# Presentation: 657828

## Results from the PHOBOS Experiment 

- at RHIC - A Perspective
- Russell Betts – UIC
- for
- The PHOBOS Collaboration

## Slide 2

## Outline of the Talk

- The PHOBOS Experiment
- The Data and Analysis
- The Nature of the Matter formed in AA Collisions
- Discovery of Simple Scaling Rules

## The PHOBOS Experiment (2004)

**Spectrometer**

**Paddle Trigger Counter**

**NIM A 499 (2003) 603**

**Octagon**

**TOF**

**SpecTrig**

**T0 counter**

**44-******** Multiplicity Detector**
**MMagnetic Spectrometer + TOF**** **

## Multiplicity and Trigger Detectors

**Octagon**

**Paddle Trigger Counter**

**Ring Counter**

**ZDC**

## Counting Particles

**Bulk of Particles p****T****<1GeV**

## Vertex Tracklets (I)

-  = 1 – 2
-  = 1 – 2

- Tracklets are two point tracks that are constrained by the event vertex.

- || < 0.04      || < 0.3

**Vertex Detector Event Display**

## Vertex Tracklets (II)

- All Pairs of Hits

## Hit Counting (I)

**Octagon, Ring and Vertex Detectors (unrolled)**
**Count Hits or Deposited Energy**

## Discriminating Background

- E (“MIP”)

- Data

- Monte Carlo

- Si

- E vs. ** in the Octagon

- From Vertex

- Not from Vertex

**

**

## Measure the Occupancy

*N*=number of tracks/pad
**=mean number of tracks/pad

- The numbers of empty and occupied pads
- determine the occupancy as a function of **,*b*

- Method: Assume Poisson
- statistics

## Energy Loss  Multiplicity

- 300 m Si

- Measured S/N = 10 - 20  <<  Landau Width
- Use Non-Hit pads - for
- 	Common-Mode Noise Suppression
- M = 240 ± 15 ± 5 ± CMN for one sensor (120 channels) at  = 0

- 0.30 - 0.40

**Energy deposited in i****th**** pad (truncated)**
**corrected for angle of incidence**

**Mean energy loss for one**
**particle traversing pad**

**RATIO OF TOTAL TRACKS**
**TO PRIMARY TRACKS**

## Elliptic Flow

- (reaction plane: ******R**)

**dN/d(********R ****) = N****0 ****(1 + 2****V****1****cos (********R****) + *****2******V******2******cos (2(************R******)***** + ... ) **

- View along beamline

- Fourier decomposition of the Azimuthal Multiplicity Distribution:

**           ****V****2**** = < cos (2(********2****)) > /********cos(2(********2****a**** ********2****b****))> )****1/2**

- Directed flow

- Elliptic flow

## Centrality Determination

## NPart and NColl (Au+Au)

- NPart and NColl (Au+Au)

**Paddle Mean**

**N****Part**

## NPart and Ncoll (d+Au)

**Multiplicity Distribution**

**Pseudorapidity**

_**Glauber Calculation**_
**Hijing 1.383**
**Hulthen w.f.**
**41mb inelastic cross-section**
**Full GEANT Simulation**

**HIJING Simulation**

**dN/d******

**Counts**

## The Data

- Au+Au

- d+Au

- Also pp at 200 GeV
- Au+Au at 62.4 GeV/u
- Spectra – Talk by Adam Trzupek

## Particle Density near Mid-Rapidity

- On the low side of nearly all predictions

## Energy per Unit Volume

- Note: Energy Density inside Proton ≈ 0.5 GeV/fm3

**Therefore total energy released in**
** ****|********| < 1 is ~2000GeV**

**Number of Particles Produced at y=0**

**Energy of Collision**

- “**Relevant” Initial Volume ~ ********R****2**** ********  ( 1 fm) ********  2**

**<E> ****~ 0.7 GeV**

**Data from: PRL 85, 3100 (2000); PRL 88, 22302 (2002);  PRL 91, 052303 (2003); arXiv:nucl-ex/0405027 **

**dN****ch****/d******

## The Energy Density is High (3-5 GeV/fm3)andMuch Larger than Inside a HadronA Description in Terms of Hadronic Degrees of Freedom is Inappropriate  

## Baryon Free at Mid-Rapidity

- PRC 67, 021901R (2003)

## Evidence from Flow

- PRL,89, 222301 (2002)
- Nucl. Phys.A715, 611c (2003)

- Approaches Hydro Limit

## Evidence from Low pT Particles

- Evidence from Low pT Particles

**In a large volume, weakly interacting system we would expect the development of particles with long wavelength**

- arXiv:nucl-ex/0401006

**PHOBOS                                 PHENIX **

## Suppression of High-pT Particles

**Au+Au**
**0-6%**
**200 GeV**

**PHOBOS d+Au 200 GeV**

**PRL 91, 072302 (2003)**

## The System is Strongly InteractingatEarly Times (2fm/c)

## No Plateau in Rapidity Distributions

- Plateau in Pseudorapidity Distributions is Misleading
- Rapidity Distributions of Pions are Gaussian

- PRL 91, 052303 (2003)
- arXiv:nucl-ex/0403050

## No Boost-Invariant Plateau for v2

-  * **PHOBOS Preliminary**** *****v****2****200**

* **PHOBOS**** *****v****2****130**

- No Boost-Invariant Plateau for v2

**PRL 89, 222301 (2002)**

## Limiting Fragmentation in pp

## Scaling – Limiting Fragmentation

**PRL 91, 052303 (2003)**

## arXiv:nucl-ex/0403033

**arXiv:nucl-ex/0403033**

** ****A Rest Frame **

**p or d Rest Frame**

## Dependence of v2 on sNN and  

**Submitted to PRL**
**arXiv:nucl-ex 0406021**

- Dependence of v2 on sNN and 

## Limiting Fragmentation in v2

- Limiting Fragmentation in v2

**Submitted to PRL**
**arXiv:nucl-ex 0406021**

## Longitudinal Scaling

- Originally expected boost-invariance in mid-rapidity region not observed.
- Scaling in fragmentation region seen in Au+Au, d+Au and (previously) in pp and p+A

## Connection between Central and Fragmentation Regions

**Nucl.Phys. A715 (2003) 65-74**

**PRL 91, 052303 (2003)**

**PHOBOS**

## How to Compare Au+Au, p+p, and d+Au

- In pp collisions, on average, approximately
- _half the energy_
- goes into the
- _leading baryon_

- A.Brenner et al.Phys.Rev.D26 (1982)1497l

- arXiv:nucl-ex/0301017
- arXiv:nucl-ex/0403033

- Au+Au higher than pp and d+Au. But........

## Universal Curve for Nch vs (s)

- When pp corrected for leading baryon

## Npart scaling for:

**N****part**** scaling for:**
******A, KA, pA, dA, AA**
**10 GeV to 200 GeV**
**N****part****  from 2 to 350**

**E178: J.E.Elias et al., Phys.Rev.D22(1980) 13**

**arXiv:****nucl****-ex/0403033**

**Phobos and E178 data**

**Preliminary**

**pp chosen to have the same available energy**** **

## NPart Scaling of Total Particle Production – Independent of Nature of Collision System

## Factorization of Energy and Centrality Dependence

**PRC 70 (2004) 021902R**

- Where are the Minijets?

## Centrality Dependence of d+Au

- arXiv:nucl-ex/0403033

##  Factorization into Geometric and Energy Parts

**SAME SEEN IN p+A AT ENERGIES 50-200 GeV**

-  Factorization into Geometric and Energy Parts

## Energy and Geometry Factorize – Independent of pT

**arXiv:nucl-ex/0405003**

**PHOBOS**

**62.4 GeV**
**200 GeV**

- Energy and Geometry Factorize – Independent of pT

## Summary and Conclusions

- High Energy Density Strongly Interacting Matter – Description in Terms of Simple Hadronic Degrees of Freedom Inappropriate. ( No Direct Evidence of Color Deconfinement or Partonic d.o.f)
- The Matter is Strongly Interacting – Not the Weakly Coupled QGP Originally Envisioned
- Simple Scaling Rules Unite the Data. Suggest Global Constraints or Universality