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.