Strangeness Production in PHOBOS

1
Strangeness Production in PHOBOS

• Conor Henderson
• Massachusetts Institute of Technology
• For the PHOBOS Collaboration
• RHIC/AGS Users Meeting, 14 May 2004
• Brookhaven National Laboratory

2
Collaboration
Birger Back, Mark Baker, Maarten Ballintijn,
Donald Barton, Russell Betts, Abigail Bickley,
Richard Bindel, Wit Busza (Spokesperson), Alan
Carroll, Zhengwei Chai, Patrick Decowski,
Edmundo García, Tomasz Gburek, Nigel George,
Kristjan Gulbrandsen, Clive Halliwell, Joshua
Hamblen, Adam Harrington, Michael Hauer, Conor
Henderson, David Hofman, Richard Hollis, Roman
Holynski, Burt Holzman, Aneta Iordanova, Jay
Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo,
Willis Lin, Steven Manly, Alice Mignerey, Gerrit
van Nieuwenhuizen, Rachid Nouicer, Andrzej
Olszewski, Robert Pak, Inkyu Park, Heinz
Pernegger, Corey Reed, Christof Roland, Gunther
Roland, Joe Sagerer, Helen Seals, Iouri Sedykh,
Wojtek Skulski, Chadd Smith, Maciej Stankiewicz,
Peter Steinberg, George Stephans, Andrei
Sukhanov, Marguerite Belt Tonjes, Adam Trzupek,
Carla Vale, Sergei Vaurynovich, Robin Verdier,
Gábor Veres, Peter Walters, Edward Wenger, Frank
Wolfs, Barbara Wosiek, Krzysztof Wozniak, Alan
Wuosmaa, 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
3
The PHOBOS Detector
Time-Of-Flight
SpecTrig
Spectrometer
T0 Detectors
4
The PHOBOS Spectrometer

• Two arms x 16 layers of silicon
• Highly-segmented in x-z plane
• Inner layers in zero-field region, outer layers
  in 2T magnetic field
• Tracking within 10 cm of interaction region

5
Particle Identification in PHOBOS

• Three techniques for Particle ID
• Stopping Particles (0.05 lt pT lt 0.2 GeV/c)
• Silicon dE/dx (0.3 lt pT lt1.3 GeV/c)
• Time-Of-Flight (0.5 lt pT lt 4 GeV/c)

6
Low-pT Stopping Particles

• Search for particles which stop in 5th
  Spectrometer layer
• Tracks identified via energy deposition pattern

• ?, K, p identified for
• 0.05 lt pT lt 0.2 GeV/c
• No B-field, so particle charge cannot be
  determined

7
Low-pT Stopping Particles
Construct a mass parameter from energy deposition
in each layer
p p
K K-
? ?-
Method calibrated by Monte Carlo simulation
8
AuAu Low-pT Particle Yields
AuAu ?sNN 200 GeV

• Yields corrected for
• Geometrical acceptance
• Reconstruction efficiency
• PID inefficiencies
• Absorption in the beam pipe
• Feed-down from weak decays
• Secondaries
• Mis-identified particles
• Ghosts

y - 0.1 0.4
arXivnucl-ex/0401006
9
AuAu Low-pT Results
AuAu ?sNN 200 GeV
PHENIX spectra for mT lt 1GeV/c2 fit with
PHENIX - open symbols PHOBOS closed symbols
-1 for mesons 1 for baryons

• This fit extrapolates smoothly to the low-pT
  points
• No enhancement of low-pT particle yields
• No evidence of unusual long-wavelength physics

Solid line fit to PHENIX spectra Dashed line
extrapolation of fit
10
Antiparticle/ Particle Ratios
Particles identified by Si dE/dx

• Invert magnetic field interchange trajectories
• Tracking efficiency and geometrical acceptance
  corrections cancel in ratio
• Antiparticle/particle ratios then just corrected
  for absorption feed-down secondaries

11
AA Particle Ratios vs. Collision Energy
K- / K p / p
Phys. Rev. C 67, 021901R (2003)
12
Baryo-Chemical Potential At RHIC
AuAu ?sNN 200 GeV
Phys. Rev. C 67, 021901R (2003)
With T 165 MeV, ?B 27?2 MeV at ?sNN 200 GeV
in AuAu
13
Particle Ratios in pp and dAu
?sNN 200 GeV
arXivnucl-ex/0309013
14
And Compared to AuAu
?sNN 200 GeV
arXivnucl-ex/0309013
15
The PHOBOS Time-Of-Flight Detector

• Two Time-Of-Flight walls
• TB at 45 deg, 5.4m
• TC at 90 deg, 4m
• 120 plastic scintillators per wall, read-out top
  and bottom
• TOF timing resolution 100 ps
• Collision start-time determined from arrays of
  Cerenkov counters (T0s) along beam-line

16
The PHOBOS Spectrometer Trigger

• High-pT tracks identified by straight-line hit
  combinations and online measurement of event
  vertex
• New scintillator walls installed (the SpecTrig)
  online trigger decision made by programmable
  electronic logic module
• Resulted in factor 15-20 enhancement in dAu
  collisions

17
Time-of-Flight Spectra Analysis

• Momentum slices are fitted to extract the yields
  of each particle species

• Yields corrected for geometrical acceptance and
  tracking efficiency no feed-down correction
  applied

18
Identified Particle pT Spectra from TOF
dAu ?sNN 200 GeV
19
Particle Composition in dAu
dAu ?sNN 200 GeV
20
Centrality Dependence in dAu
No observed variation in particle composition
with collision centrality in dAu
Peripheral 40-70
Central 0-20
20-40
21
mT-scaling in dAu ?
dAu ?sNN 200 GeV
x 2
Kaon yields scaled by 2
22
But Not in AuAu?
AuAu ?sNN 200 GeV

• Spectra normalised at pT 2 GeV/c (Different
  norm. factors for kaons and protons)
• mT-scaling violated at low mT in AuAu
• Consistent with transverse expansion of AuAu
  collision system

23
PHOBOS Strangeness Summary

• Three techniques for particle ID from low to high
  pT stopping particles Si dE/dx and
  Time-Of-Flight
• AuAu results
• Low-pT yields ? no evidence of unusual
  long-wavelength physics violation of mT-scaling
  observed
• Particle ratios indicate ?B 27?2 MeV at RHIC
• dAu results
• Particle ratios differ from AuAu at same ?
• Preliminary identified particle spectra exhibit
  mT-scaling
• pp results
• Preliminary particle ratios similar to dAu

24
Strangeness Outlook from PHOBOS

• AuAu identified particle spectra from TOF
• dAu low-pT particle yields
• Particle ratios in AuAu at 62.4 GeV
• Low pT ? yields in AuAu

25
Back-up Slides
26
Stopping Particles Acceptance
27
Low-pT Yields Compared to Models

• Event generators unable to consistently describe
  low pT yields.
• HIJING overpredicts yields at low pT.
• Ratio of measured to HIJING yields averaged over
  low pT range

28
Comparison to Hydro. Models
P. Kolb and R. Rapp, Phys.Rev. C67, 044903 (2003)
Red curves Tdec100MeV
Solid without initial transverse boost
Blue curves Tdec165MeV
Dashed with initial transverse boost
Inclusion of an initial (pre-hydrodynamic)
transverse flow better describes the spectra.
29
TOF and Spec PID Acceptances
30
dAu pbar/p compared to models
31
TOF Particle Ratios
32
Particle Composition vs.?s in p(d)A Collisions