# Presentation: 382751

##  Recent Results from PHOBOS at RHIC

- Robert Pak
*Brookhaven National Laboratory** *
- for the  PHOBOS  Collaboration
***PANIC 2002***
***Osaka, Japan***
***Sept. 30******th******, 2002***

##                   Collaboration

**	**_**BROOKHAVEN NATIONAL LABORATORY**_**	****Mark Baker, Donald Barton, Alan Carroll, Nigel George, **
**	****	****Stephen Gushue, George Heintzelman, Burt Holzman, **
**	****	****Robert Pak, Louis Remsberg, Peter Steinberg, Andrei Sukhanov**
**	**_**INSTITUTE OF NUCLEAR PHYSICS, KRAKOW**_**	****Andrzej Budzanowski, Roman Hołyński, Jerzy Michałowski, **
**	****	****Andrzej Olszewski, Pawel Sawicki, Marek Stodulski, **
**	****	****Adam Trzupek, Barbara Wosiek, Krzysztof Woźniak**
**	**_**MASSACHUSETTS INSTITUTE OF TECHNOLOGY**_**	****Maartin Ballintijn,**** ****Wit**** Busza (Spokesperson), Patrick Decowski, ****Kristjan Gulbrandsen****,**** Conor Henderson****, ****Jay Kane****, Judith Katzy, **
**	****	****Piotr Kulinich, ****Jang Woo Lee****, Heinz Pernegger, ****Corey Reed****, **
**	****	****Christof Roland, Gunther Roland, Leslie Rosenberg, **
**	****	****Pradeep Sarin****, Stephen Steadman, George Stephans, **
**	****	****Carla Vale****, Gerrit van Nieuwenhuizen, Gábor Veres, **
**	****	****Robin Verdier, Bernard Wadsworth, Bolek Wysłouch**
**	**_**NATIONAL CENTRAL UNIVERSITY, TAIWAN**_**	****Chia Ming Kuo****, Willis Lin, Jaw-Luen Tang**
**	**_**UNIVERSITY OF ILLINOIS AT CHICAGO**_**	****Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, ****Richard Hollis****, ****Aneta Iordanova****, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, ****Joe Sagerer**
**	**_**UNIVERSITY OF MARYLAND**_**	****Abigail Bickley****, ****Richard Bindel****, Alice Mignerey, **
**	****	****Marguerite Belt Tonjes**
**	**_**UNIVERSITY OF ROCHESTER**_**	****Joshua Hamblen****, ****Erik Johnson****, ****Nazim Khan****, Steven Manly, **
**	****	****Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs**

**                  ****Collaboration**

## PHOBOS Apparatus

**PHOBOS Apparatus**

**PID from**
**Spec Arms**

**p**

**K****+**

******+**

- 

- -5.4

- +5.4

**0 keV**

**500**

**4******** Multiplicity Detector**

- 

**Unrolled single event display**

## Kinematics: many ways to slice pz

_**Away from mid-rapidity:**_

**Rapidity: Generalized velocity**

**Pseudorapidity: ~ y: easier to measure**

**Feynman x: scaled p****z**

## Centrality Determination

**Centrality Determination**

** ****HIJING + GEANT**
** ****Glauber Calculation**
** ****Model Paddle Response**

**Paddle signal (a.u.)**

- Counts

**N****part**

- Counts

**more**
**central**

**Number of participants is number of nucleons that interact  **

## Particle Spectra

**Particle Spectra**

**0.2<y************<1.4**

**PHOBOS niche at low p**

**15% central**

## Why charged particle production?

- Fundamental handle on the initial state
- Upper bound on initial entropy density
- Entropy does NOT require equilibrium
- Initial gluon density
- Constrains subsequent evolution
- Freezeout entropy density

## Pseudorapidity Density Distributions

- Au+Au collisions at s = 19.6, 130, 200 GeV
- dN/d for || < 5.4 over full azimuth
- Top 50% of total cross section (Npart~ 65 - 360)
- Data at 56 GeV only for central density at || < 1

## Particle Density near Midrapidity

**Particle Density near Midrapidity**

**Constraining the Models**

**Theory 1999**

**Theory 2000**

**(2001)**

**(2000)**

**(2000)**

## Centrality Dependence of Central Density 

**Centrality Dependence of Central Density**** **

**PHOBOS Au+Au**

- 19.6 GeV
- preliminary

- 130 GeV

- 200 GeV

- Data from PRC 65 061901R (2002)

-       0.11±0.04
-       0.11±0.04
- 19.6**     **0.15±0.07

******s****NN                     ****x**
**------      -------------   **

## Limiting Fragmentation

**Extent of limiting fragmentation region grows with beam energy**
**No broad boost invariant region**

**Limiting Fragmentation**

*** **= ****- y**beam*

**dN/d********/********N****part****/2******

**PHOBOS Au+Au**

*19.6 GeV*

*130 GeV*

*200 GeV*

## Centrality Dependence of Nch

**Centrality Dependence of ********N****ch******

**Error band due to high-******** extrapolation**

**Au+Au**

**Total charged particle multiplicity per participant **
**is centrality independent **

## Remove the “Leading Proton” Effect 

**Remove the “Leading Proton” Effect**** **

******s**

******s****eff**

- Basile et al (1980-1984)

- pQCD e+e- Calculation

- (A. Mueller,
-  1983)

******N****ch******

## Comparison of Nch vs. Energy

**Comparison of ********N****ch******** vs. Energy**

**Different systems converge at high energy**

## Total Multiplicity vs. Beam Energy

**Total Multiplicity vs. Beam Energy**

- p+pp+X :

**e****+****e****-   ****Fit**

## Azimuthal Angular Distributions

**Azimuthal Angular Distributions**

**           ****= N****0 ****(1 + 2v****1****cos (********R****) + 2****v****2****cos (2(********R****)) + ... )**

- Look at emission patterns using Fourier expansion: extract v2 components from the fits.

- “head on” view of colliding nuclei

## v2 vs. centrality 

**v****2 ****vs. centrality**** **

**PHOBOS Au+Au**

** ****Suggests early thermalization**** **

**|********| < 1**

**200**

**130**

**v****2**

**<N****part****>**

**Notes:**
130 GeV errors not shown
Similar to 130 GeV results
Within errors, difficult to distinguish whether or not v2 is saturated or  scales with the increase in multiplicity as the energy increases.

## v2 vs. transverse momentum

**v****2 ****vs. transverse momentum**

**v****2**** appears to saturate for p****T ****> 2**
**Similar to results from STAR and PHENIX**

**PHOBOS preliminary**
**h****+**** + h****-**
**200 GeV Au+Au**

***track-weighted  centrality averaging***

**0<********<1.5**

***(top 55%)***

**v****2**

***17% scale error***

**Notes:**
Here we integrate over h+ and h-

## v2 vs. pseudorapidity

**v****2 ****vs. pseudorapidity**

**PHOBOS Au+Au**

** **

**v****2**

**<N****part****>~190**

**No boost invariance over a broad region**

**Notes:**
Again, little changed from 130 GeV

## v2 vs.  cf. Hydro Model

*** ***

**v****2**

**130 GeV result: nucl-ex/0205021, accepted in PRL  **

**Notes:**
Again, little changed from 130 GeV

## Not boost invariant over broad  range
Limiting fragmentation confirmed for AA
Total yields in AA scale primarily with Npart
Surprising similarity between AA and e+e-

- Limiting fragmentation confirmed for AA
- Total yields in AA scale primarily with Npart
- Surprising similarity between AA and e+e-

**Conclusions**