# Presentation: 820618

## RHIC

- 10 o’clock

**PHOBOS**

**PHOBOS**** **rising @ Brookh(e)aven

- Rudi Ganz
**U****niversity of ****I****llinois ****C****hicago**

- BRAHMS

- STAR

- PHENIX

## The PHOBOS collaboration:

**A ****“small”**** experiment with about 50 collaborators from:**
- Brookhaven National Laboratory
- Case Western Resreve Univ.
- Institute of Nuclear Physics Krakow
- Jagiellonian Univ. Krakow
- Massachusetts Institute of Technology
- Univ. of Illinois Chicago
- Univ. of Maryland
- Yale Univ.

## Outline:

- The “Philosophy” of  **PHOBOS**
- Components and Capabilities
- Current Status of **PHOBOS**** **
- Take a Walk through the Spectrometer

- PHOBOS @ RHIC

## The approach of PHOBOS

- Large coverage for charged multiplicity (without PID) on event-by-event basis
- Coverage  of low pt at mid rapidity (with PID)(Temperature characterisation)
- Charged to neutral ratio measurement
- Since the physical signal is a priori unknown requires an unbiased way of taking data  (high rate capability)
- Ready to go from the very beginning of RHIC beam
- Designed “conservatively” for simplicity and flexibility
- Uniform technology: Silicon micro strip/pad detector

- PHOBOS @ RHIC

- Ref.: C.Halliwell A comparison of RHIC experiments

## “ Conservative” technology

-  Silicon micro strip and pad detectors (300  m 1 MIP  104 keV) for multiplicity AND spectrometer detector modules (WA98 type).
-  Conventional 2 Tesla magnetic field

- PHOBOS @ RHIC

- Ref.: R.Betts in Advances in Nucl. Dynamics 2

*Silicon detector schema*

- +HV

- p+ Implant

- n+

- Polysilicon
- Drain Resistor

**Dielectric 1**

- bias bus

- signal lines

**Dielectric 2**

- vias

- metal 1

- metal 2

## The experimental Challenge

- At a nominal luminosity of L2x1026 s-1 cm-2 we expect 600 Hz “min. bias” event rate and 60 Hz (10%) central collisions
- Each central collision produces about 12000 primary particles
- Additionally secondary part.

- PHOBOS @ RHIC

## Secondary Particles in the PHOBOS detector

**Secondary Particles in the ****PHOBOS**** detector**

## PHOBOS components and capabilities

-  Vertex Finder
-  Multiplicity Area
-  Spectrometer and TOF
-  Trigger and DAQ

**PHOBOS**** components and capabilities**

- PHOBOS @ RHIC

## The Vertex Finder

**Outer Vertex Detector**
- 8 modules  192 ch. ea.

**Inner Vertex Detector**
- 8 modules  2x192 ch. ea

- PHOBOS @ RHIC

- 2 x 3072 channels

## The PHOBOS Vertex Finder

**Beam**

**Width of strip 0.03 cm => resolution 0.05 cm**
**For z=0 we expect 189(230) hits on outer **
**and 300(330) hits on inner vertex **

**45****o**

**z**

** ****12.5 cm**

** ****-12.5 cm**

**11 cm**

**5.5 cm**

**x**

- PHOBOS @ RHIC

## The PHOBOS Multiplicity array

- -0.55 m

- -5.04 m

- -2.34 m

- -1.13 m

-  0.55 m

-  1.13 m

-  2.34 m

-  5.04 m

- z

- ****

****

- ****

**  ****85% of 4******** covered**
** ****Charged part. multiplicity d****2****N/d********d******** ****  **(  is azimuthal angle to beam)  ************** and ******
** ****Distinguish multiple hits by the ****   energy deposition**

- PHOBOS @ RHIC

- Note:
- Pseudorapidity **** defined as
******ln(tan********)) **
-  polar angle with
- respect to beam

## The Octagon Multiplicity Detector

- Vertex Finder

- Pb- Converter

- PHOBOS @ RHIC

-         90 modules 64 ch. ea.
- => 5760 channels

- Spectrometer
- opening

****

**9 cm**

## The Multiplicity Ring Detectors

- PHOBOS @ RHIC

- 6 rings at distances of
- z =  1.13 m , 2.34 m and 5.04 m
- Each ring has 8 x 64 channel
- modules
- Positioned such that the whole
- pseudorapidity range is covered
- for vertices of   10 cm

## Multiplicity distribution in PHOBOS

- PHOBOS @ RHIC

## PHOBOS spectrometer

- PHOBOS @ RHIC

**PHOBOS ****spectrometer**

- Geometrical acceptance
- 1 unit in 
- eg. for vertex at z=0:
- 0.5< <1.5 and
- 2 x11o  in (azimuthal)
- =>about 1% of the particles
- are covered
- 11 planes of silicon
- detectors in each arm
- with decreasing
- segmentation

## PHOBOS particle identification

- PHOBOS @ RHIC

- high B-field region

- 5 planes in high field region
- 6 planes in low field region

- 85 % track finding eff.
- uniform over p

## PHOBOS particle identification with TOF wall

**PHOBOS particle identification with TOF wall**

**Range out ****in 7 planes:****	****55 MeV/c ******
**	****	****	****135 MeV/c K**
**dE/dx****	****	****	****550 MeV/c ******** /K**
**	****	****	****950 MeV/c K/p**
**TOF****	****CDR: ****	****	****1150 MeV/c ******** /K**
**	****	****	****1900 MeV/c K/p**

- PHOBOS @ RHIC

## PHOBOS acceptance

- PHOBOS @ RHIC

- Time of flight

## PHOBOS  reconstruction

- PHOBOS @ RHIC

## PHOBOS Trigger and DAQ

- PHOBOS @ RHIC

- Low and high trigger counter (Cerenkov; low also used as timing for TOF wall)
- Data rate is 200 Hz (30 % centrality and 18 Kbytes/event)
- => 60 - 430 TByte/year !!!
- Limited by data recording capability, data storage and CPU time for all four experiments
- MIT-designed pipeline chip provides “continuous” data flow

## PHOBOS: Physics Highlights

- Global event characteristic in almost 4
- Momentum and particle ID at Mid rapidity
- Low pt sensitivity
- High data rate capability vs. large acceptance of “large” experiments