Status of 20 GeV Au+Au Analysis
Overview:
• 20 GeV run’s details
• 2 centrality bins analyzed
• PID method
• efficiency correction
• results: yields, ratios, dN/dy
Roppon Picha
23 Oct 2002
UCD NPG meeting
RHIC Runs Overview
Refs:
• Year 2000:
– Au + Au
–p+p
@ 130 GeV
@ 130 GeV
K. Schweda, HS02
RHIC BUR 2002
2 weeks
(3) weeks
• Year 2001: (Silicon Vertex Tracker installed)
– Au + Au
– Au + Au
–p+p
@ 200 GeV
@ 20 GeV
@ 200 GeV
15 weeks
1 day
5 weeks
• Year 2002: 1st of November
– d + Au
–p+p
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@ 200 GeV
@ 200 GeV
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16 weeks
8 weeks
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20 GeV Au+Au at STAR
• Au+Au at E = 19.6 GeV per nucleon; 0.25 Tesla magnetic field
• Trigger: ZDC e&w and CTB > 15 [RHIC BUR 2002]
• Details:
– Production:
/auto/stardata/pDST/flow_pDST_production/reco/minBias/22GeVZDC/
ReversedHalfField/P02ge/2001/
– 12 runs:
• 2329: 88, 89, 91, 92, 93, 94, 100, 101
• 2330: 2, 3, 4, 5
• # minbias events = 278,452
• # events with (Nch = 0) = 106,250
• # good minbias events = 172,202
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Selections: Events
Vertex locations
vY
vZ
|vZ| < 30
vX
(vX+0.246)2+(vY-0.378)2 < 1
(center of TPC, which has
uniform acceptance)
(eliminates interactions with beam
pipe material)
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Selections: Events
Multiplicity: 2 centrality bins are studied
more central collisions
0-10% central: Nch >= 237
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more central collisions
10-30% central: 117 <= Nch < 237
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Selections: Tracks
DCA (cm)
Tracking %
The distance of closest
approach between the track
and the main event vertex
has a built-in cut at 3 cm to
eliminate background
protons. 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) to eliminate short
tracks. And the fraction of
points used in the fit was
required to be greater than
half the possible number to
avoid split tracks.
Fit points
Chi squared
rapidity: |y| < 0.5,
5 bins, bin’s width = 0.2
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Particle ID: finding dE/dx
Bethe-Bloch parametrization:
Particles to
identify: +, -, K+,
K-, p, and pbar
pK6/28/2016
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e-
masses
: 139.570 MeV
K: 493.677 MeV
p: 938.272 MeV
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PID: Fitting dE/dx distribution
• From the parameters, 4
Gaussians are constructed in
each mt-m0 slices.
• mt-m0 range: 0-1 GeV/c2,
divided into 40 slices (bins),
bin’s width = 0.025 GeV/c2.
-
• The integrations of the fits
then give the raw yields of the
particles (d2N/(2pi mt dmt dy)
as a function of mt-m0).
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e-
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K-
p-
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Efficiency
Method: O. Barannikova and J. Romero
• Plot pt distribution of Monte-Carlo embedded
tracks (mMcTracks.mPtMc) and the reconstructed
tracks (mMatchedPairs.mPtMc) from
/auto/pdsfdv41/starprod/QA/MiniDst/20GeV/RevHalfField/
• Plot the ratio of the two
• Fit the ratio to: eff = m1*exp(-(m2/pt)^m3)
Efficiency depends on particle type, rapidity, and
impact parameter. The higher the track density, the
lower the efficiency. The embedding was run for -,
K-, and pbar. We apply the same corrections for their
positive counterparts.
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Corrected yields
• corrected yield = raw yield/eff
• Fit the yields to distribution
functions: use Bose-Einstein for
pions and kaons (bosons);
Fermi-Dirac for protons and
antiprotons (fermions)
• … obtain dN/dy and
temperatures from the fits
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Pion Spectra, Au+Au 20 GeV
|y| < 0.1
Au + Au --> + + X
6/28/2016
Au + Au --> - + X
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Kaon Spectra, Au+Au 20 GeV
|y| < 0.1
Au + Au --> K+ + X
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Au + Au --> K- + X
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p, pbar Spectra, Au+Au 20 GeV
|y| < 0.1
low statistics for pbar
Au + Au --> p+ + X
6/28/2016
Au + Au --> p- + X
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Spectra shape
0-10% central
h+
h-
10-30% central
h+
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dN/dy distributions
+
K+
p+
• dN/dy increases with centrality, nearly doubles in some cases
• more uniform as a function of rapidity in peripheral collisions
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p15
Particle ratios
• From the yields --> particle ratios can be obtained. The ratios give us basic
information about the collision dynamics and the nuclear matter produced.
• K/ ratios (not studied here) --> strangeness production (70% of strange quarks go
into producing kaons)
Refs:
STAR Coll., nucl-ex/0206008
+
• K /K ratios --> kaon medium modification
F. Wang, nucl-ex/9911004
C. Roy, nucl-ex/0111017
• pbar/p ratios --> baryon transport K-
-
K+
up’s and down’s
+
p1 Au ion = 79 p, 118 n
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s
d
u
d u ds
d u
u
+
u
+
d
+
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up’s and down’s
quarks’ masses
u: 1.5 - 4.5 MeV
d: 5 - 8.5 MeV
s: 80 - 155 MeV
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Ratios: Pions
As the lightest mesons, pions are the most
abundant particles produced. Unlike K and p
ratios, the +/- ratio is not constant over mt-m0.
We found a slight excess of negative pions. This is
due to the isospin dependence in the branching
ratios. There are more neutrons (118+118) than
protons (79+79) in a collision. The direct + are
produced in pp, while direct - are produced in nn
collisions. [J. Klay, Ph.D. thesis 2001]
0-10%
y --->
-0.4
-0.2
0.0
0.2
0.4
const
0.9375
0.9941
0.9307
0.9554
0.9044
slope
0.0696
0.5118
0.116
0.4505
0.1624
The +/- ratio also allows us to calculate 10-30%
the Coulomb potential (q1q2/r). We find
y --->
that the potential decreases as the energy
const
increases. [D. Cebra, QM 2002]
slope
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* errors omitted
-0.4
-0.2
0.0
0.2
0.4
0.9337
0.9406
0.981
0.9315
0.9327
0.1031
0.0968
-0.030
0.08674
0.1309
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Ratios: Kaons and Protons
K-/K+
pbar/p
• both vary very little as a function of rapidity
• not a big difference between the two centrality bins
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Ratios: Kaons and Protons
Both (us)/(us)and
K-/K+
(uud)/(uud) increase
with collisions energy, with
the kaon’s ratio increasing
faster (more s quarks).
They approach unity at E =
200 GeV.
Sources:
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E866/917 nucl-ex 0008010
WA97 JPhys G25 (1999) 171
NA44 JPhys G23 (1997) 1865
NA49 NP A661 (1999) 45c
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Summary
• yields increase with centrality; similar shape in 2 centrality
bins; uniform over |y| < 0.5
• dN/dy increases with centrality; uniform over |y| < 0.5,
although some have curious shapes
• Both K-/K+ and pbar/p ratios uniform over |y| < 0.5;
dramatically increases with energy
• K ratio - no strong dependence on impact parameter
• pbar/p ratio slightly higher in peripheral collisions *
• To do - antiproton absorption in detector material; analysis of
other centrality bins; better understanding of the physics
involved
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