# Presentation: 677764

## Recent Results fromThe PHOBOS Experiment

**for the **
- PHOBOS **Collaboration**

**Recent Results ****from****The PHOBOS Experiment**

## 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

**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, George Heintzelman, ****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**

**The   PHOBOS   Collaboration**

## The PHOBOS Detector (2001)

**The PHOBOS Detector (2001)**

**Completed Spring 2001 **
**4******** Multiplicity Array**
**- Octagon, Vertex & Ring Counters**
** ****Two Mid-rapidity Spectrometers**
** ****TOF wall for High-Momentum PID**
** ****Triggering**
**Scintillator Paddles**
** ****Zero Degree Calorimeter**

- Cerenkov

**137000 Silicon Pad channels**

## Summary of PHOBOS Results

-  Charged Particle Multiplicity at 56 and 130 GeV. PRL 85,3100(2000)
**Results from PR2000 – ****Incomplete Second Arm**
-  Particle Ratios. PRL (in press)
-  Rapidity and Centrality Dependence of Elliptic Flow.
-  Centrality Dependence of Mid-Rapidity Multiplicity. PRL(submitted)
-  dN/d Distributions as a Function of Centrality. PRL (in press)
**Results from PR2001 – ****Complete Detector**
-  Charged Particle Multiplicity at 200 GeV. PRL (submitted)

**Summary of PHOBOS Results**

## Triggering on Collisions

**Coincidence between Paddle counters at ********t = 0 defines a valid collision.**
**Paddle + ZDC timing reject background.**
**Sensitive to 97****±3 ****% of inelastic cross section for Au+Au.**

**Triggering on Collisions**

- Negative Paddles

- Positive Paddles

- ZDC N

- ZDC P

- Au

- Au

- PP

- PN

- Paddle Counter

**Valid**
**Collision**

- ZDC Counter

## Trigger Selection - ZDC vs Paddles

**Trigger Selection - ZDC vs Paddles**

- Peripheral

## Au+Au Collision Event Display

## Particle Ratios at 130 GeV from PHOBOS Spectrometer - PRL (in press)

**********************= 1.00 ****±****0.01 (stat) ****±**** 0.02 (syst)**
**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**

## Thermal Model of Ratios

**Thermal Model of Ratios**
******B**** = 45 MeV **
**Assuming T=165 MeV**
**Redlich et al**

**Phase Diagram**
**(N Xu)**

## Flow from Azimuthal Asymmetry of Octagon Hits

**Flow from Azimuthal Asymmetry of Octagon Hits**
** ****@130 GeV**

**Preliminary**

## Pseudo-Rapidity Dependence of v2

**Pseudo-Rapidity Dependence of v****2**
**130 GeV**

**Dependence of v****2**** on ********Roughly Follows dN/d******

**Preliminary**

****

## Measuring Multiplicity

- Tracking detectors
- Tracklets in Spectrometer
- Tracklets in Vertex Detector
- Single layer detectors
- Count hits above threshold, assume Poisson-statistics
- Use deposited energy (dE/dx) in each Si-pad

## Tracklets

- Tracklet: Two-Hit Combination + Vertex Position
- >300 Tracklets/Central Event in Vertex, >100 in Spectrometer

**Vertex-Detector**

## Analog and Digital Hit-Counting

**Octagon, Ring and Vertex Detectors (unrolled)**
**Count Hits or Deposited Energy**

****

****

## Increase over SPS and over pp Data

_**First Physics Results from RHIC**_
**Tracklet Analysis of CR2000 Data**
** ****PRL 85,3100(2000)**

## }

- Npart

**Yellow band:**
**Systematic uncertainty**

- Preliminary

- dNch/d/(0.5*Npart)

- Central Collisions

- Peripheral Collisions

*}*

*}*

- Data

- Theory

**Dependence of Multiplicity on Centrality at 130 GeV Tracklets – PR2000**

## Shapes of dN/d Distributions at 130 GeV - Hit Counting

**Shapes of dN/d******** Distributions at 130 GeV - Hit Counting**

-  Shapes only weakly dependent    on centrality
-  Differ in details

## Slide 18

## Predictions for ratio of dN/d between 130 and 200 GeV

**Predictions for ratio of dN/d******** between 130 and 200 GeV**

- Ratio is easier to measure and to calculate.
- Predictions range from 1.08 to 1.32

## PHOBOS Data Taking in 2001

- Collected ~2000 min bias 130 GeV collisions
- Check Detector Calibrations
**First 200 GeV collisions seen July 18****th**
- Collected > 1M Events Since.
- Data Reconstructed and Analyzed on RCF and “pharm”

## Silicon Signals 200 vs 130 GeV

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

**Essential for Hit-Counting; Many Uncertainties**
**Drop Out in Ratio 200/130 GeV**

- low occupancy (<10%)

- high occupancy (>70%)

## Results for 200 GeV

**Results for 200 GeV**

**6% Most Central Events - Systematic Uncertainties Only**

- 1.14±0.05

## Energy Dependence

**Logarithmic Dependence ? - Energy Scan Needed.**

## Ratio 200/130 GeV

**Ratio 200/130 GeV**

**Phobos Measurement**

**Ratio 200/130**
**averaged for four **
**PHOBOS methods**

**R****200/130**** = 1.14 +/- 0.05 Moderate Increase in Energy Density?**

**Systematic Uncertainty**

## Pseudo-Rapidity Dependence of Ratio

**Pseudo-Rapidity Dependence of Ratio**

**Broadening of dN/d******** Distribution 130    200 GeV**

**200 GeV**

## Summary 

- Several Results at 56 and 130 GeV
- First results from Au+Au at 200 GeV
**dN/d********=  650 +/- 35 for 6% most central collisions**
**dN/d******** increases by 1.14+/- 0.05 from 130 GeV to 200 GeV**
- Moderate increase in energy density
- Further constraints for  theoretical scenarios
**Much more to come !!!**