# Presentation: 354831

## Results from the PHOBOS                experiment at RHIC

**Birger Back **
**Argonne National Laboratory**
**for the **
**PHOBOS Collaboration**

## PHOBOS  Collaboration

_**ARGONNE NATIONAL LABORATORY**_
_**BROOKHAVEN NATIONAL LABORATORY**_
_**INSTITUTE OF NUCLEAR PHYSICS, KRAKOW**_
_**MASSACHUSETTS INSTITUTE OF TECHNOLOGY**_
_**NATIONAL CENTRAL UNIVERSITY, TAIWAN**_
_**UNIVERSITY OF ROCHESTER**_
_**UNIVERSITY OF ILLINOIS AT CHICAGO**_
_**UNIVERSITY OF MARYLAND**_

**Birger Back, Nigel George, Alan Wuosmaa**
**Mark Baker, Donald Barton, Alan Carroll, Stephen Gushue, **
**George Heintzelman, Robert Pak, Louis Remsberg, Peter Steinberg,**
**Andrei Sukhanov**
**Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski,**
**Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek,**
**Barbara Wosiek, Krzysztof Wozniak**
**Wit Busza ****(Spokesperson),****  Patrick Decowski, Kristjan Gulbrandsen, **
**Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, **
**Johannes Muelmenstaedt, Heinz Pernegger, Corey Reed,**
**Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin,**
**Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen,**
**Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch**
**Willis Lin, Jaw-Luen Tang**
**Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly,**
**Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs**
**Russell Betts, Clive Halliwell, David Hofman, Burt Holzman,**
**Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter**
**Richard Bindel, Edmundo Garcia, Alice Mignerey**
** **

## Outline

**Apparatus / 2000 run statistics**
**Physics results**
**Charged particle multiplicity **
**Energy dependence at ****h****=0 **
**Centrality dependence at ****h****=0 **
**Pseudo-rapidity distributions **
**Total charged particle multiplicity **
**Flow of charged particles**
**Antiparticle / particle ratios**
**Summary**
**(2001 run plan )**
**(Future upgrade plans)**

## PHOBOS Detector 

** ****96000 Silicon Pad channels**
** ****4******** Multiplicity Array **
** ****Mid-rapidity Spectrometer**
** ****Scintillator Paddles +  Zero Degree**
**   ****Calorimeter for triggering**
** ****TOF wall for high-momentum PID**

## Year 2000 running

**Commissioning run: (May-July)**
**Sacrificial detectors installed**
**56 GeV:****	****	****6352 collisions**
**130 GeV:****	****12074 collisions**
**Physics run: (July-August)**
**Full complement of detectors**
**130 GeV:****	****~3.5 M collisions**
**Well over 1 TB of data written to tape**
**Essentially flawless performance of PHOBOS detector**

## Multiplicity of charged particles / tracklets

**Tracklet counting in double Si layers**

**Spectrometer**

**Vertex**

**Notes:**
To begin, I’d like to start by trying to answer, at least in part, the question of why it is we want to study dndeta. Actually, this conceptually simple observable is sensitive to all aspects of particle production in hi Collisions.  This includes things like the interplay between hard and soft processes, nuclear related effects such as shadowing or medium effects like jet quenching, dndeta is also sensitive to the dynamic evolution of the system and rescttering , and in effect represents a time integral over the then entire collision process.  so there is a lot of potentially complicated physics wrapped up in this one quantity.  Another reason to look at the pseudorapidity density is that there exist lots of data in the literature with which to compare, from a variety of systems from pp and pA to nucleus nucleus collisions, over a wide range of energies.  And the question, which I will probably not answer today but which will certainly be thought about in the near future is, how might new data from rhic fit into this global picture?

## Multiplicity at =0 vs Energy

**First RHIC physics result: Back ***et al.***,*** ***PRL 85 (2000) 3100**
**About 20 theoretical  papers (Feb 15, 2001)**

**Multiplicity at ********=0 vs Energy**

_**RESULT**_
_**S**__**s**_	_**dN/d**__**h**_
**56 GeV****	****408********12********30**
**130 GeV****	****555********12********35**
_**S**__**s**_	_**2dN/d**__**h**__**/N**__**part**_
**56 GeV****	****2.47********.10********.25**
**130 GeV****	****3.24********.10********.25**
**Ratio: ****	****1.31********.04********.05**
**@ 130 GeV **
**30% higher than p+p**
**70% higher than SPS**

- SPS

- Phobos

- p+p

## Multiplicity at =0 vs Npart

**Multiplicity at ********=0 vs N****part**

- Npart

- Good agreement with recent PHENIX data
- Good agreement with previous PHOBOS point
- Neither HIJING nor EKRT describe data well

**Yellow band:**
** ****Systematic uncertainty**

- Preliminary

- dNch/d/(0.5*Npart)

- HIJING

**Kharzeev**
**Nardi**

- EKRT

**Notes:**
HIJING: Wang & Gyulassi, PRL

EKRT:

Kharzeev:

## Multiplicity in 4(Hit counting)

**Multiplicity in 4********(Hit counting)**

**Determine dN****ch****/d******** for –5.4 < ******** < 5.4**
**Shape**
**Total multiplicity**
**Evolution vs N****part**
**Hit counting **
**Poisson occupancy corrections**

- Octagon

- Rings

- Single Si layer

## dNch/d vs Centrality

- Preliminary

**Statistical errors only   -  10-20% systematical uncertainty **

**Notes:**
So now lets look at the data.  First of all let me show some distributions for a selected set of centrality bins, here described by the fraction of the cross section they correspond to. The colors are there just to show you that the data from these disparate pieces of the experiment actually match up together quite nicely.  And just looking at these distributions you can start to get a sense of  what’s happening as the data evolve with centrality.  For instance, its clear that for the most central collisions the data in the fragmentation region are relatively lower to those at mid rapidity than for more peripheral collisions.  But lets look at these things in a bit more detail.

## Centrality Dependence of Nch(||<5.4)

**Centrality Dependence of N****ch****(|**********|<5.4)**

- Npart

- ±10% Systematic Uncertainty

- HIJING

- Nch(||<5.4)

**Notes:**
Oh yes, before we go on to looking at features of the distributions, let me first
show you the centrality dependence of the total charged particle multiplicity,
here measured within our acceptance between +- 5.5 units of rapidity.  Here the blue data points represent our data, and the red points/line is the 
prediction of HIJING.  We see that the the data are a bit higher, with a maximum
of about 4200 charged particles between +-5.5.  

## Centrality dependence of dN/dh 

**Phobos**

**HIJING**

- Central
- Mid central
- Peripheral

- Increased particle production at mid-rapidity
- Differences in shapes

## Elliptic Flow

**Elliptic Flow**

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

**Octagon**

**Ring**

****

**V****2**

**PHOBOS**** **

**STAR**

**Reaction plane**

## Anti-particle / particle ratios

**Anti-particle / particle ratios**

- Tracking in the spectrometer
- Alternate 2T magnetic fields
- Energy loss and momentum

**-**

- Central region
- not baryon-free

## dNch/d @=0 per participant
70% higher than SPS for central Au+Au
40% higher than p+p at RHIC energy
Npart evolution between HIJING and EKRT
dNch/d in 4
<Nch> = 4100 +/- 410  (||< 5.4) for 3% central
Additional particle production near =0 for central events
Wider than HIJING 
Elliptic flow
V2 up to 0.06 – close to hydrodynamic limit
larger than SPS
V2 drops  for || > 1.5
Particle ratios
p/p ratio 0.55 +/- 0.1   
Central region not yet baryon free – maybe at 200 GeV

**70% higher than SPS for central Au+Au**
**40% higher than p+p at RHIC energy**
**N****part ****evolution between HIJING and EKRT**
**dN****ch****/d******** in 4******
**<N****ch****> = 4100 +/- 410  (|********|< 5.4) for 3% central**
**Additional particle production near ********=0 for central events**
**Wider than HIJING **
**Elliptic flow**
**V****2 ****up to 0.06 – close to hydrodynamic limit**
**larger than SPS**
**V****2**** drops  for |********| > 1.5**
**Particle ratios**
**p/p ratio 0.55 +/- 0.1   **
**Central region not yet baryon free – maybe at 200 GeV**

**Summary**

## Outlook I: 2001

**100x statistics**
**Both arms completed**
**Physics:**
**low-p****T**** physics**
**Spectra**
**HBT**
**Resonances (********at low p****T******
**Event-by-Event physics**
**Energy systematics**
**Species systematics**

## Outlook II: 2003 + beyond

**Micro-Vertex**

**Transition **
**Radiation **
**Detector**

**EM-Calorimeter**

**Existing Spectrometer**
**High rate (> 0.5 kHz)**
**High Resolution**
**Add**** **
**Micro-vertex Detector**
**Narrow beam-pipe**
**ALICE prototype TRD Electron ID**
**EM-Calorimeter**

**Charm Production at RHIC**
**Measure single electrons from displaced vertices**