EM Probes at RHIC II

1
EM Probes at RHIC II

• Zhangbu Xu (BNL)
• For the RHIC-2 EM-Probes Working Group
• (co-convenors G. David, R. Rapp, XZB)

http//www.phenix.bnl.gov/WWW/publish/david/rhicii
_em Talks by members at April Workshop R. Rapp
(Nov. PAC meeting) Simulations from
individuals First Draft of Working Group Document
2
Outline

• 1. Motivations
• Chiral Symmetry Restoration
• QGP Radiation -- Temperature
• 2. Experimental Measurements
• Vector Meson In-medium Properties
• Thermal Dilepton Spectra
• Direct Photons
• 3. Challenges
• Chiral partners?
• Understanding backgrounds
• What do we learn from SPS results
• 4. Conclusions

3
Search for Chiral Symmetry Restoration
scholar.google.com 4,130www.google.com 76,700
Responsible for gt90 mass of the visible world
4
QCD Phase Diagram
A thermodynamic state is specified by a set of
values of all the thermodynamic parameters
necessary for the description of the system. ---
statistical mechanics by K. Huang Temperature
(T), chemical potential (m), pressure(P)

• Chemical/thermal Equilibrium at certain stage of
  the evolution
• At the predicted QCD phase boundary
• persistent from SPS to RHIC
• Below the phase boundary atAGS and SIS
• Can we put a point above?

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Experimental Signatures

• Vector Meson Properties
• Thermal Dileptons
• Direct Photons

6
EM Emission and Chiral Symmetry
Brown-rho, Rapp
nucl-th/0409054
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Low-Mass Vector Mesons at RHIC

• hadron liquid close to Tc r, w melt, f more
  robust?
• baryonantibaryons important

Energy Scan!!
8
Dilepton from HI
smooth transition HG ? QGP liquid? Integrated
yields from several stages
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QGP Temperature and Composition
Direct Photons
Intermed.-Mass Dileptons

• thermal window q0 , Mee 2GeV
• photons jet-QGP interactions compromise
  T-sensitivity
• less so for dileptons charm?

Energy Scan
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Detector Upgrades

• PHENIX
• Hadron Blind Detector (HBD) reject g conversion
  and Dalitz decay
• Silicon Vertex Tracking DetectorMeasure charm
  background
• STAR
• Time-of-Flight TPCTOFEMC Electron/muon
  Identification
• Heavy Flavor Tracker (HFT) TPCHFT reject g
  conversion, Dalitz decay and HF?e
• Data Acquisition System (1KHz minbias rate)

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Measuring Low Mass Electron Pairs in PHENIX

• Hardware
• Compensate magnetic field with an inner coil to
• preserve ee- pair opening angle (foreseen in
• original design ? B?0 for r ? 50-60cm)
• Compact HBD in inner region

• Strategy
• Identify signal electrons with p gt 200 MeV/c
  from
• vector mesons in the outer PHENIX detectors
• Identify low momentum electrons with p lt 200
  MeV/c
• (mainly from Dalitz pairs and conversions) in
  the HBD
• Reject pair if opening angle lt 200 mrad
• (for 90 rejection).

Requirements Electron efficiency ? 90
Double hit recognition ? 90 Modest ?
rejection 200
C. Woody, RHICII EM probes group meeting, 04/30/05
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Resulting Low Mass Pair Spectrum with the HBD

• Combinatorial background is
• reduced by more than two
• orders of magnitude
• limited by combinatorial
• background from open charm
• A precision measurement of
• charm using the Silicon
• Vertex Tracking Detector.

C. Woody, RHICII EM probes group meeting, 04/30/05
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Rates for Vector Meson Production

• Consider Run 8 (200 GeV Au x Au, 4x Design
  Luminosity)
• Lpeak 30 x 1026 cm-2 s-1
• L ave store 8 x 1026 cm-2 s-1
• 20 KHz peak min. bias rate
• 5.4 KHz avg min. bias rate
• 10 week dedicated HBD run (central field in ?
  configuration)
• eRHIC x ePHENIX 0.5 x 0.5 0.25

Lower energy 30GeVg2 44

• fs produced in h lt 0.5
• e,e- into PHENIX central
• arm acceptance
• pT,e gt 200 MeV/c

8.2 x 109 min. bias events produced N f?ee-
1.5 x 10-5 / min.bias event
fwr (ee- ? PHENIX) 1 0.9 0.7
N f?ee- 1.2 x 105 produced in PHENIX
acceptance
C. Woody, RHICII EM probes group meeting, 04/30/05
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STAR MRPC-TOFr Electron PID
electrons
Electron identification TOFr 1/ß-1 lt 0.03
TPC dE/dx electrons!!! 2? in azimuth,
etalt1
STAR Collaboration, PRL 94, 2005, 062301
Clean electron PID can be obtained up to PTlt 3
GeV/c. ? This is used to measure the semileptonic
decay of open charm. Hadron rejection power at
10-5. M. Shao et al., nucl-ex/0505026 Higher pT
TPCTOFEMC
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Reject g conversion

• Background g ? ee-
• HFT discriminates background !
• Need low mass detector
• Also DD ? ee-

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Dalitz Decay
TPC ee- invariant mass ? conversion
p0 Dalitz decay
Find the pair from Dalitz,reject both leptons
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Charm Spectrum Measurement

• Au Au, 50M central events
• D0 ? K p
• Stat. uncertainties small

• We can subtract DDbar ? ee-
• Alternative Approach reject electrons
  event-by-event
• (50 for electron pair)
• HFT resolution lt10?m

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STAR Dilepton Capability
TOF vector meson TOFHFT thermal radiation
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Rate Estimate
Assume
(pp)
(AuAu)
TOF matchPID eff 80
TPCHFT eff 60 (?)
From PDG
Acceptance N f?ee- 8 x 10-4 / min.bias event
Preliminary estimation for the requested data
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Muon ID at low pT
p
e
m
STAR Preliminary 0.15ltpTlt0.25 GeV/c, DCAlt3cm

• less Dalitz decay and g conversion (ltx10),
  Decays from p,K rejected by HFT DCA
• Dilepton mass 0.2ltmlt0.6 GeV

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Direct Photons

• Direct Measurement of direct photons
• All g known sources
• Virtual photon (via low-mass dilepton)
• gg HBT

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Virtual?Direct Photons
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Comparison to Conventional result
Conventional methodgall-gph
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gg HBT

• Thermal model for direct photon production
• Assuming l1
• EMCTPC efficiencies50(59)
• 16 million AuAu events
• Sandweiss/Chikanian (Yale)

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Challenges

• Lack of measurements of Chiral Partners
• Backgrounds on dilepton, direct photon
  measurements
• What we learn from SPS Measurements

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Chiral Partner (r ? a1)
h1 ? 3 p B.R. 0.1 h1 ? ? p B.R.
0.9 Mass 1.170 GeV/c2 G 360 MeV/c2
UrQMD minimum bias AuAu at vsNN 200 GeV
r?pp- Cocktail
P. Fachini
? 3s signal a1 ? g p ? 54M events ! Interesting
but difficult (Type B)
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Vector Meson?CSR

• Experimental observation of modification of
  Vector Spectral Function significant by itself
• Model calculation of V-A Spectral
  FunctionCalculate vector and axialvector
  spectral functions as a function of temperature
  and density (including as many of the
  constraints, Weinberg sum rule) in a chirally
  invariant model.
• Compare results with Lattice QCD
• Detailed Experimental-Thoery comparison vs
  energy/density scanPerform detailed comparisons
  of the in-medium effects on the vector correlator
  with dilepton data (centrality, excitation
  function, mass and q_t-spectra), which requires a
  realistic expansion model (e.g. hydro/transport).

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Charm Background in PHENIX Dilepton
crude charm RHIC 2
subtracted spectrum thickness of line represents
statistical error
15 at m2GeV Needs -2
QGP
Vacuum Vector mesons
R. Seto, RHICII EM probes group meeting, 04/30/05
Mass (GeV)
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Charm/Dalitz Background in STAR
DDbar ? ee- correlations can be measured
(lt-10) Dalitz rejection Inner Detector
tracking SSDHFTVertexin progress
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Direct Photons ?Temperature

• pQCDQGPHGJet
• Extract temperature difficult
• Energy Scan important
• Detailed model comparison

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Residual Correlation from pp HBT
Similar effect for virtual photons? Negligible
with HBD
Background 4x signal Subtract from known pp
HBT Sandweiss/Chikanian (Yale) in progress
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Recent Advances at SPS Power of Precision
RRWambach 99
NA60

• r -meson melting , dropping mass?
• address absolute norm., pt-dep., M gt 0.9GeV
  (4p?mm-!)
• cocktail-r (smooth signal)? vector
  dominance?
• precision data essential to rule out models

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Electromagnetic Probes at SPS
Dileptons
Medium Effects! 10 QGP
HG 4p?mm- 30 QGP
RR Shuryak 99
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What needed at the SPS?

• Experimentally
• Low-mass ee- drop. mass vs. broad., energy scan
  run time
• Cronin enhancement vs. temperature
  systematic p-A
• Isolation of charm dileptons
  vertex detector
• Redundancy
• Physics
• QGP subdominant ?!
• early thermalization (v2) ?!

RHIC II (dedicated QCDLab) has it all
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Conclusion

• EM probes are directly sensitive to the chiral
  symmetry properties and to the temperatures of
  the partonic system
• There are three major measurements related to
  the questions above
• Each with complemental/redundant techniques
• Challengesnot only confirm a theory, but rule
  out incorrect ones requires precision data in M,
  qt , centrality and vs
• RHIC (detectors) positioned to meet the
  challenges - Dalitz suppression / charm ID
  redundancy - energy-scan (006-200GeV!?)
• Theory Homework chiral theory with baryons,
  sQGP, g/dilepton components,

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Remarks
CHARM
DILEPTON

• Am I too negative? ????pessimistic troops win
• Continue pushing our detectors to their limits
• Understand the strength and weakness

• Am I too positive?I dont think of all the
  misery, but of all the beauty that still remains.
  Anne Frank
• Continue exploration of techniques and
  simulations
• Understand and reduce the background

I believe we can fly, RHIC II will provide us
with wings
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2.2 EM Emission and Chiral Symmetry
r - a1(1260) (chiral partners)
Axial-/Vector in Vacuum
pQCD cont.
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Introduction I EM-Probes -- The Basic Questions
QCD Phase Diagram

• Thermalization at RHIC ? study the phase diagram
  
• (highest) temperature of the matter
• chiral symmetry restoration (mass generation!)
• medium effects on spectral properties above
  below Tc

Inevitable consequences of QGP, link to lattice
QCD
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(ii) Heavy Partons Multiple Bound States ?
based on finite-T lattice potentials approach
to zero-binding line ? stable-mass r-resonance
Shuryak,Zahed, Brown,

• thermal parton scattering through bound states?
• composite interactions? quark scaling?

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Cocktail comparison

• Data and cocktail absolutely normalized
• Cocktail from hadronic sources
• Charm from PYTHIA
• Predictions are filtered in PHENIX acceptance
• Good agreement in p0 Dalitz
• Continuumhint for enhancement not significant
  within systematics
• What happens to charm?
• Single e ? pt suppression
• angular correlation???
• LARGE SYSTEMATICS!