Midrapidity charged hadron spectra in the Au Au collisions at sqrtsNN 19.6 GeV at STAR

1
Mid-rapidity charged hadron spectra in the AuAu
collisions at sqrt(sNN) 19.6 GeV at STAR
Daniel Cebra Roppon Picha, Juan Romero Trieu Mai,
Mark Allen, Nathan Farr, David Cherney

• University of California, Davis
• for the STAR Collaboration

2
Overview

• 19.6-GeV Introduction
• Methods
• Event selection - centrality determination -
  Track select.
• PID - Bethe-Bloch, dE/dx vs ??,Gaussian fits
• Efficiencies - Proton background - p-bar
  absorbtion
• Results
• mt-m0 spectra - dN/dy distributions
• Particle ratios - rapidity and centrality
  dependence
• Comparisons with SPS 17.2 GeV results
• Summary

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STAR Time Projection Chamber

• The 19.6-GeV was run in Nov 2001
• Magnetic field 0.25 T (half field)
• Gas P10 (90 Ar 10 CH4) at 1 atm
• Detectors SVT, TPC, FTPCs
• Trigger
• Minbias (CTB gt 15, ZDC e-w coinc.)
• Central (CTB gt 600)
• Total Event Pool
• 175466 events

from 0915am (25th) to 0626am (26th)
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Event selection

• Event cuts
• vertex X, vertex Y r lt 1 cm (over 95 pass)
• vertex Z lt 30 cm (over 25 pass)
• 43,131 events pass vertex cuts
• Track cuts
• global DCA lt 3 cm
• fit points gt 24
• 0.51 lt fit points/max points lt 1.05
• chi-squared of fit lt 2.5

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Trigger

• ZDC vs. CTB observations
• not a boomerang shape as in 130 and 200 GeV (20
  GeV spectators are more dispersed, not many
  neutrons make it to the ZDCs.)
• the discontinuity around CTB1300 comes from the
  trigger criteria

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Centrality

• Centrality determination is done offline by
  selecting on the charged particle multiplicity.
• 20-GeV Nch distribution shape is scaled to those
  from other RHIC beam energies.
• 130 and 200 GeV have the same shape. 20-GeV shows
  trigger inefficiencies in the 10-30 and 70-100
  centrality bins.

50-70
70-100
0-10
10-30
30-50
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PID by dE/dx

• dE/dx is described by the Bethe-Bloch function
• We use a 7-parameter Bethe-Bloch function to
  calibrate the dE/dx centroids as a function of ??
  ( p/m)
• Distributions of ln(dE/dx) are fitted by 4
  Gaussians for ?, K, pr, and e. (with fixed
  centroids and widths)

h, ? lt 0.5
only 3 free parameters, in most cases
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Efficiency corrections
Reconstruction efficiency is plotted as a
function of the transverse momentum. The
efficiency drops sharply below 0.15 GeV/c because
the low-momentum particles stay inside the TPC
(rcurv p/B) and dont reach the CTB.
pt distributions of embedded and reconstructed
tracks
The ratio is the efficiency.
Efficiency embedding Eric Hjort Method Olga
Barannikova
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Background protons corrections

• Interactions between fast pions and detector
  material generate secondary protons.
• background protons have low momenta, are
  negligible for mt-m0 above 0.3 GeV/c2.
• We find the ratio of background/total proton
  inside 0-3 cm DCA. And use this to calculate real
  proton yields.

Method of Schweda PRL 86, 4778 (2003)
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Transverse mass spectra
p
?
K

• thermal fits
• Bose-Einstein statistics for pions and kaons
• Fermi-Dirac for protons

pbar
K-
?-
blast wave fit parameters Tth 103 /- 4 MeV
?r 0.47 /- 0.04 c (M. Kaneta, Jul 14, 2002)
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Transverse mass spectra

• comparison between different particles
• Low antiprotons produced at this energy
• Protons yields are comparable to kaons
• Similar inverse slopes for all particles

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Comparison to published spectra
STAR 19.6 GeV Data NA49 (5 - 10) p- , Ks
PRC66, 054902 (2002) p,p-bar private
comm. NA44 (top 10) PLB388, 431
(1996) WA98 p- NPA698, 647 (2002)
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Average pt

• From the models, we can approximate midrapidity
  ltptgt

AuAu 20 GeV
O. Barannikova AuAu 200 GeV
Mean pt rises with centrality, particles mass,
and beam energy.
calculated from sqrt(ltmtgt2-mo2)
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Midrapidity dN/dy
p
?
K
The extracted rapidity densities are consistent
with the values from the SPS.
K-
?-
pbar
Note NA49 results are the averages over y lt 0.6
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dN/dy comparisons to published data
There is good agreement between the new results
and the published yields
STAR (top 10) new result NA49 (top 5) p- ,
Ks PRC66, 054902 (2002) p,p,p-bar NPA661, 45
(1999) NA44 (top 10) nucl-ex/0202019
(2002)
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Particle ratios
Particle ratios tell us about the quark contents.
top 10 central
top 10 central

• K-/K and pbar/p flat over mt-mo

pbar/p drops as collisions become more central,
suggesting pbar absorption within nuclear
material.
no strong centrality dependence in K-/K
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K/p ratios
There is very different behavior between the
positive and negative K/p ratios. K/p ratios
in pp follow the trend of the K-/p-.
NA44
K/p
PHENIX
E917
BRAHMS
NA49
K/p
STAR 130 nucl-ex 02008 (2003) NA49 PRC66,
054902 (2002) NA44 nucl-ex 0202019 (2002) E917
PLB476, 1 (2000) PHENIX PRL88, 242301
(2001) BRAHMS QM2002
K-/p-
(sNN)1/2
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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
STAR Preliminary results
E866/917 nucl-ex 0008010 WA97 JPhys G25 (1999)
171 NA44 JPhys G23 (1997) 1865 NA49 NP A661
(1999) 45c STAR PRL 86, 4778 (2003) STAR
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Particle ratios pions
?/?- ratios show Coulomb effect with
positively-charged source, ? get extra kick,
whereas ?- are pulled toward the source.
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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 WA98 QM2001 KaoS PLB420
(1998) 20 NA44 1996
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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
Phobos 19.6 GeV Result - QM2002
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Summary

• Thermal models fit spectra of pions and kaons
  well protons show visible peaks near 0.1 GeV/c2
  -- a blast wave effect.
• Flat rapidity distributions indicate incomplete
  stopping within y lt 0.5.
• AuAu at 19.6 AGeV - not baryon free.
• K-/K 0.6, no strong dependence on centrality
  or rapidity
• pbar/p ratio 0.1
• decreases in more forward rapidity bins
• decreases with centrality
• Both kaon and pbar/p ratios increase with beam
  energy our results consistent with the trend.
• Future more comparisons to SPS results higher
  rapidity results for pions
• Overall, the results confirm the consistency
  between RHIC and SPS apparatus.