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Physics Results of the NA49 exp.
on Nucleus – Nucleus Collisions
at SPS Energies
P. Christakoglou, A. Petridis, M. Vassiliou
Athens University
HEP2006, Ioannina
13-16 April 2006
Outline
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Introduction
Energy dependence of Pb+Pb collisions
transverse mass spectra
particle yields
yield ratios
Electric charge correlations
the Balance Function
system size and centrality dependence
rapidity dependence
energy dependence
Conclusions
Introduction
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Pb+Pb collisions at top SPS energy:
Initial energy density exceeds the critical value
predicted by lattice QCD
Strong collective behavior
Proposed signatures for deconfinement observed
Signatures not specific for deconfinement
SPS energy scan : 20, 30, 40, 80, 158 GeV/nucleon
To search for structure in the energy dependence
of hadron production characteristics indicating the
onset of deconfinement
The NA49 Detector
TOF+dE/dx
midrapidity
VCAL detects projectile spectators
dE/dx resolution 3-6%
TOF resolution ~60ps
dE/dx
forward rapidity
Transverse mass spectra at mid-rapidity
20 AGeV
40 AGeV
158 AGeV
(mT  m0 )GeV
Kinetic freeze-out at T~ 90–110 MeV , βτ ~ 0.45c
Energy dependence of average transverse mass
Increase of  mT  for final state particles (π,Κ,p)
slows sharply at the lowest SPS energy
Energy dependence
of the effective temperature parameter of Kaons
m
 T
dN
e T
mT dmT dy
• consistent
with approximately constant pressure
and temperature in a mixed phase system
Rapidity Spectra
Energy dependence of antibaryon/baryon ratios
The effect of the
increasing baryon density
is seen in the strong
decrease of
antibaryon/baryon ratios
towards lower SPS
energies.
Global view – Phase diagram
Statistical model describes yields
from AGS to RHIC energies
T of “hadrochemical” freeze out
increases
B decreases
hadrochemical freeze out points at SPS
energies approach the phase boundary
Energy dependence – pion yields
•
Increase of ratio with energy
gets steeper in the SPS range
•
Pion deficit changes to enhancement
compared to p+p
• SMES: statistical model of the
early stage
Increase of initial d.o.f.
between AGS and SPS
Energy dependence –ratio of K, Λ yields to pions
anti-strange quark carriers:
• sharp peak of K   
•

 yield small
hadronic models do not
reproduce
the sharp peak
strange quark cariers:
• similar peak in Λ/π
• structure in K   
Energy dependence – ratio of strange hadrons to pions
K 
ES 
 
•Strangeness to pion ratio peaks
sharply at the SPS
•The peak is not seen in p+p
collisions
•SMES reproduces the data
Suggests ONSET of
deconfinement at SPS
Electric charge correlations in A+A collisions
• Oppositely charged particles are
created at the same location of
space – time.
• Charge – anticharge particles that
were created earlier (early stage
hadronization) are separated
further in rapidity.
• Particle pairs that were created
later (late stage hadronization)
are correlated at small Δy.
• The Balance Function quantifies
the degree of this separation and
relates it with the time of
hadronization.
The Balance Function
The Balance function is defined as a correlation in y of
oppositely charged particles, minus the correlation of
same charged particles, normalized to the total number of
particles.
1  N (, P2 | , P1)  N (, P2 | , P1) N (, P2 | , P1)  N (, P2 | , P1) 

B( P2 | P1 )  

2
N (, P1)
N (, P1)

where P1: any rapidity interval in the detector
P2: relative rapidity interval
1  N  ( )  N  ( ) N  ( )  N  ( ) 

B( )  

2
N
N

• Bass-Danielewicz-Pratt, Phys.Rev.Lett.85, 2000
• D. Drijard et al, Nucl. Phys. B(155), 1979
The width of the Balance Function
The overall width of the Balance
Function (BF) in relative rapidity is
a combination of the thermal
spread and the effect of diffusion.
Due to cooling the width falls with
time (σtherm).
The effect of diffusion stretches the
BF (σδn).
If the hadronization occurred at
early times then the effect of
collisions is to broaden the BF.
On the other hand late stage
hadronization suggests
narrower BF.
k


B
i
i 0
 i
k
B
i 0
i
System size and centrality dependence - √sNN = 17.2 GeV
The width takes its
maximum value for p+p
interactions.
Data show a strong
system size and
centrality dependence.
Neither HIJING nor
shuffled data show any
sign of system size or
centrality dependence.
C. Alt et al. [NA49 collaboration], Phys.Rev. C71, 034903 (2005).
Comparison NA49 – STAR
Au+Au@ √sNN = 130 GeV
NA49 data show a
strong centrality
dependence of the order
of (17 ± 3)%.
STAR data show also
a strong centrality
dependence of the order
of (14 ± 2)%.
J. Adams et al., (STAR Collaboration) Phys. Rev. Lett. 90, 172301 (2003)
Rapidity dependence - √sNN = 17.2 GeV
Mid – rapidity (2.5 < η < 3.9)
Acceptance filter OFF
Acceptance filter ON
Forward rapidity (4.0 < η < 5.4)
Energy dependence
 
W 


shuffling

 
shuffling
data

 100%


Conclusions
•
1.
2.
3.
Central Pb+Pb collisions were studied in the SPS energy
range.
At 30A GeV :
The ratio of strangeness to pion production shows a
sharp maximum.
The rate of increase of the produced pion multiplicity per
wounded nucleon increases.
The effective temperature of pions and kaons levels to a
constant value.
These features are not reproduced by hadronic models
but find a natural explanation in a reaction scenario with
the onset of deconfinement in the early stage of the
reaction at low SPS energy.
Conclusions (cont.)
• Results from a study on :
1. system size and centrality dependence
2. rapidity dependence
3. energy dependence
of charged particles correlations, performed with the
Balance Function, are consistent with the predicted
delayed hadronization.
BACK UP
√sNN = 8.8 GeV – RAPIDITY DEPENDENCE
Mid – rapidity (1.8 < η < 3.2)
Acceptance filter OFF
Acceptance filter ON
Forward rapidity (3.3 < η < 4.7)