# Presentation: 806980

## Application of Glauber Model in d+Au Collisions

- M.L. Miller, Yale University
- (with help from K. Reygers, D. Morrison)

## Outline

- Importance of Glauber calculations
- Methods/Specifics of calculations
- Systematics
- Application in d+Au
- Summary

## RAA and High pT Suppression

- High *p**T** *suppression: post hard-scattering phenomenon

*N**coll*: total number of inelastic nucleon-nucleon interactions

| Class | Ncoll |
| --- | --- |
| 0-5%
Au+Au | 1051  71 |
| 60-80%
Au+Au | 21  7 |
| 0-100%
d+Au | 7.5  0.4 |

- PRL. **91**, 172302 (2003)

- PRL. **91**, 072304 (2003)

## Centrality: Experimental Control

**Particle production scales with increasing centrality**

## Centrality: Experimental Control

**Particle production scales with increasing centrality**

- PHENIX

## Definitions

## Glauber Model Assumptions

- Geometrical picture of nucleus-nucleus collision
- Nucleons follow straight line trajectories
*All* nucleon-nucleon interactions occur with constant probability

- Nuclear density profile:
- Woods-Saxon (Au)
- Hulthen (deuteron)

- n-n Interaction Probability:
- inel40-42 mb

- Analytic calculation (Optical)
- Monte Carlo

## Au+Au: Calculation Method

## Mapping Ncharge to Npart

- Calculated

- Measured

## Optical vs. Monte Carlo

- Primary difference in differential cross section.
- Ncoll(b), Npart(b) approximately unaffected.

- PHENIX, K. Reygers

- Test dependence on nucleon-nucleon (nn) interaction
- Hard sphere
- Gray disk
- Gaussian

## Cross Section: Systematics

- PHENIX, K. Reygers

- Au+Au, 200 GeV

- Density Profile

- n-n interaction

- n-n interaction

## <Ncoll>: Systematics

- PHENIX, K. Reygers

- Au+Au, 200 GeV, 0-10% Central

- Experimental

- n-n interaction

- Experimental

- Density profile

- n-n interaction

## <Ncoll>: Systematics

- PHENIX, K. Reygers

- Au+Au, 200 GeV, 0-10% Central

- Density profile dominant uncertainty

## <Ncoll>: Systematics

- Au+Au, 200 GeV, 70-92% Central

- PHENIX, K. Reygers

- Experimental

- n-n interaction

- Experimental

- Density profile

- n-n interaction

## Au+Au Glauber Summary

- Monte Carlo vs. Optical
- Total cross section depends on nn-interaction probability distribution
- Central collisions:
- Ncoll systematics dominated by W.S., *inel*
- TAA “self-normalizing”
- Peripheral collisions (the p+p limit):
- Ncoll, TAA systematics dominated by experimental considerations
- Autocorrelations, trigger biases become important

## “Centrality Selection” in p+p

- <*p**T*> and sum-*p**T* increase with Nch (centrality)

- STAR Preliminary

## Centrality Selection” in p+p

- Increased energy, multiplicity contained in near/away jet cones

- STAR Preliminary

- CDF: PRD, **65**, 092002 (2002)

- ||<1 , Charged hadrons only

## Avoiding “Trigger Bias” in d+Au

- Different experimental triggers at RHIC
- STAR min-bias: ZDC-Au (953% of hadronic)
- Study high *p**T* production for midrapidity
- ||<1
- STAR TPC
- Define centrality using forward region
- 2.8<<3.8 (Au fragmentation region)
- STAR Forward TPC
- Already measured for p+p at UA5
- Cross-checks:
- Forward multiplicity studied vs.  w.r.t. leading TPC hadron: Nch(2.8<<3.8 ) stable vs. leading particle *p**T*
- Tag single neutron in the ZDC-deuteron direction

## Application to d+Au

- MC distribute 197 nucleons for Au nucleus
- MC distribute proton of d
- Place neutron of d accordingly
- MC sample impact parameter distribution
- Interact nucleons
- Simulate Nch distribution in 2.8<<3.8 (Au-direction)
- Repeat

- Hulthen Distribution

- 0-20% class:   fluctuations decrease Ncoll from 16.5 to 15.1
- 0-100% class: no change

## Simulating Nch in d+Au

- Assume Npart(Au) scaling
- Use measured pp multiplicity distribution
- Include FTPC efficiency (83%)

- But, UA5 measured NSD (2 “beam-jets”)
- FTPC-Au only requires 1 “beam jet”
- Only apply UA5 Neg. Binomial for NSD events Correction factor

## Simulating Nch in d+Au

| Class | Ncoll |
| --- | --- |
| 0-100%
d+Au | 7.5  0.4 |
| 0-20%
d+Au | 15.0  1.1 |
| 1-neut.
d+Au | 2.9  0.2 |

**Selecting significantly different event classes**

- PRL. **91**, 072304 (2003)

- Simulating Nch in d+Au

## Summary

- Au+Au:
- Systematics well understood
- *inel *: sensitive to n-n interaction
- d+Au
- Glauber+UA5 agrees with forward Nch
- Robust selection of different centrality classes
- Consistency in N*coll* calc.
- PHENIX agrees with STAR
- Minbias: 7.5 vs. 8.4 diff. due to trigger efficiencies

- Geometrical approach
- Assumptions?
- No dynamics!
- Straight line trajectories
- No nucleon excitation
- No explicit *p**T* dependence
- Assumptions most robust in central Au+Au
- Correlations/fluctuations become important in p+p limit

## Centrality Selection” in p+p

## Centrality Selection” in p+p

## Centrality: d+Au Collisions

- No background subtraction
- Central: top 20% of  -3.8<η<-2.8 uncorrected multiplicity

**Back-to-back jets are *****not***** suppressed in central *****d+Au***

_**underlying event**_: *p+p* *d+Au* minbias *d+Au* central
-  _**near-side**_: correlation strength and width similar
-  _**away-side**_: *d+Au* peak broader but with little centrality dependence

## Centrality Selection” in p+p

- Increased energy, multiplicity contained in near/away jet cones

- STAR Preliminary

- ||<1 , Charged hadrons only