Peter Steinberg

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Some RHIC Advice for LHC Day-1 Physics

• Peter Steinberg
• ATLAS Week _at_ CERN 14 February 2005

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Evolution of Heavy Ion Collisions
InitialCollisions
pQCDinteractions
Hadronization
Initial Nuclei
AGS (2-5 GeV)
SPS (6-20 GeV)
RHIC (20-200 GeV)
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The News From RHIC
STAR AuAu 200 GeV
Hot, Dense Matter
Color-opaque ideal fluid
Color glasscondensate
These are properties inferred from high-pT
observables. What do we learn from just the bulk,
i.e. not from probes
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The Underlying Event
UA1 pp 900 GeV
STAR AuAu 200 GeV
Enormous change in multiplicity What are the
qualitative differences? How do we build a
heavy ion collision?
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Geometry vs. Dynamics
?sNN/2
?sNN/2
Soft Physics Hard Physics
Non-perturbative Perturbative
Long-distance Short-distance
Participants Binary Collisions
Baryon Stopping Parton-parton collisions
Hydrodynamics Jet quenching
Thermal Freeze-out Fragmentation
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The Bulk of Particles dN/dh
h
PHOBOS
RHIC program made sure at leastone experiment
had near-4p coverage Upgrades have
extendedrange of all detectors
PHOBOS_at_RHIC
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What Do Multiplicities Teach Us?
Parton distributionsNuclear Geometry Nuclear
shadowing
0 fm/c
Parton production reinteraction
2 fm/c
Chemical Freezeout Quark Recombination
Jet FragmentationFunctions
7 fm/c
Hadron Rescattering
Thermal Freezeout Hadron decays
gt7 fm/c
Independent stages Bulk physics integrates time
history
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What do Multiplicities Teach Us?
PHOBOS White Paper, nucl-ex/0410022
Even central events only can exclude
certaindynamical contributions
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Extrapolating to the.LHC
Without additional input, would extrapolatedN/dh
in AA linearly to LHC energies 7
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Soft vs. Hard Contributions
Collisions
b
Participant
Does the reaction factorize into hard and
soft?
hard should be amplified by increasingnumber
of collisions energy
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Soft vs. Hard Contributions
Collisions
b
Participant

• Factorization into hard and soft?

• Centrality dependence at h0 is energy
  independent

? Apparent factorization into energy geometry
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Centrality Evolution in 4p
Shape changes dramatically with centrality
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Participant Scaling

• p(d)A Wounded nucleon model. Nch Npart x pp
• In AA linear with Npart, but not pp
• Non-trivial correlation over full phase space
• Similar scaling not seen at mid-rapidity

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Limiting Fragmentation
PHOBOSPRL 91 (2003)
dN/dh energy-independent in rest-frame of
projectile Centrality-dependent limiting curve
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Limiting Fragmentation Ubiquitous
ybeam ? ln(?s/Mp) Mp 1GeV
yjet ? ln(?s/Mj) Mj 1GeV
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The Shape of the Underlying Event
ee- ?hadrons
pp ?hadrons
AA ?hadrons
Central events have similarshape to ee- pp
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Universality of Total Multiplicity
ee- ?hadrons
pp ?hadrons
AA ?hadrons
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What about pp?

• In pp collisions, have leading particles
• Flat distribution of
• independent of energy
• Only ½ of energy available for particle production

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Bulk Observables not messy

• Rather, they are highly constrained as a function
  of energy and centrality
• Linear scaling with Npart
• Limiting fragmentation
• Ntot scales like ee- (and pp at ?s/2) above SPS
  energy
• Reduced leading particle effect
• Baryon density suppresses entropy at lower
  energies
• All of these could be accidents
• Difficult to integrate these observables without
  a theoretical framework

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Some Limits of QCD
1/x
ColorQuantumFluid
ColorGlassCondensate
NPQCD (lattice)
pQCD
Hydrodynamics!
Q2
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Strongly-Coupled Matter
On the lattice, onlyreach 75-80
ofStefan-Boltzmann limit
In context of N4 SUSY QCDthis is the signature
ofa strongly-coupled plasma(Klebanov et al)
May point to a paradigm shiftQCD does not
predict weakly interacting QGP for accessible
T Is there strongly-coupled matter in AA
collisions? (RBRC Workshop May 14-15, 2004 RHIC
Whitepapers)
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Transverse Dynamics
130 GeV
Peripheral collisions show elliptic flow
Reasonable agreement with boost-invariant ideal
hydro in more central events
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Longitudinal Dynamics

• Strong interactions are very strong
• Short range in space ? long range in rapidity
• Complete stopping? explosion

• Strong interactions are relatively weak at tlt1
  fm/c
• Coulomb cross sections12 1/s12 exp(-(y1-y2))
• Partial stopping
• plateau in rapidity two fragmentation regions

Carruthers Duong-van 1973
ISR 53 GeV PISA/SUNYSB 1972 (unpub.)
Energy
y
duck or rabbit?
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Pseudorapidity Dependence
PHOBOSnucl-ex/0406019
19.6, 62,130,200 GeV
Limiting fragmentation works almost too well
for v2(h) Is there really a hydro limit? No
boost-invariance - challenge for 3D hydro
calculations
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Fermi-Landau Approach

1. Early thermalization
2. Blackbody EOS
3. Multiplicity formula
4. Npart (volume) scaling

Nch2.2s1/4(Carruthers)
(Landau- Fermi)
Evolution fromenergy packed into
Lorentz-contractedvolume thermalized
?s
Multiplicity evolutionresults from huge e
R/?s
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Gaussian Rapidity Distributions
Compilation by C. Blume, NA49SQM2004, Cape Town
Landaus hydrodynamics predicts magnitude (in
1950s!) (trend appears to be slightly different
than expected)
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Total Width ? Limiting Fragmentationfrom
Landau Hydrodynamics
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Total Width ? Limiting Fragmentationfrom
Landau Hydrodynamics
Clearly the limiting curve goes non-linear ?
difficult to extrapolate directly from RHIC data
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Using Universality
JETSET
PYTHIA(_at_ ?s/2)
dN/dh 8000 corresponds to 40! 3000 correspoinds
to 15
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For Ntot as well
JETSET PYTHIA well below Landauso maybe
midrapidity will be larger
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Heavy Ion Models
(central)
Agreement is interesting very different
rapidity distributions
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What Can We Do with ATLAS?
Calorimetry
Tracking
R
?-chambers
Barrel
Diffraction/Proton Tagging Region
EndCap
RP
Tracking
ZDC/TAN
FCAL
LUCID
TAS
y
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Luminosity for Soft Physics
Many RHIC results have been made withlow
integrated luminosity LHC Day 1 1025 x
1110-24 100 Hz Sufficient for multiplicity
soft spectral measurements
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Forward Coverage
Unfortunately,no LHC experimentachieves full
forwardcoverage overlappingwith RHIC.
Tracking Cal
FCAL
Due to theoreticaluncertainties, difficult to
extrapolatelimiting fragmentationall the way to
h0at the LHC.
Backgrounds incalorimeters may be challenging
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PHOBOS Backgrounds

• GEANT Study with HIJING _at_ 130 GeV
• Large backgrounds can be subtracted on average
  (even up to 30-40)
• Fluctuations a more difficult problem

Reconst.
dN/dh
True
130 GeV AuAu
h
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A Day-1 Program

• Magnet off
• Focus on Inner Tracker (no TRT) calorimeters
• No need for curved tracks (PHOBOS used 2-point
  tracklets)
• Bulk observables
• Centrality measures
• dN/dh and Nch in pp and AA vs. Npart
• Integrated elliptic and directed flow vs. h
  (Wosiek)
• ET measurements also useful
• Physics payoff
• Discriminate between dynamical models
• Study relationships (e.g. universality) between
  pp and AA?
• Ideally, we can get higher energy pp
• Does boost invariance emerge at LHC Bjorkens
  revenge?
• Insight into relevance of hydrodynamic approaches
  (rapidity distributions elliptic flow)
• Gain insight into collective longitudinal
  dynamics?

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Extra Slides
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Dynamical Models vs. Data
HIJING Hard Soft
ParticleDensitynear y0
Large variation inpredictions
RHIC Data
Hadron Transport
pp Data
Compilation by V. Topor-Pop
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Centrality Dependence vs. pT
Are particles gained goingfrom peripheral to
central?
Are particles lost goingfrom central to
peripheral?
Whatever the case, itappears to be
energy-independent(PRL 62.4 200 GeV)
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Total Multiplicity _at_ LHC
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Extrapolation Games
(Mueller 1983)
pQCD Landau agreefor current data? win-win
for LHC
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Adding in pp data
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Comparisons of AA vs. ee-/pp
Similar trend at mid-rapidity
Broad agreement inangular distributions
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Fragmentation Functions in ee-
RPP2002
DGLAPevolution
DGLAP analysis sufficient for multiplicity in
ee-
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pQCD ? Limiting Fragmentation

• Theoretical analysis from 1989 on rapidity
  distributions in jets
• Gaussian rapidity distributions (s?ln(s) ?
  Landau-like)
• Limiting fragmentation (x-scaling)

K. Tesima, ZPC4743-50,1990
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Feynman Scaling in pp ee-

• Yields invariantwith xF (or xp)

Cooper Schonberg 1973
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Statistical Models and ee-/pp
F. Becattini, hep-ph/9701275

• Statistical models also describe ee- and pp
• Temperature constant vs. ?s
• Typically baryochemical potential mB assumed to
  be 0

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Net Baryon Density in AA
ee-
Net baryon density (p-p) at mid-rapidity
increaseswith decreasing beam energy (as pions
decrease) ? Buildup of a baryon chemical
potential mB Nuclear Stopping ? mechanism not
understood
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Baryon Density Suppresses Entropy
PAS, J. Cleymans, et al (Preliminary)
Thermal-Statistical phenomenology predicts
suppressionof entropy density in presence of
large mB