QM2002 Poster Version updated 13Jul02

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QM2002 Poster - Version updated 13-Jul-02
Rapidity (dN/dy) and Transverse Mass (mt)
Disributions for p/-,K/-, Proton and
Anti-Proton Production from AuAu Collisions
atsqrt(sNN) 19.6 GeV
Daniel Cebra University of California, Davis For
the STAR Collaboration
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Abstract
Rapidity (dN/dy) and transverse Mass (mt)
distributions for p, p-, K, K-, p, p- from
central AuAu collisions at sqrt(sNN) 19.6 GeV
are studied as a function of rapidity for -0.4 lty
lt0.4 using the Solenoidal Tracker At RHIC. The
mt-m0 spectra for these particles are used to
determine the temperature and radial flow of the
the emitting source and the yield of each
particle species. Using the mid-rapidity p-bar/p
ratio, and also the K-/K ratio which is
sensitive to the valence quark content of the
kaons, one can infer the net-baryon density.
These data are of particular interest since this
energy and baryon density regime is comparable to
that for fixed target PbPb collisions at 158 AGeV
(sqrt(sNN) 17.2 GeV) at the CERN Super Proton
Synchrotron (SPS). The systematic trend of the
data from the baryon rich environment in 40 AGeV
fixed PbPb collisions at the SPS (sqrt(sNN) 8.8
GeV) to the baryon depleted region at RHIC (AuAu
at sqrt(sNN) 130,200 GeV) will be examined.
Mid-rapidity pion ratios are used to image the
residual positive charge of the pion emitting
source and comparison is made to corresponding
results from the the SPS, AGS, SIS, and RHIC
(AuAu collisions at sqrt(sNN) 130,200 GeV). The
shapes and magnitudes of the rapidity density
plots for p, p-, K, K-, p, p- are contrasted
with those at higher and lower energy, and the
scaling behavior for these distributions is
examined in the context of the gluon saturation
model.

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A Typical 19.6 GeV event. Note that the track
multiplicity is generally lower than STAR events
at 130 or 200 AGeV.
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The STAR AuAu 19.6 GeV Data Set

• The 19.6 GeV data set was taken on the final
  day of year-two heavy-ion running (0915
  25-Nov-01 to 0626 26-Nov-01). The detector was
  run at HALF FIELD.
• 12 data runs were recorded (2329088,...91-94,...1
  00-101, 2330002-2330005)
• Minimum bias trigger (ZDC coinc.CTBgt15 .or.
  No ZDCCTBgt600)
• The production data is the pico-DSTfrom
  /auto/stardata/starspec3/flow_pDST_production/reco
  /minBias22GeVZDC/ReversedHalfField/P02ge/2001/
• With no event selection cuts, we have Nevents
  175466.
• Selecting on collisions with vertices within 30
  cm the center of the detector reduces this to
  42412.
• Total number of good top 10 centrality events
  5106.

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Event Vertex Cuts
Vertex Z location
Vertex X and Y Location
Note The beam diamond is quite broad.
The beam diamond for the 20 GeV run was fairly
large. We imposed event vertex cuts to restrict
our analysis to events with vertices within the
center 30 cm of the detector, where the
acceptance is the most uniform. We selected
events with vertices within the center of the
beampipe to eliminate contributions from triggers
on secondary interaction with the beam pipe
material.
Before Cuts
Note The beam pipe is evident.
25.1 of events pass this cut
After Cuts
96.3 of events pass
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Triggering for the 19.6 GeV dataset
The STAR trigger was not optimized for 19.6 GeV
Collisions. The correlation between ZDC and CTB
that had been observed in the 130 and 200 GeV
data sets was not seen in the lower energy
collisions. To approximate a minimum bias
trigger, events were selected if the satisfied
either a minimum ZDC threshold or a minimum CTB
threshold.
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Centrality Determination
80 60 50 40
10
20
30
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The raw mid-y (hlt0.5) charged particle
multiplicity is shown on the left for the three
RHIC collision energies. On the right the same
data are scaled both horizontally and vertically
to estimate the best positions for the centrality
bias cuts. The cut lines correspond to
percentages of the total minimum bias cross
section. (Note the 20 GeV trigger had
inefficiencies for collisions in the impact
parameter range corresponding to 10-30)
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An Alternate Approach to Centrality Determination
Limit to Correlation
The Multiplicity distributions are scaled to
the point where the correlation between mid-h and
higher-h particle yield starts to break
down. The Cut locations determined with this
method are indistinquishable to those determined
in the previous method.
Put 10 Cut Here
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Track Quality Cuts
In addition to event quality cuts, individual
track quality cuts were also imposed on the data.
The distance of closest approach between the
track and the main event vertex was limited to 3
cm. The c2 of the helix fit to the found points
was limited to 2.5. The total number of points
used in the fit was required to be greater than
25 (out of a maximum of 45). And the fraction of
points used in the fit was required to be greater
than half the possible number to avoid split
tracks.
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Particle Identification
Particle identification is achieved through
relative ionization in the TPC gas. The plot to
the right shows a scatter plot of the ltdE/dxgt of
a track versus the log of its total momentum.
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Fits to the dE/dx distributions
We use a Bethe Bloch parametrization with three
free parameters to account for detector effects.
We determine these values with the well IDed
particles and fine tune the parametrization for
each condition.
Fix the centroids and wdiths rigorously.
Only free parameters in the fits are the
amplitudes. This extends the fitting range beyond
previous analyses. Fit for e, p, K, p.
p e K p
Kaons and protons
electron
pions
u 2.8 ln (dE/dx) 1.5
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Particle Ratios - AuAu _at_ 19.6 GeV
Still need to consider proton background!
The first observables are the particle ratios.
These often require the fewest corrections.
STAR PRELIMINARY RESULTS
p/p-
K-/K
p-/p
R 0.611 /- .013 stat
R 0.093 /- .003 stat /- .004
sys
Pion Ratio has structure as a function of mt-m0.
Ratio is independent of mt-m0
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Pion Ratios - Imaging the Source
A comparison of the pi/pi- ratios at different
energies. From this comparison, we can find the
Coulomb potential assuming a spherically
symmetric geometry. As can be seen, the
potential decreases as the collision energy
increases. This is evidence of the depletion of
the initial baryons in the central region.
E866 PRC57 (1998) R446 KaoS PLB420 (1998) 20
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Kaon and Proton Ratios
Results are consistent with expectations
Proton and kaon ratios for central events both
increase with collisions energy. This illustrates
the reduction in the net baryon content and/or
the baryon chemical potential across this
collision energy range from the AGS to the top
energies at RHIC. At 19.6 GeV the collisions
produced Baryon-rich matter.
K-/K
Publication in prep J. Berger
STAR Preliminary results
PRL04778
E866/917 nucl-ex 0008010 WA97 JPhys G25 (1999)
171 NA44 JPhys G23 (1997) 1865 NA49 NP A661
(1999) 45c
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Estimated Efficiency Corrections
TPC efficiency is influenced by many effects,
including event multiplicity, vertex location,
and detector configuration. We determined our
efficiency for this analysis by simulating tracks
in GEANT and then embedding those tracks into
full events.
These events are then analyzed and found tracks
are associated with those that were embedded.
This allows us to accurately determine the
efficiency for a given detector configuration. In
these figures, we show the embedded track
distribution for midrapidity pions and in red the
found track. The ratio of found to embedded is
then the efficiency. Similar analysis is done for
all other particle species and rapidity bins.
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Efficiency Corrected Mid-rapidity Spectra
K-
p-
p-
A blast wave model simultaneous fit to these
spectra yields Tfo 104 /- 4 MeV br 0.47
/- 0.12
p
K
p
STAR PRELIMINARY RESULTS
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dN/dy near mid-rapidity
p-
p
The yields of the various particle species near
mid-rapidity are consistent with similar
measurements at the SPS. The pions and positive
protons demonstrate yields which are constant as
a function of rapidity near mid-rapidity. The
yields of the kaons and p-bars are peaked at
mid-rapidity.
K-
K
p
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Mid-rapidity K to p ratios
R0.111 /- 0.003 stat /- 0.029 sys
R0.187 /- 0.004 stat /- 0.031 sys
Figure from NA49 Beam request
19.6 GeV Preliminary STAR result
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dN/dh as a Function of Centrality
19.6 GEV STAR PRELIMINARY RESULT
The yields of charged particles as a function of
centrality and rapidity show a dip at
mid-rapidity for central events and are flat for
peripheral events.
300 200 100
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dN/dh Collision Energy Systematic
The charge particle yield at mid-rapidity for
central events (top 5 or 6) is consistent with
the trends established by AGS, SPS, and higher
energy RHIC data.
Phobos PRL 85 (2000) 3100 Phobos nucl-ex 018009
(2002) PHENIX PRL 86 (2001) 3500 STAR(130)
nucl-ex 106004 (2001) BRAHMS QM2001 NA49 WA98
nucl-ex 0008004 (2000) WA97/NA57
CERN-EP-2000-002 E866/917 PRC59 (1999) 2173 E877
PRC51 (1995) 3309
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v1 vs. Eta
Most Peripheral
Semi-Peripheral
Directed Flow results for the 19.6 GeV Dataset
STAR Preliminary Results
Semi-Central
Most Central
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v2 vs. Eta
Semi-Peripheral
Most Peripheral
Elliptic Flow Results for the 19.6 GeV dataset.
STAR Preliminary Results
Most Central
Semi-Central
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Conclusions

• Studies of Pion ratios demonstrate the reduction
  of source charge consistent with a transport of
  the incident baryons beyond mid-rapidity.
• Kaon and Proton ratios are consistent with SPS
  trends. The 19.6 GeV collisions create
  baryon-rich matter.
• The temperatures and radial flow are consistent
  with other RHIC results, but suggest temperatures
  below those previously reported for SPS
  collisions.
• dN/dys show magnitude and shape consistent with
  SPS results. The protons exhibit an elongation
  along the beam axis consistent with incident
  baryon transport. The produced kaons and p-bars
  are peaked at mid-rapidity. The pions, which have
  a strong delta resonance contribution are flat at
  mid-rapidity similar to the protons.
• Kaon to pion ratios consistent/low wrt SPS.
• dN/dh are consistent wrt SPS and RHIC results.
• Preliminary Flow results.
• Generally - agreement with SPS results.