strangeness in quark matter 2006, los angeles, 26-31 marchladislav Šándor1 strangeness production...
TRANSCRIPT
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 1
Strangeness production at the CERN SPS
Ladislav ŠándorSlovak Academy of Science
Institute of Experimental PhysicsKošice, Slovakia
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 2
Plan of talk
• Why strangeness ?• Flavour equilibration• Strangeness enhancements• Search for the onset of deconfinement• Collective flow and thermal freeze-out• Nuclear modification factors
selected results on strangeness production from SPS ion experiments, particularly NA49 and WA97/NA57
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 3
Why strangeness ?
• Strange quarks/antiquarks not present in the initial state of heavy-ion reaction, all the strangeness of final state produced in the course of interaction
• Thus, the strange hadrons bear an important information on the collision dynamics
• Specific predictions (J.Rafelski, B.Mueller) allowing to consider strange and particularly multi-strange baryons as promising probes signalling the QGP creation
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 4
Flavour equilibration ?
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 5
Thermal model fits
P.Braun-Munzinger, I.Heppe,J.Stachel, PL B465 (1999) 15
high degree of thermal equilibration even for rare multi-strange particles
Question: is thermal and chemical equilibrium achieved for hyperons ?
Applying a statistical model which assumes equilibrium and testingdata against model predictions gives a way to answer this question
A realistic model assuminggrand canonical ensemble applied to SPS mid-rapiditydata on particle ratios inPb-Pb collisions at 158 GeV
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 6
Thermal model fits (2)
Interesting work done in recent years in development of statistical hadronization models and their application to growing experimental data
• F. Becattini et al., Phys. Rev. C69 (2004) 024905• J. Letessier, J. Rafelski. nucl-th/0504028• T. Renk. J.Phys.G: Nucl.Part.Phys., 30 (2004) 1495
Remarkable success of statistical hadronization model in describing data on particle yields in a wide energy range
prediction of particle ratios obtained from comprehensive analysis of the SPS data for several relevant QGP observables in the framework of a model of the space-time evolution of the collisions
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 7
Thermal model fits (3)
A comprehensive analysis of data over a broad energy range (√sNN = 2.7 – 200 GeV); A.Andronic, P.Braun-Munzinger, J.Stachel. nucl-th/0511071
Statistical quality of fit at 158 A GeVnot satisfactory : inhomogeneous freeze-out scenario ?
(D. Zschiesche, nucl-th/0505054)
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 8
Energy dependence of thermal parameters
similarity of chemical freeze-out at SPS and RHIC
SPS RHIC
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 9
Strangeness enhancements
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 10
First SPS signals on enhancement
WA85 experiment, OMEGA spectro- meter, S-W and p-W data at 200 A GeV , central rapidity, pT > 0.9 GeV/c
Λ and anti-Λ production enhanced by a factor 1.7 with respect to negatives when going from p-W to S-W collisions
S.Abatzis et al., Nucl. Phys. A 525 (1991) 445c
NA35 streamer chamber experiment studying S-S and p-S collisionsat 200 A GeV observed ~ twofold increase of strange particle (K0,Λ) production with respect to negatively charged hadrons when going from p-S to S-S interactions
J. Bartke et al., Z. Phys. C 48 (1990) 191; R. Stock et al., Nucl. Phys. A 525 (1991) 221c
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 11
WA85 and WA94 results
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 12
NA57 Pb-Pb data at 158 A GeV
NA57 data confirm
the global enhance-
ment pattern first
observed by the
WA97 in narrower
centrality region
Significant centrality dependence of strangeness enhancements for all hyperons except for
_ Strangeness enhancement:
WA97 data: F. Antinori et al., Nucl.Phys. A661 (1999) 130c
F. Antinori et al., J.Phys.G 30 (2004) S129-S138
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 13
... and at 40 A GeV/c
systematic
errors:
10% for , 15% for
Most peripheral
class accessible
in NA57 :
<Nwound> = 62 4
(95 % confidence level)
Enhancements still there at 40 A GeV/c with the same hierarchy as for higher energy data: E() < E()
[ ]
F. Antinori et al., J.Phys.G 30 (2004) S717-S724
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 14
… and recent NA49 results
NA49 and NA57results qualitativelyconsistent
Important contributionto the knowledge of centrality dependence
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 15
Energy dependence of enhancements
enhancements in the most central collisions
(classes 3-4) are larger at 40 GeV than at 160 GeV
steeper increase with centrality at lower energy 40 GeV
160 GeV
1
10
100
qualitative agreement with canonical suppression prediction (A.Tounsi & K.Redlich, hep-ph/0111159)
NA57 data
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 16
Energy dependence (cont.)
Agreement of data with the model only qualitative…Need for more refined model development and predictionsfor 200 GeV Au-Au and LHC …
STAR data: J.Phys.G 31 (2005) S1057
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 17
The onset of deconfinement ?
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 18
Where is the point of the onset of deconfinement ?
Important NA49 activity :
• Search for a threshold by varying the energy of the largest collision system (central Pb+Pb reactions)
• SPS energy scan: 20,30,40,80,158 GeV/nucleon
• The energy dependence of several observables shows anomalies at lower SPS energies
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 19
Ratio of K, Λ yields to pions
• sharp peak of K+/π+
ratio
• smooth rise of anti-Λ/π
• similar peak in Λ/π ratio• structure in K–/π–
hadronic models donot (yet ?) reproduce the sharp peak
)(2
1 ><+><=>< −+± πππ
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 20
Inverse slope parameters
• inverse slope parameter T of K+ and K- constant at SPS
T/m - e
dydmm
dN T
TT
∝
• approximately constant pressure and temperature in mixed phase ?
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 21
Ratio of strange hadrons to pions
Strangeness to pion ratio peaks sharply at the SPS
SMES explanation: - entropy, number of s,sbar quarks conserved from QGP to freezeout - ratio of strange/nonstrange d.o.f. rises rapidly with T in hadron gas - Es drops to predicted constant level
above the deconfinement threshold
onset of deconfinement at SPS ?
hadr
onic
mixed
partonic
M.Gazdzicki, M.Gorenstein. Acta Phys.Pol.B 30 (1999) 2705
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 22
New phenomenon or conventional physics ?
A number of recent theoretical activities aiming at more conventional explanation :
If the “horn” signals the onset of deconfine-ment, one is expecting strong event-by-event fluctuations, search for is going on …
There are indications for a change of behaviour at lower SPS energy, howeverthe relation of observed effects to theonset of deconfinenent is still unclear
B. Tomášik, E. Kolomeitsev. nucl-th/0512088 ,J. Letessier, J.Rafelski. nucl-th/0504028 ,J. Cleymans et al., nucl-th/0510283,J. Cleymans et al., Phys. Lett. B 615 (2005) 50 …
Interesting result, waiting fair confirmation …
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 23
Transverse spectra,thermal freeze-out scenario
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 24
Transverse mass spectra
Shape of spectra close to exponential:
T/m - e
dydmm
dN T
TT
∝
T – apparent temperature (free parameter of fit) reflecting both the transverse collective flow and thermal motion. SPS data suggest approximately linear dependenence of T on particle mass
NA44 data – L.G.Bearden et al.,PRL 78 (1997) 2080
NA57, 158A GeV
The same shape of hyperonand anti-hyperon spectra
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 25
T versus particle mass
Presented at SQM2003, Atlantic Beach
WA97 and NA57 data compatible, SPS and RHIC trends similar
To disentangle contributions from a pressure driven collective flow (transverse velocity <βT>) and thermal motion (freeze-out tempera-ture Tfo) one needs to invoke
models A hydrodynamically inspired blast-wave approach is widely used (Schnedermann, Sollfrank & Heinz, PR C48 (1993) 2462)
Relativistic hydrodynamics at early deconfined phase plus UrQMD transport at hadronic stageS.A.Bass, A.Dumitru. PR C61:064909,2000
~
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 26
Blast-wave fits, NA57 data
Pb-Pb 40 A GeV/c
NA57, JPG30 (2004) 823
G
n
GS RrR
rr ≤⎥
⎦
⎤⎢⎣
⎡=⊥ )( ββ)(tanh)( 1 rr ⊥
−= βρ
Pb-Pb 158 A GeV/c
Model : thermalization plus hydro-dynamicaltransverse flow description
rdrT
pI
T
mKmA
mym
N GR TTTj
TT
j ∫ ⎟⎠
⎞⎜⎝
⎛⋅⎟⎠
⎞⎜⎝
⎛⋅=0 01
2 sinhcoshdd
d ρρ
F. Antinori et al, J. Phys. G: Nucl.Part.Phys, 30 (2004) 823; 31 (2005) S127
Fitswithn=1
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 27
Simple blast-wave fits, NA49 data
158A GeV 30A GeV 20A GeV
kinetic freeze out at Tfo 120 –130 MeV, <βT > 0.4 – 0.5 at SPS
Reasonable description of spectra at all SPS energies
Taken from : The NA49 collaboration, CERN-SPSC-2005-041, Nov. 2005
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 28
Freeze-out of multistrange hyperons
NA49 data on Ω productionC.Alt et al., PRL 94 192301 (2005)
The data favor a low transverse expansion velocity and high freeze-out temperature (Tfo= 170 MeV, <βT> = 0.2 , fit B2)
indication for early freeze-out of the at top SPS energy ?Analysis by G.E.Bruno (NA57)
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 29
High(er) transverse momenta
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 30
Nuclear modification factors at SPS
It seemed that QGP signatures such as “jet quenching” (parton energy loss in the dense deconfined medium) is absent at SPS energies
M.M. Aggarwal, et al.,WA98 Collaboration, Eur. Phys. J. C 23 (2002) 225;X.N. Wang, Phys. Rev. Lett. 81 (1998) 2655.
Re-analysis of WA98 data using more realistic pp-reference changed the situation
D. d’Enterria. Phys.Lett.B 596(2004) 32
Suppression consistentwith behaviour at RHIC
What about strangeparticles ??
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 31
NA57 results on RCP for K0, Λ
dydpNd
dydpNd
N
NpR
tP
tC
Ccoll
PcolltCP /
/)(
2
2
×=
F.Antinori et al., Phys,Lett.B 623 (2005) 17
Similar patterns as at RHIC, described by calculations involving parton energy loss
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 32
Recent NA49 data
From talk of T. Schuster (this conference)
Apparently a consistent picture at SPS is developing, needs further analysis
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 33
Comparison of NA57 and STAR data
NA57
• Similar K0- relative pattern observed at SPS and RHIC energy
• RCP values higher by ~0.5 at SPS, no significant suppression observed
STAR data from: Phys.Rev.Lett. 92 (2004) 052302
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 34
Conclusions We have learnt quite a lot till now, analysis continuing (see talks of
M. Mitrovski, T. Schuster, H. Helstrup, J. Milosevic)
Thermal model fits of data at the top SPS energy show high degree of chemical and thermal equilibration, even for rare multi-strange hyperons
Enhancement of strangeness observed by WA97, NA57 and NA49 in central Pb-Pb collisions with respect to p-Be (p-p) reaction, increasing with strangeness content of particle (QGP prediction)
Important energy scan performed by the NA49 experi-ment. Several anomalies found in energy dependence of some observables (strangeness/pions ratio, inverse slopes of kaons, …) indicating possible onset of deconfi-nement ? in central Pb-Pb collisions at about 30 A GeV
Strangeness in Quark Matter 2006, Los Angeles, 26-31 March Ladislav Šándor 35
Conclusions (cont.)
The analysis of transverse mass spectra of strange particles in Pb-Pb at the top SPS energy suggests that after a central collision the system expands explosively and then it freezes-out when the temperature is ~120 -140 MeV, with an average transverse flow velocity of about one-half of the speed of light. An indication for possible early freeze-out of multi-strange hyperons
A similar tendency in strangeness production at SPS and RHIC observed (chemical freeze-out temperature, strangeness enhancements, transverse flow, high pT spectra, …)