# Presentation: 455402

## Recent Results from PHOBOS at RHIC

- Edmundo García
- University of Illinois at Chicago
- Lima, Julio 2004.
- On behalf of the PHOBOS Collaboration

## Outline

- Introduction:
- RHIC and PHOBOS
- Where is RHIC in the phase diagram for hadronic matter?
- Characteristics of medium produced at RHIC
- Global observables of RHIC collisions
- Final Remarks

##                       Collaboration

- Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski,
- Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell,
- Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman,
- Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan,
- Piotr Kulinich, Chia Ming Kuo, Willis Lin, Steven Manly, Alice Mignerey,
- Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park,
- Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Helen Seals,
- Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz, Peter Steinberg,
- George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale,
- Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger,
- Frank Wolfs,  Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch
- 	ARGONNE NATIONAL LABORATORY	BROOKHAVEN NATIONAL LABORATORY
- 	INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW	MASSACHUSETTS INSTITUTE OF TECHNOLOGY
- 	NATIONAL CENTRAL UNIVERSITY, TAIWAN	UNIVERSITY OF ILLINOIS AT CHICAGO
- 	UNIVERSITY OF MARYLAND	UNIVERSITY OF ROCHESTER

-                       Collaboration

## Physics

**atoms**

**particles**

**nucleus**

**qgp**

**energy/density**

**time**

## Relativistic Heavy Ion Collider

**AGS: ********s****NN**** = 4.8 GeV**

**SPS: ********s****NN**** = 17 GeV**

-  **RHIC: pp, dAu, AuAu**
******s****NN**** : 20 to 200 GeV**

**Relativistic Heavy Ion Collider**

-  **LHC ********s****NN**** = 5.6 TeV**

##        The Detector

- 4π Multiplicity Detector
- 2 Arm Multiparticle Spectrometer

- Trigger Detectors
- Calorimeters   NZDC, PZDC,SPECCal
- TOF

-        The Detector

**TOF**

**pCal**

**T0**

**specCal**

**paddle counters**

**cerenkov counters**

**spec trigger**

**spectrometer**

**multiplicity rings**

## RHIC: Production of a “baryon free” – “high energy density” medium

- Models prior to RHIC

- <Npart>/2 = number of participant nucleon pairs

## Study of geometrical dependence of collision’s at  RHIC: Flow

**Reaction ****plane ****(********R****) **

- x

- z

- y

**x **(defines R)

**y**

**z**

**Study of geometrical dependence of collision’s at  RHIC: ****Flow**

- Initial azimuthal asymmetry

- Is the azimuthal asymmetry preserved when measured in the detectors?

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

## Strong elliptic flow signal: Medium is highly interactive

**Nucl. Phys. A715 (2003) 611-614**

**at mid rapidity**

- peripheral

- central

## Few low momentum particles : Medium is highly interactive

**Few low momentum particles ****: ****Medium is highly interactive**

**arXiv:nucl-ex/0401006**

- In a large volume, weakly interacting system one would expect the development of particles with long wavelength producing an excess in the yields (coherent particle production)

**Central 200 GeV **
**Au+Au **
**at mid rapidity**

## Suppression of high-pT particle yield

- Multiplicity yield per participant pair divided by a fit to the invariant  cross-section for proton antiproton collisions (200 GeV UA1)

**p****T **** ****(GeV/c)**

*No increase in particle production*
* **above p**T** ~ 2 GeV/c for all*
* **centralities : yield suppression*

***Initial State***
***Coherence?***

***Interaction in ***
***Dense Medium?***

**PHOBOS, nucl-ex/0302015**

- Suppression of high-pT particle yield

## Is suppression an initial or final state effect? 

- Is suppression an initial or final state effect?

- Initial state

- Final state

- How to discriminate? turn off final state  d+Au collisions

- From STAR:  BNL colloquium June 18

- strong modification of Au wave function (gluon saturation)

- partonic energy loss in dense medium generated in collision

## Suppression of high-pT particles: Medium is highly interactive and parton density is high.

**AuAu 200 GeV**

**PHOBOS dAu 200 GeV**

- PRL 91, 072302 (2003)

***From Glauber***
***(HIJING 1.383)***

***41mb (same as ***
***for Glauber)***

***From UA1, using***
***Pythia to go from***
***|************| < 2.5 to 0.2 < ************ < 1.4***

***PHOBOS data***

## Slide 14

## RHIC

- High energy density medium
- Baryon Free medium
- Medium created at RHIC strongly interacts with high pt partons.
- What other global observables define RHIC collisions so far?
- Scaling behaviors
- Limiting fragmentation

## “Participant” Scaling

**“****Participant” Scaling**

**N****coll****= # of NN collisions: ****~N****4/3**

**N****part****/2 =  # of participating nucleons: ****N**

**“****Collision” Scaling**

**N****part****/2  =  # of participating**
** ****pairs of nucleons:  ****= 1**

**N****coll**** = # of NN collisions:  ****= 1**

**Wang,Gulassy   Phys. Rev. Lett 86(2001) Kharzeev,Nardi Phys. Lett. B 501(2001) **

## Au+Au centrality dependence allows only about 10% Ncoll scaling at mid rapidity

**Au+Au centrality dependence allows only about 10% N****coll**** scaling at mid rapidity**

- Au+Au

- 0

- 200

- 400

- 0

- 10

- 20

- dN/d/(Npart/2)

- <Npart>

- 200 GeV - ||<1

- peripheral

- central

- p+p

- Binary Collision(Ncoll) Scaling

- Participant
- (Npart) Scaling

## All RHIC energies show a similar Npart dependence

**All RHIC energies show a similar N****part**** dependence**

**19.6 GeV ****preliminary**

**130 GeV**

**200 GeV**

- Data is normalized by p+p value for each energy.

- Au+Au

- peripheral

- central

**Participant **(Npart) **scaling**

**Binary collision scaling**

## Total charged multiplicity scaling with Npart

- Shaded band is uncertainty on extrapolation procedure
- Errors include contributions from Nch and Npart scaling

- Open symbols are UA5 data at 200 GeV and results from an interpolation at lower energies

- PHOBOS:  nucl-ex/0301017

## Npart scaling for asymmetric collisions:

**N****part**** scaling for ****asymmetric**** collisions:**

- arXiv:nucl-ex/0403033

## Limiting Fragmentation

**Limiting Fragmentation**

- UA5, Z.Phys.C33, 1 (1986)

**dN/d******

- selected systematic errors

- 0 – 6 % central

- 35 – 40 % central

- PHOBOS Au + Au

**PRL 91, 052303 (2003)**

## Rest frame of A

**Rest frame of A**

**Rest frame of p or d**

## Elliptic flow:

**Elliptic flow:**

**PRL 91, 052303 (2003)**

## Final Notes

- RHIC collisions
- Produce a high energy density medium
- Baryon Free medium
- Medium created at RHIC strongly interacts with high pt partons.
- Global observables (not clearly understood yet)  that define RHIC so far:
- <Npart> scaling behaviors
- Limiting fragmentation
- ...
- ...

## A “coffee napkin” Estimate ofEnergy Density at Mid-rapidity.

- Total energy released in =1 is ~ 1000 GeV

****Initial Energy Density Estimate,******* *****~ 5 GeV/fm****3**

- Max initial overlap volume

- Look at all produced particles in a Central ‘Head-on’ Collision

-  = 1

## ’

******’**

******’**

**dN/d********’/<N****part****>/2>**

- PHOBOS Au + Au

- systematic errors not shown

- 200 GeV

- 19.6 GeV

- systematic errors on error bars

## Strong elliptic flow signal: Medium is highly interactive

**Nucl. Phys. A715 (2003) 611-614**

**At mid rapidity**

**200 GeV Au+Au PHOBOS preliminary**

- 0 <  < 1.5

- 0-55% central, h+ + h-