# Presentation: 130117

## V Latin American Symposium on Nuclear PhysicsBrasil, Setembro 2003

- Edmundo García
- University of Illinois at Chicago
- for the PHOBOS collaboration

- Recent Results From the Phobos Experiment at RHIC

## Collaboration (May 2003)

- Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Bruce Becker, Russell Betts,
- Abigail Bickley, Richard Bindel, Andrzej Budzanowski, Wit Busza (Spokesperson), Alan Carroll,
- Patrick Decowski, Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen,
- Stephen Gushue, Clive Halliwell, Joshua Hamblen, Adam Harrington, Conor Henderson,
- David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Erik Johnson,
- Jay Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo, Jang Woo Lee, Willis Lin, Steven Manly,
- Alice Mignerey, Gerrit van Nieuwenhuizen, Aaron Noell, Rachid Nouicer, Andrzej Olszewski,
- Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Louis Remsberg, Christof Roland,
- Gunther Roland, Joe Sagerer, Pradeep Sarin, Pawel Sawicki, Iouri Sedykh, Wojtek Skulski,
- Chadd Smith, Peter Steinberg, George Stephans, Andrei Sukhanov, Ray Teng,
- Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Robin Verdier, Gábor Veres, Bernard Wadsworth, Frank Wolfs,  Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch, Jinlong Zhang
- 	ARGONNE NATIONAL LABORATORY	BROOKHAVEN NATIONAL LABORATORY
- 	INSTITUTE OF NUCLEAR PHYSICS, KRAKOW	MASSACHUSETTS INSTITUTE OF TECHNOLOGY
- 	NATIONAL CENTRAL UNIVERSITY, TAIWAN	UNIVERSITY OF ILLINOIS AT CHICAGO
- 	UNIVERSITY OF MARYLAND	UNIVERSITY OF ROCHESTER

## Relativistic Heavy Ion Collider

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

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

-  **RHIC: pp, dA, AA**
******s****NN**** : 20 to 200 GeV**

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

**Notes:**
Aerial view of the relativistic heavy ion collider
Located about 60 miles east of Manhattan in Long Island
There are 4 experiments running at RHIC, you hear about star this morning, there is also phenix a large collaboration 
and two small detectors phobos and brahms.
On the right hand side I show the center of mass nucleon nucleon collision energy of the machines leading historically our field form the AGS cyclotron to to RHIC to the LHC collider.
With RHIC we have enter a new energy regime, so we expect dramatic changes in the physics quantities that we measure

## RHIC in nuclear matter phase diagram

**A+A central collisions**

- Au + Au 200 GeV:
-  >5 GeV/fm3  (0.5 GeV/fm3 inside protons)
- 27 MeVwith~170 MeV

**dN****ch****/d********||********|<1****/<1/2 N****part****>**

## 4π Multiplicity Detector

- 2 Arm Multiparticle Spectrometer
- Trigger Detectors
- Zero Degree Calorimeters   (PCAL ZDC)
- TOF

**AuAu Run**

**dAu Run**

## dN/dEta

**dN****ch****/d****h**

**19.6 GeV***

**130 GeV**

**200 GeV**

- PHOBOS: Phys. Rev. Lett. 91,052303(2003)

- centrality

-  Broad range of collision energy measured
-  Large pseudorapidity coverage
-  Comprehensive coverage in centrality

- Typical systematic errors

- AuAu

| Centrality | <Npart> | <Ncoll> |
| --- | --- | --- |
| 0 - 6% | 344+-11 | 1050 |
| 6 - 15 % | 276+-9 | 780 |
| 15 – 25% | 200+-8 | 500 |
| 25 – 35% | 138+-6 | 300 |
| 35 – 45% | 93+-5 | 175 |
| 45 – 55% | 64+-4 | 107 |

## Centrality determination

**Multiplicity distribution**

**Counts**

_**Glauber Calculation**_
**Hijing **
**Full GEANT Simulation**

- centrality bins

- <Npart>
- <Ncoll>

- AuAu

- dAu

## 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

## “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) **

## pT spectra for Au+Au @ 200 GeV

## Centrality dependence vs. pT

**PHOBOS, nucl-ex/0302015**

- Similar centrality dependence at
- pT = 0.45 and 4.25 GeV/c

- Centrality dependence vs. pT

- Yield per participant pair divided by a fit to most peripheral bin as a function of the number of participants

## Yield Relative to ppbar

- 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**

## 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

## d+Au pT spectra

- PHOBOS d+Au: nucl-ex/0306025

**All syst. uncertainties: 90% C.L.**

| Centrality
Selection | <Npart> | <Ncoll> |
| --- | --- | --- |
| 0 – 20% | 15.5 +-0.9 | 14.6+-0.9 |
| 20 – 40 % | 10.9+-0.9 | 9.7+-0.8 |
| 40 – 70 % | 6.7+-0.9 | 5.4+-0.8 |
| 70 – 100 % | 3.3+-0.7 | 2.2+-0.6 |

## Nuclear modification factor

- PHOBOS d+Au: nucl-ex/0306025

- central Au+Au

***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***

## Centrality dependence of RdAu

- Centrality dependence of RdAu

- For pT = 4 GeV/C:
- Data disfavor initial state
- interpretation of
- high-pT suppression

- Initial state parton saturation model
- predicts (hep-ph/0210333)
- decrease of ~25 % from
- <Ncoll> _>_ 6 to central collisions

- PHOBOS d+Au: nucl-ex/0306025

**All syst. uncertainties: 15 – 20 at 90% C.L.**

## Conclusions

- Data disfavors the “initial state” parton saturation interpretation of high-pT hadron suppression
- Yield quenching of high pT in AuAu can not be accounted by initial state interactions that should be also present in dAu.
- The suppression of the inclusive yield observed in central Au+Au collisions is consistent with final-state interactions with a dense medium generated in such collisions.
- Similar evidence was corroborated by the other 3 experiments at RHIC
- BRAHMS: Strong suppression of  high pT yield  in central collisions
- PHENIX: Jet Suppression
- STAR: strong suppression of the inclusive yield and back-to-back correlations at high pT

## Slide 18

## Slide 19

## Backup transparencies from here

## Systematic Uncertainties

- Background subtraction on tracklets < ±5%
- Uncertainty on  due to model differences < 5%
- Total contribution due to feed-down correction < 4% (typically 1%)
- Total fraction lost due to stopping particles < 5%
- Both are corrected via MC normalization
- Total uncertainty on dN/d is ±8%
- Npart
- Loss of trigger efficiency at low-multiplicity <10%
- Uncertainty on Npart  <1%
- Uncertainty in modeling paddle fluctuations
- Uncertainty on Npart  <6%
- ( dN/d / Npart )130 / ( dN/d / Npart )56
- Many uncertainties cancel in the ratio

## Initially released energy density >5GeV/fm3 

- Initially released energy density >5GeV/fm3
- Note: energy density inside proton ≈ 0. 5GeV/fm3

- Total energy released ~2000GeV

- Max. initial overlap volume

## Trigger

- ZDC N

- ZDC P

- Au

- Au

- PP

- PN

- CN

- CP

- Event Selection (average number of participants <Npart>)

## Transverse Momentum and Centrality dependence of particle ratios

**Preliminary**

**Preliminary**

- PHOBOS 200 Au+Au

- Errors bars are systematic uncertainties

- 10% most central

## Particle Spectra – low pT

** ****(Ullrich QM2002)**

**Centralities:**
-    Brahms 10 %
-    Phenix 5 %
-    Star 5%
-    Phobos 15%

**nucl-ex/0210037 July 2003**

## Physics

- atoms

- particles

- nucleus

- qgp

**energy/density**

**time**

## Slide 27