR and AGS data.Comment: 4 pages, 2 figures, Proceedings for the XXII International Conference
on Ultra-relativistic Nucleus-Nucleus Collisions (Quark Matter 2011), 22-28
May 2011, Annecy, Franc
Gamma-based clustering via ordered means with application to gene-expression analysis
Discrete mixture models provide a well-known basis for effective clustering
algorithms, although technical challenges have limited their scope. In the
context of gene-expression data analysis, a model is presented that mixes over
a finite catalog of structures, each one representing equality and inequality
constraints among latent expected values. Computations depend on the
probability that independent gamma-distributed variables attain each of their
possible orderings. Each ordering event is equivalent to an event in
independent negative-binomial random variables, and this finding guides a
dynamic-programming calculation. The structuring of mixture-model components
according to constraints among latent means leads to strict concavity of the
mixture log likelihood. In addition to its beneficial numerical properties, the
clustering method shows promising results in an empirical study.Comment: Published in at http://dx.doi.org/10.1214/10-AOS805 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Correspondence between HBT radii and the emission zone in non-central heavy ion collisions
In non-central collisions between ultra-relativistic heavy ions, the
freeze-out distribution is anisotropic, and its major longitudinal axis may be
tilted away from the beam direction. The shape and orientation of this
distribution are particularly interesting, as they provide a snapshot of the
evolving source and reflect the space-time aspect of anisotropic flow.
Experimentally, this information is extracted by measuring pion HBT radii as a
function of angle with respect to the reaction plane. Existing formulae
relating the oscillations of the radii and the freezeout anisotropy are in
principle only valid for Gaussian sources with no collective flow. With a
realistic transport model of the collision, which generates flow and
non-Gaussian sources, we find that these formulae approximately reflect the
anisotropy of the freezeout distribution.Comment: 9 pages, 8 figure
Constituent quark scaling violation due to baryon number transport
In ultra-relativistic heavy ion collisions at \roots\approx200 GeV, the
azimuthal emission anisotropy of hadrons with low and intermediate transverse
momentum ( GeV/c) displays an intriguing scaling. In particular,
the baryon (meson) emission patterns are consistent with a scenario in which a
bulk medium of flowing quarks coalesces into three-quark (two-quark) "bags."
While a full understanding of this number of constituent quark (NCQ) scaling
remains elusive, it is suggestive of a thermalized bulk system characterized by
colored dynamical degrees of freedom-- a quark-gluon plasma (QGP). In this
scenario, one expects the scaling to break down as the central energy density
is reduced below the QGP formation threshold; for this reason, NCQ-scaling
violation searches are of interest in the energy scan program at the
Relativistic Heavy Ion Collider (RHIC). However, as \roots is reduced, it is
not only the initial energy density that changes; there is also an increase in
the net baryon number at midrapidity, as stopping transports entrance-channel
partons to midrapidity. This phenomenon can result in violations of simple NCQ
scaling. Still in the context of the quark coalescence model, we describe a
specific pattern for the break-down of the scaling that includes different flow
strengths for particles and their anti-partners. Related complications in the
search for recently suggested exotic phenomena are also discussed.Comment: 7 pages, 2 tables, 2 figures. Wording sharpened. Two tables added, to
quantify the estimate of stopped quark fraction
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Lessons Learned and Next Steps in Energy Efficiency Measurement and Attribution: Energy Savings, Net to Gross, Non-Energy Benefits, and Persistence of Energy Efficiency Behavior
This white paper examines four topics addressing evaluation, measurement, and attribution of direct and indirect effects to energy efficiency and behavioral programs: Estimates of program savings (gross); Net savings derivation through free ridership / net to gross analyses; Indirect non-energy benefits / impacts (e.g., comfort, convenience, emissions, jobs); and, Persistence of savings
Beyond the RCT: Integrating Rigor and Relevance to Evaluate the Outcomes of Domestic Violence Programs
Programs for domestic violence (DV) victims and their families have grown exponentially over the last four decades. The evidence demonstrating the extent of their effectiveness, however, often has been criticized as stemming from studies lacking scientific rigor. A core reason for this critique is the widespread belief that credible evidence can derive only from research grounded in randomized control trials (RCTs). Although the RCT method has its strengths, we argue that it is rarely an optimal—or even a possible—approach for evaluating multifaceted DV programs. This article reviews the reasons that RCT is a poor fit for such programs and argues that a more inclusive conceptualization of credible evidence is critical to expanding our knowledge base about how DV programs affect survivors’ safety and well-being
Femtoscopy and energy-momentum conservation effects in proton-proton collisions at 900 GeV in ALICE
Two particle correlations are used to extract information about the
characteristic size of the system for proton-proton collisions at 900 GeV
measured by the ALICE (A Large Ion Collider experiment) detector at CERN. The
correlation functions obtained show the expected Bose-Einstein effect for
identical particles, but there are also long range correlations present that
shift the baseline from the expected flat behavior. A possible source of these
correlations is the conservation of energy and momentum, especially for small
systems, where the energy available for particle production is limited. A new
technique, first introduced by the STAR collaboration, of quantifying these
long range correlations using energy-momentum conservation considerations is
presented here. It is shown that the baseline of the two particle correlation
function can be described using this technique.Comment: Hot Quarks 2010 conference proceedings, to appear in Journal of
Physics: Conference Series (JPCS
A Content Analysis of Youth Internet Safety Programs: Are Effective Prevention Strategies Being Used?
ABSTRACT: Almost half of youth in the U.S. report receiving internet safety education (ISE) in their schools. Unfortunately, we know little about what educational messages make a difference in problems such as cyberbullying, sexting, or online predators. To consider directions for improving effectiveness, a content analysis was conducted on materials from four ISE programs. Results indicate that ISE programs are mostly not incorporating proven educational strategies. Common ISE messages have proliferated without a clear rationale for why they would be effective. It is recommended that program developers and other stakeholders reconsider ISE messages, improve educational strategies, and participate in evaluation. The field must also consider whether ISE messages would be better delivered through broader youth safety prevention programs versus stand-alone lessons
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