# Presentation: 504044

## Neutral Pions in 200 GeV Cu+Cu Collisions

- Stefan Bathe                                                            UC Riverside                                                              for the PHENIX Collaboration

- DNP/JPS 2005, Maui, September 20

**Notes:**
Changed title 

## Hard Scattering and RAA

- Hard Scattering and ***R***AA

- Hard processes
- yield scales with *N*coll
- small cross section
- incoherent superposition
- Nuclear Modification Factor *R*AA

- absence of nuclear effects: *R*AA=1 at high *p*T
- Strong suppression in central Au+Au

**strong**
**suppression**

- Phys.Rev.Lett.91:072301,2003

## 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

## System-Size Dependence

- Vary centrality
- Vary nucleus size

- different geometrical shape
- more precise centrality determination for small systems

- Au+Au

- Cu+Cu

## Npart(x,y)

- ellipticity different

- Transverse *N*part density for similar <*N*part> in Au+Au and Cu+Cu

## PHENIX Experimental Setup

- Central spectrometer arms |**| < 0.35
-  and 0, via 2-decay
- Electromagnetic calorimeter (EMCal)
- Lead scintillator calorimeter (PbSc)
- Lead glass calorimeter (PbGl)

## 0 Spectra in 200 GeV Cu+Cu

- 56 M minimum-bias events
- 1.9 M high-*p*T events recorded, 2.2 B sampled
*p*T range 1-17 GeV/*c*
- Also measured 62.4 GeV and 22.5 GeV
- need *p+p* reference

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions
*R*AA ~ 0.4 for central collisions

## 0 RAA in 200 GeV Cu+Cu

- 0 *R*AA in 200 GeV Cu+Cu

*R*AA ~ 1 in peripheral collisions
*R*AA ~ 0.4 for central collisions
- Charged-hadron *R*AA larger in central collisions

- Charged hadrons:
- see Carla Vale’s talk, BF11

## Comparing Au+Au and Cu+Cu 

- Comparing Au+Au and Cu+Cu

- Suppression 2x stronger in central Au+Au, but:

******0**

******0**

******0**

## 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

## Model Comparison

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

- density and path-length dependend

## Corona Jet Production

- V. Pantuev,
- hep-ph/0506095

- Different surface-to-volume ratio, different absorption for same *N*part

## Summary and Outlook

- High-*p*T 0 (h+-) suppression in central Cu+Cu at 200 GeV
- Suppression similar to that in Au+Au for same *N*part
- Consistent with pure density and path-length dependence
- Hints for shape-dependent suppression mechanism for same system size
- More information from reaction-plane dependence

## Slide 23