# Presentation: 703836

## Charged Hadron Spectra and Ratios in d+Au and Au+Au Collisions from PHOBOS Experiment at RHIC

**Charged Hadron Spectra and Ratios in d+Au and Au+Au Collisions from PHOBOS Experiment at RHIC**

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

***4th Budapest Winter School on Heavy Ion Collisions***
***(December 1st-3rd 2004) in Budapest, Hungary***

**nucl-ex/0410022, 2004**

## PHOBOS Collaboration

- Abigail Bickley, Richard Bindel, Andrzej Budzanowski,Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski, Edmundo García, Tomasz Gburek, Nigel George,
- Kristjan Gulbrandsen, Steve Gashue, Clive Halliwell, Joshua Hamblen, Adam Harington,
- Michael Hauer, George Heintzelman, Conor Henderson, David Hofman, Richard Hollis,
- Roman Holynski, Burt Holzman, Aneta Iordanova, Erik Johnson, Jay Kane, Judith Katzy,
- Nazim Khan, Wojtek Kucewicz, Piotr Kulinich, Chia Ming Kuo, Jang Wo Lee, Willis Lin,
- Steven Manly, Don McLeod, Alice Mignerey, Rachid Nouicer , Gerrit van Nieuwenhuizen,
- Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Louis Remsberg,
- Mike Reuter, Christof Roland, Gunther Roland, Leslie Rosenberg, Joe Sagerer, Pradeep Sarin,
- Paweł Sawicki, Helen Seals, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz,
- Peter Steinberg, George Stephans, Andrei Sukhanov, Jaw-Luen Tang, Marguerite Belt Tonjes,
- Adam Trzupek, Carla Vale, Robin Verdier, Gábor Veres, Edward Wenger, Frank Wolfs,
- Barbara Wosiek, Krzysztof Wozniak, Alan Wuosmaa, Bolek Wyslouch, Jinlong Zhang
- 	ARGONNE NATIONAL LABORATORY	BROOKHAVEN NATIONAL LABORATORY
- 	INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKÓW	MASSACHUSETTS INSTITUTE OF TECHNOLOGY
- 	NATIONAL CENTRAL UNIVERSITY, TAIWAN	UNIVERSITY OF ILLINOIS AT CHICAGO
- 	UNIVERSITY OF MARYLAND	UNIVERSITY OF ROCHESTER

## PHOBOS Detector

**PHOBOS Detector**

**T0 counter**

**Spectrometer**

**SpecTOF**

**TOF**

**multiplicity, vertex and calorimeter **
**detectors are not labeled**
***(see Russell Be******t******ts talk)***

**Magnet**

**Paddle Trigger**
** ****counter**

**Paddle Trigger**
** ****counter**

**T0 counter**

## pT and PID Measurement in PHOBOS Spectrometer

**   ****track curvature in B field  => p,charge **
**   ****dE/dx in Si, ToF                 => mass**
**	****	**
**  ****p****T**** = 0.03 - 0.2 GeV/c **
**       ****low-p****  ****particles stop **
**       ****in silicon wafers           => p, mass**
**       ****B field negligible  =>   no charge ****	****	****	****	****   identification****	****	****  **

**p****T**** and PID Measurement in PHOBOS Spectrometer**

**10 cm**

**PHOBOS Spectrometer**
** **dipole magnetic field of 2T at maximum
-  16 layers of silicon wafers
-   fine/optimal  pixelization, precise dE measurement
-   collision vertex close to spectrometer
-   near mid-rapidity coverage

**70 cm**

## PID Measurement in PHOBOS Spectrometer

**<dE/dx>**

**p****T**** > 0.2 GeV/c**

**p****T**** = 0.03 - 0.2 GeV/c**

**E****tot**** = ******** dE****i ****, i=A, ... ,E**
**M****p****i**** ****= E****i**** dE****i****/dx **
**M****p ****= < M****p****i**** >**

******/K separation:  p****T   ****< ~0.6 GeV/c**
**p(p) separation: p****T****  < ~1.2 GeV/c **

- 

**K**

- p

## Energy Dependence of Antiparticle to Particle Ratios

**Energy Dependence of Antiparticle to Particle Ratios**

**p/p**

-  **K****–****/K****+**

**A+A central, near mid-rapidity**

**particle ratios increase with energy**
**net baryon density is **
**rapidly decreasing**

**GOOD CONDITIONS FOR QGP FORMATION**

**at ********s****NN**** = 200 GeV in Au+Au central collisions:**
**	****baryochemical potential:**** ********B**** = 27 ******** 2 MeV**** ****	****	****	****energy density: ******** ****=**** ****~ 5 GeV /fm****3 ****, ********0 ****= 1 fm, nucl-ex/0410022**

##  Charged Hadron Transverse Momentum Distributions 

**in AA collisions “BULK” of hadrons is produced at low transverse momentum **
**“****TAIL” of transverse momentum distribution at high-p****T**** originates from hard partonic scatterings **

**0.2<y************<1.4**

** ****Charged Hadron Transverse Momentum Distributions **
**in Au+Au collisions at ********s****NN ****= 200 GeV**

**centrality: 0-15%**
**mid-rapidity**
****** ****PRC RC in press**
**    ****nuc-ex/0401006**
****** ****PLB 578 (2004) 297**

**PLB 578 (2004) 297**

**TAIL**

**BULK**

**invariant yields**

**particle density**

## High-pT Probes

- parton

**nucleus**

**t = - ****a few**** fm/c**

- parton

**t = 0 fm/c**

**hard partonic**
**scattering **

**t = + ****a  few **** fm/c**

**hadronization**

**jet of hadrons**

**leading**** h****adron**** **
**of high p****T**

**t = + ****a  few**** fm/c**

**scattered partons **
**pass through**
**hot and dense**
**medium**

**Hard partonic scatterings occur early in AA collision. Scattered**
**partons can probe the dense and hot medium created in AA collision **

**dete****c****tor**

-  **nucleus**

**if scattered partons loose energy then the number**
**of leading hadrons will be suppressed (”jet quenching”)**

**High-p****T**** Probes**

## Nuclear Modification Factor  RAA 

**R****AA**

**„****hard collisions”**

**„****soft **
**collisions”**

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

**Nuclear Modification Factor  R****AA**

- RAA=1 (Ncoll scaling), lack of nuclear effects, small cross section for hard partonic scattering

- Ncoll - number of binary inelastic NN interactions in AA

*,m,*

- NN data:
- p+ p      (UA1) at  200  GeV
- p+ p      (ISR)  at  62.4 GeV

## RAuAu for Charged Hadrons in Au+Au Collisions 

**at ********s****NN  ****= 200 GeV **** **

**suppression of high-p****T**** hadron production is observed**
**strongest effect is seen in most central collisions**

## High-pT Suppression 

**Final state effects****?**

**energy loss in medium**

**initial state effects possible in d+Au**
**no final state effects**
**no suppression in d+Au collisions indicates that final state effects are responsible for suppression in Au+Au **

**central Au+Au:**

**d +Au:**

**Initial state effects****?**

**gluon saturation:**
**suppression of high parton density (g+g-> g)**
**Color Glass Condensate**

**d+Au at 200 GeV is a control experiment**

## RdAu for Charged Hadrons, sNN = 200 GeV 

**R****dA****u**** ****for Charged Hadrons, ********s****NN ****= 200 GeV **

**d+Au control experiment **
**indicates that suppression of particle production in central Au+Au collisions at ********s****NN ****= 200 GeV ****is a consequence of final state effects**

**PRL 91 (2003)  072302**

**R****dAu**

**Au****+A****u**

**mid-rapidity, 0.2<y********<1.4**

**medium created in Au+Au collisions is strongly interacting**

**no suppression in d+Au collisions**

## Low-pT Spectra of Identified Charged Particles in Central Au+Au at sNN  = 200 GeV

** ****no enhancement in low****-****p****T**** yields for pions is observed **
** ****flattening of (p+p) spectra down to very low p****T****, consistent**
** ****with transverse expansion of the system**

**|****T= 229 MeV for ****(********+****+********-****)**** **
**    ****  ****293 MeV for (K****+****+ K****-****) **
**      ****392 MeV for (p + p) **

**PRC RC in press**
**nucl-ex/0401006**

****

**medium created in Au+Au collisions is strongly interacting**

##  RAuAu for Charged Hadrons at sNN  = 62.4 GeV  

**nucl-ex/0405003 (Au+Au, 62.4 GeV) **

**RHIC Physics Run 2004**

**R****AuAu**** at 62.4 GeV is significantly higher than at 200 GeV for all centralities within the studied p****T**** range**

**R****AuAu**

## Energy Dependence of RAA

**nucl-ex/0405003**

**central Pb+Pb and Au+Au collisions, near mid-rapidity**

**R****AA ****> 1    at ********s****NN ****= 17.2 GeV **
**R****AA ****< 0.2 at ********s****NN ****= 200  GeV**

**smooth evolution of R****AA**** with energy**

## pT(GeV/c)

- pT(GeV/c)
- yields normalized by Npart weakly depend on centrality

**Nuclear Modification Factor R****AA****Npart**

**0     1     2     3      4**

**    ****1     2     3      4**

**      ****1     2     3     4**

**    ****1     2     3     4**

**N****coll ****scaling**** **

- 45-50%

- 25-35%

- 15-25%

- 0-6%

- Npart - number of participating (wounded) nucleons in AA

**nucl-ex/0405003, Au+Au:**** ******** 62.4 GeV,**** **
**	****	****      ******** ****200 GeV**

## Factorization of Energy and Centrality Dependence of RAANpart at sNN  = 62.4 and 200 GeV

**yield per participant (or R****AA****Npart****) changes by less than 25% for both energies in centrality range from 60 to 340 participants.**
**centrality evolution is the same at both energies:**

**R****AA****Npart**** = R****PC****Npart**** (N****part****) * f(********s****NN ****)**

**Factorization of Energy and Centrality Dependence**** ****of R****AA****Npart** **at ********s****NN  ****= 62.4 and 200 GeV**

**nucl-ex/0405003**

## Summary

**Au+Au: **
**almost net-baryon free environment, energy density  ~ 5 Ge****V****/fm****3**
**strong suppression of high-p****T**** charged hadron yields in central collisions at 200 GeV (~ 5 times at p****T ****~ 5 GeV) **
**no evidence for enhanced production of very low-p****T**** pions**
**flattening of p+p spectra at low-p****T****, strong radial flow in the system, **
**R****AuAu**** at 62.4 GeV is significantly higher than R****AuAu**** at 200 GeV**
**factorization of energy and centrality dependence of R****AuAu****Npart**
**approximate N****part**** scaling of hadron yields **
**d+Au:**
**no suppression of charged hadron yields at high-p****T**** (at mid-rapidity) suppression in central Au+Au is final state effect**

## Conclusions

**STRONGLY INTERACTING, HIGH DENSITY **
**AND ALMOST NET-BARYON FREE **
**MEDIUM IS CREATED AT THE HIGHEST RHIC ENERGY IN CENTRAL Au+Au COLLISIONS**

- particle ratios          high-pT suppression     low-pT spectra

## Triggering on Collisions & Centrality 

- Coincidence between Paddle counters  at t = 0 defines a valid collision
- Paddle + ZDC timing reject background

- Central

- Peripheral

** **HIJING +GEANT
-  Glauber calculation
-  Model of paddle trigger

- Data

- Data+MC

## RdAu as a Function of Pseudo-rapidity( = - ln tan(/2))

**nucl-ex/0406017, PRC in press**

**nucl-ex/0406017, PRC in press**

**positive ******** is in deuteron direction**
**with increasing ********,  R****AA****  decreases**
**model constraints: Color Glass Condensate**

## mT Scaling in d+Au vs Au+Au

**PRC in press**
**nucl-ex/0401006**

**d+Au**

**Scale uncertainty: 15%**

**Not feed-down corrected**

**Au+Au**

**  ****Spectra **
**normalized**
** ****at 2 GeV/c**

**m****T**** Scaling in d+Au vs Au+Au**

## RAA at low energy (fixed target experiments) 

**R****AA ****at low energy**** (****fixed target experiments****)**** **

**Initial state effects**

**R****SAu**

**R****PbPb**

**R****pA**

**Cronin effect**

**E****lab****  = 200 AGeV, ********s****NN**** = 19.4GeV**

**Pb+Pb: E****lab**** =158 AGeV, ********s****NN**** = 17.3 GeV**

**R****SS**

**m****ultiple scatterings**
**p****T**** broadening****  => **** R****AA ****>1*** *

## Factorization of RAA(sNN,centrality)

**For b<10.5 fm:**

**Centrality ******** N****coll**

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

- nucl-ex/0405003

## Theory Calculations

- 38

- Cronin Effect:
- X.N. Wang, Phys. Rev **C61**, 064910 (2000).
- Attributed to initial state multiple scattering.
- Implemented by Q2(pt) dependent Gaussian kt broadening

- Energy loss applied:
- M. Gyulassy, I. Vitev, X.N Wang
- and B.W. Zhang; nucl-th/0302007

- dE/dxo is the only free parameter.
- It is determined by fitting to
- STAR central RAA(pt)