Counting%20on%20QCD:%20Multiplicity%20Measurements%20in%20High-Energy%20Collisions

1
Counting on QCDMultiplicity Measurements in
High-Energy Collisions

• Peter Steinberg
• Chemistry Department
• Brookhaven National LaboratoryPhysics Department
  Colloquium
• December 4, 2001

2
Acknowledgments

• Ideas presented here are based on work and
  discussions with
• Mark Baker (BNL), Wit Busza (MIT), Sean Kelly,
  Jamie Nagle (Columbia)and Dima Kharzeev (BNL)

3
Two Faces of the Strong Force
pion
proton
proton

• Fundamental
• Quarks are held together by exchanging colored
  gluons
• V1/r at short distance
• Vkr at long distances
• We say that quarks and gluons are confined in
  hadrons mesons baryons

• Residual
• Hadrons are made of confined quarks and gluons
  with net zero color
• Hadrons interact by exchanging other hadrons
• A substantially weaker, non-confining force (van
  der Waals)

4
Phases of QCD Matter

• We have strong interaction analogues of familiar
  phases
• Nuclei behave like a liquid
• Nucleons are like molecules
• Quark Gluon Plasma
• Ionize nucleons with heat
• Compress them with density
• New state of matter!

5
Lattice QCD calculations

• Gluons are charged photons
• Non-Abelian gauge theory
• Perturbative QCD (pQCD) only applicable at large
  momentum transfer
• Teraflop-scale computers simulate equilibrium
  QCD
• Non-perturbative
• Ising model calculation
• Predict phase transition

QCDSP
(F. Karsch, hep-lat/0106019)
hadrons ?quark/gluon
6
Heavy-Ion Collisions
VNI Simulations Geiger, Longacre, Srivastava,
nucl-th/9806102
1
2
3
4
Colliding Nuclei
Parton Cascade
Hadron Gas Freeze-out
HardCollisions

• Entropy produced as system evolves
• Where does most of it come from?
• Initial, partonic or hadronic stage?

7
SppS Collisions
UA1, 900 GeV
proton
anti-proton
?s 200, 546, 900 GeV
10s of particles
8
RHIC Collisions
?sNN 130, 200 GeV
Gold
Gold
(center-of-mass energy per nucleon-nucleon
collision)
1000s of particles
9
Learning by Counting

• Larger systems produce more particles
• But can carefully counting
• the number of final state particles
• help us understand
• the nature of the entire history of the
  collision?
• Many things to consider
• Geometry how many elementary collisions?
• Interactions how are particles produced?
• Time evolution how do the interactions change?

10
Rapidity

• Hadronic collisions are characterized by limited
  transfer of transverse momentum
• Most particles we observe carry only small
  fraction of (anti)proton longitudinal momentum (x
  pz/pz,max)
• Rapidity variable increases dynamic range
  (xlt.1)
• Lorentz boost changes y by a constant

11
Pseudorapidity

• Rapidity requires complete characterization of
  4-vector
• Conceptually easy, but requires a spectrometer
• Experiments with high multiplicities and limited
  resources use pseudorapidity
• dN/dh related to dN/dy, but not the same,
  especially for slower particles

?
beam axis
12
Pseudorapidity Distributions in pp
UA5, ZPC 33 (1986) / CDF, PRD 41 (1990)
All charged particles
dN/dh
Anti-proton fragmentation region
Protonfragmentation region
mid-rapidity plateau
h
h
1
-1
2
-2
0
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Hard Soft Processes

• Soft processes (pT lt 1 GeV)
• Color exchange excites baryons
• Baryons decay to soft particles
• Varies with number of struck nucleons
• wounded nucleon model
• Hard processes (pT gt 1 GeV)
• Gluon exchange in a binary collision creates jets
• Jets fragment into hadrons, dominantly at
  mid-rapidity

(mini)jet
(mini)jet
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Multiple Collisions with Nuclei

• Nuclei are extended
• RAu 6.4 fm (10-15 m)
• cf. Rp .8 fm
• Geometrical model
• Binary collisions (Ncoll)
• Participants (Npart)
• Nucleons that interact inelastically
• Spectators (2A Npart)
• pA Npart Ncoll 1
• (Npart 6 for Au)
• AA Ncoll ? Npart4/3

b
1200
Ncoll
Npart
400
b(fm)
9
0
18
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Measuring Centrality

• Cannot directly measure the impact parameter!
• peripheral
• central

Spectators
Participants
Zero-degreeCalorimeter
Paddle Counter
Spectators
spectators studied with zero-degree calorimeters,
and participants via monotonic relationship with
produced particles
16
Measurables

• In nucleon-nucleon (NN) collisions, we study the
  height of the plateau
• In AA collisions, normalize by number of
  participant pairs to compare to NN

(mid-rapidity)
17
AA normalized to equivalent NN
PRL 87 (2001)
pp
Central AA
fpp(s)
(CDF/UA5)

• Each effective nucleon-nucleon collision in
  central collisions of nuclei produces 40 more
  particles than pp!

18
dN/dh Theory
2½ years ago, predictions Varied by a factor of
2 Actual RHIC data landssquarely on the low
side! Models that work better have less
contribution from hard processes
Eskola, QM2001
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Predicted Energy Dependence

• Ratio R200/130 less sensitive to overall scale
• Data favors models where hard processes do not
  dominate!

Hard
Soft
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Interlude Energy Density

• Q Is there a connection between height of
  plateau and the energy density (cf. lattice)

Bjorken Estimate
to
R
PRL 87 (2001)
(iff R1.18A1/3 to 1fm/c)
PHENIX finds a constant amount of transverse
energy(ET) per particle, implying
e(200 GeV) 4.6 x 1.14 5.2 GeV/fm3
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Hard Soft, Redux
What about non-central events?
We already expect that charged particle production
can have two components
Fraction from hard processes
proton-proton multiplicity
We can tune the relative contribution by varying
the collision centrality
But is this description unique?.
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QCD Structure of the proton
electron
Q2
proton
xP
xpz/pz,max
Scale breaking

• At large Q2, we probe smaller and smaller partons
  within the proton
• QCD predicts scale-breaking at low-x from gluon
  splitting

Scale invariance
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QCD at very low x
30!
1.5
sea
valence

• Structure functions rise rapidly at low-x
• More rapid for gluons than quarks

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Parton Saturation

• Gluons recombine at a critical density
  characterized by saturation scale Qs2
• Below this scale, the nucleus looks black to a
  probe

• Gluons below x1/(2mR) overlap in transverse
  plane with size 1/Q

t
Scale depends on volume(controlled by
centrality!)
Colored Glass Condensate
McLerran, Venugopalan, Kharzeev, Dumitru,
Schaffner-Bielich
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Two Component vs. Saturation
PHOBOS, nucl-ex/0105011 / PHENIX, PRL 86 (2001)

2C
Kharzeev/Nardi PLB507, 2001
UA5
But there is more information...
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Multiplicity Measurements in 4p
dE/dx
-5.4
5.4
500 keV
60 keV
Single-event display
27
Consequences of Parton Saturation

• Saturated initial state gives predictions about
  final state.
• Nh c x Ng

Kharzeev Levin, nucl-th/0108006
PRL 87 (2001)
m22Qsmr, pTQs , l.25 from HERA F2 data
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Saturation Works at 200 GeV
L. McLerran, DNP 2001
h
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Implications of Saturation

• Saturation models describe the data
• Initial state parton density might be high enough
  to reach saturation regime
• Large initial-state energy density
• Even larger than Bjorken estimate
• 18 Gev/fm3 _at_ 130 GeV (Kharzeev/Nardi)
• Agreement in pseudorapidity (i.e. not y)
• Angular distribution of emitted gluons maps
  directly onto final state hadrons
• Prediction of local parton hadron duality
• LPHD (Dokshitzer, Mueller, Ochs, Khoze, et al)

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Limiting Fragmentation
4
900 GeV
rest frame
3
546 GeV
A
B
200 GeV
2
53 GeV
1
A
A
UA5
0
B falls apart
B _at_ rest
0
-2
-4
-6
h-ybeam
Energy independent

• UA5 observed clear limiting fragmentation in
  pbar-p

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Limiting Fragmentation in AuAu
nucl-ex/0108009

• PHOBOS central AuAu data shows limiting behavior
• Different than UA5 data at 200 GeV
• Not surprising
• Limiting fragmentation should vary with colliding
  system

200 GeV
130 GeV
UA5 200 GeV
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Central AuAu vs. pp at 200 GeV

• pbar-p and AuAu differ only by a scaling of 1.3
  everywhere
• Suggests geometrical picture of NN collisions in
  pp and AA
• Hadronic dynamics?
• What about rescattering?

Peripheral NN collision
Central NN Collision
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Rapidity distributions in ee-
DELPHI, PLB459 (1999)
QCD string fragmentation
q
q
Jet axis
Extends pp, AA correspondence to ee- collisions
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Limiting Fragmentation, Redux
DELPHI, PLB459 (1999)
ee-
AuAu
Limiting fragmentation seen even in ee- but
nothing to fragment except QCD strings
themselves
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What have we learned?

• Rapidity distributions have similar features in
  all symmetric systems
• electron-positron, proton-antiproton,
  nucleus-Nucleus
• Rising to h2, then a plateau to midrapidity
  (h0)
• Limiting fragmentation seems to be the
  fragmenting of the QCD strings themselves
• Universal behavior
• QCD evolution phase space
• Are AA collisions as confusing as we thought?
• Does local parton-hadron duality allow QCD to
  predict even the output of heavy-ion collisions?

36
Tentative Conclusion

• Can we really count on QCD to help explain the
  complicated dynamics of entropy production in a
  heavy-ion collision all the way to freeze-out?
  Yes, if

1
2
3
4
Colliding Nuclei
Parton Cascade
Hadron Gas Freeze-out
HardCollisions
QGP? / Fragmentation
Gentle Freeze-out
Geometry/Saturation
LPHD
QCD
37
UA5 Experiment
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Comparison to pp and models
PRL 87 (2001) forthcoming
Systematic error not shown
Central
AMPT(rescattering)
HIJING
Peripheral
Scaled UA5 200 GeV data
h ? h (Y130/Y200) dN/dh fpp(s)
130 GeV
Ybeam
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pA Rapidity Distributions
NA5 DeMarzo, et al (1984)

• Behavior of AuAu distributions similar to pA at
  lower energies
• Useful to count slow protons to measure
  centrality of pA collision

Target regionRapid rise withnuclear thickness
Mid-rapidity Saturation
Beam region pA is below pp