# Presentation: 885820

## First Results fromthe PHOBOS Experiment at RHIC:

- Judith  Katzy, MIT

**Charged Particle Multiplicity Near Mid-Rapidity ****in Central Au+Au Collisions ****at ********s=56 and 130 AGeV**

- Judith Katzy DPF2000

## PHOBOS Collaboration

_**ARGONNE NATIONAL LABORATORY**_
**Birger Back, Nigel George, Alan Wuosmaa**
_**BROOKHAVEN NATIONAL LABORATORY**_
**Mark Baker, Donald Barton, Mathew Ceglia, Alan Carroll, Stephen Gushue, George Heintzelman,  Hobie Kraner ,Robert Pak,Louis Remsberg,  Joseph Scaduto, Peter Steinberg, Andrei Sukhanov**
_**INSTITUTE OF NUCLEAR PHYSICS, KRAKOW**_
**Wojciech Bogucki, Andrzej Budzanowski, Tomir Coghen, Bojdan Dabrowski, Marian Despet, Kazimierz Galuszka, Jan Godlewski , Jerzy Halik, Roman Holynski,  W. Kita, Jerzy Kotula, Marian Lemler, Jozef Ligocki, Jerzy Michalowski, Andrzej Olszewski********, Pawel Sawicki , Andrzej Straczek, Marek Stodulski, Mieczylsaw Strek, Z. Stopa, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak, Pawel Zychowski**
_**JAGELLONIAN UNIVERSITY, KRAKOW**_
**Andrzej Bialas, Wieslaw Czyz, Kacper Zalewski**
_**MASSACHUSETTS INSTITUTE OF TECHNOLOGY**_
**Wit Busza*****,**** Patrick Decowski, Piotr Fita, J. Fitch, C. Gomes, Kristjan Gulbrandsen, P. Haridas, Conor Henderson, Jay Kane , Judith Katzy , Piotr Kulinich, Clyde Law, Johannes Muelmenstaedt, Marjory Neal, P. Patel,  Heinz Pernegger, Miro Plesko, Corey Reed, Christof Roland, Gunther Roland, Dale Ross,  Leslie Rosenberg, John Ryan, Pradeep Sarin, Stephen Steadman, George Stephans, Katarzyna Surowiecka, Gerrit van Nieuwenhuizen,  Carla Vale, Robin Verdier****,**** Bernard Wadsworth, Bolek Wyslouch**
_**NATIONAL CENTRAL UNIVERSITY, TAIWAN**_
**Yuan-Hann Chang, Augustine Chen, Willis Lin, JawLuen Tang**
_**UNIVERSITY OF ROCHESTER**_
**A. Hayes, Erik Johnson, Steven Manly, Robert Pak, Inkyu Park, Wojtech Skulski, Teng, Frank Wolfs**
_**UNIVERSITY OF ILLINOIS AT CHICAGO**_
**Russell Betts, Christopher Conner, Clive Halliwell, Rudi Ganz, Richard Hollis, Burt Holzman,, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter**
_**UNIVERSITY OF MARYLAND**_
**Richard Baum, Richard Bindel, Jing Shea, Edmundo Garcia-Solis, Alice Mignerey**

- Judith Katzy DPF2000

## Relativistic Heavy Ion Collider

**13 June**: 1st Phobos AuAu Collisions @ s = 56   AGeV
**24 June**: 1st Phobos AuAu Collisions @ s = 130 AGeV
**     ****2001**:                   AuAu Collisions @ s = 200 AGeV

- species: pp , AuAu
- energies: 30-200AGeV

- Judith Katzy DPF2000

## RHIC physics goal

-      quark gluon plasma
-      deconfinement phase transition
-      chiral symmetry restoration

_**RHIC physics goal**_

**Study QCD**

- Judith Katzy DPF2000

## Nucleus - Nucleus Collisions  

- Hadronic scattering
- via exchange

**RHIC: ********s = 53-200 GeV**

- Partonic scattering
- via gluon exchange?
- Rescattering? Minijets?

**Nucleus - Nucleus Collisions  **
**- the scattering process **

**AGS**: s = 4.8 GeV

**SPS**: s = 17 GeV

- d N/ dis sensitive to the scattering process (hard vs soft)

- Judith Katzy DPF2000

## Energy density  in Au Au Collisions

**Energy density  in Au Au Collisions**

- 

- 

-  d N

-  d N

-  d y

-  d 

- Spectator
- nucleons

- Participating
- nucleons

******nergy **
**density?**

- Produced
- (primary)
- particles

-  y = 1/2 * ln( E+pL/E-pL)
- ln (tan 

- Judith Katzy DPF2000

## PHOBOS Detector

- Judith Katzy DPF2000

## Commissioning run 

- Commissioning run
-     set-up

- Judith Katzy DPF2000

## PHOBOS Trigger

- Au

- Au

- PP

- PN

**First Collisions**:  June 13

**PHOBOS Trigger**

- ZDC P

- ZDC N

- Negative Paddles

- Positive Paddles

- Judith Katzy DPF2000

## Examples of events

- Hits in VTX

- Hits in SPEC

- Tracks in SPEC

- 130 AGeV

- 56 AGeV

- 130 AGeV

- Judith Katzy DPF2000

## Monte Carlo Simulations

- Very good under-
- standing of detector
- response

**Monte Carlo Simulations**

- generator: HIJING 1.35
- simulations: Geant 3.21

- Silicon Detector Response

- Judith Katzy DPF2000

## Centrality Determination

- events with highest signal in paddle counters
- corresponds to 6% most central events of inelastic cross section

- Data

- 

- Judith Katzy DPF2000

## Inelastic cross section from spectator neutrons

****

****

- ZDC

- ZDC

******not in scale)**

-  m

- Confirms
- centrality measurement
- paddle counter acceptance
- for periphal collisions

- 130 AGeV

- ultiplicity

- Forward  ns

- Judith Katzy DPF2000

## Extracting Npart

- Select 6% most central events based on paddles gives

- Npart

- Events/bin

- MC

**b**

******part**

- Judith Katzy DPF2000

## Vertex Distribution

- X

- Z

- Z

- Y

- X

- Fiducial volume
- -25cm < Z < 15 cm

- Judith Katzy DPF2000

## Event Statistics

**56 AGeV**
- Collision Events : 6352
- Central Events : 382
**Central Events (–25 < z < 15) : 103**
**130 AGeV**
- Collision Events : 12074
- Central Events : 724
**Central Events (–25 < z < 15) : 151**

- Judith Katzy DPF2000

## Tracklets Reconstruction

**  ****Two hit combinations that point to the vertex**
-   **Vertex tracklets d********<0.1, d******** = ********2 **** - ********2, ****, **
**    ****Spectrometer tracklets ****dR<0.02 ,  dR = ******** (dh****2**** – df****2)**
**  ****Redundancy essentially eliminates feed-down, secondaries, random noise hits**

- z

- y

- z

- x

- Judith Katzy DPF2000

## Uncorrected dN/d

- Final result extracted by integrating over Z

- Spectrometer

- Vertex

- GeV
-  GeV

- Judith Katzy DPF2000

## Final Results for dN/d

- Judith Katzy DPF2000

## Comparisons with SPS and pp 

- Comparisons with SPS and pp

- Submitted for publication

-  **dN/d******** obtained at RHIC is 70 %  higher then at SPS**
-   => increase of energy density by 70%
-  **dN/d********per participating nucleon obtained in AuAu significantly   **
**   ****higher  then in pp collisions**
-   => AuAu collisions differ from simple superposition of pp

- Judith Katzy DPF2000

## The Central region of PHOBOS

- Reconstruct charged particle multiplicity
-   in || < 5.5 over 4event by event
-  search for events with high multiplicity (e.g.excess of entropy in QGP)
-  search for fluctuations (e.g. dcc)

- Judith Katzy DPF2000

## arm of the spectormeter

- arm of the spectormeter

- Spectrometer:
-  measure pt spectrum down to very low momentum
-   ->sensitive to collective  phenomena over large volumes and DCC
-  reconstruct size of volume with high energy density and lifetime (HBT)
-   -> excess of entropy
-  reconstructmass and width -> look for unusual medium effects

- Judith Katzy DPF2000

## Status of PHOBOS

- Complete Detector Installed July 5 - 13
- 25000 Collisions Recorded at 130 AGeV
-   more to come during August 2000

_**Status of PHOBOS**_

- Judith Katzy DPF2000