Very Low pT in x

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Very Low pT in x
Past, Present, and Prospects
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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 Holynski,
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 Wozniak, Shaun Wyngaardt,
Bolek Wyslouch ARGONNE NATIONAL
LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITU
TE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS
INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL
UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT
CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF
ROCHESTER
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Kinematic Regions at RHIC
Physics Topics Bulk - Particle production, sets
the stage Tail - Jet quenching, probes
medium Head - Sensitive to global properties
BULK
HEAD
TAIL
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The PHOBOS Detector (2001/2002)
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The Low pT Part
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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.

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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.
F
E
D
Xcm
C
B
A
.
.
Beam pipe
0 10 20 Z cm
.
Single stopping particles from GEANT simulations
are used to calibrate the method.
Zcm
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Comparison of dE/dx for low momentum pions
(red) PR01 data (black) HIJING events
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Particle Measurements at Very Low pT
Summary of data reconstruction
? ?
AuAu ?sNN200 GeV (RHIC Run II) Centrality 15
most central events Vertex -7cm lt Zvtx lt
15cm Event statistics 2,070,000 Total
reconstructed tracks (?? ??) 14,000 (K?
K?) 1,900 (pp) 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
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Published Result AuAu 200 GeV 0-15
PHENIX - open symbols PHOBOS closed symbols
PRC 70 051901 (R)
Used PHENIX because 0-15 data published
Log scale!
Solid curves Fits to PHENIX only Dashed curves
Extrapolations
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Published Result (Blown Up)
PRC 70 051901 (R)
????????? AuAu 15 central
Extrapolation of fit to PHENIX data for pTgt0.25
GeV/c
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Comparison to Cramer et al.
Data is (???????????? AuAu 0-15 central
Cramer, et al. PRL 94, 102302 private comm.
Fit to modified data
Star data, fixed fit
PRL curve, data fit problem


Estimated yield 0-5
nucl-ex 0506008
Phobos 0-15 data
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Procedure to Estimate 0-5 Yield

• PHENIX pion data for pT250-450 MeV/c
• STAR pion data for pT126-214 MeV/c (estimate 15
  numbers using 5-10, 10-20 data)
• PHOBOS charged hadron data for pT250-450 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
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Near-Term Future
dAu 200 GEV
Study system size dependence
AuAu 62.4 200 GEV
Study energy centrality dependence ?
Important to determine where any interesting
properties turn off
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Far-Term Future
AuAu 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

Cramer, et al. PRL
R0 (fm)
KT (MeV/c)
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Far-Term Future
AuAu 200 GEV high statistics data from RHIC
Run 4
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.
R0 (fm)
KT (MeV/c)
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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