Investigating the QGP with the ALICE dimuon arm

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Investigating the QGP with the ALICE dimuon arm

• Physics Case Studies

Bruce Becker
SRCNS, July 2004
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outline

• The QGP
• What is it ?
• why are we interested ?
• How can we measure it ?
• Recent developments and interpretations
• AuAu from RHIC
• AuAu in the light of dAu
• ALICE Dimuon spectrometer
• What QGP signals can we measure with her ?
• Heavy meson suppression
• Normalisation
• (event plane )
• Conclusion

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Nuclear matter is a little wierd

• Nuclear matter is difficult to think about
• What does it look like ?
• What does it feel like ?
• Can you eat it ?
• Instead, rely on what the properties of the
  theory and our intuition
• Nuclear matter descriebd by QCD
• BUT QCD is not solveable in a non-perturbative
  way
• Can't describe the nucleus ground state with QCD
• Rely on models and phenomena

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Sounds good, but...

• Where is the evidence from theory ?
• Take a hint QCD is a confining theory.
• MIT Bag Model tries to explain it
  phenomenologically.
• Pressure in a bag of fermions
• Countered by vacuum pressure (fudge)
• In equilibrium, energy of system is
• Minimzing wrt R.
• For QGP
• Gives critical temperature of

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What about the real QCD ?

• Can use the Lattice to investigate the full
  properties of QCD
• Use Lagrangian to calculate partition function
• With N flavours and local guage invariance

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Thermodynamic observables
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Lattice results
Phase transition !
KarschQM '01 (hep-ph/0103314)
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QCD predicts a phase transtion !

• Bag model phenomenalogically predicts phase
  transition.
• Simuations of LQCD show a massive increase in
  number of d.o.f near T170 MeV.
• Depending on nature of the nuclear matter,
  transition is 1st or 2nd order
• How could we see it in the lab (universe ?)
• Smash nuclei !!
• Increase density increase pressure
• Use beam energy to deposit a lot of energy in a
  small volume, increases energy density.

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How could we tell ?

• When asking questions of a state of matter, there
  are 2 ways
• Bulk observables
• Transverse energy
• Thermalisation
• Chemical equilibration, strangeness production
• Collective properties
• Flow
• Effect on (calibrated) probes
• Calibrations have to be done with pQCD should
  be calculable --gt high q2
• Jet quenching
• Heavy meson production/suppression

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Centrality

• More central collissions, more energy deposited.
• Ncoll and Npart calculated with a Glauber Model
• Basic assumptions
• Equal fractions of multiplicity relate to equal
  fractions of cross-section
• Use Glauber to relate Npart , Ncoll to b

PA Ncoll 11 Npart 12
b
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Bjorken model

• How much energy in HI collisions ?
• Bjorken model predicts energy density
• from transverse energy and
• Formation time

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Global Observables

• Thermalisation, equlibration
• Chemical equlibration
• Yields and ratios of particle species
• Thermal equlibration
• Low pT spectra (Bolztmann)
• Application of hydrodynamic flow

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Strangeness production / enhancement

• Strangeness production
• QGP copiuously, via gluon fusion
• Low threshold
• Smaller timescale (1-3 fm)
• Hadron gas hadronic scattering
• higher threshold
• Production rate may be similar...
• But timescale is much longer than 10 fm
• Anti-strangeness production should be increased
• Low energy levels of light quarks already filled
  at finite baryon density
• Fermi energies of light quarks higher than bare
  mass of ssbar pair more energetically
  favourable than qqbar

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Flow definition
Directed Transverse flow
y
3rd flow component (anti - flow)
x
Squeeze out
Spherical flow
Elliptic flow
v2
V2 can be calculated assuming hydrodynamic
transport requires initial thermal equilibrium
!
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Hard probes

• High pT jets
• Deconfined medium with large gluon density
• QCD predicts gluon bremstrahhlung
• High pT jets supressed, only leading partons from
  surface escape
• High q2 probe can be calibrated with pQCD !

Central AuAu _at_ RHIC PbPb _at_ SPS
? deconfined QCD matter
pp, pA High pt (q2) scattering in QCD vaccum
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Heavy meson suppression

• Colour confinement relies on large long-distance
  potential
• Zero-temperature qqbar potential
• As temperature increases nonzero q-qbar
  potential
• Put V into Schrodinger equation, solve some
  states are unbound !
• QGP melts the tightly bound J/Psi Ups mesons !

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Hadronic suppression

• normal (hadronic) suppression can also produce
  a suppression of heavy mesons, through 2
  mechanisms
• Co-mover absorbtion
• Glauber absorption
• Distinguish between them with respect to the
  event plane.

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Ratios of quarkonia

• QGP affects different species of quarkonia
  differently
• Tightly bound lower mass higher TD
• Stepwise suppression pattern !
• hadronic/nuclear effects should affect all
  members of quarkonia family equally
• Use ratios to cancel out nuclear effects !

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Quarkonia in a hot, expanding QGP
Isentropic Bjorken expansion
Entropy proportional to T3
Time at which temperature drops below
dissociation temp
Higher pT mesons feel QGP for shorter time
max pT at which they can be suppressed.
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Temporal and Spatial effects

• Interplay between temporal and spatial effects in
  the suppression of quarkonia
• Suppression greater if feel the QGP for longer
• Lower pT
• Larger QGP

R RPb
R RS
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Probing the nature of the QGP

• Want to know more information, not just if QGP is
  formed, but what the nature of it is
• nature of QGP in this case means description of
  screening mass
• Limiting cases (a la Gunion Vogt)
• Parametrisation from SU(N) Lattice simulations
• Estimation from pQCD (parton gas)

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Predictions for ratios of spectra

• Can use pT spectra to distinguish models of QGP
  formation

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So much for the theory...what happens in the
REAL world !?
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Chemical Equilibration
Statistical model fit to RHIC (STAR) data mB
50 MeV, T Tc
Braun-Munzinger Magestro Redlich Stachel,
hep-ph/0105229.
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Strangeness production
Enhancement of strange baryons
Manzari et. al (QM '02) Nucl Phys A 715
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Flow at RHIC
Bulk PQCD Hydro
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Hard probes

• High q2 processes calibrated with pQCD ! Good
  probes...
• High pT (6-10 GeV !) probes measured well at
  RHIC.
• High PT b2b jets seen in pp collisions not
  present in AuAu !
• Compare pp to AuAu/PbPb etc using so-called
  nuclear modification factor

High pT production calibrated with pQCD
Bulk production (non-perturbative)
?
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Nuclear modification factor RAA
Assuming ncoll or npart scaling...
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Nuclear modification in AuAu
SPS Large enhancement at moderate pT
RHIC huge (x5 !) suppression !

• What causes the large suppression at high pT ?
• Most likely explanation jet suppression
• Especially taken with STAR's b2b high pT results
• But ! Could also be an initial-state effect...

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PHOBOS RAA
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Other interpretations of high pT suppression

• What if jets aren't suppressed, but just
  underproduced ?
• Theoretical limit of high energy pQCD - Colour
  Glass Condensate
• Colour consists of deconfined gluons
• Glass acts like
• solid on short time scales but
• liquids on long time scales
• Condensate gluons have a very high density

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CGC predictions

• CGC is the theoretical limit of high energy QCD
  (aka saturation)
• Initial-state modification, not final state
• Predicts
• Slight suppression of high pT
• particle yield
• monojets from gluon
• fusion
• Explains
• jet suppression
• One-side high pT IAA

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Jets and jet suppression
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Is the suppression initial state or final state ?
Initial state?
Final state?
gluon saturation
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dAu run results - Jets
Away-side jet re-appears !
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dAu results nuclear modification factor - RdAu
PHENIX dAu data
PHENIX AuAu data
Preliminary Data
Final Data
High pT suppression clearly a final-state effect !
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So much for that...what about heavy quarkonia
suppression ?
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Evidence for J/Psi suppression ?
Drell-Yan reference (QGP independent !)
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Heavy meson suppression (2)

• Other explanations, nuclear effects
• Nuclear absorbtion
• Binary scaling
• SPS data fits absorption
• Explains all but most central PbPb at SPS.

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Nuclear effects 2

• An initial state effect
• Modified PDF's in AB c.f nn collisions
• Nucleon vs Nuclear PDFs
• At RHIC dominant mechanism for particle
  production is gluon fusion look at
• Nuclear PDF
• Nucleon PDF

RHIC
SPS
LHC
Anti-shadowing R gt 1 (enhancement)
Shadowing R lt 1 (suppression)
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Nuclear effects 3

• Other nuclear effects, like co-movers used to
  reproduce the SPS data
• All successful to some degree, QGP not the only
  explanation for J/psi in central PbPb.
• Nuclear absorbtion always there, but how what is
  Glauber to co-mover ratio ?

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Looking forward to the LHC...

• Much higher
• Energy density
• Initial gluon density
• Heavy quarkonia production rates
• Jet cross-section
• Particle multiplicity
• Theoretical problems
• Description of initial state (CGC ? Minijet rate
  ?)
• Extent, lifetime of QGP (10fm ? 100 fm ?)
• Nature of QGP (chiral symmetry ?)
• Something of a terra incognita
• (but physicsts love a good mess !)

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ALICE Pour répondre à ce défi
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ALICE is being installed now... (november 2003)
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Dimuon spectrometer
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Observables with the dimuon spectrometer

• Measures muons with good accuracy across wide
  range of pT (0.8 GeV - gt 10 GeV)
• Very efficient trigger good acceptance of rare
  signals
• Good mass resolution (50-150 MeV)
• Heavy quarkonia identification
• Rejection of low pT background.
• Can measure ratios of quarkonia families
• Spatial resolution quite good
• In same phase space as PMD (on opposite side of
  IP)
• PMD gives reaction plane, dimuon spectrometer
• Can do v2 analysis !

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ALICE dimuon trigger

• ALICE dimuon L0 trigger rate is 2kHz for PbPb
  bandwidth limited
• 90 efficient for PbPb highest quarkonia rate
• Largest event sizes
• L0 efficiency drops
• Data rate could be increased for lighter systems
• Efficiency drops
• HLT would increase acceptance
• and saturate available bandwidth
• Reject more low pT bkgnd
• Interesting to study e.g.
• Mid-central PbPb
• ArAr
• Compare to pp at ALICE

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ratios
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Upsilon suppression pattern

• Observing a suppression pattern with centrality
  (a la NA50) is direct evidence of QGP creation
  (sequential melting)
• But ! No well-defined reference ! sDY lt sJy
• Regeneration of Y, Y' is possible from feed-down
  (doesn't happen at SPS)
• Reference has to be independent of QGP formation
• Direct photons ? Too few in detector's phase
  space
• Total B cross-section ? (not well understood and
  large uncertainties at Tevatron
• ??
• Have to study suppression relative to many
  references
• No Golden Reference !

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A full study is necessary !

• As a function of
• Centrality identify suppression or enhancement
  patterns
• System size disentangle normal from
  anomalous suppression
• Quarkonium species Survival probabilities probe
  temperature directly (insensitive to nuclear
  abs.)
• PT disentangle various QGP models
• And relative to
• Open charm, beauty problem of the reference
• Reaction plane seperate Glauber and co-mover
  absorbtion
• Other QGP signals
• NO SMOKING GUN AT LHC !

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conclusion

• Glimpses of QGP at SPS
• Probably created at RHIC many complementary
  signs, but
• Still some tricky questions / surprises
• Extent of CGC ?
• Thermalisation... too fast ? Ubiquity of thermal
  model...
• Quarkonia reference ?
• Etc, etc etc...
• One thing for sure the QGP created at RHIC will
  be well studied by by ALICE !
• Maybe even understood... one day... maybe...

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Azimuthal anisotropy

• Determinig how much anisotropy with event plan
  from PMD

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Heavy meson azimuthal anisotropy

• If heavy mesons flow, they have to thermalise
• Nuclear suppression of quarkonia