# Presentation: 932972

## High pT Spectra and RAA: What have we learned and where do we go next?

- Jennifer Klay

-  High pT Particle production in p+p/A+A
-  Quantifying nuclear effects – RAA/RCP
-  Initial vs final state nuclear effects
-  Pseudorapidity/√sNN dependence
-  What have we learned?/Summary

- Lawrence Livermore National Laboratory

## Fundamental Goal of High pT Studies

**To understand partonic interactions within a dense colored medium**

- Fundamental Goal of High pT Studies

## High pT Particle Production

**Jet: A localized collection of hadrons which come from a fragmenting parton**

**Parton Distribution Functions**
**Hard-scattering cross-section**
**Fragmentation Function**

**a**

**b**

**c**

**d**

- High pT Particle Production

*“**Collinear factorization”*

## How do they do?

- Barnafoldi, Levai, Papp, Fai and Zhang, nucl-th/0212111

- X.N. Wang, PRC 61 (2000) 064910

## High pT Particle Production in A+A

**(pQCD context...)**

- High pT Particle Production in A+A

## Modified PDFs: “Shadowing”

- Emel’yanov, Khodinov, Klein, & Vogt, PRC 61 (2000) 044904

- A=200

***Shadowing ***~ suppression of the structure functions vs. x in nuclei compared to free nucleons

## Initial State Multiple Scattering: “Cronin Effect”

## Partonic Energy Loss

- Energy loss  
- softening of fragmentation  suppression of leading hadron yield

- X.N. Wang, PRC 61 (2000) 064910

## Nuclear Modification Factor RAA/RCP

-  R < 1 in regime of soft physics
-  R = 1 at high-pT where hard scattering dominates  A+B = A*B(p+p)

- Nuclear Modification Factor RAA/RCP

- Quantify deviations from expected behaviour in p+p collisions:

- <Nbinary>/inelp+p

- (Nuclear Geometry)

## High pT Hadron Spectra at RHIC: The Raw Materials

- Particle species...	h±, π0, Λ, K0s, p,...
- Multiple energies...	62.4, 130, 200 GeV
- Multiple beams...	p+p, d+Au, Au+Au
- Large phase space coverage...	Midrapidity/Forward
- More to come...	Cu+Cu/Si+Si, Higher Statistics...

## PHENIX Au+Au 130 GeV

- PLB 561 (2003) 82-92

- PRL 88 (2002) 022301

## The First Big RHIC Result

- PHENIX, PRL **88**, 022301 (2001)

**Au+Au Collisions at 130 GeV**

- **Parameterized reference based on world’s data (no p+p at 130 GeV)

## Reference Spectrum for 130 GeV

**UA1 ****Measured at 200 GeV**** for |********| < 2.5**
***Reference spectrum at 130 GeV***** is an *****interpolation***** ****of many data sets over a wide range of ********s**

_**Power Law Parameters:**_
**200 GeV  A = 310±18, p****0**** = 1.77±0.09, n = 12.08±0.37**
**130 GeV  A =267±6, p****0**** =1.90±0.09,    n = 12.98±0.9**

## STAR p+p and Au+Au 200 GeV

**Much bigger data sample**

- Reference spectrum ***measured*** at RHIC

- Eh, what’s a few more GeV/c, give or take?

- PRL **91** (2003) 172302

## STAR RAA and RCP at 200 GeV

- STAR RAA and RCP at 200 GeV

- A few more GeV/c reveals a new feature for h±: RAA/RCP are ~flat for pT > 6 GeV/c

- RAA at 200 GeV

- PRL **91** (2003) 172302

## PHENIX High pT π0 Suppression

**Binary scaling**

**Participant scaling**

- π0 are pretty much always flat 
- Particle dependence at intermediate pT is INTERESTING!

- PRL 91, 072301 (2003)

- √sNN = 200 GeV

## Baryons vs. Mesons

- Significant proton-pion and lambda-kaon differences, but the “baryon enhancement” ends by pT ~ 5-6 GeV/c

- PRL **92** (2004) 052302

- PRC 69, 034910 (2004)

- PRC 69, 034909 (2004)

## Establishing suppression as a final state effect

## d+Au inclusive yields relative to binary-scaled p+p

- Suppression of the inclusive yield
- in central Au+Au is a final-state effect

-  PRL 91, 072304 (2004)

- d+Au  Enhancement

- Au+Au  Suppression

## The Second Big RHIC Result

- 072302/3/4/5

## Baryon Enhancement a final state effect too!

- nucl-ex/0309012

- PHENIX Central Au+Au

- STAR d+Au

- STAR p+p

-  proton/charged hadron consistent in d+Au, p+p, e+e- (gluon jets)
-  baryon enhancement in central Au+Au is final state effect

## What have we learned so far?

- Is it partonic or hadronic?
- What is the eta/√sNN dependence?

## Final state hadronic rescattering?

- <L/> = 1

- <L/> = 2

- <L/> = 3

- Gallmeister, Greiner and Xu, PRC 67, 044905

-  Cannot describe flat RAA

## BRAHMS d+Au and Au+Au 200 GeV

- Au+Au: PRL 91, 072305 (2003)

- d+Au: nucl-ex/0403005

**R****dAu**

**R****CP****(dAu)**

**R****CP****(2.5-4.0 GeV/c)**

**η****=0**

**η****=2.2**

**R****AA**

**R****AA**

**R****CP**

**(40-60%)**

**(0-10%)**

**η****=0**

**η****=1**

**η****=2.2**

**η****=3.2**

- No strong eta dependence in Au+Au
- Could CGC dynamics govern forward rapidity in d+Au?

## Where does suppression start?

- X.N. Wang, nucl-th/0307036

**Is there a transition from enhancement to suppression?**

**?**

- Primary motivation for running Au+Au at 62.4 GeV

## Important Aside: Reference Matters...

- D. d’Enterria, nucl-ex/0403055

- Vitev-Gyulassy Energy loss with dNglue/dy = 400-600*

- At the SPS (√s = 20 GeV) no good p+p->π0/h reference...

- * Compared to dNglue/dy = 200-300 in PRL **89** 252301 (2002)

## Where does suppression start?

- X.N. Wang, nucl-th/0307036

## Reference Spectrum at 62.4 GeV: ISR Results

- Reasonable agreement (within
- ~30% norm. uncertainties)
- among h+/- parametrizations

- +/-25% uncertainty

- Compiled References: π0, h±

## PHOBOS Au+Au 62.4 GeV

- nucl-ex/0405003

- Exciting results (first to be submitted), but what about higher pT?

## STAR Au+Au 62.4 GeV

- ISR Reference Compilation

## STAR RAA at 62 GeV

**Splitting at intermediate p****T**** but ~convergence above ~6 GeV**

## STAR RCP  Central/Peripheral

**Moderate splitting at intermediate p****T**** but ~convergence above ~6 GeV**

***Not corrected for weak decay feeddown yet...***

## Summary I

- We have a very comprehensive set of data from RHIC that will keep growing

- From p+p:
-  We have good agreement with NLO pQCD – an established baseline
-  Good reference spectra are *essential* for understanding A+B collisions

- From d+Au:
-  Consistency with Cronin effects seen at lower energies (enhancement)
-  Suggestions of possible CGC dynamics at forward rapidity (BRAHMS)
-  No initial state suppression observed (No CGC at mid-rapidity high-pT at RHIC)

## Summary II

- From Au+Au:
-  High pT Suppression is large and due to final state effects
-  Consistent with Jet Quenching via partonic energy loss with initial gluon densities of order dNg/dy ~ 1000
-  Interesting production mechanisms at work at intermediate pT ~ 2-6 GeV/c
- Coalescence/recombination models are promising explanation
-  Precision Identified particle measurements extremely important
-  Energy independence of suppression for √s = 62 – 200 GeV?
-  Experiment  Theory feedback driving exciting progress in the field

- RHIC Physics IS exciting: 2 PRL Covers, (both featuring results on high pT spectra)

## What’s on the horizon?

- _High statistics Run IV Au+Au ___     PID’ed spectra PHENIX/STAR π0:   pT ~ 20 GeV/c...?
- STAR Λ/K0s, STAR/PHENIX π/k/p:  pT ~ 10 GeV/c...?

*Well into the fragmentation regime...*

## Fin

## Parton distribution functions

- NLO Global fits to data

- Many different flavors... MRST, GRV LO, CTEQ4M,5M,6 etc...

## Fragmentation Functions

- Kniehl, Kramer, Potter,   Nucl. Phys. **B** 582, 514 (2000)

- Kniehl, Kramer, Potter,   Nucl. Phys. **B** 597, 337 (2001)

## Nuclear Collision Geometry

- Au+Au

- Glauber Model Calculation:

- Number of participants: number of incoming nucleons (participants) in the overlap region
- Number of binary collisions: number of inelastic nucleon-nucleon collisions

## STAR Au+Au 130 GeV

- **Parameterized reference based on world’s data (no p+p at 130 GeV)

## Recombination/Coalescence Models

**Recombination + Fragmentation: ****Fries, Muller, Nonaka, Bass, PRC 68 (2003) 044902; PRL 90 (2003) 202303** ** **

## Energy dependence

- I. Vitev and M. Gyulassy,
- Phys.Rev.Lett.** 89** (2002)

- Steeper spectra at SPS  Larger impact of Cronin effect

- Between SPS and RHIC: Factor ~2 in εBj and factor ~2 in dNglue/dy

- dNglue/dy = 400-600

- D. d’Enterria, nucl-ex/0403055

## STAR RAA at 62.4 GeV

## Why the big eta difference?

**Doh!  The reference matters...  How about R****CP****?**