EM Probes at RHIC II
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Transcript of EM Probes at RHIC II
RHIC II Workshop, 11/11/05 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_emTalks by members at April Workshop
R. Rapp (Nov. PAC meeting)Simulations from individuals
First Draft of Working Group Document
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1. Motivations: a) Chiral Symmetry Restoration
b) 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
Outline
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Search for Chiral Symmetry Restoration
scholar.google.com: 4,130www.google.com: 76,700
Responsible for >90% mass of the visible world
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QCD Phase DiagramA thermodynamic stateA 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. HuangTemperature (T), chemical potential (, pressure(P) …
1. Chemical/thermal Equilibrium at certain stage of the evolution
2. At the predicted QCD phase boundary
3. persistent from SPS to RHIC
4. Below the phase boundary atAGS and SIS
5. Can we put a point above?
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Experimental Signatures
• Vector Meson Properties• Thermal Dileptons• Direct Photons
The Lost Sword 刻舟求剑 (Fable, 《呂氏春秋》 ~250B.C.)
RHIC
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EM Emission and Chiral Symmetry
at Tc: Chiral
Restoration
)Im(Ims
dsf AV
2
)Im(Imsds)qq( AVs 2 [Brown-rho, Rapp…]
nucl-th/0409054
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Low-Mass Vector Mesons at RHIC
• hadron liquid close to Tc: , “melt”, more robust?
• baryon+antibaryons important Energy Scan!!
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Dilepton from HI
smooth transition HG → QGP liquid?Integrated yields from several stages
R. Rapp, hep-ph/0010101
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QGP Temperature and Composition
• thermal window: q0 , Mee≈ 2GeV
• photons: jet-QGP interactions compromise T-sensitivity• less so for dileptons; charm?
Direct Photons Intermed.-Mass Dileptons
Energy Scan
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Detector Upgrades
• PHENIX – Hadron Blind Detector (HBD)
reject conversion and Dalitz decay– Silicon Vertex Tracking Detector
Measure charm background
• STAR– Time-of-Flight
TPC+TOF+EMC Electron/muon Identification– Heavy Flavor Tracker (HFT)
TPC+HFT reject conversion, Dalitz decay and HFe
– 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 e+e- pair opening angle (foreseen in original design B0 for r 50-60cm)• Compact HBD in inner region
Strategy • Identify signal electrons with p > 200 MeV/c from vector mesons in the outer PHENIX detectors
• Identify low momentum electrons with p < 200 MeV/c (mainly from Dalitz pairs and conversions) in the HBD
• Reject pair if opening angle < 200 mrad (for ~ 90% rejection).
HBD
HBD
Requirements * Electron efficiency 90% * Double hit recognition 90% * Modest rejection ~ 200C. 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
• ’s produced in || < 0.5• e+,e- into PHENIX central arm acceptance• pT,e > 200 MeV/c
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)• RHIC x PHENIX = 0.5 x 0.5 = 0.25
N e+e- = 1.2 x 105 produced in PHENIX acceptance
8.2 x 109 min. bias events produced
N e+e- = 1.5 x 10-5 / min.bias evente+e- PHENIX1 : 0.9 : 0.7
C. Woody, RHICII EM probes group meeting, 04/30/05
Lower energy 30GeV2 = 44
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STAR MRPC-TOFr Electron PID
Electron identification: TOFr |1/ß-1| < 0.03 TPC dE/dx electrons!!!2 in azimuth, |eta|<1
electrons
Clean electron PID can be obtained up to PT< 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/0505026Higher pT: TPC+TOF+EMC
STAR Collaboration, PRL 94, 2005, 062301
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Reject conversionBackground: e+e-
HFT discriminates
background !
Need low mass detector
Also: DD e+e-
e+e-
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Mass(e+e-)<0.15 GeV/c2
TPC e+e- invariant massγ conversion π0 Dalitz decay
Dalitz Decay
pT (GeV/c)
Ang
le
Find the pair from Dalitz,reject both leptons
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Charm Spectrum Measurement
• Au + Au, 50M central events
• D0 K +
• Stat. uncertainties small
•We can subtract DDbar e+e-
•Alternative Approach: reject electrons event-by-event(~50% for electron pair)•HFT resolution <10m
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STAR Dilepton Capability
TOF: vector meson TOF+HFT: thermal radiation
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Rate Estimate1|/ dydn 15.0 02.0Assume: (pp)
5107)( eeBr 4103)( eeBr From PDG:
MeV15 MeV8300|/ dydn (AuAu)
TOF match+PID eff ≈ 80% TPC+HFT eff ≈ 60% (?)
Au+Au #events for ω
with 3σ signal
#events for Φ with 3σ signal
TOF+TPC 7M 2M
TOF+TPC+HFT 200K 100K
Preliminary estimation for the requested data
Acceptance: N e+e- = 8 x 10-4 / min.bias event
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Muon ID at low pT
e
STAR Preliminary:0.15<pT<0.25 GeV/c, DCA<3cm
less Dalitz decay and conversion (<x10), Decays from ,K rejected by HFT DCADilepton mass: 0.2<m<0.6 GeV
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Direct Photons
• Direct Measurement of direct photons
All – known sources
• Virtual photon (via low-mass dilepton)
HBT
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VirtualDirect Photons
S/B=~1
0
0
direct
data
incl.
direct
*
*
RR
RR
calculated from Dalitz formula
measuredRdata ÷
R
R
Rdirect
incl.
direct
measured with EMCal
Here we are…
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Comparison to Conventional result
0
0
direct
data
incl.
direct
*
*
RR
RR
incl.
direct
( + 1 )
Conventional method:
all-
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HBT
• Thermal model for direct photon production
• Assuming =1• EMC+TPC efficiencies:
50%+(5—9%) • 16 million Au+Au 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 ( a1)
UrQMD minimum bias Au+Au
at √sNN = 200 GeV
h1 3 π B.R. = 0.1h1 ρ π B.R. = 0.9Mass = 1.170 GeV/c2
Γ = 360 MeV/c2
3σ signal a1 π± 54M events ! Interesting but difficult (Type B)
Cocktail
P. Fachini
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Vector MesonCSR
• Experimental observation of modification of Vector Spectral Function significant by itself
• Model calculation of V-A Spectral Function:Calculate 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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subtracted spectrumthickness of linerepresents statistical error
Vacuum Vector mesons
QGPQGP
15% at m=2GeVNeeds ~+-2%
R. Seto, RHICII EM probes group meeting, 04/30/05
crude charm RHIC 2
Charm Background in PHENIX Dilepton
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Charm/Dalitz Background in STAR
DDbar e+e- correlations can be measured (<+-10%)Dalitz rejection: Inner Detector tracking: SSD+HFT+Vertexin progress
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Direct Photons Temperature
• pQCD+QGP+HG+Jet• Extract temperature
difficult • Energy Scan important• Detailed model
comparison
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Residual Correlation from HBT
Background = 4x signalSubtract from known ±± HBTSandweiss/Chikanian (Yale) in progress
0
0
0
e+
e-
0
e+
e-
Similar effect for virtual photons?Negligible with HBD
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drop. mass (norm.)
[RR+Wambach ’99]
• -meson “melting” , dropping mass?• address: absolute norm., pt-dep., M > 0.9GeV (4→!)• “cocktail-“ (+smooth signal)? vector dominance? • precision data essential to rule out models
Recent Advances at SPS: Power of Precision
drop. mass (norm.)drop. mass (norm.)
NA60
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Electromagnetic Probes at SPSDileptons
MediumEffects!10% QGP
[RR+ Shuryak ’99]
HG: 4→
30% QGP
[Turbide,RR+Gale’04]
Direct Photons
[Liu+ RR’05]
Central Pb(158AGeV)+Pb
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What needed at the SPS?
Experimentally:• Low-mass e+e: 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• Challenges:
not only confirm a theory, but rule out incorrect ones requires precision data in M, qt , centrality and √s
• RHIC (detectors) positioned to meet the challenges: - Dalitz suppression / charm ID; redundancy - energy-scan (006-200GeV!?)
• Theory Homework: chiral theory with baryons, sQGP, dilepton components, …
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Remarks
• Am I too negative? 哀兵必胜“ pessimistic” troops win
• Continue pushing our detectors to their limits
• Understand the strength and weakness
• Am I too positive?I don’t 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
CHARM DILEPTON
I believe we can fly, RHIC II will provide us with wings
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2.2 EM Emission and Chiral Symmetry
~ “- a1(1260)” (chiral partners)
Axial-/Vector in Vacuum
pQCD cont.
)Im(Ims
dsf AV
2
at Tc: Chiral
Restoration
)T(fMqxdd
dN Bee23
2
44
Imem ~ [ImD+ImD/10+ImD/5]
• Low-Mass Dilepton Rate:
-mesondominated!
• Axialvector Channel: ± invariant mass-spectra ~ Im Da1(M) ?!
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T=1.4Tc
[Tokyo]
Spectral Functions
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
Introduction I: EM-Probes -- The Basic Questions
Inevitable consequences of QGP, link to lattice QCD
1.0 T/Tc
m‹qq›-
[Bielefeld]
Chiral CondensateQCD Phase Diagram
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(ii) Heavy Partons + Multiple Bound States
→ based on finite-T lattice potentials approach to “zero-binding line” ~ stable-mass-resonance
[Shuryak,Zahed, Brown, …]
• thermal parton scattering through bound states?• composite interactions? quark scaling?
Dilepton Radiationratio to pert. qq rate
Mee/mq
_
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Cocktail comparison
• Data and cocktail absolutely normalized
•Cocktail from hadronic sources•Charm from PYTHIAPredictions are filtered in PHENIX acceptance
•Good agreement in 0 Dalitz•Continuum:hint for enhancement not significant within systematics
•What happens to charm?•Single e pt suppression•angular correlation???
• LARGE SYSTEMATICS!