strangeness production at the cern sps

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


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


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


Flavour equilibration ?


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


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


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)


Energy dependence of thermal parameters

similarity of chemical freeze-out at SPS and RHIC

SPS RHIC


Strangeness enhancements


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


WA85 and WA94 results


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


... 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


… and recent NA49 results

NA49 and NA57results qualitativelyconsistent

Important contributionto the knowledge of centrality dependence


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


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


The onset of deconfinement ?


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


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


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 ?


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


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 …


Transverse spectra,thermal freeze-out scenario


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


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

~


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


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


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)


High(er) transverse momenta


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 ??


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


Recent NA49 data

From talk of T. Schuster (this conference)

Apparently a consistent picture at SPS is developing, needs further analysis


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


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


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, …)

strangeness production at the cern sps

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sps data

pw data

thermal model fits p

statistical model

ss interactions

collisions thermal model

strangeness production

model predictions