# Presentation: 774259

##   Phobos at RHIC                  

- Edmundo Garcia
- University of Illinois at Chicago
- for the
- PHOBOS Collaboration

- IV Latin American Symposium on Nuclear Physics
- Mexico City
- Septiembre 2001

## 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**

- Bolek Wyslouch, MIT

## Outline	

**Introduction****	**
**The detector**
**Trigger System**
**Multiplicity Detector**
**Spectrometer**
**Physics Measurements****	**
**Multiplicity**
**Particle Ratios**
**Flow**
**Final Notes**

- Bolek Wyslouch, MIT

## Physics

- atoms

- particles

- nucleus

- qgp

**energy/density**

**time**

**Physics**

- Bolek Wyslouch, MIT

## Relativistic Heavy Ion Collider

**AGS: ********s = 4.8 GeV**

**SPS: ********s = 17 GeV**

-  **RHIC: pp, pA, AA**
******s : 30 - 200 GeV**

**Relativistic Heavy Ion Collider**

-  **LHC ********s = 5.2 TeV**

- Bolek Wyslouch, MIT

## RHIC collisions

- PHENIX

- STAR

- PHOBOS

- BRAHMS

- Two nuclei approach   relativistically contracted

- Hard collisions take place during first stages of reaction

- Interactions of produced particles act at soft and hard scales

- Final particles freeze out towards the detectors

**RHIC collisions**

- Bolek Wyslouch, MIT

## Experiment

**Experiment**

- Bolek Wyslouch, MIT

## One of the “small” RHIC experiments,  size  (6 x 6 x 3 m), and people (50 scientist)
Designed to be able to examine and analyze a very large amount of minimum bias interactions (high trigger rate capability)
Measurements 
Multiplicity and angular distribution of charged particles
 < 5.3 over 4  coverage event by event
Particle spectra
0.5 <  < 1.5  and 2 x 11o in   (azimuthal)
Covers about 1% of particles 
Capable to reconstruct low momentum particles ( 55 MeV/c ) 

**Designed to be able to examine and analyze a very large amount of minimum bias interactions (high trigger rate capability)**
**Measurements **
**Multiplicity and angular distribution of charged particles**
** ********< ****5.3 over 4**** ******** ****coverage event by event**
**Particle spectra**
**0.5 < ******** < 1.5  and 2 x 11****o ****in ********  (azimuthal)**
**Covers about 1% of particles **
**Capable to reconstruct low momentum particles ( 55 MeV/c**** ********)**** **

- pseudorapidity****ln (tan))

- rapidity** y** = 1/2 * ln [( E + p)L/ (E - pL)]

**Phobos**

- Bolek Wyslouch, MIT

## Phobos Trigger

- ZDC N

- ZP

- Au

- Au

- PP

- PN

- CN

- CP

- paddle counters

**Valid**
**Collision**

- cerenkov counters

- zdc

**good**
**vertex**

**Phobos Trigger**

- Bolek Wyslouch, MIT

## Centrality measurement

- Centrality. ****  number of spectator neutrons in ZDC
- number of spectator neutrons in ZDC **=** f (Epaddles)
- Centrality  ****Epaddles

**Centrality measurement**

- charged spectators

- charged spectators

- Bolek Wyslouch, MIT

## Centrality selection

- peripheral

- central

- 6%

**Centrality ****selection**

- Bolek Wyslouch, MIT

## Phobos silicon

**Phobos silicon**

- Bolek Wyslouch, MIT

## Position Sensitive Silicon Detectors

- Strip

- Drift

- Pad/Pixel

- Bolek Wyslouch, MIT

## Multiplicity and vertex detectors

- Multiplicity and Vertex Detector

**Run 5374, Event 79495 **

- 

- vertex

- octagon

- rings

**Multiplicity and vertex detectors**

- Bolek Wyslouch, MIT

## Spectrometer

**Spectrometer**

******-**

**K****-**

**pbar**

**pid**

- Bolek Wyslouch, MIT

## Multiplicity measurement

**Use energy deposited in **
**each Si pad**

**Hit Counting, Analog and Tracklet methods agree to better than ±4%**

**Multiplicity measurement**

**Count hits above threshold, assume Poisson statistics**

**Analog information**

**Counting**

**Tracking detectors**

**Count three-point tracklets: two layers of Si and event vertex match**

**Single Layer Silicon**

- Bolek Wyslouch, MIT

## Energy Systematics: 3-200 GeV

- Bolek Wyslouch, MIT

## Pseudo-Rapidity Dependence

**200 GeV**

- 6% central

**Pseudo-Rapidity Dependence**

- Bolek Wyslouch, MIT

## Centrality Dependence at =0 

- Kharzeev/Nardi

- Bolek Wyslouch, MIT

## Anti-particle / particle ratios

- z

- x

- 10 cm

- y

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

**Determine ratio of ********-****/********+****, K****-****/K****+****, p/p**
**Compare to lower energy (AGS/SPS) results**

- 70 cm

- Bolek Wyslouch, MIT

## Particle Ratios at 130 GeV from PHOBOS

**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**

**Particle Ratios at 130 GeV from PHOBOS**

- Bolek Wyslouch, MIT

## Elliptic Flow

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

- Elliptic flow

- reaction plane

****

- **R**

- Bolek Wyslouch, MIT

## ~4 Elliptic Flow @ 130 GeV

- Peripheral

**b**

- Central

**b**

**Normalized Paddle Signal**** **

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

**PHOBOS ****Systematic**** error ~ 0.007**

**V****2**

**Centrality dependence of V****2**

**|********|<1.0**
****sNN=130GeV

**preliminary**

**preliminary**

**~4******** Elliptic Flow @ 130 GeV**

**SPS  17 GeV**

- Bolek Wyslouch, MIT

## Multiplicity
Increase in the slope of dn/d as a function of the energy when compared to ppbar
dn/d increase with energy from 130 to 200 GeV is modest and concentrated at mid-rapidity
Other measurements:
Elliptic flow: Strong and -dependent
K-/K+ ratio does not follow linear extrapolation from lower energy
 Antiproton/proton ratio increasing towards unity (approaching a baryon-free regime)
Centrality dependence of dN/d.
No conclusive/clear signal of QGP (so far), it seems that any finding will take a systematic scan of all QGP probes at RHIC energies 

- Increase in the slope of dn/d as a function of the energy when compared to ppbar
- dn/d increase with energy from 130 to 200 GeV is modest and concentrated at mid-rapidity
- Other measurements:
- Elliptic flow: Strong and -dependent
- K-/K+ ratio does not follow linear extrapolation from lower energy
-  Antiproton/proton ratio increasing towards unity (approaching a baryon-free regime)
- Centrality dependence of dN/d.
- No conclusive/clear signal of QGP (so far), it seems that any finding will take a systematic scan of all QGP probes at RHIC energies

**Summary**

- Bolek Wyslouch, MIT

## Acceptance

**multiplicity detector**

**spectrometer**

- Bolek Wyslouch, MIT

## 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%**
******N****part******
**Loss of trigger efficiency at low-multiplicity <10%**
**Uncertainty on ********N****part ******** <1%**
**Uncertainty in modeling paddle fluctuations**** **
**Uncertainty on ********N****part ******** <6% **
**( dN/d******** / ********N****part******** )****130**** / ( dN/d******** / ********N****part******** )****56**
**Many uncertainties cancel in the ratio**** **

**Systematic Uncertainties**

- Bolek Wyslouch, MIT

## Particle Ratios (Chemistry)

- _stat_            _sys_

- Bolek Wyslouch, MIT

## Phobos silicon

- produced by ERSO, Taiwan

- AC coupled Pad (p-implant + metal 1 pad)
- polisilicon bias resistor
- metal 2 readout line
- contact hole metal 1- metal 2

**Phobos silicon**

- Bolek Wyslouch, MIT