Georgios Tsiledakis

1
Scale Dependence of Mean Transverse Momentum
Fluctuations at Top SPS Energy measured by the
CERES experiment

• Georgios Tsiledakis
• Univ. of Heidelberg
• for the CERES collaboration

CERN Heavy Ion Forum, 24 May 2006
2
Outline

• ltpTgt Fluctuations indications of a phase
  transition
• Measures of mean pT fluctuations
• Previous results from SPS
• The CERES experiment
• Recent results from run2000 at 158AGeV/c Pb-Au
• ? ltpTgt fluctuations
  resolved in Dh-Df
• ? Centrality
  dependence
• ? Elliptic flow
  contribution
• ? Two-particle
  correlation analysis (c)
• Conclusions and outlook

3
ltpTgtEbE Fluctuations Search for signatures of
the QCD phase transition

• Transverse momentum

They might be sensitive to

• 1st order Phase Transition
• Formation of QGP droplets
• 2nd order phase transitions
• Location of Critical Point of QCD phase diagram

• Lattice QCD There is a critical point...but
  uncertainty in location is large.
• M. Stephanov (hep-ph/0402115)
• Experiments Can contribute...
• Observables pT fluctuations

but also to

• HBT and Coulomb correlations
• Collective flow
• Resonance decays
• (Mini-) jet production

S. Mrowczynski (Phys. Lett. B314 (1993) 118) M.
Stephanov, et al. (Phys. Rev. Lett. 81 (1998)
4816)
4
Event-by-Event mean pT distributions(CERES
collaboration, Nucl. Phys. A727(2003)97-119)

• Event-by-event MpT distributions in real
  events are slightly wider than those in mixed
  events
• Evidence for the non-statistical (dynamical) mean
  pT fluctuations

Real mixed
Real /mixed
6.5 central, 2.2lthlt2.7, 0.1ltpTlt1.5 GeV/c
5
Look at pT fluctuations vs

• Collision Energy
• Critical point
• Centrality
• Reaction mechanisms
• Systematics is not straightforward
• Different acceptance
• Different variables
• Different phase-space region
• Here
• Translate to SpT

6
Measure of mean pT fluctuations

• DMpT variance of MpT dist.
• Dp2T variance of inclusive pT dist.
• ltNgt mean multiplicity
• pT inclusive (event-averaged) mean pT

• Normalized dynamical fluctuation

• 0 for purely statistical fluctuation
• gt 0(lt 0) with positive/negative two-particle
  correlation or dynamical EbyE fluctuation

Dimensionless measure
SpT

• Proportional to mean covariance
• of all particle pairs / event
• Robust under change of multiplicity due to
  changes in beam energy and acceptance

7
Previous results Collision energy dependence

• Dynamical fluctuations of 1 are observed at
  SPS, which are similar to RHIC data, independent
• of beam energy.
• No indication for the critical point or phase
  transition is observed.

No evidence for non-monotonic change or enhanced
fluctuations prediction at the critical point
2 (Stephanov, PRD60(1999)14028)
(CERES collaboration, Nucl. Phys.
A727(2003)97, J.Phys.G30(2004)S1376)
8
Previous results Centrality dependence

• Baselineextrapolation from pp measurement
• 12 measured in pp_at_ISR
• (Braune, PLB123(1983)467)
• Non-monotonic dependence of NpartSpT2
• Little collision energy dependence
• Maximum in semi-central events
• Consistent with the pp extrapolation in central
  and peripheral

CERES
NpartSpT2 0.03(const)
(CERES collaboration, Nucl. Phys.
A727(2003)97, J.Phys.G30(2004)S1376)
9
Comparison to STAR and NA49 data
STAR preliminary
NA49
CERES
G. Westfall, QM2004
nucl-ex/0208020 Phys. Rev. C70 (2004) 034902

• Similar fluctuations at each centrality at SPS
  and RHIC energies

similar non-monotonic dependence
10
How to identify the source of the fluctuations
Fluctuations were characterized so far by one
single (integral) number. More sensitivity is
needed.
Learning more...

• Look at more differential measures to
  discriminate among various correlation sources
• the scale dependence of momentum correlations
  carries information about their origin and may
  allow a separation of different contributions
• Fluctuations at low/high pT
• 2-particle correlators
• Substructures from different charge combinations
• Physics (HBT, Coulomb)
• Detector effects

11
CERES Experiment
PbAu collisions at 158 GeV per nucleon
acceptance 8 lt q lt 15 2.2 lt h lt 2.7 Df2p
Multiplicity Counter
12
Tracking selection and parameters

• Track Selection
• TPC tracks (no. of hits gt 12 out of 20)
• Target cut (projection of TPC track on the
  primary vertex lt 10cm)
• Momentum resolution
• Dp/p ((0.0152(0.016p)2)1/ 2 at 158 AGeV
• Full f Acceptance
• Tracking efficiency
• Better than 85 at pTgt0.05 GeV/c
• 2-particle resolution
• 5 mrad in TPC

• 10M events analyzed for the present analysis
• 0.1 lt pT lt 1.5 GeV/c

13
Centrality determination
fraction of the inelastic cross section
Multiplicity Counter pulse height distribution
Centrality selection Multiplicity Counter (158
GeV) Number of participant nucleons is estimated
with a geometric nuclear overlap (Glauber) model
0 -gt impact parameter0
Centrality classes
14
Measurement of pT Fluctuations using the Average
Momentum Correlator

• To quantify dynamical pT fluctuations
• We define the quantity lt?pt,1?pt,2gt.
• It is a covariance and an integral of 2-body
  correlations.
• It equals zero in the absence of dynamical
  fluctuations
• Defined to be positive for correlation and
  negative for anti-correlation.

S. Voloshin. V. Koch. H. Ritter, PRC60 (1999)
but ltDpt,1 Dpt,2gt allows to select pairs by
charge sign, Df, Dh,...
/ 2
/ 2
where
15
Inclusive analysis Differential analysis

• 0 lt Df fi fj lt 180
• 24 bins in Df

ltDpt,1Dpt,2gt 22.71(32) MeV2 s2pT,dyn
21.98(44) MeV2 SpT 1.04(1)
n events 107 central

• 0 lt Dh hi hj lt 0.5
• 5 bins in Dh

• We divide the Dh-Df space into
• 120 bins
• For each bin we calculate the
• ltDpt,1Dpt,2gt value
• We construct the ltDpt,1Dpt,2gt
• and the SpT map

scale-independent results consistent with
previous data

• This approximation holds well
• The SpT 1 agrees with the
• previous measurements

CERES preliminary
CERES preliminary
16
Mixed event analysis
mixed pairs
true pairs
ltDpt,1Dpt,2gt -0.05/- 0.31 MeV2
ltDpt,1Dpt,2gt 22.71/- 0.32 MeV2
declination of signal with increased
?? reproduced with event mixing trivial effect
of pt(?) dependence?

• We correct by subtracting the maps of the mixed
  pairs from
• that of true pairs

CERES preliminary
17
ltDpt,1Dpt,2gt as a function of Df and Dh
short range correlations narrower and weaker for
unlike sign pairs ? HBT and Coulomb?
0.1 lt pT lt 1.5 GeV/c
away-side correlations elliptic flow, jets?

• dominant short-range component
• weak back-to-back structure

• 10M events analyzed, 8 centrality

CERES preliminary
18
ltDpt,1Dpt,2gt as a function of Df and Dh
ltDpt,1Dpt,2gt 22.71/- 0.32 MeV2

• average value consistent
• rich structure in Dh-Df
• origin?

Overall fit
CERES preliminary
19
Normalized dynamical fluctuation SpT
rich structure ? averaging over ?? and ?? is
dangerous
away-side peak Invariant on Dh ( back-to-back
jet fragments ? )

• On average SpT 1.04 (agrees well with
  previous studies)

CERES preliminary
20
ltDpt,1Dpt,2gt for different charge combinations
() ()
(-) (-)
() (-)

• there are differences

• at Df lt 7.5 deg. and small Dh () () and (-)
  (-) similar strength and shape
• ? origin could be the same (HBT, Coulomb?)
• () (-) peak weaker and narrower
• ? Coulomb, resonances, ee-?
• away-side component (flow, minijets?)
• What is the proton contribution ?

CERES preliminary
21
Centrality dependence

• Correlations are positive
• Decrease with centrality

CERES preliminary
STAR preliminary
Scale lt?pt,1?pt,2gt by ltNpartgt to remove 1/N
correlation dilution

• similar centrality dependence at SPS and RHIC

22
Centrality dependence for real and mixed events
REAL events
Dh 0 - 0.5
MIXED events
23
Centrality dependence
Pb-Au 158 AGeV/c 0ltDhlt0.5 CERES preliminary
lt?pt,1?pt,2gt (MeV2/c2)
Df

• angular dependence increases towards peripheral
  collisions

24
Centrality dependence
Df
region around ?? 45o is not affected by the
elliptic flow, HBT, Coulomb, jets ? look here
for the critical point

• The most interesting region is at 30 lt Df lt 60
  

Where the strongest contributions come from?
what about elliptic flow?
25
Elliptic flow contribution
Introduces a cos(2Df) modulation

• We introduce the measure fi,j

pT GeV/c

• We calculate the average momentum correlator for
• mixed pairs that have flow

J. Milosevic QM2005
p elliptic flow measured by CERES at lts/sgeogt
8.9
Subtracting the map of mixed pairs from the map
of mixed pairs with flow we can evaluate the
elliptic flow expressed in units of MeV2
CERES preliminary
26
Elliptic flow contribution
Mixed-event analysis
Monte Carlo
inputpt dependent measured V2 values(CERES)
The CERES elliptic flow introduces a modulation
in the Dh-Df space with a maximum amplitude of
7.6 MeV2
27
Elliptic flow contribution
Pb-Au 158 AGeV 0-8
Measured ltDpt,1Dpt,2gt
CERES
elliptic flow

• Measured elliptic flow cannot explain the
• observed rich structure in Dh-Df space
• What is the origin of the away-side peak?
• low or high pT effect?

28
pt-pt correlations

• cumulant pt variable x(pt)

Counts
x(pt)
pt (GeV/c)
pt (GeV/c)
Inclusive pT distribution
Cumulant pT variable c
29
azimuthal dependence
0 lt Df lt 30 30 lt Df lt 60
60 lt Df lt 90
Pb-Au 158 AGeV central
all pairs
90 lt Df lt 120 120 lt Df lt 150
150 lt Df lt 180

• High pT correlations
• at 120 lt Df lt 180

CERES preliminary
30
azimuthal dependence for different charges
()() (-)(-)
()(-)
Pb-Au 158 AGeV central
0 lt Df lt 30
()() (-)(-)
()(-)
150 lt Df lt 180
pT fluctuations are dominated by high pT effects
also at SPS
CERES preliminary
31
Pairs that have only the CERES elliptic flow
0 lt Df lt 30 30 lt Df lt 60
60 lt Df lt 90
90 lt Df lt 120 120 lt Df lt 150
150 lt Df lt 180
all pairs
CERES preliminary
32
Pairs that have only elliptic flow with
v23v2CERES
0 lt Df lt 30 30 lt Df lt 60
60 lt Df lt 90
90 lt Df lt 120 120 lt Df lt 150
150 lt Df lt 180
all pairs
CERES preliminary
33
Summary and conclusions

• For the first time at SPS, the
  charge-dependent mean pT
• fluctuations have been analyzed as a
  function of the angular pair separation, Df, and
  of the separation in pseudo-rapidity, Dh.
• Several contributions identified ? averaging
• over ?? and ?? hides underline physics
• A short range correlation peak (HBT, Coulomb,
  2-track resolution) narrower and weaker for
  unlike-sign pairs
• A broad away-side peak, independent on Dh
  (mini-jets?) comes from high-pT correlations
• The measured elliptic flow cannot explain
  neither the structure nor the magnitude of pT
  fluctuations
• Consistency between fluctuations and
  2-particle correlations
• Fluctuations in the region 30 lt Df lt 60 are
  cleaner
• and centrality independent ? look here for
  the critical point
• at other energies

34
Outlook

• Scale dependent correlation analyses
  significantly increase the sensitivity to
• critical fluctuations

corrected SpT ?
Collision energy dependence
Apply the appropriate kinematic cuts and pT
selections to exclude contributions from known
sources...
35
CERES Collaboration
D. Adamova, G. Agakichiev, D. Antonczyk, A.
Andronic, H. Appelshäuser, V. Belaga, J.
Bielcikova, P. Braun-Munzinger, O. Busch, A.
Castillo, A. Cherlin, S. Damjanovic, T. Dietel,
L. Dietrich, A. Drees, S. Esumi, K. Filimonov, K.
Fomenko, Z. Fraenkel, C. Garabatos, P. Glässel,
G. Hering, J. Holeczek, V. Kushpil, B. Lenkeit,
W. Ludolphs, A. Maas, A. Marin, J. Milosevic, A.
Milov, D. Miskowiec, R. Ortega, Yu.
Panebrattsev, O. Petchenova, V. Petracek, A.
Pfeiffer, S. Radomski, J. Rak, Ravinovich, P.
Rehak, W. Schmitz, J. Schukraft, H. Sako, S.
Shimansky, S. Sedykh, J. Stachel, M. Sumbera, H.
Tilsner, I. Tserruya, G. Tsiledakis, T. Wienold,
B. Windelband, J.P. Wessels, J.P. Wurm, W. Xie,
S. Yurevich, V. Yurevitch
NPI ASCR, Rez, Czech Republic GSI Darmstadt,
Germany Frankfurt University, Germany Heidelberg
University, Germany JINR Dubna, Russia Weizmann
Institute, Rehovot, Israel SUNY at Stony Brook,
USA CERN, Switzerland BNL, Upton, USA Münster
University, Germany MPI Heidelberg, Germany
36
Back-up slides
37
Rapidity range dependence of Mean pT fluctuations

• Monotonic increase in FpT, decrease and
  saturation in SpT
• SpT agrees at each Dh bin at 3 beam energies
• We use SpT to compare data with different Dh

FpT dependent on multiplicity, acceptance size,
beam energy.
(CERES collaboration, Nucl. Phys. A727(2003)97)
38
Short-range correlations (SRC)
C2(qinv)
all
acceptance 0 lt Df lt 7.5 deg. , 0 lt Dh lt 0.1
p-,p-
HBT Coulomb
1
qinv (GeV/c)
p-,p
qinv (GeV/c)
Coulomb resonances

• The peak at the origin is probably contaminated
• by HBTCoulomb

1
qinv (GeV/c)
D.Antonczyk p - correlations for CERES
39
Dependence on acceptance
40
ltNgt lt?pt,1 ?pt,2gt vs Centrality
region around ?? 45o is not affected by the
elliptic flow, HBT, Coulomb, jets ? look here
for the critical point

• The most interesting region is at 30 lt Df lt 60
  

Where the strongest contributions come from?
CERES preliminary
41
Elliptic flow contribution
Dh 0 - 0.1
Measured ltDpt,1Dpt,2gt
Elliptic flow 7.6cos(2Df)

• The elliptic flow cannot explain the
• observed rich structure in Dh-Df space
• What is the origin of the away-side peak?
• low or high pT effect?

CERES preliminary
42
Elliptic flow contribution
Dh 0 - 0.1
Elliptic flow 7.6cos(2Df)

• The CERES elliptic flow cannot explain the
  observed rich structure in Dh-Df space
• What is the origin of the away-side peak? low or
  high pT effect?

43
Two-particle correlations (Df dependence)
0 lt Df lt 30 30 lt Df lt 60
60 lt Df lt 90
90 lt Df lt 120 120 lt Df lt 150
150 lt Df lt 180
/ pairs

• High pT correlations
• at 120 lt Df lt 180

CERES preliminary
44
Two-particle correlations (Df dependence)
0 lt Df lt 30 30 lt Df lt 60
60 lt Df lt 90
90 lt Df lt 120 120 lt Df lt 150
150 lt Df lt 180
-/- pairs

• High pT correlations
• at 120 lt Df lt 180

CERES preliminary
45
Two-particle correlations (Df dependence)
0 lt Df lt 30 30 lt Df lt 60
60 lt Df lt 90
90 lt Df lt 120 120 lt Df lt 150
150 lt Df lt 180
/- pairs

• High pT correlations
• at 120 lt Df lt 180

CERES preliminary
46
(No Transcript)