1. 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

2. RHIC Runs Overview Au + Au @ 130 GeV 2 weeks p + p @ 130 GeV (3) weeks
Refs: K. Schweda, HS02
RHIC BUR 2002
Year 2000:
Au GeV 2 weeks
p + p @ 130 GeV (3) weeks
Year 2001: (Silicon Vertex Tracker installed)
Au GeV 15 weeks
Au + 20 GeV 1 day
p + p @ 200 GeV 5 weeks
Year 2002: 1st of November
d + Au @ 200 GeV 16 weeks
p + p @ 200 GeV 8 weeks
9/22/2018
NPG Meeting

3. 20 GeV Au+Au at STAR # good minbias events = 172,202
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
9/22/2018
NPG Meeting

4. Selections: Events Vertex locations vY vZ vX |vZ| < 30
(center of TPC, which has uniform acceptance)
(vX+0.246)2+(vY-0.378)2 < 1
(eliminates interactions with beam pipe material)
9/22/2018
NPG Meeting

5. Selections: Events Multiplicity: 2 centrality bins are studied
more central collisions
more central collisions
0-10% central: Nch >= 237
10-30% central: 117 <= Nch < 237
9/22/2018
NPG Meeting

6. Selections: Tracks Fit points DCA (cm) Tracking % Chi squared
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.
rapidity: |y| < 0.5,
5 bins, bin’s width = 0.2
DCA (cm)
Tracking %
Chi squared
9/22/2018
NPG Meeting

7. Particle ID: finding dE/dx
Bethe-Bloch parametrization:
Particles to identify: +, -, K+, K-, p, and pbar p-
masses
: MeV
K: MeV
p: MeV K- e- -
9/22/2018
NPG Meeting

8. 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 = 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). - e- K- p-
9/22/2018
NPG Meeting

9. 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.
9/22/2018
NPG Meeting

10. 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
9/22/2018
NPG Meeting

11. Pion Spectra, Au+Au 20 GeV |y| < 0.1 Au + Au --> + + X
9/22/2018
NPG Meeting

12. Kaon Spectra, Au+Au 20 GeV |y| < 0.1 Au + Au --> K+ + X
9/22/2018
NPG Meeting

13. p, pbar Spectra, Au+Au 20 GeV
|y| < 0.1
low statistics for pbar
Au + Au --> p+ + X
Au + Au --> p- + X
9/22/2018
NPG Meeting

14. Spectra shape h+ h- h+ h- 0-10% central 10-30% central 9/22/2018
NPG Meeting

15. dN/dy distributions + K+ p+ p- - K-
dN/dy increases with centrality, nearly doubles in some cases
more uniform as a function of rapidity in peripheral collisions p- - K-
9/22/2018
NPG Meeting

16. Particle ratios K- u K+ - d - u s + d u d u d + u s + p- d K- +
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)
K-/K+ ratios --> kaon medium modification
pbar/p ratios --> baryon transport
Refs: STAR Coll., nucl-ex/
F. Wang, nucl-ex/
C. Roy, nucl-ex/ K- u K+ - d - u s +
up’s and down’s d
up’s and down’s u d u d + u s +
quarks’ masses
u: MeV
d: MeV
s: MeV p- d K- +
1 Au ion = 79 p, 118 n -
9/22/2018
NPG Meeting

17. 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 ( ) 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 the Coulomb potential (q1q2/r). We find that the potential decreases as the energy increases. [D. Cebra, QM 2002]
10-30%
* errors omitted
y --->
-0.4
-0.2
0.0
0.2
0.4
const
0.9337
0.9406
0.981
0.9315
0.9327
slope
0.1031
0.0968
-0.030
0.1309
9/22/2018
NPG Meeting

18. 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
9/22/2018
NPG Meeting

19. Ratios: Kaons and Protons
Both (us)/(us)and (uud)/(uud) increase with collisions energy, with the kaon’s ratio increasing faster (more s quarks). They approach unity at E = 200 GeV.
K-/K+
Sources: E866/917 nucl-ex
WA97 JPhys G25 (1999) 171
NA44 JPhys G23 (1997) 1865
NA49 NP A661 (1999) 45c
9/22/2018
NPG Meeting

20. 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
9/22/2018
NPG Meeting