Transcript Document

Large-pt Kaon spectra from
200GeV AuAu collisions at RHIC
Ming Yao (For The STAR Collaboration)
University of Science and Technology of China
STRANGENESS IN QUARK MATTER 2007
strangeness in quark matter 2007
Outline
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Motivation
PID method
Results
Conclusions
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Why charged Kaon
• K+ : u sbar 494MeV/c
• K－ : -ubar s 494MeV/c
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Kaons carry a large fraction of produced strangeness.
Measurements of Kaon distributions and yields will allow to study the strangeness
freeze-out dynamics.
The yields of Kaon at the intermediate transverse momentum will allow us to
address the issue of coalescence of the s quarks with u quarks.
Comparison with other particles can tell us about the Kaon production mechanism
and add to the understanding of freeze out dynamics in heavy ion collisions .
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STAR detectors: TPC & MRPC-TOF
Time Projection Chamber
A new technology (TOF) ---Multi-gap Resistive Plate Chamber
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Tracking
Ionization energy loss (dE/dx)
Coverage -1<<1
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Good timing resolution (<100ps)
One tray for now, 120 trays in the
future
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What do we have
TPC -- dE/dx vs. p
TOF -- 1/beta vs. p
Kaon identification carried out using the following measurements :
(a) ionization energy loss in gas of TPC vs. momentum
(b) velocity of the particles vs. momentum using TOF
We will focus on the intermediate pT region
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Predicted m2 method
Assumption: TOF timing behavior is pT independent at intermediate pt range
1. Cut on dE/dx & m2 to select pure
pion sample
2. Comparing the time resolution in 2
pT bins
0.5<pT<0.6
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pion sample
0.9<pT<1.1
Two time resolutions
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The t distribution
describes TOF timing
behavior and it is
essentially pT
independent.
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t distribution (TOF timing behavior)
If we identify one particle as Pion (0.9<pT<1,1), we can get the particle’s time
of flight (t0) from its momentum and path length:
t0=f(p,L)
TOF also measures the particle’s time of flight (t1);
Let:
t=t1-t0
So t distribution describes the TOF behavior since the t0 uncertainty is very
small.
Select pion sample
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(m2) Prediction 1.1<pT<5.0
‘real’ value
p
Primary
Track
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Path
Length
m0
Predicted value
Time
of Flight
+t
(randomly)
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Time
of Flight
 & m2
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Fitting real data to extract raw yield
Parameters:
 -- pion raw yield
 -- kaon raw yield
 -- proton raw yield
m(); m(k); m(p) ---predicted m2 distribution
n(); n(k); n(p) ---n distribution
Fit function for m2 distribution:
f(,,)= *m()+*m(k)+*m(p)
Least 2 method fitting
Fit function for n distribution:
g(,,)= *n()+*n(k)+*n(p)
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m
n
χ   {(Fi  f(α, β, γ)) /Fi }   {(G j  g(α, β, γ))2 /G j }
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2
i 1
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j1
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K+
K+
Kaon invariant yield
Kaon invariant yield
Kaon inv_yield at different centralities
K–
K–
STAR Preliminary
STAR Preliminary
AuAu 200GeV
J. Adams et al. , Phys. Rev. Lett. 92, 112301 (2004).
0.1-0.7GeV/c
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K－/K+ at different centralities
K－/K+ ratio
STAR Preliminary
K－/K+ ratio
0-10%
STAR Preliminary
10-20%
AuAu 200GeV
K－/K+ ratio
STAR Preliminary
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20-40%
K－/K+ ratio
STAR Preliminary
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40-80%
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K/  at different centralities
K/ 
AuAu
K–/ ―
200GeV
K+/ +
STAR Preliminary
More s quark are produced at central collision.
K+/ + ->
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u sbar/ u dbar
K–/ ― -> ubar s/ ubar d
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K/  compared with 62 GeV
K–/ ―
K+/ +
STAR Preliminary
STAR Preliminary
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K－/K+ compared with 62 GeV
K－/K+
K－/K+
STAR Preliminary
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STAR Preliminary
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Rcp compared with 62 GeV
STAR Preliminary
K+
K－
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K+
K－
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Conclusions
• The Time-of-Flight detector, based on MRPC technology, has greatly
enhanced particle identification capability in STAR. The measured Kaon
spectra are extended up to pT ~ 5 GeV/c.
• The transverse momentum spectra for charged Kaon at 0.3 < pT < 5 GeV/c
at mid-rapidity in 200 GeV Au+Au collisions has been measured by the
STAR experiment at RHIC.
• More strange quarks are produced at more central collisions at low and
intermediate pT. Consistent with Ks measurements.
• In 200GeV collision, K/  ratios vs. pT are similar at 200 and 62.4 GeV
Au+Au collisions.
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Thank you