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STAR

Strangeness production and Cronin effect in d+Au collisions at √sNN = 200 GeV in STAR

Xianglei Zhu (Tsinghua U / UCLA)For the STAR Collaboration

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Motivation I : Strangeness enhancement• Strangeness enhancement

in A+A collisions at RHIC energy, as a QGP signature (see. A. Timmins’ talk)

• d+Au strangeness data will connect peripheral A+A and p+p, supplying the baseline for the study of strangeness enhancement in the deconfined matter.

Redlich, JPG27Redlich&Tounsi, 02

From A. Timmins

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Motivation II : Cronin effectCronin effect in p+A, d+Au collision [Cronin1975], seen in nuclear modification factor RAA or RCP

• < 1 for pT < Px

• > 1 for pT > Px

• approach 1 again when pT →

p

q

h

A

Traditional models [Accardi hep-ph/0212148]:

Multiple parton/hardon scatterings in initial state

Recombination/Coalescence:[Hwa&Yang 03, 04]

Final state effect, modification of hadronization

What is the mechanism of Cronin effect? • traditional models: do not explicitly predict particle type dependence, but centrality dependence.• final state models: predict particle species dependence of Cronin effectMeasurements of particle type and centrality dependence of Cronin effect seem helpful to understand the effect.

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Cronin effect in d+Au STAR

• Cronin effect seen with identified particle RdAu

• For 2<pT<5GeV/c, RdAu of proton seems higher than pions.• With topological reconstruction, strange hadrons have good PID up to high pT! Suitable for the study of particle type dependence of Cronin effect.

STAR, PLB637, PLB616

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Outline

• K0s, L, X spectra in most central d+Au 200GeV

• Strangeness enhancement in most central d+Au

• Baryon enhancement at intermediate pT (L/ K0s)

• Cronin effect, particle type dependence of RdAu in most central d+Au

• Summary

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STAR Run 08 d+Au Experiment SetupTime Projection Chamber

Forward TPCs

•Vertex Position Detector to confine primary vertex within ±30 cm, together with east ZDC as minimum bias trigger

•No inner silicon detectors

•pion, kaon, proton and electron : identified using ionization energy loss in TPC.

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The data set STAR Run 8 d+Au 200GeV Minimum Bias data

Event Selection: |VertexZ| < 30cm ~ 32M events after the cuts.

Centrality definition in d+Au: Provided by uncorrected # of

charged particles @ east Forward TPC (Au side, -3.8<<-2.8),‘refMult’ in the right plot

Currently, only the most central (0-20%) events are well definedby East FTPC.

STAR Preliminary

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Strangeness reconstruction in STAR• Strange particles are reconstructed with their secondary TPC tracks

through weak decay topology, thanks to TPC full azimuthal coverage.

cm89.7, cp

cm68.2 ,0 cKs cm91.4, cp

cm46.2, cKpK

K0s, |y|<1

0.4 <pt< 0.6Λ, |y|<1

0.4 <pt< 0.6

Ξ, |y|<1

0.6 <pt< 1.0

STAR Preliminary STAR PreliminarySTAR Preliminary

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Spectra of K0s, Λ, Ξ

• Λ(Λbar) corrected for weak decay feed-down from Ξ’s

• Spectra can be fitted with Levy function [Wilk&Wlodarczyk 00], low pT – exponentialhigh pT – power law

fitting parameters:

percentage of dN/dy from extrapolation at low pT:K0

s 10% ; Λ 17.5%; Ξ 29%

d+Au 200 GeV, 0-20%

dN/dy n T χ2/ndf

K0s 1.21±0.02 11.7±0.2 0.215±0.002 34.86/12

Λ 0.402±0.008 20.9±1.3 0.270±0.005 10.81/10

Ξ 0.058±0.003 14.0±1.3 0.275±0.011 3.52/5

STAR Preliminary

statistical error only

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Strangeness enhancement• Λ and K0

s enhancement factor is almost the same in most central d+Au, lie in a trend with peripheral Cu+Cu and Au+Au

• Ξ yield is consistent with that from most peripheral Cu+Cu.

• strangeness enhancement factor is proportional to the strangeness content in d+Au too

dN/d

y / <

Npa

rt>

rela

tive

to p

p

STAR Preliminarystatistical error only for d+Au data

Number of Participants Npart

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Λ/K0s

• Baryon enhancement at intermediate pT at STAR measured with Λ/K0

s

• Most central d+Au data is close to peripheral Au+Au

STAR Preliminaryp+p data is from STAR PRC75

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Nuclear modification factor RdAu

STAR Preliminary

π,K,p data is from STAR, PLB616, 637, statistical error only; φ data is from STAR arXiv:0809.4737;p+p K0

s and Λ is from STAR PRC75

•

• K0s agree with charged K at low pT

• Λ agrees with proton at intermediate pT (2 – 4GeV/c).

• Particle type (baryon/meson) dependence of RdAu for pT from 2.0 to 4 GeV/c; φ meson RdAu also falls into the meson band, for high statistical φ data, see C. Jena and X. Zhang’s posters

ppinelbindAu

TTppineldAu

TTTdAu

NT

dydppdT

dydppNdpR

/

)2/(

)2/()(

2

2

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Summary• Measured productions for 3 identified particles(K0

s, L, Ξ) in most central (0-20%) d+Au collisions at RHIC

• Strangeness (K0s, L, Ξ) enhancement in central d+Au.

Especially, we see Ξ enhancement factor is also large in most central d+Au

• L/K0s ratio in dAu is close to Au+Au most peripheral (60-

80%)

• d+Au Nuclear modification factor seems to show particle type (baryon/meson) difference (K0

s ,,φ vs. p, L), indicating that the hadronization scheme may have an impact on the Cronin effect

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backups

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Comparison of K0 to charged K spectra

• Good agreement of K0s and charged

K (from TPC dEdx) spectra at low pT.

STAR Preliminary

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RAA in low energy p+A collisions

s =27.4GeV

P.B Straub,PRL 68, 452(1992)Rw/Be in p+A collisions

W: tungsten Be: beryllium

s =38.8GeV

Rw/Be : Mesons (2 quarks):

kaon and pion ~ 1.5; Baryons (3 quarks):

proton ~ 2.5

Particle-type dependence ??

~1.4

~1.5

~2.5