# Presentation: 050069

## Overview of Resultsfrom PHOBOS experiment at RHIC

**Andrzej Olszewski**
**Institute of Nuclear Physics, Krak****ów, Poland**
**for the**** **
**PHOBOS Collaboration**

**Notes:**
From runs in 2000/2001

## PHOBOS at RHIC

## PHOBOS Collaboration

_**ARGONNE NATIONAL LABORATORY**_
_**BROOKHAVEN NATIONAL LABORATORY**_
_**INSTITUTE OF NUCLEAR PHYSICS, KRAKOW**_
_**MASSACHUSETTS INSTITUTE OF TECHNOLOGY**_
_**NATIONAL CENTRAL UNIVERSITY, TAIWAN**_
_**UNIVERSITY OF ROCHESTER**_
_**UNIVERSITY OF ILLINOIS AT CHICAGO**_
_**UNIVERSITY OF MARYLAND**_

**       ****Birger Back, Nigel George, Alan Wuosmaa**
**        ****Mark Baker, Donald Barton, Alan Carroll, Joel Corbo, Stephen Gushue, Dale Hicks, Burt Holzman,Robert Pak, Marc Rafelski, Louis Remsberg, Peter Steinberg, Andrei Sukhanov**
**        ****Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak**
**        ****Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, Michel Rbeiz, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch**
**        ****Chia Ming Kuo, Willis Lin, Jaw-Luen Tang**
**        ****Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly,Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs**
**        ****Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer**
**        ****Richard Bindel, Alice Mignerey**

**Notes:**
Collaboration of institutions from USA, Poland, Taiwan

## The PHOBOS Detector (2001)

**The PHOBOS Detector (2001)**

** **4 Multiplicity Array
- - Octagon, Vertex & Ring Counters
-  Mid-rapidity Spectrometer
-  TOF wall for high-momentum PID
-  Triggering
-  Scintillator Paddles Counters
-  Zero Degree Calorimeter (ZDC)

**137000 silicon pad readout channels**

**1m**

**Notes:**
Multiplicity Array. Spectrometer – silicon detectors
TOF, Trigger – plastic detectors

## Central Part of the Detector

- (not to scale)

**0.5m**

**Notes:**
Shape, size, size of pixels
Basic functions – tracking, vertexing, particle detection

## PHOBOS in PHOTOS

- Octagon Detector

- Ring Counter

- _Silicon pad sizes_
- Octagon Detector:       2.7 x 8.8 mm2
- Vertex Detector:   0.5 x (12-24) mm2
- Ring Counter:    (5x5) - (10x10) mm2
- Spectrometer:   (1x1) - (0.5x19) mm2

- ~ 25 cm

- Spectrometer

- Vertex Detector

**Notes:**
Description of the module
Pad shapes and sizes

## PHOBOS Running Summary

**Commissioning****:**** ****(May-July)**
**Part of silicon installed**
**Au+Au collisions at **
**	********s****NN**** = 56 GeV and 130 GeV**
**First published result****s**** on**** **
**dN****ch****/d********|****|********|<1****, ********s****NN****=****56 and 130 GeV**
**Physics run: ****(July-August)**
**1 spectrometer arm setup**
**Au+Au collisions at ********s****NN**** = **
**	****130 GeV:  ~3.5 M collisions total**

**Year 2000 running**

**Essentially flawless performance of PHOBOS detector**

**Commissioning: ****(mid-July)**
**Add 2nd spectrometer arm**
**Au+Au collisions**
**	********s****NN ****= 130 GeV and 200 GeV**
**	****First published result on **
**	****dN****ch****/d********|**|******|<****1****,**** ********s****NN****=**** 200 GeV**
**Physics run:  ****(mid-August********)**
**2 spectrometer arm setup**
**Au+Au collisions at ********s****NN**** = 200 GeV:  ~3.5 M collisions by end of August**

**Year 2001 running**

## Results to Date

**Versus energy**
**Central**** collisions, ********0****:**
******s****NN**** = 56 and 130 GeV**** **
**    ****PRL 85 (2000) 3100**
******s****NN**** = 200 GeV****,**** **
**    ****submitted to PRL**
**Versus centrality****:**
******s****NN**** ****=**** ****130 GeV****,**** ********0**
**   ****submitted to PRC**
**Versus angle and centrality**
******s****NN**** ****= 130 GeV****, ****|********|<5.4**** **
**    ****PRL 97 (2001) 102303**

_**Elliptic flow**_
**Versus angle and**** ****centrality**
******s****NN****= 130 GeV, |********|<5.3**** **
**        ****QM2001, to be submitted soon**** **

**Results to ****D****ate**

_**Particle r**__**atios**_**  p/p, K****-****/K****+****, ********-****/********+**
**Central**** collisions:**
****** ****s****NN**** = 130 GeV**
**      ****PRL 97 (2001) 102301**

**_**

## Triggering on Collisions

**Triggering on Collisions**

- Negative Paddles

- Positive Paddles

- ZDC N

- ZDC P

- Au

- Au

- PP

- PN

- Paddle Counter

**Valid**
**Collision**

- ZDCCounter

**Notes:**
A few words on ZDC and Paddle detectors

## Selecting Collision Centrality

- Larger signal = more central collision.

- _Central Collision_:  Large Npart

- _Peripheral Collision_:  Small number of participating nucleons

- “side” view of colliding nuclei

- “side” view of colliding nuclei

- 3<||<4.5

- PN

- PP

- b

## Centrality Determination

**% ********(Paddle mult.)      % ********(N****part****)**

**HIJING + GEANT**

**(3<|********|<4.5)**

**% Error on N****part**

**Analysis is limited to events with N****part ****> 70**

**Centrality ****D****etermination**

**N****part**

**Notes:**
Absolute error decreases, but relative error increases
Lowest Npart > 70

## Charged Particle Density 

**Charged Particle Density **

_**Measurement**_** **

** ****Energy Dependence **
** ****System Size **
** ****Angular Dependence**

_**Context**_

_**Study**_** **
**Energy/entropy density production**
**Response to properties of  nuclear/partonic medium**
- Saturation
- Jet quenching
**Importance of hard and soft processes**
**Re-scattering effects**
**Long-range particle correlations**
**Memory of the initial geometry in the final state**

**Charged Particle Density**
**Event Anisotropy  - Flow**

## Tracklets

**Tracklet: Two-hit combination + vertex position**
**>300 tracklets/central event in Vertex, >100 in Spectrometer**

**Vertex**
**detector**

**Notes:**
In spectrometer, 1-2 and 5-6 planes used as independent tracklet reconstruction sets

## Analog and Digital Hit-Counting

- _Digital_
- Count hits above
- energy threshold,
- assume Poisson-
- statistics in the
- distribution of hits
- among the pads
- _Analog_
- Use deposited
- energy (dE/dx) in
- each pad to estimate
- number of particles
- that crossed the pad

**Hits in Octagon, Ring and Vertex**
**for single event**** **

## Charged Particle Density

**Charged Particle Density**

_**Four counting methods**_** **
**Tracking detectors - ************ measurements**
**Tracklets in Spectrometer**
**Tracklets in Vertex detector**
**Single layer detectors - 4******** ****measurements**
**Use deposited energy (dE/dx) in each Si-pad**
**Count hits above threshold, assume Poisson-statistics**

_**All four measurements corrected for**_**:**
- secondary particles, feed-down from weak decay, stopping particles
_**Systematic uncertainty**_**:**
- from 4.5% (Tracklets in Spectrometer) to 10% (Hit counting)

**All**** four methods deliver consistent results - final results averaged**

## Charged Particle Density at 0 

_**PHOBOS first measurements **_
**- charged particle density**** **
**- ****in mid-rapidity**
**- ****for 6% of the most central events**

**dN****ch****/d********|****|********|<1****(****56 GeV)****   = 408 ******** 12(stat) ******** 30(syst)**
**dN****ch****/d********|****|********|<1****(****130 GeV) ****= 555 ******** 12(stat) ******** 35(syst) **
**dN****ch****/d********|****|********|<1****(****200 GeV)**** = 650 ******** 35(syst)**

## dNch/d|||<1 vs Energy

**dN****ch****/d********|****|********|<1**** vs Energy**

- AGS

- PHOBOS 56

- RHIC 130

- SPS

- p+p

**_**

**Preliminary**

- PHOBOS 200

- nucl-ex/0108009
- Submitted to PRL

## dNch/d|||<1: Ratio 200/130 GeV

**dN****ch****/d********|****|********|<1****: Ratio 200/130 GeV**

**R****200/130**** = **
**1.14 +/- 0.05(sys)**

**New results for 200 GeV**

- nucl-ex/0108009

**dN****ch****/d********|****|********|<1**** = **
**650 ******** 35 **
**dN****ch****/d********|****|********|<1****/********0.5N****part******
** ****= 3.78 ******** 0.25**

## Multiplicity at =0 vs Centrality

**Multiplicity at** ******=0 vs Centrality**

******s****NN ****=130GeV**

- nucl-ex/0105011

## Multiplicity in 4 - Centrality Dependence

**Multiplicity in 4******** - ****C****entrality ****D****ependence**

**The width of the distribution changes with centrality**

******s****NN**** = 130GeV**

**PRL 97 (2001) 102303**

## Multiplicity in 4 - Centrality Dependence

** ****3% most central collisions**
**      ****<N****ch****> = 4200 ******** 470**

**(dN****ch****/d********)/(0.5N****part****)**** **

**central(0-6%)**

**peripheral(35-45%)**

**Multiplicity in 4******** - ****C****entrality ****D****ependence**

**central(0-6%)**

**Total N****ch****(|********| ******** 5.4)**

** ****Additional particle**
**  ****production near ********=0**

**PRL 97 (2001) 102303**

## Change in dN/d with Energy

**200 GeV - 6%**

**130 GeV - 6%**

**UA5 200 GeV**
**(NSD)**

**First attempt to compare **
**dN/d******** shape for Au+Au**
**at 130 and 200 GeV**
**‘****Limiting  fragmentation’**
** ****check by plotting dN/d******
** ****with ******** ******** ******** - Y****beam**
**Agreement for AA in the**
**fragmentation region**
**Different slope when **
**compared to  pp**

- Systematic errors not shown

## Azimuthal Angular Distributions

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

- V2 d**etermines to what extent the initial state spatial/momentum  anisotropy is preserved in the final state****.**** **

## Centrality Dependence of V2

-  Anisotropy increases for peripheral collisions
-  Large V2 signal compared to lower energy

**Centrality Dependence of V****2**

**V****2**

**Normalized Paddle Signal**** **

**SPS**

-  (**STAR : Normalized N****ch**** )**** **

** ****|********| < 1.0 **
******s****NN****=130GeV**

**PHOBOS Systematic error ~ 0.007**

**17 GeV**

**Preliminary**

## V2 (elliptical flow) vs 

** **Averaged over centrality
-  V2 drops for || > 1.5

## Microscopic viewpoint:
    Antiproton/proton ratio determined by: 
Baryon stopping; 
Pair production; 
Absorption in nuclear medium

**    ****Antiproton/proton ratio determined by: **
**Baryon stopping; **
**Pair production; **
**Absorption in nuclear medium**

**Why Measure Antiparticle/Particle Ratios?**

*  **Thermodynamic viewpoint****:***
-    **Particle ratios can be used to estimate hadro-chemical potentials**

## Anti-particle / particle Ratios

- p

- K+

- +

- K-

- p

- -

- 70 cm

**Tracking in the spectrometer**
**Alternate 2T magnetic fields**
**Energy loss and momentum**

## Results for Ratios

**K********/K****+**** = 0.91 ****±**** 0.07 (stat) ****±**** 0.06 (syst)**
**p/p     = 0.60 ****±**** 0.04 (stat) ****±**** 0.06 (syst)**

**     ****K****-****/K****+**** vs Energy                             p/p vs Energy**

**Results for ****R****atios**

**Higher values of K****-****/K****+**** and **
**p/p than at lower energies**

**PRL 97 (2001) 102301**

## Results for Ratios

- B=45±5 MeV, which is
- much lower than that observed
- at SPS (~240-270 MeV)

- Assumes freezeout temp ~170 MeV in statistical model of Redlich (QM01)

**Results for ****R****atios**

**PRL 97 (2001) 102301**

## Charged Particle Densities
(Entropy)
dNch/d at 0 per participant
First look at Au+Au at 200 GeV 
	- increase in density by 14% compared to 130GeV
Logarithmic increase with energy from AGS to RHIC
Npart evolution stronger then linear, indicates increasing
	 contributions from hard processes
dNch/d in 4
Additional particle production concentrated 
	near 0 for central events
Decreasing width with increasing centrality
On average 4200 particles in central collisions at 130GeV

**(Entropy)**
**dN****ch****/d******** at ********0 per participant**
**First look at Au+Au at 200 GeV **
**	****- increase in density by 14% compared to 130GeV**
**Logarithmic increase with energy from AGS to RHIC**
**N****part ****evolution stronger then linear, indicates increasing**
**	**** ****contributions from hard processes**
**dN****ch****/d******** in 4******
**Additional particle production concentrated **
**	****near ********0 for central events**
**Decreasing width with increasing centrality**
**On average 4200 particles in central collisions at 130GeV**

**Summary 1**

## Summary 2

**Elliptic flow**
**Increase of elliptic flow (V****2****) for more peripherial events**
**Increase of flow effect with increasing energy**
**V****2 ****at mid-rapidity up to 0.06 **
**V****2**** drops  for |********| > 1.5**
**Particle ratios**
**K****-****/K****+****   and  p/p significantly higher than at AGS or SPS **
********** ****~ 45 MeV vs 270 MeV at SPS**
**p/p between HIJING and RQMD predictions**
**Central region closer to baryon free state**

## Outlook: Year 2001

**100x statistics**
**Physics:**
**low-p****T**** physics**
**Spectra**
**HBT**
**Resonances (********at low p****T******
**Event-by-Event physics**
**Energy systematics**
**Species systematics**

## The End

## Silicon Signals 200 vs 130 GeV

**Signal shapes are (almost) identical:**

**Methods developed for 130 GeV good also at 200 GeV**

## dNch/d at mid-rapidity: ||<1

**Tracklets ******** three-point tracks **
**two-hit combinations+measured event vertex**

******x ****~ 450 ********m**
******y ****~ 200 ********m**
******z ****~ 200 ********m****    **

**Spectrometer**

**Vertex Detector**

-   **D = (********2 ****+ ********2****) ****1/2**** < 0.015**

**|********| < 0.04**** , ****|********| < 0.3**

**The measurements in the two different Si pad detectors**
**(different location, granularity, acceptance, systematics)**

- dNch/d at mid-rapidity: ||<1

## Tracklet analysis

**TOTAL systematic errors:**
**   ****4.5% ****(Spectrometer)****           7.5% ****(Vertex)**

**FINAL RESULTS**
**Combined Vertex and Spectrometer measurements**
**(weighted by the inverse of their total systematic error)**

## Hit Counting method

**Evaluate number of particles per hit pad            **
**    ***** Assume Poisson statistics P(N)=********N****e****-********/N!**
**        ******** ****- determined by the measured ratio (p) of occupied**
**             ****to empty pads:  ******** = ln(1+p) **
**     *****  Perform a multi-Landau fits to ********E (********,N****part****) spectra**
**        ****(convoluted with gaussian)**
**Calculate acceptance                                  **
**Fold in a final background correction (MC)           **** **
**      ****F****Bkg****(********,N****part****) =(dN****ch****/d********)****MC****Truth****/(dN****ch****/d********)****MC****Reconstructed**

**N****hits****(********,N****part****)**
**N****tr/hit****(********,N****part****)**
**A(********,Z****vtx****)**
**F****Bkg****(********,N****part****)**

## Analog method

**Get energy deposited in each Si pad                    ********E(********,N****part****)**
**Divide this energy by the average energy per track   ****<********E(********)>**
**Correct for the fraction of primaries                    ****f****prim****(********)****   **
**<********E> and f****prim**** are obtained from HIJING+GEANT simulations**

**Hit Counting and Analog methods agree to within 5%**
**(systematic errors in each method are  ********10%)**

## Secondary particles (+2%)
Little material between interaction point and sensitive volume

Antiproton absorption in detector (+8%)
From GEANT simulations 

Feed-down from weak decays (-2%)
Reduced by tracking within 10cm of vertex
Further limit by distance-of-closest-approach cut on tracks

**Little material between interaction point and sensitive volume**
- Antiproton absorption in detector (+8%)
**From GEANT simulations **
- Feed-down from weak decays (-2%)
**Reduced by tracking within 10cm of vertex**
**Further limit by distance-of-closest-approach cut on tracks**

**Corrections to the Raw Numbers**

## Slide 40