# Presentation: 354112

## Measurement of Large Transverse Momentum Hadrons and Constraintson Medium Opacity Parameters

- ICHEP 2008
- Philadelphia, PA
- 31 July 2008

## High-pT Particle Production

- Domain of hard scattering processes
- Partonic collisions characterized by large Q2 (~pT2)
- Cross-sections are factorizable

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08

## High-pT and Heavy Ion Collisions

- p+p
- Fragmentation into QCD-vacuum
- A+A
- Occurs early in reaction phase
- Probe for evolution of later hot and dense phase

- Time

- Space

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08

## Do we observe hard-scattering?

- PHENIX,  PRD**76**(2007)051006(R)

- NLO-pQCD precision agreement!

- D.Winter: High pT Hadrons and the Medium

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## Hard-scattered partons in Au+Au

- p+p data scaled by assuming independent N+N collisions, Ncoll, agrees well with peripheral Au+Au

**Central data exhibit suppression!**

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08

## Models of the Medium

- Suppression currently attributed to:
- Energy loss due to coherent medium-dependent gluon bremsstrahlung
- Collisional energy loss
- Existing models:
- Describe geometry and evolution of medium differently
- Properties of the medium represented by a (single) parameter
- Examples:

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08

## High-pT measurement with PHENIX

- Tracking and Matching (DC, PC)
- charged hadrons

- EMCAL (PbSc + PbGl)
-  , 0, Ks

- EMCAL --- measure energy deposit of electrons and gammas
- DC --- obtain momentum of charged particles

- PC --- measure hit position of charged particles

- h+

- h-

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08

## 0 measurement in Au+Au and Cu+Cu

- _RHIC Run4 Au+Au data_
- Sampled 241 b-1
- 1972 x 241 b-1 ~ 1 pb-1 p+p equivalent
- 1.5 B events recorded

- _RHIC Run5 Cu+Cu data_
- Sampled 3 nb-1
- 632 x 3 nb-1 ~ 1 pb-1 p+p equivalent
- 2.2 B events sampled
- 59 M minimum bias events
- 1.9 M high-pT triggered

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08

## RAA – Nuclear Modification Factor

- RAA ~ 0.2 for pT > 5.0 GeV/c

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08

## RAA measurement in PHENIX

- Little if any pT dependence
- Consistent with most energy loss models, except where semi-opaque medium assumed
- Suppression pattern similar across centralities
- Sensitive only to Npart (not geometry)?

- 0-5%

-  arXiv:0801.4020

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## RAA and Energy Loss

- pT>5 : a = 0.58±0.07, S0 = (8.3±3.3)x10-3
- pT>10 : a = 0.56±0.10, S0 = (9.2±4.9)x10-3
- GLV & PQM predict a ≈ 2/3
- RAA vs. centrality does not saturate

- Fit Npart dependence:

- Energy loss increases with pT

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08

## Quantitative Analysis

- Fit the data by varying the model parameters, and determine the most likely range of values (at 1 and 2 levels)
- Careful categorization of uncertainties
- “Type A”: point-to-point uncorrelated [err bars]
- “Type B”: point-to-point correlated [boxes]
- “Type C”: globally correlated (normalization factors)

- NB: Includes only experimental uncertainties!

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## Example: PQM and transport coeff

- Monte Carlo using quenching weights from BDMPS
- Realistic geometry and static medium
- No initial-state mult. scattering or modified PDFs
- q-hat = color charge density  parton-medium cross-section
- <q-hat>: measure of average squared pT transferred from parton to medium

-  PRC77 064907 (2008)

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## PQM: constraints at 1 and 2 

- (Fairly) Clear min in the modified 2:

- Given infinite identical experiments, fraction of experiments having a worse 2 than real experiment

- (1 and 2 standard deviations)

- D.Winter: High pT Hadrons and the Medium

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## Summary of Constraints

- Assumes perfect model with a single parameter
- Experimental uncertainties only

- PQM    <q> = 13.2          GeV2/fm

- +2.1
- - 3.2

- ^

- GLV    dNg/dy = 1400

- +270
- - 150

- WHDG    dNg/dy = 1400

- +200
- - 375

- ZOWW   0 = 1.9          GeV/fm

- +0.2
- - 0.5

- All parameters are constrained to 10-20% at 1 std dev!

- D.Winter: High pT Hadrons and the Medium

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## What about a simpler approach?

- Basically consistent with flat RAA within one std dev contour
- Indicates energy loss proportional to pT

- D.Winter: High pT Hadrons and the Medium

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## Summary

- PHENIX has measured high-pT hadrons in Au+Au 200 GeV collisions with unprecedented statistics and precision
-  0s over 1 < pT < 20 GeV/c
- Most central RAA ~ 0.2 (above pT = 5 GeV/c) and constant to highest pT
- RAA suggests energy loss is proportional to pT
- Essentially constant in pT (for pT>5 GeV/c)
- Integrated RAA does not saturate with Npart
- Data is consistent with predicted Sloss  Napart
- High-pT spectra follows same power law from p+p to Au+Au (n ≈ 8.1)
- Unprecedented ability to constrain models
- Quantitative analysis using complete set of experimental uncertainties
- Eg. <q-hat> = 13.2 +2.1 -3.2 (PQM)
- Eg. dNg/dy = 1400 +270 -150 (GLV)

- 10 – 20 % level!

- D.Winter: High pT Hadrons and the Medium

- 31 Jul 08