High Energy Dilepton Experiments

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High Energy Dilepton
Experiments
Experiments _at_ RHICFuture Outlook
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RHIC

• RHIC Relativistic Heavy Ion Collider
• located at Brookhaven National Laboratory

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RHIC and its experiments

• whats so special about RHIC?
• its a collider
• no thick targets!
• detector systematics do not depend on ECM!
• pp vs 500 GeV (polarized beams!)
• AA vsNN 200 GeV (per NN pair)

• experiments with specific focus
• BRAHMS (until Run-6)
• PHOBOS (until Run-5)
• multi purpose experiments
• PHENIX
• STAR
• all experiments are crucial!!

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PHENIX in practice
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PHENIX in principle

• 3 detectors for global event characterization

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Low mass ee- prospects _at_ RHIC

• 2 scenarios _at_ SPS profit from high baryon density
• dropping r mass
• broadening of r
• what to expect at RHIC?

• baryon density almost the same at SPS RHIC
  (although the NET baryon density is not!)

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ee- theoretical guidance at RHIC
R. Rapp nucl-th/0204003
e-
e

• in-medium modifications of vector mesons persists
• open charm contribution becomes significant

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The founding fathers view

• before 1991
• proposals for various experiments at RHIC
• STAR, TALES, SPARC, OASIS, DIMUON
• except for STAR everything else is burned down
• from the ashes rises PHENIX
• Pioneering High Energy Nuclear Interaction
  eXperiment
• 1991 PHENIX conceptual design report
• philosophy
• measure simultaneously as many observables
  relevant for QCD phase transitions as you can
  imagine
• all but one low-mass dielectrons
• why no dielectrons?
• included in first TALES proposal
• considered to be too difficult for PHENIX
• a lot of work can make impossible things happen

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Au-Au collision as seen in PHENIX
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PHENIX tracking particle ID
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PHENIX measures dielectrons

• first attempt from 2002 Au-Au Run
• S/B 1/500 (!) for minimum bias events
• not enough statistics
• Au-Au data taken in 2004
• 100x statistics
• photon conversions reduced by factor 2-3
• expect background reduction by 2

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Low-mass ee- pairs the problem

• electrons/event in PHENIX
• Ne (dN/dh)p0 (BRCONV) acc
  f(pTgt0.2GeV)
• 350 (0.0120.02)
  0.50.7 0.32 1.3
• combinatorial background pairs/event
• B ½ ½Ne2e-N 0.1
• expected signal pairs/event (mgt0.2GeV, pTgt0.2
  GeV)
• S 4.210-4
• ?signal/background
• as small as 1/ few hundred
• depends on mass
• what can we do to reduce the background?

? Signal to Background S/B 1 / 250
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Conversion/Dalitz rejection?

• typically only one leg of the pair is in the
  acceptance
• acceptance holes
• soft tracks curl up in the magnetic field
• only (!) solution
• catch electrons before they are lost
• need new detector and modification of magnetic
  field

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Consequences of poor S/B

• how is the signal obtained?
• unlike-sign pairs F
• combinatorial background B (like-sign pairs or
  event mixing)
• ? S F B
• statistical error of S
• depends on magnitude of B, not S
• DS v2B (for SltltB)
• background free equivalent signal Seq
• signal with same relative error in a situation
  with zero background
• Seq S S/2B
• example S 104 pairs with S/B 1/250 ?
  Seq 20
• systematic uncertainty of S
• dominated by systematic uncertainty of B
• example event mixing with 0.25 precision
  (fantastic!)
  ? 60 systematic
  uncertainty of S (for S/B 1/250)

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Combinatorial background

• ingredients for the battle plan
• PHENIX 2 arm spectrometer
• dNlike ? dNunlike ? different shape ? need event
  mixing
• analyze pairs
• unlike sign (N-) and like sign (N and N--)
• produce mixed events
• unlike-sign pairs (B-) and like-sign pairs (B
  and B--)
• normalize mixed events properly (2vNN--)
• and be careful to
• do the pair analysis identically in real and
  mixed events
• mix only events with the same topology
  (centrality, vertex)
• remove detector artifacts
• remove unphysical correlations
• use like sign pairs as cross check for the
  normalization
• two years later ..

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Background shape like sign
RATIO
--- Foreground same evt N --- Background
mixed evt B

• small signal in like sign pairs at low mass
• from double conversion or Dalitzconversion
• normalize B and B to N and N for m gt 0.7
  GeV
• ? very good agreement in shape

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Background normalization 2vNN- -
TOTAL SYSTEMATIC ERROR 0.25
--- Foreground same evt --- Background mixed evt

• uncertainties
• statistics of N and N-- 0.12
• different pair cuts in like and unlike sign 0.2

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Conversion rejection

• artifact of PHENIX tracking
• assume that all tracks originate from the vertex
• off vertex tracks ? wrong momentum vector
• ? conversions are reconstructed with m?0 (mr)
• ? need to be removed since affect low-mass region
• ? how?

• conversions open in a plane perpendicular to
  the magnetic field

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Subtraction of cross pairs

• p0?g g
• ee-
• ee-

unlike cross like cross unlike 4-body
X
Data like Monte Carlo Cross Like Cross Unlike
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Raw unlike-sign mass spectrum

• put it all together
• a powerful cross check
• additional converter ?2.5 times more
  combinatorial background

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Cocktail comparison

• low-mass continuum enhancement
• intermediate mass continuum PYTHIA agrees with
  data?

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Comparison with theory

• calculations for minimum bias collisions
• our favorite scenarios
• thermal radiation from QGP is included in
  addition
• clear enhancement above cocktail
• large uncertainties
• ? not conclusive regarding in-medium r
  modification

R.Rapp, Phys.Lett. B 473 (2000) R.Rapp,
Phys.Rev.C 63 (2001) R.Rapp, nucl/th/0204003
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Reference dielectrons in p-p

• very good agreement of data and cocktail
• PYTHIA does NOT describe the charm contribution
  (was seen for single electrons as well)

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Comparison p-p vs. Au-Au

• binary scaling of p-p data to compare with Au-Au
• suppressed charmonia, charm, f, w, p0
• enhanced low-mass continuum

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Yield in different mass ranges
0-100 MeV p0 dominated scales
approximately with Npart 150-750 MeV
continuum scaling? 1.2-2.8 GeV charm
dominated scales with Ncoll

• study centrality dependence
  of yields in these regions

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Centrality dependence

• p0 production scales approximately with Npart
• expectation for low-mass continuum
• if in-medium enhancement is related to pp or qq
  annihilation
• ? yield should scale faster than Npart (and it
  does)

• charm is a hard probe
• total yield follows binary scaling (known from
  single e)
• intermediate mass yield shows the same scaling

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Summary

• sorry, no conclusion yet!
• PHENIX at RHIC
• first dielectron measurements in HI collisions at
  a collider
• despite low signal/background ratio
• reasonably good statistics
• unprecedented accuracy of combinatorial
  background calculation
• observations at low dielectron mass
• enhancement relative to the cocktail and to p-p
• not enough precision to distinguish between
  models
• enhancement increases faster than Npart with
  centrality
• observations at intermediate dielectron mass
• PYTHIA doesnt describe data in p-p collisions
• PYTHIA does a reasonable job in min. bias Au-Au
  collisions
• just a coincidence?
• room for thermal radiation?
• can these measurements be improved by collecting
  (much) more statistics with the existing
  apparatus?

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(Near) future precision ee- measurement

• identification of dielectrons with small opening
  angle BEFORE one of the legs is lost
• electron ID before the magnetic field
• full acceptance electron detector
• ?new field configuration
• ? HadronBlindDetector (HBD)

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Hadron Blind Detector (HBD)

• Dalitz rejection via opening angle
• identify e in field
  free region
• veto signal e with partner
• HBD concept
• windowless CF4 Cherenkov detector
• 50 cm radiator length
• CsI reflective photocathode
• triple GEM with pad readout
• construction/installation 2005/2006 (repair 2007)

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Future

• dielectron measurements in high energy HI
  collisions
• go to even higher energy, i.e. maximum
  temperature ? LHC
• go back to lower energy, i.e. maximum baryon
  density ? FAIR
• stay at RHIC
• HBD (and
  
  silicon vertex
  
  upgrades) for
  
  improved
  
  experiments
  
  
  at maximum
  
  RHIC energy
• low energy
  
  program, i.e.
  
  use RHIC as
  
  
  a storage ring
  
  instead of an
  
  accelerator

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Projections for RHIC high energy

• impact of the HBD modified B field at top
  energy
• recorded collisions
• 109
• 1010

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Projections for RHIC low energy

• collision rates decrease with decreasing beam
  energy
• 40 Hz _at_ 8.6 GeV/u
• 2 weeks run time gives 50M events
• HBD eliminates sys. uncertainty
• electron cooling in RHIC can increase the
  collision rate by a factor 10
  ? 500M events in 2 weeks
• ?very promising!!!