Peter Steinberg
Systematics of Charged Particle Production in 4 with the PHOBOS Detector at RHIC
Peter A. SteinbergBrookhaven National Laboratory
George Washington University, 16 Nov 2001
Peter Steinberg
Systematic Measurements• Do Nucleus-Nucleus
collisions show collective behavior• Energy (or particle) density • Scaling with centrality• Hard and soft processes
contribute• Rapidity plateau• Effect of initial geometry on
final state
Participants
Spectators
Spectators
pp collisions
pA collisions
We study this with systematics of charged particle production:
Energy, Rapidity, Centrality, Azimuthal angle
Peter Steinberg
Centrality• Nuclei are extended
• RAu ~ 6.4 fm (10-15 m)
• Impact parameter (b) determines • Npart – 1 or more collisions
• Ncoll – binary collisions
• Proton-nucleus:• Npart = Ncoll + 1 (2 = 1+1 in pp)
• Nucleus-Nucleus • Ncoll Npart
4/3
Participants
Spectators
Spectators
pp collisions
pA collisions
b
b
Ncoll
NpartUseful quantities to compareAu+Au to N+N collisions!
Peter Steinberg
Soft & Hard Particle Production• Soft processes (pT < 1 GeV)
• Scales with number of participants• Color exchange leads to excited
nucleons that decay • Create rapidity plateau
• Hard processes (pT > 1 GeV)• pQCD can calculate jet cross sections• Scales with number of binary
collisions• QCD evolution leads to narrower
distribution around y=0
collpppartpp NxnNnxddN
)1(
minijet
minijet
Peter Steinberg
RapidityUseful single-particle observable:
3
3
dpdE
dypdd
pEpEy
Tz
z2
3
ln21
dydn
pdd
T
2
2
0
5
1 0
1 5
2 0
2 5
3 0
-1 -0.5 0 0.5
cos
)(cosddn
0
0 .2
0 .4
0 .6
0 .8
1
1 .2
1 .4
1 .6
-6 -4 -2 0 2 4 6
y
dydn
0
0 .2
0 .4
0 .6
0 .8
1
1 .2
1 .4
1 .6
-6 -4 -2 0 2 4 6
y
dydn
Kinematics:Change of variables
Dynamics:Particle distributions are expected to be“boost invariant”
Peter Steinberg
Pseudorapidity• Rapidity requires complete characterization of 4-vector
Conceptually easy, but requires a spectrometer• Experiments with high multiplicities and limited
resources use “pseudorapidity”
• dN/d ~ dN/dy for y<2. Easily seen from Jacobian (dy = d)
2tanln
TTT dyddN
ymm
dddN
pp
22
2
cosh1
tanh(y)1
-1-5 5y
222 mpm TT where
Peter Steinberg
UA5 Experiment
Peter Steinberg
Energy Dependence in pp• Feynman’s postulate of
boost invariance• dn/dy plateau is energy
independent• Requires F2 ~ 1/x
• Pure parton model!• No QCD evolution
• Violations of scaling at SppS energies
• No plateau!• Models like HIJING can
reproduce this behavior• What about Au+Au
Peter Steinberg
RHIC & Experiments
• Nucleus-Nucleus (Au+Au) collisions up to sNN = 200 GeV
• Polarized proton-proton (pp) collisions up to sNN = 450 GeV
Peter Steinberg
PHOBOS Experiment @ RHIC• Large acceptance to
count charged particles• Small acceptance, high-
resolution spectrometer• Focus is on simple
silicon technology, timely results
Peter Steinberg
PHOBOS Collaboration (Nov 2001)ARGONNE NATIONAL LABORATORY
BROOKHAVEN NATIONAL LABORATORY
INSTITUTE OF NUCLEAR PHYSICS, KRAKOW
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
NATIONAL CENTRAL UNIVERSITY, TAIWAN
UNIVERSITY OF ROCHESTER
UNIVERSITY OF ILLINOIS AT CHICAGO
UNIVERSITY OF MARYLAND
Birger Back, Alan Wuosmaa
Mark Baker, Donald Barton, Alan Carroll, Joel Corbo, Nigel George, Stephen Gushue, Dale Hicks, Burt Holzman, Robert Pak, Marc Rafelski, Louis Remsberg, Peter Steinberg, Andrei Sukhanov
Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak
Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, Michel Rbeiz, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch
Chia Ming Kuo, Willis Lin, Jaw-Luen Tang
Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly,Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs
Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer
Abigail Bickley, Richard Bindel, Alice Mignerey
Peter Steinberg
The full PHOBOS DetectorMid-rapidity Spectrometer
~4 Multiplicity Array
TOF
135,000 Silicon Pad channels: spectrometer + multiplicity
Cerenkov Trigger Paddles
Peter Steinberg
Multiplicity Measurements in 4
-5.4 +5.4
Single-event display
Vertex “tracklets” – 3 point tracks
500 keV
60 keV
dE/dx
Peter Steinberg
Phobos acceptance (zvtx=0)
Peter Steinberg
Measuring CentralityCannot directly measure the impact parameter!
but can we distinguishperipheral collisions from
central collisions?
“Spectators”
Zero-degreeCalorimeter
“Spectators”specpart 2 NAN
Paddle Counter
Can look at spectators with zero-degree calorimeters, and participants via monotonic relationship with produced particles
Peter Steinberg
Centrality Selection
• HIJING predicts paddle signal (3<<4.5) to be monotonic w/ Npart
• Spectator matter measured in ZDC anti-correlates
• Expected if partspec NAN 2
Cut on fractions of total cross section to estimate Npart
Central 6%
Npart~341
Peter Steinberg
• Estimating 96% when really 90% overestimates Npart
• We stop around Npart~100• Species scan might help
Uncertainty on Npart
• Error of fraction of total cross section determined by knowledge of trigger efficiency• “Minimum-bias” still has bias• Affects most peripheral events
% Error on Npart
Npart
Peter Steinberg
Energy Dependence near =0Errors are dominated by systematics
AGS/SPS points extracted by measured dN/dy and <mT>
New data at 200 GeV shows a continuous near-logarithmic rise at mid-rapidity
fpp(s) =
Peter Steinberg
Ratio of dN/d at 200 & 130 GeV 90% Confidence Level
Hard scattering dominant contribution
Limited role of hard scattering
Peter Steinberg
Parton Saturation• Gluon distribution rises
rapidly at low-x• Gluons of x~1/(2mR)
overlap in transverse plane with size 1/Q
• At “saturation” scale Qs2
gluon recombination occurs
• In RHIC Au+Au collisions, saturation occurs at a higher Qs
2 (thus higher x)
Saturation describes HERA data!
3/1222 , AQxxGQQ sAsss Scale depends on volume
Peter Steinberg
Particle Density vs. Centrality
Is this picture unique?…
UA5 (pp)
EKRT
2
2
log82.2
QCD
s
part
QddN
N
22 2~ GeVQs22 3.1~ GeVQs
.~2 constddN
N part
KN
Peter Steinberg
UA5
KN
2C
Two Component Model
collpppartpp NxnNnxddN
)1(
What if we move away from mid-rapidity?
09.0,25.2 xnpp
Peter Steinberg
Pseudo-rapidity Distributions• Using Octagon and Ring
subdetectors• Measure out to ||<5.4• Corrections
• Acceptance• Occupancy• Backgrounds (from MC)
• Systematic errors• 10% near =0• Higher near rings
Back
grou
nd C
orr.
HIJING Simulation
130 GeV: PRL 87 (2001) forthcoming
Peter Steinberg
Consequences of Parton Saturation
4)2/1(
2
22
22211ln
sinh
cosh ys
QCD
ysy
opart
TT
esQyeQe
sscN
ypm
yddN
• Saturated initial state gives predictions about final state.
N(hadrons) = c N(gluons) (parton-hadron duality)
Describes energy, rapidity, centrality dependence of charged particle distributions
Kharzeev & Levin, nucl-th/0108006
m2=2Qsm, pT=Qs ~.25 extracted from HERA F2 data
Kharzeev & Levin, nucl-th/0108006, input from Golec-Biernat & Wüsthoff (1999)
Intriguing! Suggests simple path from initial to final state…
Peter Steinberg
Comparison to pp and models
Peripheral
Central
Scaled UA5 200 GeV data
HIJINGAMPT
(rescattering)
Ybeam
(Y130/Y200)dN/d = fpp(s)
PRL 87 (2001) forthcomingSystematic error not shown
130 GeV
Peter Steinberg
Centrality Dependence vs. • Nch = 4200 ± 420 for central
events• HIJING good to 10%
• Above 3-4 decreases vs. Npart
• “Crossover” not seen in HIJING,• Models with rescattering do
better job
PRL 87 (2001) forthcoming
Peter Steinberg
pA: Rapidity Distributions
• Several new features relative to pp1. Peak of distribution
shifts backwards2. Depletion forward of
beam rapidity3. Cascading near target
rapidity – rapid increase
NA5 DeMarzo, et al (1984)
Peter Steinberg
Centrality Dependence: pA
• NA5 showed ratio of multiplicites produced in rapidity regions in pA vs. pp vs, R = dN/dy|pA / dN/dy|pp vs. (np)• Large enhancement in target rapidities• At central rapidity, ratio seems to saturate to 3 (cf. AQM)• At forward rapidity, energy degradation leads to less particle
production than pp
Peter Steinberg
Limiting Fragmentation
UA5, Z.Phys.C33, 1 (1986)
130 GeV
200 GeV
UA5 200 GeV
• True in central AA• Difference to pp not surprising• Depends on colliding system
• UA5 observation of ‘limiting fragmentation’
- Ybeam = ln xF + ln (MN/pT)
Peter Steinberg
Limiting Fragmentation, contd.• Central A+A is 40% higher
than pp at RHIC energies• At 200 GeV, Simple linear
scaling by 30% agrees (within systematics) over the whole distribution!• Higher pT in A+A vs. p+p should
correct p+p by at least 5%
• Detailed balancing of jets and rescattering in A+A??• Complicates interpretation of
central + fragmentation region in pp and central-AA
Peter Steinberg
Conclusions• Systematics of charged particle production have been
explored by the PHOBOS experiment• Energy, Centrality, Rapidity
• Broad features of particle production are consistent with our previous understanding of hadronic interactions• pp and pA collisions are very instructive• Limiting fragmentation• Change in scaling behavior at high-
• Some mysteries, however• Same shape for pp and central Au+Au
• Theoretical models are assimilating new data• Energy dependence (influence of hard processes)• Parton saturation
Peter Steinberg
Why Rapidity?• Proton-proton cross section dominated by soft processes w/
limited pT • Up to ISR energies, it was observed that
• The energy dependence becomes weak• Transverse and Longitudinal dynamics factorize
• But if y = ½ ln(E+pz/E-pz) , dy = pL/E
• iff we assume F1(x) is constant at low x (NB, dy = x dx)• which is true if structure functions go as 1/x
TTLT
pFxFspxFEpdpd
ddpdE 212
2
3
3
,,
.22
2 constdydBdypdpFd TT
Peter Steinberg
Proton-proton collisions• Fits to Woods-Saxon
• dn/dy=C(1+exp(y-yo)/)-1 ~.59
• High-multiplicity events at low-energy:• shows narrowing effect of jets
Peter Steinberg
Hit counting technique
1. Count hits binned in , centrality (b) 2. Calculate acceptance A(ZVTX) for that event3. Find occupancy in hit pads O(,b) by counting empty to hit channels assuming
Poisson statistics 4. Fold in a background correction factor fB(,b)
dNch
d=hits
O(,b) ×fB(,b)A(ZVTX)
Rings Octagon Rings
Low E
High E
Vertex
Vertex
Spec
Spec
PoissonStatistics
E depositionIn multiplicity detectors for one event.