# Presentation: 511260

## Very Low pT in               x

- Past, Present, and Prospects

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

##                     Collaboration (March 2005)

- Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel,
-  Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane,Piotr Kulinich, Chia Ming Kuo,
- Wei Li, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer,
- Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov,
- Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres,
- Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs,  Barbara Wosiek, Krzysztof Woźniak,
- Shaun Wyngaardt, Bolek Wysłouch
- 	ARGONNE NATIONAL LABORATORY	BROOKHAVEN NATIONAL LABORATORY
- 	INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW	MASSACHUSETTS INSTITUTE OF TECHNOLOGY
- 	NATIONAL CENTRAL UNIVERSITY, TAIWAN	UNIVERSITY OF ILLINOIS AT CHICAGO
- 	UNIVERSITY OF MARYLAND	UNIVERSITY OF ROCHESTER

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## Kinematic Regions at RHIC

**TAIL**

**BULK**

**HEAD**

- Physics Topics:
- Bulk - Particle production, sets the stage
- Tail - Jet quenching, probes medium
- Head - Sensitive to global properties

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## The PHOBOS Detector (2001/2002)

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## The Low pT Part

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## The Technique

- Simple in principle, Si gives dE/dx which sums to total energy
- Combination of dE/dx and ETot gives mass
- Complicated in practice
- Energy in last plane is unreliable (decays, interactions, etc.)
- Need to iterate that energy until (dE/dx)i and Ei consistent
- Many corrections: decays, secondaries, ghosts, tracking 	efficiency, PID efficiency, mis-identified particles, etc.

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## Particle Measurements at Very Low pT

**X[cm**]

**A**

**B**

**C**

**D**

**E**

**F**

**Z[cm]**

**Beam pipe**

-  **0        10        20   Z [cm]**

**   ****.**

**   ****.**

**   ****.**

- Particle Measurements at Very Low pT

- For now: Only find particles stopped in the 5th spectrometer plane

- Reconstruction is based on the analysis of the measured energy depositions in the first 5 layers.
- Fewer than 5 planes per track leads to more background.
- Beyond that, magnetic field becomes a serious issue.

- Single stopping particles from
- GEANT simulations are used to
- calibrate the method.

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## Comparison of dE/dx for low momentum pions

**Comparison of dE/dx for low momentum pions**
**         ****(red)  PR01 data****       ****(black) HIJING events**

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## Particle Measurements at Very Low pT

**Summary of data reconstruction**

- Au+Au  sNN=200 GeV (RHIC Run II)
- Centrality: 15% most central events
- Vertex:     -7cm < Zvtx < 15cm
- Event statistics: 2,070,000
- Total reconstructed tracks:
**   ****(************ ****+**** ************)****	****14,000**
**   ****(****K********+**** ****K********)****	****1,900**
**    ****(****p+p****)****	****1,000**

******+**** + ********–**

- Dominant background:
- Feed-down, ghost,
- mis-identified K& p

- pT-dependent corrections from 0.69 to 0.78

- Use HIJING simulations with yields tuned to our measurements

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

**Notes:**
Raw number of reconstructed tracks per 1000 events:
DATA   6.9    0.94   0.50
HIJING 8.4     2.0    2.3

## Published Result  Au+Au 200 GeV 0-15%

**PHENIX - open symbols**
**PHOBOS –closed symbols**

- Solid curves: Fits to PHENIX only
- Dashed curves: Extrapolations

*Log scale!*

- PRC 70
- 051901 (R)

- Used PHENIX
- because  0-15% data published

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## Published Result (Blown Up)

-   Au+Au 15% central

- Extrapolation of fit
- to PHENIX data
- for pT>0.25 GeV/c

- PRC 70
- 051901 (R)

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## Comparison to Cramer et al.

- Comparison to Cramer *et al*.

- PRL curve,
- data fit problem

- Star data,
- fixed fit

- Fit to modified
- data

- Phobos 0-15% data

- Estimated yield 0-5%

- Data is (****  Au+Au 0-15% central

- Cramer, *et al*.
- PRL 94, 102302
- & private comm.

- nucl-ex
- 0506008

- {

- }

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## Procedure to Estimate 0-5% Yield

- PHENIX pion data for pT=250-450 MeV/c
- STAR pion data for pT=126-214 MeV/c (estimate 15% numbers using 5-10, 10-20% data)
- PHOBOS charged hadron data for pT=250-450 MeV/c
- PHOBOS summed charged hadron data
- These ratios range from 16.2 to 20.1%
- Our extrapolation band spans 15-20%

- Of course, the final answer is to have the data...

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

## Near-Term Future

**d+Au 200 GEV**

-  — Study system size dependence

**Au+Au 62.4 & 200 GEV**

- — Study energy & centrality dependence
- ✔ Important to determine where any interesting properties “turn off”

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

**Notes:**
Currently we have 21 pipi pairs in one arm, 23 pipi pairs in 2 arms
4 piK, 4pip, 1KK.

## Far-Term Future

**Au+Au 200 GEV – high statistics data from RHIC Run 4**

- — Study low pT yields vs many event parameters
- Could identify negatives using excess energy in last plane
- — Attempt to get BE correlation function at very low mT
- Momentum and angular resolutions are encouraging

- R0 (fm)

- KT (MeV/c)

- Cramer, *et al.*
- PRL

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

**Notes:**
Currently we have 21 pipi pairs in one arm, 23 pipi pairs in 2 arms
4 piK, 4pip, 1KK.

## Far-Term Future

**Au+Au 200 GEV – high statistics data from RHIC Run 4**

- R0 (fm)

- KT (MeV/c)

- We will try, but the reality is that this data set is unlikely to yield usable pair statistics using very low pT stopping particles.
- There are several other options to explore to see how low in KT Phobos can reach.

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005

**Notes:**
Currently we have 21 pipi pairs in one arm, 23 pipi pairs in 2 arms
4 piK, 4pip, 1KK.

## Conclusions

- i             low pT data can play an important role in evaluating model(s) of the  exotic systems formed at RHIC
- More data for yields will be available soon
- Looking forward to detailed comparison to theory(ies)
- Very low KT HBT would likely require a dedicated experiment

- George Stephans

- Femtoscopy BNL Workshop 6/21/2005