Heavy Ion Physics: the ALICE program

1
Heavy Ion Physicsthe ALICE program
Preliminary version

• Raimond Snellings

1. Physics motivation and the focus of our
group 2. The NIKHEF hardware contribution to
ALICE 3. Current status and our ambitions at
NIKHEF
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QCD at extreme conditions

• Lattice QCD predicts a phase transition to a
  quark gluon plasma at energy densities of about 1
  GeV/fm3 and at a temperature of about 170 MeV
• The quark gluon plasma is a state of matter
  expected to have existed in the early universe
  about 1 microsecond after the Big Bang
• Heavy-ion collisions provide experimental access
  to the properties of QCD matter at extreme
  temperature and density (the equation of state at
  the QGP phase transition and in the QGP phase)
• Spontaneous chiral symmetry restoration
• The origin of our mass
• deconfinement
• The building blocks of QCD, quarks and gluons,
  become quasi free

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The focus of our group

• The properties of the QCD Equation of State above
  Tc
• dp/de calculable in lattice QCD
• observables collective motion of low transverse
  momentum particles as function of mass
• The color density of hot and dense QCD matter
• Induced soft gluon radiation by partons
  traversing the medium
• observables medium modifications of jets and
  heavy particle production

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Heavy ion physics needs a reference

• QGP properties are calculable from first
  principles in lattice QCD
• However currently our observables are not
  completely calculable from first principles (i.e.
  contributions from cold hadronic matter)
• A reference measurement is needed and can be
  provided by elementary collisions (pp and pA)
• pA certainly not before 2010
• Or by collision geometry
• Centrality dependence
• Azimuthal dependence

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Non central A-A collisions
Non central collisions break the azimuthal
symmetry! Observables, like the collective motion
and the medium modification of jets, become
azimuthally dependent. These are currently
studied at STAR by our group
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Azimuthal dependence of particle yield (elliptic
flow)
Phys.Rev.Lett.86402-407,2001 e-Print Archive
nucl-ex/0009011 TOPCITE 100 Cited 265 times

• Strong elliptic flow observed at RHIC
• Agreement with hydrodynamic model calculations
  for non-peripheral collisions
• Mass dependence shows sensitivity to the EoS,
  heavy mass particles are particularly sensitive
• Day 1 measurement

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Big impact!
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Parton energy loss in hot and dense matter
Radiated gluons decohere due to multiple
interactions with the medium This energy loss
depends on the path length and gluon density at
the early phase
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High-pt azimuthal correlations

• Clear back to back azimuthal correlation in pp
  and dAu collisions
• Disappearance of the back to back correlation in
  central AuAu collisions
• Color density more than 50 times larger than in
  cold nuclear matter!

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Jets versus the reaction plane

• Energy loss dependence on path length!

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The analysis of elliptic flow and jet
correlations are closely connected

• Elliptic flow and jets, both sources of azimuthal
  correlations between the particles
• Azimuthal correlations due to jets need to be
  understood in order to study flow
• Azimuthal correlations due to flow need to be
  understood to study jets

• At large transverse momenta largest contribution
  to azimuthal correlations still due to elliptic
  flow
• After flow correction jet like signature clearly
  visible

• Sophisticated analysis of multiparticle
  correlations allow to disentangle the flow
  component from the jets

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The QGP observables we study versus the reaction
plane in ALICE

• Collective motion of low pt particles versus the
  reaction plane (elliptic flow)
• Test of quark gluon plasma Equation of State
  properties, dp/de (calculable in lattice QCD)
• Order of the phase transition
• Open charm particularly interesting test if
  heavy masses participate in the hydrodynamic
  behavior
• Jet correlations versus the reaction plane
• Detailed test of medium induced parton energy
  loss, jet quenching mechanism (length and gluon
  density dependence)
• Open charm particularly interesting detailed
  test of jet quenching mechanism (dead cone effect)

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Why heavy-ions at the LHC?
SPS(17) RHIC(200) LHC(5500)
dNch/dy 400 700 3000-8000
eGeV/fm3 (t0 1 fm/c) 2.5 3.5 - 7.5 15 40
Vf fm3 103 7103 2104
tQGP fm/c 1 1.5 4 4 10
t0 1 0.5 0.2

• Larger, longer lived QGP phase
• Observables get largest contribution from the QGP
  phase
• Higher energies provide access to abundant hard
  probes (high-pt jets, charm, ..)

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Calculated elliptic flow and the QGP properties
at the LHC

• (black line) QGP contribution to the observable,
  increases with colliding energy
• (red dots) total observed signal QGP hadron
  phase
• At the LHC about 80 of the integrated flow
  signal generated in the QGP phase!

Hirano, private communication
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The best suited detector at the LHC for
heavy-ions ALICE

• Ideally suited for these correlation with the
  reaction plane measurements
• Full azimuthal coverage
• Particle reconstruction and identification from
  100 MeV/c to tens of GeV/c
• The key detectors are the TPC and the ITS (with
  the NIKHEF SSD contribution)

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The Alice ITS

• Main vertex 15 µm in central PbPb
• Vertex charm, strange decays 50 µm
• ?p/p (pTgt1 GeV, with TPC) 14-gt3
• Particle ID (dE/dx)

• Strong contribution to outer layers (SSD)
• project leader SSD (6 labs)

• FE module

• Support and cooling

• Endcap

• ADC

SSD

• DAQ

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NIKHEF ALICE hardware activities (SSD)

• Design of SSD support (with Turin)
• Design ladder frames (with St. Petersburg)
• Design of SSD cooling system (with CERN)
• Design of front-end modules (with Kharkov and
  Strasbourg)
• Design ladder cabling (with Kharkov)
• Design SSD cabling (industrial production)
• Design and production of front-end module test
  equipment
• Design and production of EndCap electronics
• Design and production of read-out modules
• Ladder assembly (with Nantes)
• Final SSD assembly
• Bottom line ITS project on schedule and NIKHEF
  SSD contribution will finish on time (2006)

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ALICE group current manpower

• Utrecht and NIKHEF Amsterdam
• Amsterdam manpower
• Staff physicist 3
• PhD students 2
• Amsterdam infrastructure
• Fraction of mechanical and electronics workshop
• Ladder assembly room
• Utrecht manpower
• Staff physicist 4
• Post-doc 1
• PhD students 5
• Students 2
• Utrecht infrastructure
• Fraction of the faculty mechanical and
  electronics workshop
• mechanical and electronic workshop of the SAP
  department (4 fte)
• Assembly room

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ALICE group current physics activities

• Have strong role in STAR EMC analysis
• 1 fte staff, 1 post-doc, 3 PhD's (until 2009) and
  2 students
• Had a leading role in correlation analysis with
  the reaction plane in STAR
• Effort is scaled down to 1 PhD (until 2007) and
  0.2 fte staff
• Have a coordinating role in correlation analysis
  with the reaction plane in ALICE (Physics
  Performance Report)
• Effort 4 fte staff and 2 PhD
• Will increase further with 3 PhDs and 1 post-doc

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Summary

• NIKHEF ALICE hardware effort on track for timely
  delivery!
• NIKHEF had and has a big impact in STAR physics
  program
• Important preparation for the ALICE physics
  program
• NIKHEF has a strong effort in physics analysis in
  ALICE,
• observables identified which test the initial
  gluon density of the created system and the QCD
  Langrangian at the phase transition and in the
  quark gluon plasma phase
• observables have in common correlations with the
  reaction plane
• observables like elliptic flow and first jet
  correlations with the reaction plane are day one
  physics with big impact
• observables like charm flow and charm energy loss
  provide more detailed constrains and are a longer
  term effort

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Extra
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ALICE neutral Kaon flow
E. Simili

• Full simulations, using charged particle tracks
  from ITS and TPC to determine reaction plane and
  calculate neutral Kaon elliptic flow

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Flow in non-central collisions elliptic flow

• Different flow in and out of the reaction plane
  the main component elliptic flow
• Unambiguous signature of collective motion
• The driving force of elliptic flow dominates at
  early times (self quenching)
• Largest contribution comes from QGP phase
• Large magnitude of elliptic flow signature of
  hydrodynamic behavior (local thermalization -gt
  LQCD)

P.F. Kolb and U. Heinz, in Quark Gluon Plasma,
nucl-th/0305084