Gábor I. Veres
Massachusetts Institute of Technology
for the Collaboration
International Workshop on Hot and Dense Matter in Relativistic Heavy Ion Collisions
March 24-27, 2004, Budapest
Hadron pT Spectra by PHOBOS from 0.03 to 6 GeV/c
Gábor I. Veres
Collaboration (March 2004)
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 Hołyński, 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, Michael Ricci,
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, Siarhei Vaurynovich, Robin Verdier, Gábor Veres, Edward
Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch
ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY
NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER
Gábor I. Veres
Outline
Hadron pT-spectra: why and how to study them?
The extremes: low and high pT
Small (d+Au) to large (Au+Au) colliding systems
Flavour dependence – charged and identified hadron spectra. Particle ratios.
Importance in the details: centrality and rapidity dependence
Gábor I. Veres
Low pT
Probing long distances
Radial flow
“soft” physics (Npart?)
Longitudinal and Transverse Dynamics
High pT
Suppression, E loss, quenching…
Initial/final state effects (different systems)
“hard” physics (Ncoll?)
dN
ch/d
Gábor I. Veres
The PHOBOS Detector (2001)
137000 Silicon Pad Channels
1m
12m Be Beampipe
Spectrometer
Octagon
Vertex
Ring Counters
Paddle Trigger Counter
Čerenkov Counter
DX magnet DX MagnetZDC ZDC
NIM A 499, 603-623 (2003)
Au+Au
Gábor I. Veres
Octagon & Vertex
Spectrometer armRing
Silicon Detectors
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dN/dpT
PID Mass +Charge
0.03 0.2 1.0 pT, GeV/c
up to 6 GeV/c
Stopping in Si dE/dx in Si ToF+Si Si
Mass
near mid-rapidity
Z
ToF
Spectra and PID in
Charge
Gábor I. Veres
Yields of (+ + ) , (K+ + K), (p + p)
pT = 30 – 200 MeV/c (depending on particle mass)
I. Hadrons in the low pT range
Negligible B field + multiple scattering:
charge sign cannot be measured
Probing long distances (truly non-perturbative QCD regime) Radial flow DCC (enhanced pion yields??) Dynamical fluctuations at phase transition??
Gábor I. Veres
PHOBOS Capability of Low pT Measurements
z
-x
10 cm
y
70 cm B=2T
Drawback:
Advantages: Sensitive detector layers close to the IP Little material between the IP and Si layers High segmentation of the Si detectors
small acceptance of the spectrometer
Gábor I. Veres
Mass measurements
(‘energy-range’ method)
Cuts on dE/dx per plane mass hypothesis
X[c
m] AB
CDE F
Z[cm]
Beam pipe
0 10 20
0
10
20
Z [cm]
Search for particles stopping in
the 5th spectrometer plane
A B C D E
dE
/dx
Ek= 8 MeV
P Ek=21 MeV
K Ek=19 MeV
Cuts on Eloss (Ek=kinetic energy)
momentum hypothesis
• Eikin=dEi+dEi+1+dEi+2…
• Mip = dE/dxi * Ei
kin m (1/2) ( m2)
Corrections acceptance, efficiency absorption, background
silicon plane
Finding very low pT particles
MC
Gábor I. Veres
Test of the method:
Reconstruction of lowmomentum MC particles
Au+Au sNN=200 GeV 15% central
MC
Measuring particle mass
p+p
K++K–
++ DATA
Gábor I. Veres
Au+Au sNN=200 GeV 15% central
-0.1< y <0.4
Invariant yields (Au+Au)
+
+
K+
+K–
p+p
nucl-ex/0401006, submitted to PRL
Momentum and energy:
From carefully calibrated MC
Yields: binned in pT and
corrected
Gábor I. Veres
Comparison to the Spectra Measured at Intermediate pT Range
PHENIX - open symbolsPHOBOS –closed symbols
Log scale!
Fit PHENIX spectra (nucl-ex/0307022)
for mT<1 GeV/c2:+1 for baryons-1 for mesons
Fits: solid curvesExtrapolations: dashed curves
Tfit: + - 0.2290.005 K + K- 0.2930.010 p + p 0.3920.015
Extrapolation of the fits to low pT agrees with our low-pT yields.Tfit increases with mass consistent with the collective transverse expansion
1/[exp(mT/Tfit)±1]
Gábor I. Veres
Model Comparisons
Event generators are not able to consistently describe low pT yields.HIJING overpredicts all yields
Gábor I. Veres
I. Low pT: Summary
No enhancement of low pT yields is observed (compared to extrapolations of intermediate pT spectra).
Spectra flatten at low pT transverse expansion Constraints for models and integrated yields.
Centrality dependence of the low pT yields
Negatively charged particle yields
Attempt to measure BE correlations at very low mT
Future (high statistics Au+Au run):
Gábor I. Veres
II. High-pT spectra. Tracking
x10 cm
1
2
By
z
Bea
m
1) find straight tracks in the field-free region
2) curved tracks found in B field by clustering in (1/p, ) space
3) Pieces matched
4) Momentum fit using the full track, and detailed field map
5) Quality cuts, DCA cuts
Very clean track sample with high efficiency
Gábor I. Veres
High-pT spectra: acceptance
Acceptance Momentum resolution
2001 Au+Au run (200 GeV): Data Sample:
7.8 M minimum bias Au+Au events (2004: over 200 M)
32 M reconstructed particles
Gábor I. Veres
Data+MC
Npart
Triggering on Collisions & Centrality
HIJING + GEANT Model of paddle trigger
Data
Centrality Determination
% (dNch signal) % (Npart, Ncoll, b)
3% uncertainty in TOT (trigger efficiency)
less than 10% uncertainty in Npart for Npart>100
Paddle Signal (a.u.)
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“Participant” Scaling
Ncoll= # of NN collisions: ~A4/3
L~A1/3
Npart/2 ~ A
“Collision” Scaling
Why Centrality Matters?
Ncoll
Npart
b [fm]
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PHOBOS-Spectra @ 200GeV Au+Au
Spectra corrected for
Acceptance/efficiency
Ghost tracks
Momentum resolution
Variable bin width
Secondaries, feed-down
At 200 GeV, min. bias p+p reference data exists (UA1)
(Ge
V/c
)-2
0.2<y<1.4
x 10-1
x 10-2
x 10-3
x 10-4
x 10-5
0.2<y<1.4
(h++h-)/2
Phys.Lett. B 578 (2004) 297
Gábor I. Veres
Scaled Au+Au Spectra / p+p-Fit
344 ± 11
276 ± 9
200 ± 8
138 ± 6
93 ± 5
65 ± 4
<Npart>
0-6%
6-15%
15-25%
25-35%
35-45%
45-50%
Centrality
Centrality range:
<b> from 10 to 3 fm
<> from 3 to 6
Phys.Lett. B 578 (2004) 297
Gábor I. Veres
Evolution with Centrality (Au+Au)
gradual change of shape
peak develops at 1.5 GeV/c
Phys.Lett. B 578 (2004) 297
Spectra normalized to a fit to the pT spectrum at Npart = 65 (most peripheral bin)
Low and high pT: approximate scaling with Npart
Gábor I. Veres
Is Suppression an Initial or Final State Effect?
Strong suppression of hadron yields at high pT!
high density strongly interacting matter (final state)? ORmulti-partonic effects in the nuclear wave-function
(initial state)?
Turn off final state to discriminate between the two scenarios d+Au collisions
Gábor I. Veres
Predictions for d+Au
Vitev, Phys.Lett.B 562 (2003) 36Vitev and M.Gyulassy, Phys.Rev.Lett. 89 (2002)
Kharzeev, Levin, McLerran, Phys.Lett.B 561 (2003) 93
“~30%suppression of high pT particles”(central vs peripheral) 16% increase central vs peripheral
Parton Saturation(initial state)
pQCD (final state)
Gábor I. Veres
PHOBOS Results from d+Au
Centrality <Npart> <Ncoll>
70-100% 3.30.7 2.20.6 40-70% 6.70.9 5.40.8 20-40% 10.90.9 9.70.8 0-20% 15.51.0 14.60.9
Cronin Effect in d+Au vs. Centrality
6% mostcentral Au+AuPhys.Rev.Lett. 91, 072302 (2003)
peripheral
central
PRL 91, 072302 (2003)
Gábor I. Veres
No high-pT suppression at y~0 in d+Au
Initial state effects may show up at HIGHER rapidities?!
(small-x region of the Au nucleus is probed)
Cronin Effect as a Function of (d+Au)- all centrality bins together -
Cronin Effect as a Function of (d+Au)- all centrality bins together -
Evolution of RdAu with
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II. Inclusive pT spectra: Summary
Charged hadron spectra measured in d+Au and Au+Au collisions vs. pT and centrality
High-pT suppression in Au+Au observed (compared to Ncoll scaling)
Control experiment: d+Au spectra
Suppression is not an initial state effect (strongly interacting quark-gluon liquid?)
Latest findings show suppression at high rapidities in d+Au!
Gábor I. Veres
Antiparticle/particle ratios as a function of Npart and pT
Identified particle spectra
III. Identified Hadrons
Flavour dependence of the effects shown Baryon transport in small and large systems Properties of the system at chemical freezeout Scaling features of different species (mT)
Important to compare more elementary (d+Au) and heavy ion (Au+Au) collisions
Motivation:
Gábor I. Veres
pT (GeV/c)0.05 0.5 5.0
Stoppingparticles dE/dx TOF
Particle ID from low to high pT
PHOBOS PID Capabilities
1 2 3 4 50p (GeV/c)
30
40
50
60
70
1/v
(ps/
cm)
0 5 10 15 20 25ETOT (MeV)
0
1
MP (
10-3
Ge
V2/c
m)
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z
-x
10 cm
y
70 cmReversible 2T magnetic field
Two symmetric spectrometer arms
Independent measurements Acceptance & efficiency corrections cancel
B=2T
Antiparticle to particle ratios in Au+Au
Careful corrections for feed-down, absorption in the material, secondaries
Gábor I. Veres
<–>/<+> = 1.025 ± 0.006(stat.) ± 0.018(syst.)<K–>/<K+> = 0.95 ± 0.03(stat.) ± 0.03(syst.) <p>/<p> = 0.73 ± 0.02(stat.) ± 0.03(syst.)
Au+Au sNN = 200 GeV, 12% most central
High precision measurements
Corrections to the measured ratios : +3.7% absorption +0.7% secondary negligible -1.2% feed-down
p/p K-/K+
-/+
Result: ratios at sNN = 200 GeV Au+Au
Phys.Rev.C 67, 021901R, 2003
Gábor I. Veres
d+Au: =Ncoll
/Npart
d
Particle Ratios Using dE/dx PID
Submitted to Phys.Rev.C, nucl-ex/0309013
PHOBOS 200 GeV
Mean number of collisions per projectile nucleon <>
...p/p Compared to Models (p+p, d+Au)
200 GeV
PHOBOS Preliminary
Mean number of collisions per projectile nucleon <>
Gábor I. Veres
Particle Ratios Using dE/dx PID
PHOBOS 200 GeV
Subm. to Phys.Rev.C nucl-ex/0309013
Au+Au: =Ncoll
/(Npart
/2)d+Au: =N
coll/N
partd
Mean number of collisions per projectile nucleon <>
Gábor I. Veres
Identified spectra in d+AuOnly ToF wall can identify above 1 GeV momentum in PHOBOS
Many experimental challenges to solve
1.) high pT-reach desired
2.) high collision rate (10-100 kHz)
and low multiplicity in d+Au, p+p
x150 – x500
improving TOF RESOLUTION:
new start time detector
increased distance from interaction point
improving STATISTICS:
new high-pT trigger system (x15 – x50)
DAQ upgrade (x10)
Gábor I. Veres
SPECTRIG
T0
T0
mini-pCal
pCal
Moved TOF walls far (5 m) from IP New, on-line high pT Spectrometer Trigger
New start-time (T0) Čerenkov detectors On-line vertexing and ToF start time
Forward proton calorimeters on Au and d sides DAQ upgrade (x10 higher rate!)
Response to Importance of High
PT Studies
Upgrades in PHOBOS for the d+Au run (2003)
Au+Au
d+Au, p+pTOFTOF
Gábor I. Veres
Trigger detectors (d+Au)
Segmented scintillator detectors at 45 and 90 degrees from beam line
Combined with the ToF walls:
selects events with particle hitting ToF and SpecTrig walls enhances high-pT (straight) tracks: “online” tracking decision-making in 50 ns
SpecTrig
ToF
rejectedaccepte
d
Gábor I. Veres
High statistics d+Au track sample
1 2 3 4 50
p (GeV/c)
30
40
50
60
70
1/v
(ps/
cm) p
K
positives, 1.6<p<1.8 GeV/c
p
K
Particle/Antiparticle Ratios using the TOFd+Au
T
per projectile nucleon <>
Identified pT -spectra in d+Au
Not feed-down corrected
Scale uncertainty: 15%
Particle Composition in d+Au
Not feed-down corrected
Comparison: Low Energy d+Au
Cronin, PRD 11, 3105 (1975)-0.1< <0.2y
y0.2< <1.2-0.5< <-0.2y
lab=3.26
Not feed-down corrected
Identified mT-spectra in d+Au
Scale uncertainty: 15%m
T=m +p
T
2 2 2
Not feed-down corrected
Identified mT-spectra at 200 GeV
Subm. to Phys.Rev.Lett.nucl-ex/0401006
d+Au
Scale uncertainty: 15%
Not feed-down corrected
Au+Au
Spectra normalized at 2 GeV/c
200 GeV
2
PHOBOS Preliminary(no feed-down corrections)
Identified mT-spectra at 200 GeV
d+Au
Scale uncertainty: 15%
Not feed-down corrected
Au+Au
Spectra normalized at 2 GeV/c
200 GeV
Subm. to Phys.Rev.Lett.nucl-ex/0401006
Gábor I. Veres
III. Identified Hadrons: Summary
PHOBOS has PID coverage from 0.03 to 3.5 GeV/c in pT
First PID ratios and spectra shown from the PHOBOS TOF
Surprisingly small centrality dependence of p/p ratios
Approximate mT scaling in d+Au as opposed to Au+Au
Particle composition in d+Au: similar pT-dependence to lower energy data (but different overall proton fraction)
Gábor I. Veres
OutlookFuture of the PHOBOS hadron spectra program:
Further RAA measurements
Energy scan (63 GeV, …?)
Species scan (Cu+Cu ? Si+Si ?)
Identified spectra in p+p and Au+Au at 200GeV
Charged spectra at very low pT with charge separation
…and many other observables besides spectra!
Gábor I. Veres
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