DNP/JPS ’05 Stefan Bathe 1

Neutral Pions in 200 GeV Cu+Cu Collisions

Stefan Bathe UC Riverside

for the PHENIX Collaboration

DNP/JPS 2005, Maui, September 20

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Hard Scattering and RAA• Hard processes

– yield scales with Ncoll

• small cross section

• incoherent superposition

• Nuclear Modification Factor RAA

• absence of nuclear effects: RAA=1 at high pT

• Strong suppression in central Au+Au

no effect

strongsuppression

TNNfAA

TAAAA

d/d

d/d0

0

pT

pNR

Phys.Rev.Lett.91:072301,2003

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Cause for Suppression

• Initial- or final-state effect?– Non-suppression in d+Au

– Non-suppression of direct photons in Au+Au

• final-state effect

Phys.Rev.Lett.91:072303

Phys.Rev.Lett.94:232301,2005

• To learn more– study system-size

dependence

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System-Size Dependence• Vary centrality

• Vary nucleus size– different geometrical

shape– more precise centrality

determination for small systems

Au+Au Cu+Cu

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Npart(x,y)

ellipticity different

Au+AuA=197

60-94% 40-50% 20-30% 0-10%

<Npart>

Cu+CuA=64

6.3 19.5 45.5 114

80-92.2% 60-80% 50-60% 30-40%

6.4 25.4 53.0 98.2

centrality

centrality

Transverse Npart density for similar <Npart> in Au+Au and Cu+Cu

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PHENIX Experimental Setup

• Central spectrometer arms || < 0.35– and 0, via 2-

decay– Electromagnetic

calorimeter (EMCal)• Lead scintillator

calorimeter (PbSc)• Lead glass calorimeter

(PbGl)

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0 Spectra in 200 GeV Cu+Cu

• 56 M minimum-bias events

• 1.9 M high-pT events recorded, 2.2 B sampled

• pT range 1-17 GeV/c

• Also measured 62.4 GeV and 22.5 GeV– need p+p reference

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions• RAA ~ 0.4 for central collisions

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0 RAA in 200 GeV Cu+Cu

• RAA ~ 1 in peripheral collisions• RAA ~ 0.4 for central collisions• Charged-hadron RAA larger in central collisions

Charged hadrons:see Carla Vale’s talk, BF11

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Comparing Au+Au and Cu+Cu

Similar behavior for Au+Au and Cu+Cu for

same Npart

Suppression 2x stronger in central

Au+Au, but:

0

0

0

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Model Comparison• Models

– parton energy loss through gluon radiation

– density and path-length dependend

• Models describe data well

Dainese, C. Loizides, G. Paic,Eur.Phys.J.C38:461,2005

I. Vitev,Phys.Rev.Lett.89:252301,2002

X. N. Wang,Phys.Lett.B595:165,2004

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V. Pantuev,hep-ph/0506095,corona jet production

density and path-length dependend

Model Comparison

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Corona Jet Production

V. Pantuev,hep-ph/0506095

• Different surface-to-volume ratio, different absorption for same Npart

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Summary and Outlook• High-pT 0 (h+-) suppression in central Cu+Cu

at 200 GeV

• Suppression similar to that in Au+Au for same Npart

• Consistent with pure density and path-length dependence

• Hints for shape-dependent suppression mechanism for same system size

• More information from reaction-plane dependence

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