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star focus
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STAR Focus features are short articles intended to cover a variety of
aspects of the STAR Experiment.
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star focus:
Long range rapidity correlations
Highlights from the STAR papers:
Long range rapidity correlations and jet production in high energy nuclear collisions
and
Growth of Long Range Forward-Backward Multiplicity
Correlations with Centrality in Au+Au Collisions at
sqrt(sNN) = 200 GeV
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Submitted for publication to Physical Review C and Physical
Review Letters respectively.
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The STAR experiment has now reported two interesting
results on long range correlations
in rapidity. One of the experimental observation
is from a correlation study in azimthal angle and
pseudorapidity for produced charged hadrons with
respect to a particle with larger transverse momentum. Such
studies revealed a jet-like
correlation at small pair phase space separation
(in azimuth and pseudorapidity - near side) which
seems to be unmodified in central Au+Au
collisions relative to d+Au and a significant
correlated yield in central Au+Au collisions at
large pair separation in pseudorapidity (the
RIDGE). The ridge is observed in Au+Au collisions
and not observed in d+Au collisions (See figures).
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star focus:
Photon multiplicity measurements at forward
rapidity
Highlights from the STAR paper:
Center of mass energy and system-size dependence of photon
production at forward rapidity at RHIC.
Submitted for publication to Physics Letters B.
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Several interesting features of the dependence
of particle density in pseudorapidity have been
observed in Au+Au collisions from the experiments
at the Relativistic Heavy-Ion Collider (RHIC).
Particle production is found to follow a unique, collision
energy independent, longitudinal
scaling in p+p and d+Au, as well as in heavy-ion
collisions. Such longitudinal scaling is also
found to be independent of collision centrality
for photons. The total charged particle
multiplicity (integrated over the full
pseudorapidity range) per average number of
participating nucleon (< Npart >) pair is found
to be independent of collision centrality by
PHOBOS experiment. However, at mid-rapidity
(|eta| <1), the PHENIX experiment showed that
charged particle multiplicity per < Npart > is
observed to increase from peripheral to central
collisions. The charged particle production
scales with a combination of < Npart > and average
number of binary collisions < Nbin >. These clearly
indicates that the mechanism of particle
production could be different in different
pseudorapidity regions. It is believed that
the scaling of particle multiplicity with
< Npart > indicates the dominance of soft
processes in particle production, whereas
scaling with average number of binary collisions
(< Nbin >) indicates the onset of hard processes
(pQCD jets).
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star focus:
J/Psi production at high transverse momentum
Highlights from the STAR paper:
J/psi production at high transverse momentum in p+p and
Cu+Cu collisions at sqrt(sNN) = 200 GeV
Submitted for publication to Physical Review Letters.
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Suppression of the c-cbar bound state
J/Psi meson production in relativistic
heavy-ion collisions arising from J/Psi
dissociation due to screening of the c-cbar binding
potential in the deconfined medium has
been proposed as a signature of Quark-Gluon
Plasma (QGP) formation. Measurements of high
transverse momentum J/Psi production in A+A
collisions relative to p+p collisions (Nuclear
modification factor) can tell us which of
the following physical scenario is possible:
(a) If nuclear modification factor is less
than one it could be due partonic energy loss
in dense matter, here the J/Psi formation then
likely proceeds through a channel carrying
color.
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star focus:
K/pi Fluctuations at Relativistic Energies
Highlights from the STAR paper:
K/pi Fluctuations at Relativistic Energies
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Submitted for publication to Physical Review Letters.
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Strangeness enhancement has been predicted to be one
of the important signatures of the formation of the quark
gluon plasma (QGP). The study of dynamic fluctuations in
event-by-event K/pi ratio may produce information
concerning
QCD phase transitions and may lead to the observation of
the critical point in the QCD phase diagram. Studying
fluctuations
of particle ratios has few advantages, it has been argued
that
considering fluctuations of the multiplicity ratio
eliminates
the effect of volume fluctuations, further fluctuation of
the
particle ratio like K/pi could be sensitive to the particle
numbers at chemical freeze-out and not at kinetic
freeze-out.
STAR experiment has recently reported the results on beam
energy dependence of event-by-event K/pi ratio fluctuations
at RHIC.
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star focus:
D* meson in jets
Highlights from the STAR paper:
Measurement of D* Mesons in Jets from p+p Collisions at
sqrt(s) = 200 GeV.
Submitted for publication to Physical Review D Rapid Communications.
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Studies by the ALEPH, L3 and OPAL Collaborations
of the D* +/- meson content in jets show that
the production from Z0 decays in e++e-
collisions is dominated by D* mesons that carry
large fractions of the jet momenta, consistent
with the jets being produced from primary c
(anti-)quarks. In pbar + p collisions at 630 GeV
and 1.8 TeV, the UA1 and CDF Collaborations have
observed D* +/- mesons in jets with transverse
energies larger than 40 GeV. Their fractional
momenta are found smaller, consistent with a
different production mechanism in which the D*
mesons originate from gluon splitting into c cbar
pairs.
At top RHIC energy, heavy quarks can still be
produced via gluon splitting. Perturbative QCD
suggests that these contributions are small, and
that the majority of the heavy quarks originate
from gluon-gluon fusion. These expectations,
however, have not until now been confronted with
data at RHIC. The STAR experiment presents the
first measurement of charged D*
mesons in
inclusive jets produced in p + p collisions at
a center of mass energy of 200 GeV at RHIC which
addresses the above issue. The charged D*
andidates were identified through the decay
sequence D*+ --> D0 pi+, D0 --> K-pi+ and
its charge conjugate. The D*+ and D*- yields of
184 +/- 44 and 169 +/- 45 were obtained in inclusive
jets with 11.5 GeV mean transverse energy.
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star focus:
Observation of Two-source Interference in
STAR
Highlights from the STAR paper:
Observation of Two-source Interference in the
Photoproduction
Reaction Au Au -> Au Au rho0.
Submitted for publication to Physical Review Letters.
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In ultra-peripheral relativistic heavy-ion collisions, a
photon
from the electromagnetic field of one nucleus can fluctuate
to a
quark-antiquark pair and scatter from the other nucleus,
emerging as
a Rho0. The Rho0 production occurs in two well-separated
(median impact parameters of 20 and 40 fermi for the cases
considered
here) nuclei, so the system forms a 2-source
interferometer.
At low transverse momenta, the two amplitudes interfere
destructively, suppressing Rho0 production. The produced
Rho0s decay almost immediately
at two well-separated points, so any interference must
develop
after the decay, and involve the pi(+) pi(-) final state.
Since
the pions go in different directions, this requires an
entangled
pi(+)pi(-) wave function which cannot be factorized
into separate pi(+) and pi(-) wave functions; this is an
example
of the Einstein-Podolsky-Rosen paradox (for more details on
this
paradox look at reference below).
The figure shows the the uncorrected midrapidity minimum
bias
Au+Au 200 GeV dN/dt spectra as a function of t(perp) =
(pT*pT).
These data are compared simulations based with and without
interference. The measured dN/dt spectrum is roughly
exponential,
but with a significant downturn for t(perp) < 0.0015
GeV*GeV,
consistent with the predicted interference (dashed
histogram).
The no-interference histogram is almost exponential (solid
histogram), dN/dt ~ exp (-kt(perp)), where k is related
to the
nuclear radius.
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star focus:
Phi Meson and Strangeness Enhancement at RHIC
Highlights from the STAR paper:
Energy and system size dependence of phi-meson
production in Cu+Cu and Au+Au collisions.
Submitted for publication to Physics Letters B.
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In a Quark-Gluon Plasma, thermal s and sbar
quarks can be produced by gluon-gluon interactions. These
interactions could
occur very rapidly and the s-quark abundance
would equilibriate. During hadronisation, the s
and sbar quarks from the plasma coalesce to form
phi-mesons. Production by this process is
not suppressed as per the OZI (Okubo-Zweig-Izuka)
rule. This, coupled with large abundances of
strange quarks in the plasma, may lead to a
dramatic increase in the production of phi-mesons
and other strange hadrons relative to non-QGP
p+p collisions.
Alternative ideas of canonical suppression of
strangeness in small systems as a source of strangeness
enhancement in high energy
heavy-ion collisions have been proposed for
other strange hadrons (e.g Kaon, Lambda,
Cascade, Omega). The strangeness conservation laws require
the production of an sbar-quark for
each s-quark in the strong interaction. The main
argument in such canonical models is that the energy and
space time extensions in smaller systems may not be
sufficiently large.
This leads to a suppression of strange hadron
production in small collision systems. These
statistical models fit the data reasonably
well. According to these models, strangeness
enhancement in nucleus-nucleus collisions,
relative to p+p collisions, should increase with
the strange quark content of the hadrons.
This enhancement is predicted to decrease with increasing
beam energy.
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star focus:
STAR readiness for proposed Beam Energy Scan
Program : Results from Au+Au collisions at 9.2 GeV
Highlights from recent data taken by STAR with the lowest
beam
energy collisions at RHIC - Au+Au collisions at 9.2 GeV.
These were presented for the first time at the
International Conference
on Strangeness in Quark Matter, 2008, Beijing China.
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One of the main aim of high energy heavy-ion collisions
is to map the QCD phase diagram. The goal being to locate
the QCD phase boundary (separating matter with hadronic
degrees of freedom from matter with quark gluon degrees of
freedom) and the QCD critical point (where
the first order phase transition ends). The phase
diagram is plotted as temperature versus baryon chemical
potential. These quantities can be changed by varying the
colliding beam energy to map the phase diagram. The
temperature
and baryon chemical potential can be measured from the
produced
particle spectra and ratios. Then one looks for signatures
for different phases and for the QCD critical point. In
addition
STAR also would like to study the beam energy which
corresponds
to onset of several interesting observations seen at top
RHIC
energy (Au+Au 200 GeV) : Number of constituent quark
scaling of
elliptic flow parameter for produced hadrons, enhanced
correlated yields at large delta_eta for delta_phi ~ 0
(Ridge)
and the suppression of high transverse momentum hadron
production
in heavy ion collisions.
In order to achieve the above goals STAR has proposed a
beam
energy scan program at RHIC spanning beam energies from
5 GeV to 50 GeV. As a first step towards achieving this
goal,
recently STAR collected data from a test run for Au+Au
collisions
at 9.2 GeV. The events for this test run was collected at a
rate
of 0.7 Hz. The first results were presented at the SQM2008.
Here
we discuss only a small subset of the results.
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star focus:
Hadronic resonance measurements in d+Au collisions
Highlights from the STAR paper
Hadronic resonance production in d+Au collisions at 200 GeV
at RHIC accepted for publication in
Physical Review C.
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The particle identification capability of
the Time Projection Chamber in STAR and
its large acceptance enables us to measure many
hadronic resonances produced in the high energy collisions.
Resonances are strongly decaying particles with lifetimes x
velocity of light that are of the order of the size of the
hot and dense medium produced in heavy-ion collisions. The
in-medium effects related to the high density
and/or high temperature of the medium can modify the
properties of short-lived resonances, such as
their masses, widths, and even their spectral shapes. STAR
experiment has recently reported the
measurement of the following resonances for colliding beam
energy of 200 GeV. These are
reconstructed from their hadronic decay channels using
invariant mass technique in d+Au
collisions - rho(770), K*(892), Delta(1232)++, Sigma(1385),
Lambda(1520).
One interesting feature was observed in the
transverse mass distribution of these resonances
measured at midrapidity. As shown in the figure,
they seem to follow a generalized scaling in
d+Au collisions between transverse mass range of 1 - 2
GeV/c2. Such a scaling could be envisaged
within the idea of saturation of gluon density
in the nucleus for high energy collisions. However such
scaling has been observed in p+p
collisions at ISR, SppbarS, RHIC energies. Also
the resonances in d+Au collisions do not show any
difference in the shape of the transverse mass distribution
between baryons and mesons at higher transverse mass.
Differences were earlier
observed for non-resonant particles along
baryon-meson lines.
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star focus:
Identified hadron measurements in STAR using the Time Projection Chamber
Highlights from the STAR paper
Systematic Measurements of Identified Particle Spectra in
p+p, d+Au and Au+Au Collisions from STAR submitted
recently to Physical Review C.
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STAR experiment has recently reported systematic
measurements of identified particle spectra in
pp, d+Au and Au+Au collisions. Along with
reporting several interesting results for the
above collision systems at different energies we
have also presented in detail the particle
identification procedure in STAR Time Projection
Chamber and the various correction factors
associated with the extraction of the yield and
shape parameters for the transverse momentum
spectra of produced hadrons.
In this focus article we present two results from
this work. For Au+Au collisions,mean pT, which
characterizes the slope of the transverse
momentum spectra are found to increase
significantly with increasing collision
centrality or decreasing impact parameter of the
collision. The trends are similar at 62.4 GeV,
130 GeV, and 200 GeV, and mean pT qualitatively
agree with each other at the same dNch/dy.
This suggests that the kinetic freeze-out
properties in Au+Au collisions are rather
energy independent for the measured collision
energies.
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star focus:
Transverse Single Spin Asymmetries measurements in RHIC
Highlights from the STAR paper Forward Neutral Pion
Transverse Single Spin Asymmetries in p+p Collisions at sqrt(s)=200 GeV submitted recently to Physical Review Letters.
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The production of particles with high transverse
momentum from polarized proton collisions at high
energies is sensitive to the quark and gluon
spin structure of the proton.
One challenge to theory has been to understand
the sizable azimuthal asymmetry of particles
produced in collisions of transversely polarized
protons, known as analyzing power (AN) or
transverse single spin asymmetry (SSA) for
inclusive pion production in polarized p + p
collisions over a broad range of collision energies and in
semi-inclusive deep inelastic
scattering (SIDIS) from transversely polarized
proton targets.
Spin-correlated transverse momentum dependent
(TMD)distribution functions (Sivers effect),
in conjunction with initial- or final-state color-charge
interactions, can explain large AN.
These functions describe parton orbital motion
within the proton, and so are important to
explore to understand the structure of the
proton.
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star focus: Beam-Energy and System-Size Dependence of Dynamical Net Charge at RHIC
Highlights from the STAR paper Beam-Energy and System-Size Dependence of Dynamical Net Charge submitted recently to Physical Review C.
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Anomalous transverse momentum and net charge event-by-event
fluctuations have been proposed as indicators of the
formation of a quark gluon plasma (QGP) in
high-energy heavy ion collisions.
In particular, Koch et
al. [1] have estimated that entropy conserving
hadronization of a plasma of quarks and gluons should
produce a final state characterized by a dramatic reduction
of the net charge fluctuations relative to those observed in
a hadron gas. Published STAR measurements indicate the
fluctuations observed in Au + Au collision at center of
mass energy of 130 GeV are little suppressed relative to
those observed in p + p collisions [2]. STAR found the
measured fluctuations in this collision system and energy
are in qualitative accord with expectations based on hadron
gas models. It is thus interesting to study the magnitude of
the fluctuations as a function of the colliding system size
by varying both collision centrality and colliding nuclei.
There is also a possibility that final state interactions
may partly wash out the expected suppression through
collision and diffusion processes [3].
Best conditions to
observe the predicted suppression may not be at 130 GeV.
It is,therefore, of great interest to carry out a study of
the system size,
and beam energy dependence of the net
charge fluctuations.
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star focus: System-size independence of directed flow at RHIC
Highlights from the STAR paper System-size independence of directed flow at the Relativistic Heavy-Ion Collider submitted recently to Physical Review Letters.
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Directed flow refers to collective sidewards
deflection of particles and is characterized
by a first-order harmonic (v1) of the Fourier
expansion of particle's azimuthal distribution
w.r.t. the reaction plane in heavy-ion collisions.
STAR has recently submitted to Physical Review
Letters multiple differential measurements of v1
for Au+Au and Cu+Cu collisions at center of mass energies
of 200 and 62.4 GeV as a function of pseudorapidity (eta),
transverse momentum, and collision
centrality. We find that directed flow violates
the "entropy-driven" multiplicity scaling which
dominates all other soft observables. STAR has
reported an intriguing new universal scaling of
the phenomenon with collision centrality.
Neither Boltzmann/cascade nor hydrodynamic models
are able to explain the measured trends.
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star focus: Charmed hadron production at low transverse momentum at RHIC
Highlights from the STAR paper Charmed hadron production at low transverse momentum in Au+Au collisions at RHIC submitted recently to Physical Review Letters.
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Charm quarks are likely to be produced only in
the early stages and can be a unique tool to
probe the partonic matter created in relativistic
heavy-ion collisions at RHIC energies. Studies of
the number of binary collision (calculated using
Glauber model) scaling of the total charm cross
section from d+Au to Au+Au collisions can be used
to test if charm production is exclusively at the initial
impact of colliding heavy ions.
The total charm production cross section is also
an important input in models of J/Psi production
via charm quark coalescence in a Quark Gluon
Plasma.
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star focus: indication of conical emission at RHIC
Highlights from the STAR paper Indications of Conical Emission of Charged Hadrons at RHIC submitted recently to Physical Review Letters.
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Experimental observation of jet-quenching studies in STAR
revealed: on the away side of a high transverse momentum
(pt)
trigger particle the correlated yield is strongly
suppressed
at pt > 2 GeV/c while at lower pt the yield is enhanced
and
the correlated hadrons appear to be partially equilibrated
with the bulk medium and are broadly distributed in
azimuth.
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Read more...
| Posted Jun 13, 2008
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star focus: Half-Day Symposium: STAR Highlights and Future, Brookhaven National Laboratory, May 5th, 2008
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At this event we celebrate Tim Hallman’s leadership as Spokesperson of the STAR experiment at RHIC for the past six years. We chronicle the experiment’s success, and look forward towards its bright future.
Sponsored by the Department of Physics, Brookhaven National Laboratory
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Posted Apr 25, 2008
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star focus: jets in nuclear collisions
Part 1 of a series on STAR analysis topics
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In high energy p+p collisions, the hard scattering of quarks and gluons early in the collision
leads to the production of jets,
narrow streams of particles that allow physicists
to detect and understand the scattering. In nuclear collisions at RHIC,
jets instead
serve as a penetrating probe of the extremely dense nuclear matter formed in the collision.
Comparing characteristics of jets in nuclear collisions to jets in p+p
collisions has uncovered special properties of dense nuclear matter at RHIC.
Read more...
| Posted Aug 3, 2006
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star focus: beamtime!
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STAR's preparations are underway for the 2006 RHIC beamtime, thanks to
special
funding for this year's experimental operations. This year's "beamtime" is
the sixth annual RHIC run, with
about 15 weeks of colliding polarized proton beams scheduled to begin in early
March. STAR collaborators come to BNL during the experiment to serve weekly
shifts as part of six-member shift teams, and each member has specific tasks
to perform while on shift.
More...
RHIC '06 operations plan
| Posted Feb 2, 2006
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star focus: quark matter 2005
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The 18th International Conference on Nucleus Nucleus Collisions
(Quark Matter 2005)
was held in Budapest, Hungary,
August 4-9. STAR presented new results on several fronts,
summarized in two experimental
summary talks on the conference's
first day and a special focus
talk on the last day. STAR contributed 15 parallel talks and
many posters.
STAR at QM 2005
| Posted Aug 11, 2005
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star focus: let's meet in warsaw
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This summer, STAR will hold its semi-annual collaboration meeting
in Warsaw, Poland, preceeding the Quark Matter 2005 conference in Hungary.
The collaboration meeting will be hosted
by Warsaw University of Technology (Politechnika Warszawska), a STAR member institute.
Meetings before major conferences give collaboration members the chance to review
results, share posters and practice talks.
Meetings away from BNL also allow institutions to open their
doors to fellow collaborators.
Read more...
| Posted May 31, 2005
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star focus: rhic lessons
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As part of a joint venture between BNL and JINR (Dubna) called the
Online Science Classroom, a series of
RHIC Lessons
have been created by STAR Collaborators in Dubna. These lessons introduce many
aspects of the research carried out by STAR, from the process of colliding different
beams of particles to the challenges of studying the formation of a quark-gluon
plasma. The lessons require Macromedia Flash Player.
RHIC Lessons
| Posted March 5, 2005
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star focus: graduate students
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One of the primary missions of STAR is to provide rigorous training for
graduate students, who account for much of the hands-on detector and analysis
efforts for the experiment. As a result, STAR has produced an impressive number of graduate
degrees over a wide range of thesis topics. Click here for a list of graduate thesis titles, with links
to many thesis files.
Read more...
| Posted January 21, 2005
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star focus: physics results
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Nearly four years have passed since STAR's
first publication,
on the observation of evidence for
strong early expansion in Au+Au collisions at RHIC, appeared in Physical Review Letters.
Since then, 23 other Physical Review Letters have followed, more than any
nuclear physics experiment in history. A growing list of brief summaries of our
publications is available on our website.
Physics results summaries
| Posted November 19, 2004
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star focus: star and the data grid
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As the amount of data recorded by the STAR Experiment accumulates, the data storage
and data processing needs grow as well. STAR is an active participant in the
Particle Physics Data Grid, a project to manage
and distribute data and data analysis tasks across a large network of computing
facilities around the country.
Read more...
| Posted September 24, 2004
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star focus: regional meetings
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STAR Collaborators comprise a community of scientists and technicians from
13 countries on four continents, and getting everyone together isn't always
an easy task. That's one reason behind STAR Regional Meetings, smaller
gatherings of STAR members a bit further from RHIC than the BNL physics building.
Regional Meetings were held in China and Russia in 2003. Next up: Bhubaneswar, India,
in October.
Read more...
| Posted August 1, 2004
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