Searching for the Quark Gluon Plasma with PHENIX

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Searching for the Quark Gluon Plasma with PHENIX
Sarah Rosendahl Division of Experimental
High-Energy Physics
FFD student seminar series contribution
2004-04-21
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OUTLINE

• Quark-Gluon Plasma where-abouts
• What is QGP?
• Why search for it?
• Experimental signals of the QGP
• RHIC and PHENIX
• Some PHENIX Results
• What lies ahead?
• Summary

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The Big Bang and the QGP

• Quark-Gluon Plasma (QGP) is thought to have
  existed shortly (lt?10-6s) after the Big Bang (BB)
• ...when it was still too hot and dense to form
  hadrons

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QGP Phase Diagram

• QGP may form at high temperature and/or density
• Big Bang, neutron stars, and RHIC (AuAu
  collisions)
• Boundaries between nucleons get dissolved
• Characteristics of RHIC collisions
• T above 175 MeV (1.5x1012K) and very small
  baryon density
• Closer to BB characteristics than earlier
  experiments

BB
RHIC
Big Bang and RHIC
Neutron stars
Normal nuclear matter
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BUT WHY..?

• Big Bang in all its glory, but is that the only
  thing were chasing after...?

• QCD complex and not fully understood
• why do quarks need to form colour neutral bound
  states?
• why is chiral symmetry spontaneously broken?
• high-energy hadron collisions needed to study
  quark and gluon interactions (hard scatterings)
• nucleus-nucleus collisions needed to study the
  lattice QCD prediction about a new state of
  nuclear matter the QGP

• Why talking about Big Bang at all?
• phase transition in BB from QGP to hadron gas
  but goes both ways can show BB theory at early
  times is ok
• and - pinning down QGP thermodynamic properties
  (if it can be proven to exist) and decay
  products, upon expansion, would contribute key
  data to astrophysics studies.

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QGP

• Differences between QGP(BB) and QGP(m-Bang)..?
• QGP in Big Bang Dt ? 10-6 s
• If QGP (or hot dense matter) at RHIC Dt ? 10-22
  s will thermal equilibrium be reached?
• Deconfined quarks lead to colour charged medium
• compare to ordinary plasma that is
• electrically charged

• Effects on particle production?
• Important as QGP cannot be studied directly
• Many ideas, but...
• ..so, it is necessary to study as many
  observables as possible...

Hadron gas
QGP
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QGP SIGNALS

• Hard probes and soft probes...

• Soft production from hadron interactions etc.
• Scales with the number of participants
• Hard scattering parton-parton interactions
  (high pT)
• Jets, direct photons...
• Scales with number of binary collisions

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QGP SIGNALS

• Some particle observables...
• J/y suppression
• in QGP the ordinary vacuum is melted
• colour screening hinder the quarks from binding
• however... there are also predictions of
  enhancement
• Mass shifts in e.g. F
• chirality restoration makes quark masses
  decrease
• F(1019MeV) -gt KK- (987MeV)
• Strangeness and charm enhancement
• enhanced probability of finding heavy quarks due
  to gluon collisions
• Jet quenching
• jets passing through the medium are suppressed

Individual J/y
QGP
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QGP SIGNALS

• Some global observables...
• Collective flow
• magnitude of anisotropic flow is sensitive to
  the degree of thermalization at the collision's
  earliest moments
• Particle spectra
• gives information on the temperature and
  thermalization process of the system
• pT fluctuations
• near a tri-critical end-point in the QCD phase
  diagram, event-by-event fluctuations in average
  pT could increase significantly
• Charge fluctuations
• reduced net charge fluctuations in plasma due to
  fractional electric charges of quarks and more
  evenly spread charges

Flow collision slightly off-center ? pressure
gradients perpendicular to reaction plane during
expansion ? anisotropic distribution of final
particles with respect to reaction plane
Henrik Tydesjö 5/5!
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RHIC

• Relativistic Heavy Ion Collider (RHIC)
• _at_Brookhaven National Laboratory (BNL)
• _at_Long Island, New York
• satellite photo taken in the 80's when the ring
  was under construction

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RHIC

• RHIC ring 4 experiments
• Pioneering High Energy Nuclear Interaction
  eXperiment (PHENIX)

• 3.8 km circumference
• AuAu, dAu and pp
• Top energies (each beam)
• 100 GeV/ nucleon AuAu.
• 250 GeV polarized pp.

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WHY dAu?

• Control experiment!
• Initial state effects like color glass
  condensate (gluon saturation in relativistic
  nuclei)
• should be visible in dAu as well as AuAu!
• Final state effects medium effects like QGP
• only visible in nucleus-nucleus collisions!

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RHIC

• More RHIC facts
• beam speed 99.995 of c
• the beam consists of 57 bunches each
  consisting of billions of nucleons
• thousands of collisions every second (lots of
  data...)
• T gt 1012 K, Ecm 200 GeV
• ..but as ions are so small it is about the same
  force of impact as that of two mosquitos
  colliding
• less than 1g gold in 20 years
• 1740 superconducting magnets to keep beam in
  place
• ..cooled with liquid helium to 4.5 K
• PHENIX weighs 3000 ton

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THE PHENIX DETECTOR

• Central magnet and muon magnets
• Two spectrometer arms
• DC, PC, TEC (tracking)
• RICH, EMC, TOF (PID)

• Two muon arms
• MuID, MuTr
• Central detectors
• MVD, BBC, ZDC

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COLLISION SIMULATION

• Lorentz contracted gold nuclei at 99.995 of c
  collides head on
• ..producing thousands of particles

• RED initial nucleons
• BLUE created baryons
• YELLOW mesons

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EVENTS IN PHENIX

• AuAu central event with no magnetic field
• lines are reconstructed tracks pointing back to
  vertex
• dAu with the magnet on

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CENTRALITY

• A correlation between ZDC energy sum and BBC
  charge sum is used
• Low ZDC (few spectator neutrons) high BBC
  signal (many participants) is a central event
• Low percentage means a central event (the 5
  most central events are in the red region)

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JET QUENCHING

• Hard scatterings (HS) produce jets of particles
• In the presence of a colour-deconfined medium,
  the partons strongly interact losing much of
  their energy via gluon radiation
• HS near the border of the collision zone can
  then lead to a jet in only one direction
• The away-side jet is quenched.

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JET QUENCHING RESULTS

• dAu similar to peripheral AuAu

• Away side jet strongly suppressed in central
  AuAu events

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NUCLEAR MODIFICATION

• Nuclear modification factor
• RAA 1 Scale with Ncoll
• RAA gt 1 Cronin effect
• RAA lt 1 Suppression (at pT gt2GeV/c)

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HIGH pT RESULTS

• Centrality dependence (all hadrons p, K, p..)

• Central events show suppression in AuAu
• Enhancement (Cronin effect) in dAu

High pT suppression is clearly a final state
effect!
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HIGH pT RESULTS

• RCP (ratio of central to peripheral yield)
• only mesons are suppressed in central events
• strangeness has no effect

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J/y RESULTS

• J/y ? ee-, m m- observed
• pp

• dAu (m m-)

Focus on J/y studies in data from current, full
luminosity AuAu run!
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WHAT LIES AHEAD?

• PHENIX Run 4 AuAu at Ecm 200 GeV finished
• full luminosity and complete detector system
  makes studies of J/y and other rare signals
  possible

• Analyses to come...
• Direct photons as a control and jet-g
  correlations
• J/y, (y', ?) yields
• F yield
• More on jets (where did the energy/momentum
  go?)
• More on flow
• More on everything... ?
• More AuAu runs (until at least 2007...)

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WHAT LIES AHEAD?

• PHENIX was designed to find the QGP the future
  experiment ALICE at LHC is designed to explore
  its characteristics (will be finished in 2007)
• Weakly interacting
• probes become
• accessible

• pp collisions
• Lmax 5x1030 cm-2s-1
• Event rate gt1 kHz

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THE ALICE DETECTOR
Forward detectors PMD, T0, V0, ZDC, FMD
Specialized detectors HMPID, PHOS
MUON Spectrometer
Central tracking system ITS TPC TRD TOF
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COLLISION SIMULATION
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SUMMARY

• The search for the new phase of matter the
  quark-gluon plasma - is on its way with

PHENIX_at_RHIC (and the other RHIC experiments...)

• It might have been found (jet quenching
  results!), but more than one observable need to
  show the right characteristics
• J/y suppression is one signal that is going to
  be investigated now with the more statistics
• Others are
• ...too many... sigh

• ALICE_at_LHC
• will study QGP in more detail using PbPb
  collisions with start in 2007 (if it goes as
  planned)

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COLLISION SIMULATION

• Not moving pictures are also good.