PowerPoint Presentation

SQM2004, Cape Town, Sept. 16, 2004

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Cronin Effect for the identified particles from200 GeV d+Au collisions

Xiangzhou Cai

Shanghai INstitute of Applied Physics (SINAP)

Chinese Academy of Sciences

for the STAR Collaboration

(Presented by Yu-Gang Ma, SINAP)

Outline

•Introduction and Motivations

•Spectra, fit function and comparison

•Rcp, RdAu, particle dependence of Cronin effect

•Summary

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IntroductionIntroduction

AndAnd

MotivationMotivation

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Some Definitions

Initial-state effect: The effect happens before the hard scattering.

Final-state effect: The effect happens after or at the hard scattering.

The behavior of the many-body systems we study (such as p-A, A-B collision)can be “calibrated” with a “reference system” like p-B or p-p.

Similarly, the central collision can be “calibrated” by theperipheral collision in the following way:

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PHENIX

1)Particle dependence of RAA/Rcp andv2 from Au+Au collisions is observed.How about RAA/Rcp in dAu? particletype or mass dependence? (Rcp forKs, , , and p).

2) The Cronin effect has beenconsidered as due to initial partonscattering. Should the Cronin effectbe influenced by the final stateparticle formation dynamics?

3) Recombination models predict theparticle type dependence of the Rcpat intermediate pT in AuAu collisions.

Rcp & v2 @ 200GeV Au+Au

m~1019 MeV/c2 ; m~1116 MeV/c2; mKs~498 MeV/c2

PHENIX: PRL91, 182301(03)

STAR: PRL92, 052302(04) nucl-ex/0306007

Models: Greco et al, PRC68, 034904(03)

Saturation at intermediate pT

Baryon and meson difference

Baryon

Meson

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Analysis detailsAnalysis details

AndAnd

Spectra of identifiedparticlesSpectra of identifiedparticles

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Centrality definition of dAu@200GeV

1)dE/dx identify stable charged particles

in a certain momentum range.

2)Unstable particles identified by decay

topology or event mixing method.

Multiplicity FTPC East in d+Au collisions

40-100%

20-40%

0-20%

Three Multiplicity Bins are defined bythe Nch per event in FTPC East

After cut: ~ 10 Million events

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Event Selection:

|VertexZ| < 50cm, with Primary vertex found, good run

After cuts, # of Events ~ 10M

Decay mode:

Ks =>+ - (68.6%)

-   - (99.9%)

p+ + - (63.9%)

=>+ - (49.1%)

K* =>  (100%)

Ks, ,  are reconstructed usingtopology cuts, like decay length, dca-v0-primV

Daughter tracks are NOT identifiedwhen pt>1.1 GeV/c, but v0 can beidentified at much higher pT.

event mixing

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•Ks and  are V0 particles:

decay length: Ks = 2.69 cm  = 7.89 cm

•In TPC, neutral Ks and  are reconstructed from chargedparticles: p, K and  (See above sketch).

Topology Cuts (See the right sketch)

•|vertexZ|<50cm

•DcaV0: between two daughter tracks < 0.7cm

•DcaImpact (distance between V0 and Primary vertex) <0.75 cm () , and < 0.6cm (Ks)

•Decay length (distance between primary vertex and V0decay point) > 2 cm (Ks and )



-

Ks and  reconstruction & Topology cuts

PrimaryVertex

-

+

PrimaryVertex

Decay point

DcaV0

Decay len

DcaImpact

Track 1

Track 2

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Reconstruction of 

Dca Lambda Daughters

Dca Xi To Prim Vertex





Decay Length Xi

 Reconstruction by the topologyof the decay:    + -

 p + -

 Selection by:

• geometrical cuts

• dE/dx pid

 Efficiency and acceptancecorrection done using the embeddingMonte-Carlo technique

Javier Castillo

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  K+ K- Branching Ratio = 0.49

Both K+ and K- come from the same event

Signal

K+ and K- come from different event

Background

Mixed event is supposed to contain everythingof significance to the correlation analysis exceptthe correlation itself.

Calculate the invariant mass of every possible

K+K- pairs and accumulate the signal to

reconstruct in each (y, pt) bin.

Event Mixing Method

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Measurements in dAu collisions

Mass = 1019.4±0.5MeV/c2, FWHM=7.31.1 MeV/c2

K+K- pair invariant mass

background subtracted



For 40~100% centrality bin at|y|<0.5 and 0.4<pt<1.3GeV/c.Red line is the same-eventdistribution. Black line is thenormalized mixed-eventdistribution.

Invariant mass distribution of  meson

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Invariant mass plots

|y|<1

0.4 <pt< 6.0

|y|<1

0.4 <pt< 6.0

|y|<1

0.6 <pt< 5.0

|y|<0.5

0.4 <pt< 1.3



• Withoutbackgroundsubtraction

• Ks, ,  : topologycuts;

• : event mixing

• The quality ofsignals are prettygood.

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Spectra for MinBias production in dAu

exp fit covering low ptend and power-law fitcovering high pt region.

Double exponential fitcan reproduce theexperimental data betterthan other two funtions.

Comparison of different Fits for  Spectra

Double exponential fit:

T1: ~300MeV; T2: 1.0~1.5GeV;

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Spectra and fits: Ks

pT: 0.4 – 6 GeV/c. cross point: pT~(2~4)GeV/c2.

With efficiency correction (including vertex efficiency). Statistical errors only.

the Lambda spectra are corrected for Xi feeddown.

Recombination model may fit spectra well … TT(low pt)+TS(middle pt)+SS(high pt)

 double exp fit

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Spectra : k, p

•The spectra in d+Au collisions are harder than those in p+pcollisions

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p+p

peripheral

central

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dN/dy vs. <Nch>

• , Ks, , increase with <Nch> indAu and AuAu collisions.

dAuMinbias

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<pt>:  shows no dependence of <Nch>within error bar, but  and  are different.

dAu Minbias

STAR PreliminarySTAR Preliminary



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Cronin effectCronin effect

AndAnd

Recombination modelRecombination model

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Comparison with Recombination Model (I)

R.C. Hwa et al., nucl-th/0403001; R.C. Hwa et al., nucl-th/0406066

Recombination model can reproduce the  spectra in d+Au collisions.

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Comparison with Recombination Model (II)

R.C. Hwa et al., nucl-th/0403001; R.C. Hwa et al., nucl-th/0406066

Recombination model can reproduce the p spectra in d+Au collisions.

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Rcp of , Ks, , @ dAu 200 GeV

•Mesons (Ks, ) have the same Rcp for dAu

•Baryons (, ) have the same Rcp too, but higher than mesons.

Particle production at intermediate pT region is dividing by the particle’s type, notthe mass. Similar particle dependence has been observed in Au+Au collisions.

Such dependence is indicative of hadron formation dynamics such asrecombination/coalescence.

TTTT

TSTS

SSSS

TTTTTT

TTS+TSSTTS+TSS

SSSSSS

STAR:  behaves like mesons, despite of the large mass: ReComb prediction

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RdAu of  comparing with  K p

•Particle production at intermediate pT region is sortedby the particle’s type, not the mass

• Low pT, RdAu <1

High pT, RdAu >1

Px~=1 GeV/c

• RdAu(p)> RdAu(,,K)

• RdAu of  is closer to thatof  and k than that of p

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Comparison with pA collision

s =27.4GeV

P.B Straub,PRL 68, 452(1992)

Rw/Be at pA collisions W: tungsten Be: beryllium

s =38.8GeV

the particle dependence hasalso been observed previouslyat lower energy.the particle dependence hasalso been observed previouslyat lower energy.

Rw/Be :Rw/Be :

Mesons (2 quarks): Mesons (2 quarks):

Kaon and  ~ 1.5;Kaon and  ~ 1.5;

Baryons (3 quarks): Baryons (3 quarks):

proton ~ 2.5proton ~ 2.5

Particle-type dependence!Particle-type dependence!

~1.4

~1.5

~2.5

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1)Measure the productions for various particles (Ks, k, p) in dAu collisions @200GeV.

2)Double exponential function can fit the , Ks,  and spectra better than others.

3)RdAu and Rcp in dAu are grouped into mesons andbaryons. It indicates that the particle production isdividing by particle type rather than particle mass.

4)It indicates that the initial parton scattering modelalone cannot explain the observed particle dependence.The hadron formation dynamics play an important role.Recombination picture provides a possiblehadronization scheme for the particle dependence.

SummarySummary

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The EndThe End

Thank you!Thank you!