# Presentation: 675058

## Results from the PHOBOS experiment 

**Adam Trzupek**
***The Henryk Niewodniczański Institute of Nuclear Physics***
***Polish Academy of Sciences***
*** ******Kraków, Poland****** ***

***The 2007 Europhysics Conference on High Energy Physics Manchester, England, 19-25 July 2007   ***

**Results from the PHOBOS experiment **
**at RHIC**

## PHOBOS Collaboration

- Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts,
- Richard Bindel, Wit Busza (Spokesperson), Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Joshua Hamblen, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova,  Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen,
- Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Christof Roland,
- Gunther Roland, Joe Sagerer, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier,
- Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs,  Barbara Wosiek,
- Krzysztof Woźniak, Bolek Wysłouch
**	****ARGONNE NATIONAL LABORATORY****	****	****BROOKHAVEN NATIONAL LABORATORY**** ****	****INSTITUTE OF NUCLEAR PHYSICS PAN****	****MASSACHUSETTS INSTITUTE OF TECHNOLOGY**
**	****NATIONAL CENTRAL UNIVERSITY****	****	****UNIVERSITY OF ILLINOIS AT CHICAGO**
**	****UNIVERSITY OF MARYLAND****	****	****UNIVERSITY OF ROCHESTER**

## Outline

**PHOBOS detector**
*Data: p+p, d+Au, Cu+Cu, Au+Au at               20 – 200 GeV*
**Charged particle multiplicities**
*Factorization of e**nergy and centrality dependence in **A**u+**A**u and **C**u+**C**u collisions*
**Azimuthal anisotropy of produced particles in ****A****u+****A****u and ****C****u+****C****u collisions**
*Participant eccentricity scaling*
**p****T**** ****- S****pectra**** of i****dentified particle****s**
*Very low p**T** data – a handle on radial flow*
**Summary**

## PHOBOS Detector 

**PHOBOS ****Detector**** **
** ****Multiplicity ****Detector (Octagon, Rings)**
** ****-5.4  < ******** < 5.4 ,  0 < ******** < 2 ********	**

**1m**

- Octagon

- Ring Counters

- Ring Counters

## PHOBOS Detector 

**1m**

- ZDC

- Spectrometer

- TOF

- ZDC

- Triggering

- Triggering

**PHOBOS ****Detector**** **
** ****Multilayer**** ****S****pectrometer,**** ****TOF**** **
**midrapidity,**** ****0.03 GeV/c< p****T****< 5 GeV/c****  **

## Charged hadron dNch/d distribution (PHOBOS)

- PRL  91 (2003) 052303,  PRC 74 (2006) 021901, PRC 72 (2005) 031901

**d+Au**

- centrality

- preliminary

- preliminary

**Au+Au**

**Cu+Cu**

- Energy dependence:
- Height increases
- Width increases (in  space)
- Centrality dependence:
- Height increases
- Species dependence:
- Same systematic trends

## Au+Au and Cu+Cu at the same Npart (       = 200 GeV)

**For the same ****N****part**** ****(****system ****size) ****dN****ch****/d******** shape is very**** ****similar for** **Au+Au**** ****and**** ****Cu+Cu**** collisions**** **

**Cu+Cu**** central**
**Preliminary**
**3-6%, N****part**** = 100**
**Au+Au**** midcentral**
**35-40%, N****part ****= 99**

**Cu+Cu**** midcentral**
**Preliminary**
**15-25%, N****part**** = 61**
**Au+Au**** peripheral**
**45-55%, N****part ****= 56**

**N****part ****- number of participating nucleons**

- PHOBOS: NPA 774 (2006) 113

## Charged particle yields in Au+Au and Cu+Cu at midrapidity

- PRC 74 (2006) 021901, NPA 774 (2006) 113

**PHOBOS**** preliminary**

** ****No centrality dependence for N****part**** > 40**
** ****Energy and centrality dependences of **
**   ****charged hadron yields factorize**

**Particle density per participant pair**

**Ratio of charged hadron yield at 200 GeV **
**to yields at lower energies (200/X)**

**Increase in particle production per participant with N****part**

**s**

## Charged Particle pT Spectra

- PRL 94, 082304 (2005), PRL **96 **(2006) 212301

**Ratio of charged hadron yields at 200 and 62.4 GeV**

**Charged Particle p****T**** Spectra**

**<p****T****> = 0.25 GeV/c**

**<p****T****> = 1.25 GeV/c**

**<p****T****> = 2.5 GeV/c**

**<p****T****> = 3.38 GeV/c**

**<p****T****> = 3.88 GeV/c**

**Au+Au**** **
**Cu+Cu**
**p+p**

- midrapidity

**No centrality dependence for ****p****T  ****=  0.2 – 4 GeV/c**
**F****actorization of energy and centrality dependenc****e**** is valid at different transverse moment****a****. **

## Extended longitudinal scaling

**Energy independence of charged particle yields from moderate to high rapidities**

- NPA 774 (2006) 113

**Extended longitudinal scaling**

## Azimutal anisotropy of produced particles

**Pressure gradients lead to azimuthal anisotropy**
**Elliptic flow is the second harmonic in the Fourier expansion of azimuthal particle distribution**

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

## v2 in Au+Au and Cu+Cu ( dependence)

**Centrality: 0-40%**

- PRL 98 (2007) 242302,
- PRC 72 (2005) 051901,
- PRL 94 (2005) 122303

** ****broad ******** range**
** ****several energies**

** ****for ****Cu+Cu ****v****2**** is ****large**** and ****grow****s**** with energy**
** ****shape**** (in ********) for ****Au+Au**** and**** ****Cu+Cu**** similar**

**0-40%, charged particles**

**Au+Au**

**Cu+Cu**

##  v2 in Au+Au and Cu+Cu (centrality dependence)

- Charged particles, || < 1

**d****ecreases with centrality**
**for ****central collisions ****v****2**** is non-zero ****(larger in Cu+Cu****)**

- PRL 98 (2007) 242302,
- PRC 72 (2005) 051901

## Standard and Participant eccentricity 

- b

- Au+Au

******0**

- Initial overlap
- geometry

- Visible in final measured particle azimuthal angular distributions

**S****tandard eccentricity****:**

- minor axis along b, (bx)

**Participant**** eccentricity****:**

- for the same b, interaction points vary from event-to-event

- minor axis not along b, (bx)

## Does using                make a difference? YES

**200 GeV**

**Does using****  ****          ****    ****make a difference?**** ****YES**

-                    **increases for smaller systems**
** ****For central Cu+Cu:**
-                                          **>>**

- PRL 98 (2007) 242302

## Eccentricity scaled v2 in Au+Au and Cu+Cu 

**v****2**** scaled by p****articipant eccentricity**

**Cu+Cu**

**Au+Au**

-            **unifies average v****2**** i****n ****Au+Au**** ****and ****Cu+Cu**** **

**v****2**** scaled by s****tandard eccentricity**

**Cu+Cu**

**Au+Au**

**200 GeV**

- PRL 98 (2007) 242302

## Eccentricity scaled v2 in Au+Au and Cu+Cu 

**v****2**** scaled by p****articipant eccentricity**

**Cu+Cu**

**Au+Au**

-            **unifies average v****2**** i****n ****Au+Au**** ****and ****Cu+Cu**** **

**v****2**** scaled by s****tandard eccentricity**

**Cu+Cu**

**Au+Au**

**200 GeV**

- PRL 98 (2007) 242302

## pT dependence of v2/

**Au+Au**** and ****Cu+Cu**** at matched N****part**

- nucl-ex/0701051

**midrapidity**

## Pseudorapidity dependence of v2  /

**Au+Au**** and ****Cu+Cu**** at matched N****part**

**Pseudorapidity**** dependence of v****2****  ****/**

**           ****unifies v****2****(********) in ****Au+Au**** and ****Cu+Cu**

**More information on the dynamical evolution can be obtained from identified particle p****T**** spectra**

## PHOBOS Particle Identification 

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

**0.03**

**0.****3**

**3****.0**

**Stopping**
**particles**

**dE/dx**

**TOF**

**PHOBOS Particle Identification**** **

**E****loss**** ****(MeV)**

**1/v (ps/cm)**

- PRC 70 (2004) 051901, PRC 75 (2007) 024910

**p+p**

******+****+********-**

**p (GeV/c)**

****

****

**p**

**p**

## Identified particle pT -spectra, Au+Au at 62.4 GeV

**Identified particle ****p****T**** ****-****spectra, ****Au+Au at 62.4 GeV**

-  **Smooth evolution with **
**   ****centrality**** **
** ****Proton spectra are harder**
** ****  ****than**** ****the meson spectra**

**Time-of-Flight measurement**
**extends p****T**** reach to 3 GeV/c**
**for protons**

- y0

- PRC 75 (2007) 024910

## Particle production at very low pT

-  **Unique low-p****T**** coverage of PHOBOS**

- y0

- PHOBOS Au+Au 62.4 GeV

** ****No enhanced production at very low p****T**
** ****p****T****-**** ****spectra consistent with transverse expansion of the system**

- PRC 75 (2007) 024910

## mT -scaling in d+Au vs. central Au+Au

**Lack of m****T**** scaling in ****central ****heavy ion collisions**

**m****T**** -**** Scaling ******** the same slope of m****T**** –spectra**

- PRC 70 (2004) 051901, PRC 75 (2007) 024910

## Summary

**dN****ch****/d******** for ****Au+Au and Cu+Cu **
**Similar at the same N****part**
**Factorization of centrality and energy dependence**
**Extended longitudinal scaling**
**Elliptic Flow**
**v****2**** for ****A+A**** is large**** and ****continues to grow ****with ****energ****y**
**Participant eccentricit****y is ****relevant for the azimuthal anisotropy**
**Scaling of v****2****/ ********part  ****for Cu+Cu and Au+Au**
**p****T**** -Spectra of Identified Particles**
**No enhanced production at very low p****T ****in central Au+Au collisions**
**Lack of m****T**** scaling in central Au+Au collision consistent with transverse expansion of the system**

**The collision geometry controls the dynamical evolution of heavy ion collisions**