Status of 20 GeV Au Au Analysis

1
Status of 20 GeV AuAu Analysis

• Overview
• 20 GeV runs 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
Refs K. Schweda, HS02 RHIC BUR 2002

• Year 2000
• Au Au _at_ 130 GeV 2 weeks
• p p _at_ 130 GeV (3) weeks
• Year 2001 (Silicon Vertex Tracker installed)
• Au Au _at_ 200 GeV 15 weeks
• Au Au _at_ 20 GeV 1 day
• p p _at_ 200 GeV 5 weeks
• Year 2002 1st of November
• d Au _at_ 200 GeV 16 weeks
• p p _at_ 200 GeV 8 weeks

3
20 GeV AuAu at STAR

• AuAu at E 19.6 GeV per nucleon 0.25 Tesla
  magnetic field
• Trigger ZDC ew and CTB gt 15 RHIC BUR 2002
• Details
• Production /auto/stardata/pDST/flow_pDST_producti
  on/reco/minBias/22GeVZDC/ReversedHalfField/P02ge/2
  001/
• 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

4
Selections Events
Vertex locations
vY
vX
vZ
vZ lt 30 (center of TPC, which has uniform
acceptance)
(vX0.246)2(vY-0.378)2 lt 1 (eliminates
interactions with beam pipe material)
5
Selections Events
Multiplicity 2 centrality bins are studied
more central collisions
more central collisions
0-10 central Nch gt 237
10-30 central 117 lt Nch lt 237
6
Selections Tracks
Fit points
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 lt 0.5, 5 bins, bins width 0.2
DCA (cm)
Tracking
Chi squared
7
Particle ID finding dE/dx

• Bethe-Bloch parametrization

Particles to identify ?, ?-, K, K-, p, and pbar
p-
masses ?? 139.570 MeV K? 493.677 MeV p?
938.272 MeV
K-
e-
?-
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), bins 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).

?-
e-
K-
p-
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/RevHalfFi
  eld/
• Plot the ratio of the two
• Fit the ratio to eff m1exp(-(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.
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

11
Pion Spectra, AuAu 20 GeV
y lt 0.1
Au Au --gt ? X
Au Au --gt ?- X
12
Kaon Spectra, AuAu 20 GeV
y lt 0.1
Au Au --gt K X
Au Au --gt K- X
13
p, pbar Spectra, AuAu 20 GeV
y lt 0.1
low statistics for pbar
Au Au --gt p X
Au Au --gt p- X
14
Spectra shape
0-10 central
h
h-
10-30 central
h
h-
15
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

p-
?-
K-
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Particle ratios

• From the yields --gt 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) --gt strangeness
  production (70 of strange quarks go into
  producing kaons)
• K-/K ratios --gt kaon medium modification
• pbar/p ratios --gt baryon transport

Refs STAR Coll., nucl-ex/0206008 F. Wang,
nucl-ex/9911004 C. Roy, nucl-ex/0111017
K-
u
K
?-
d
?-
u
s
?

• ups and downs

d
ups and downs
u
u
d
d
?
u
s
?
quarks masses u 1.5 - 4.5 MeV d 5 - 8.5 MeV s
80 - 155 MeV
p-
d
K-
?
1 Au ion 79 p, 118 n
?-
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
(118118) than protons (7979) 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 ---gt -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 ---gt -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 0.08674 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

19
Ratios Kaons and Protons
Both (us)/(us)and (uud)/(uud) increase with
collisions energy, with the kaons ratio
increasing faster (more s quarks). They approach
unity at E 200 GeV.
K-/K
Sources 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 lt 0.5
• dN/dy increases with centrality uniform over y
  lt 0.5, although some have curious shapes
• Both K-/K and pbar/p ratios uniform over y lt
  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