Identified Particle Transverse Momentum Distributions from Au Au Collisions at 62'4 GeV per Nucleon

1
Identified Particle Transverse Momentum
Distributions from AuAu Collisions at 62.4 GeV
per Nucleon Pair

• Conor Henderson
• 27 June 2005
• MIT PhD Thesis Defence

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Outline

• Introduction to Heavy-Ion Physics
• The proton/pion puzzle at intermediate pT
• The PHOBOS Detector
• Track reconstruction and particle ID
• Obtaining pT spectra
• Results and conclusions

3
The QCD Phase Diagram

• Strongly-interacting matter can exist in a
  variety of phases
• Asymptotic freedom of QCD implies a stable
  quark-gluon plasma

4
A Relativistic Heavy-Ion Collision

• Heavy-ion collisions attempt to create and study
  this QGP phase

5
Heavy-Ion Collision Centrality
Central collision
Peripheral collision
6
Npart and Ncoll Scaling
Number of participating nucleons, Npart
8 Number of nucleon-nucleon collisions, Ncoll 16

• What to expect for a nucleus-nucleus collision?
• Npart scaling - Wounded Nucleon Model
• Ncoll scaling - Independent superposition of
  nucleon-nucleon collisions

7
Transverse Momentum Spectra
PHOBOS
PRL 94, 082304 (2005)
AuAu 62.4 GeV
8
High-pT Particle Suppression

• High-pT particle yields found to be suppressed
  significantly below Ncoll scaling from pp
  collisions

9
Jet Quenching in Deconfined Medium
I. Vitev, QM2004

• Parton energy loss significantly greater in a
  dense deconfined medium
• Results in suppression of high-pT particles

10
Proton/Pion Puzzle

• Proton/pion ratio at intermediate pT found to be
  remarkably high in 200 GeV AuAu collisions

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Not seen in dAu at 200 GeV

• dAu collisions did not show such a high
  proton/pion ratio

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Proton/Pion Puzzle II

• Protons and pions also observed to have very
  different centrality-scaling in 200 GeV AuAu
  collisions

13
Parton Recombination Models
R Hwa, QM2004
R Fries, QM2004

• Predicts parton recombination to dominate over
  fragmentation at intermediate pT
• Results in baryon/meson enhancement

14
Baryon Stopping Jet Quenching
Vitev, Gyulassy PRC 65, 041902, 2002

• Arises from pion suppression due to
  jet-quenching plus large baryon contribution from
  baryon stopping (transport of baryons from beam
  to mid-rapidity)
• Suggested mechanism is gluon junctions baryon
  number is traced by junction, not by valence
  quarks - easier to transport over large rapidities

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Motivation for this Thesis

• Identified particle pT spectra from AuAu
  collisions at 62.4 GeV will investigate the
  proton/pion puzzle at a new collision energy
• This will help to understand the processes which
  govern particle production at intermediate pT in
  the complex system formed in heavy-ion collisions

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The Relativistic Heavy-Ion Collider
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Collaboration (June 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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The Detector 2004
TOF Walls
T0 Detectors
SpecTrig
Spectrometer
Paddle Trigger
NIM A 499, 603-623 (2003)
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Event Trigger
Au
Au
Paddle Triggers

• Good events are selected by timing
  characteristics
• A vertex trigger is used to further enhance the
  selection of useful events

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Centrality Bins used in this Analysis
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The PHOBOS Spectrometer

• Silicon sensors
• Outer layers in 2T magnetic field
• High segmentation in bending direction
• Tracking within 10 cm of interaction point
• Coverage near mid-rapidity

x
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A Spectrometer Event
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Track Reconstruction

• Road-following algorithm finds straight tracks in
  field-free region
• Curved tracks in B-field found by clusters in
  (1/p, ?) space
• Match pieces, fit to determine best momentum

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Silicon dE/dx Particle Identification
p
K
?
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Time-of-Flight Detectors

• Two TOF walls, at 90 deg. and 45 deg. to
  beam-axis
• 120 plastic scintillators per wall, PMT read-out
  top and bottom
• Start time provided by T0 Cerenkovs
• Total timing resolution 140 ps

TOF identification ?t d(E1 - E2)/p
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TOF Particle Identification
p
K
?
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Corrections Applied to Raw Data

• Geometrical Acceptance
• Tracking Efficiency
• Momentum resolution
• Ghost tracks
• Feed-down from weak decays
• Secondary particles
• Effect of Silicon and TOF Dead channels

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TOF and Spectrometer Data Synthesis

• Spectrometer and TOF cover different kinematic
  ranges
• Synthesis procedure used to present the data
  in a detector-independent way

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Identified Particle pT Spectra, AuAu at 62.4 GeV
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Proton/Hadron Fraction at 62.4 GeV

• Protons are the dominant hadron species at
  intermediate pT

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Proton/Hadron Fraction vs. Energy

• Proton/hadron fraction actually found to be
  higher at 62.4 GeV than at 200 GeV

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Centrality Dependence at 62.4 GeV

• Protons and mesons exhibit different centrality
  dependence

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Centrality Dependence at 200 GeV

• Similar centrality dependence seen at 200 GeV

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Proton/Pion Puzzle at 62.4 GeV

• The proton/pion puzzle does not suddenly appear
  at 200 GeV - essentially the same features are
  also present at 62.4 GeV AuAu collisions
• This will provide important data for explanations
  of the puzzle
• No theoretical predictions for 62.4 GeV yet
• But the baryon junctions model must also describe
  baryon stopping at 62.4 GeV - we can probe this
  too using the present data

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Energy Dependence of Proton Spectra

• More low pT protons at 62.4 than at 200 GeV
• Baryon stopping must dominate low-pT proton
  yields

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Integrating pT Spectra to get dN/dy

• Integrating pT spectra gives total particle
  yield, dN/dy

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Net Protons vs. Npart

• Interesting linearity with Npart
• May constrain models of baryon transport

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Net Protons vs Beam Rapidity
(0-5)

• Fits smoothly into rapidity dependence of baryon
  stopping

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Summary

• Identified particle pT spectra have been measured
  for AuAu collisions at 62.4 GeV using the PHOBOS
  Spectrometer and TOF
• The proton/pion puzzle seen at 200 GeV is present
  also at 62.4 GeV - will be interesting to see if
  models can explain this energy dependence
• Net proton yields near mid-rapidity also
  presented - should further constrain model based
  on baryon stopping

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Backup slides
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The Synthesis Procedure
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A Synthesized Example
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Acceptance and Efficiency Correction

• Largest correction is geometrical acceptance and
  tracking efficiency
• Obtained from Monte Carlo simulations of PHOBOS
  detector

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Feed-down from Weak Decays

• Feed-down from weak decays makes
  detector-dependent distortion of primary pT
  spectra
• Correction obtained by simulating ? and ? decays
  - find relative probability of reconstructing
  daughter proton relative to primary proton with
  same pT
• Input physical values to estimate correction
• Cross-check using a track quantity,
  distance-of-closest-approach to event vertex,
  which is shown to have sensitivity to feed-down
  products

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Antiparticle/Particle Ratios

• Antiproton/proton ratio can be used to estimate
  baryochemical potential, ?B
• p/p ? exp(-2?B/T)
• Obtain
• ?B ? 80 MeV
• (if T 165 MeV)

p/p 0.38 ? 0.03 (sys.)
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Antiparticle/Particle Ratios vs. Energy

• These first results at 62.4 GeV fit smoothly
  into the energy evolution of antiparticle/particle
  ratios

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mT Spectra and mT-Scaling?

• Suggested that particle spectra should scale
  with mT
• Prelim results from dAu agreed with this but
  AuAu at 200 GeV clearly violated mT-scaling
• These results at 62.4 GeV do not support it
  either

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Lattice QCD Phase Transition

• Lattice QCD calculations predict a phase
  transition from hadrons to QGP

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Factorization of Energy and Centrality Dependence

• Centrality dependence of
• vs. pT is the same at 62.4 and 200 GeV
• Challenge to collision models to reproduce this

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Proton Centrality Dependence
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Antiproton/Hadron Fraction
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Antiproton/Hadron Fraction vs Energy
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Proton/Hadron Fraction vs. Energy
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Proton and Antiproton Yields vs Npart
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Charged Particle Multiplicity

• Multiplicity can be used to estimate the energy
  density
• Found to be above the Lattice QCD critical value
  for a QGP

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Vertex-Finding

• PHOBOS vertex detector is able to determine
  event vertex with resolution of 0.1mm
• Other sub-detectors have worse resolution but
  can provide additional information
• A composite Selected Vertex is created from
  all the info available

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Silicon Sensors in PHOBOS

• Based on principle of semiconductor pn-junctions
  - reverse-biased to increase depletion zone
  sensitive volume
• Charged particle traversing sensor creates
  electron-hole pairs - collected by electrodes as
  signal in sensor

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Paddles/ZDC Correlation
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Track-Fit Probability
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DCA
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TOF Residuals
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K, Proton Momentum Reconstruction
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Lambda reconstruction - Found
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Lambda reconstruction - Not Found
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Various Lambda/p Ratios
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Centrality Determination

• Need an observable which closely correlates with
  Npart
• For PHOBOS, Paddle Signal is good measure

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Centrality Determination II

• Then divide the observed Paddle Signal
  distribution in bins
• Monte Carlo simulations determine Npart and
  Ncoll for these bins

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Tracking Efficiency and Momentum Resolution

• Single-track reconstruction efficiency 90
• Momentum resolution better than 5 for pT lt 8
  GeV/c