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Electromagnetic Measurements at RHIC. Hideki Hamagaki Center for Nuclear Study University of Tokyo. Prologue. EM probe = penetrating probe? correct EM probes provide information on when and where they are produced - PowerPoint PPT Presentation
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Electromagnetic Measurements at RHIC
Hideki Hamagaki
Center for Nuclear StudyUniversity of Tokyo
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 2
Prologue• EM probe = penetrating probe?
– correct– EM probes provide information on when and where they
are produced– EM probe? = leptons or photons; promptly produced
without intermediate state (hadron resonances); thermal photons/pairs, direct photons, Drell-Yan pairs, pairs from annihilation (quark-level and hadron-level),
• Physics quantities with EM measurements have many other sources
– hadron’s leptonic or photonic decays (as I will show you later)
– some of hadrons work as penetrating probes, depending on their life-time and when and where they are produced;
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 3
Prologue -- cont.• J/
– production: initial stage, and hadronization stage (recombination)
– life-time: c = 2264 fm; decay far outside the source
– long life time: 0, , D and B
• low-mass vector mesons; , , – production: creation in hadron phase + coalescence in the
freeze-out stage– life-time: c() = 1.3 fm, c() = 23.3 fm, c() = 46.2 fm– a part of and decays inside the source (modifications),
and the rest outside the source (free decay)– (recreation; thermalized): ee (escape from
the source) is dominated by the decay inside, can be used as a fast clock, and is a “pseudo-penetrating probe”
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 4
Outline of My Talk
• Photon measurements• Lepton measurements
– Single electron measurements leptonic decay of open quarks
– J/ measurements– Thermal pairs and vector mesons
• Summary and Outlook
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 5
Direct photons• Direct photon is a unique probe, which provides direct
information of its birth, because of penetrating property• photons can come from every stage of collisions, and can
have various origins• direct photons = not from “hadron decay”
direct photons
photons from hadron decay
non-thermalinitial hard scattering
pre-equilibrium
thermalQGP
Hadron gas
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 6
Quick Look of Various Photon Sources
• for thermal and hard photon measurements, hadron decay is a non-trivial background source– strong suppression of high pT
hadrons would improve the ratio, in particular, for hard direct photons
• a window for QGP thermal photons; pT = 1 ~ 3 GeV/c
thermal
decay
hard
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 7
Success in the 1st round• Direct photon in high pT
region in Au+Au collisions– suppression of high pT 0
yield makes the 0 ratio larger
– comparable to a pQCD calculation (Ncoll scaling), which means;
no strong initial state effect; modification of structure function
0 suppression is the final state effect
• 2nd round thermal photons
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 8
Is thermal photon yield suppressed?
• Comparison of the present result with calculations with kT broadening and jet QGP bremsstrahlung– fast quarks passing through QGP, which is a
significant photon source for pT < 6 GeV/c
• At present, it is too early to claim anything significant
• Gluon plasma (GP) and photon yield– chemically non-equilibrium state; GP
QGP– hotter than QGP ( smaller degeneracy)
• hot glue scenario?– If GP stays long, it will have impact to
photon production in hot matter; photon yield will be related to the evolution from GP to QGP
– Other way to say: photon suppression is the signature of GP; native no-QGP hadron picture can be ruled out
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 9
Single electrons are from many sources
• Photonic– Photon conversions
• from decay mesons• from direct photons
– Dalitz decays of 0, , ’, , • Non-photonic
– leptonic decay of D & B– Thermal lepton-pairs– Di-electron decays of ,,– Kaon decay (Ke3 0e)
• Leptonic decay of D & B is dominant among the “signals”
– Today’s signal is tomorrow’s BG
• Subtraction is a tricky business
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 10
Extraction of non-photonic componentsNe
0
1.1% 1.7%
Dalitz : 0.8% X0 equivalent
0
With a converter in
W/O converter Conversion at the detectors
0.8%
Non-photonic
1
1
RNNRN
RNNN
NNRNNNN
sim
C
e
NC
esimNp
e
sim
NC
e
C
eP
e
Np
e
P
esim
C
e
Np
e
P
e
NC
e
without Conv.with Conv.
En
trie
s/N
evt
En
trie
s/N
evt
pT[GeV/c]Mininum Bias Au+Au in sNN=200GeV
N(tot)/N(photonic)
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 11
Centrality dependence of electron yield
• Ncoll scaling seems to work well• NAuAu(pT;b) = Npp(pT)Ncoll
= 0.938+-0.075(stat) +- 0.018(sys) = 0.958+-0.035(stat) with p-p data included
• Binary scaling seems to work well
• keen interest has been in the behavior of heavy quarks in dense matter
– thermalization? – energy loss?
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 12
Hints of communication
• v2 for non-photonic electron– prediction based on a quark
coalescence model v2(D) ≈ v2(light) + v2 ( charm )
– “thermal” is preferred
• RAA(pT) for non-photonic single electrons in Au+Au
– significant reduction at high pT– suggest sizable energy loss!
Year-4 RUN results with high statistics are definitely needed to confirm these
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 13
Quarkonium in HI collisions
• Novel idea of J/ suppression– by Matsui and Satz (1986; before
experimental results)– a good probe of deconfinement
• suppresion due to Debye screening in deconfined phase
• History in Brief– observation of suppression in S + A
turned out to be similar to p + A
– anomalous suppression observed in Pb + Pb
a result to evident QGP at SPS
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 14
Recent Progress in Theory (I)
• Lattice-QCD told us;
– confining potential starts to disappear at low temperature far below TC
– J/ does not melt easily• M. Asakawa, T. Hatsuda; Phys.
Rev. Lett. 92 (2004) 012001
– Impact to the scenarios• Is dissociation dynamical or
static?
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 15
Recent Progress in Theory (II)• Enhancement of J/ yield
– basic idea is to add recombination of charms to scenario of the direct production with subsequent suppressions
– statistical hadronization model• coalescence in the final stage
– L. Grandchamp et al., Nucl.Phys. A715 (2003) 545
– A. Andronic et al., Nucl.Phys.A715 (2003) 529
– kinetic formation model• reproduction in QGP +
coalescence in the final stage– R.L. Thews et al., Phys. Rev. C 63
(2001) 054905
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 16
J/ in Au-Au collisions at RHIC
• results from Year-2 RUN– very poor
• but is already inconsistent with large enhancement scenarios
– e.g. kinetic formation model, cf. PRC 63, 054905 (2001)
• In Run-4, 240 b-1 recorded with improved detector performance
– ~ 100 times more J/signals expected than in Run-2
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 17
Thermal radiation and low-mass vector mesons
• Still missing at RHIC
• Experimentally, combinatorial background is very large and must be subtracted properly
• Large physics background comes from charm.– Charm production is
measured with ~15% accuracy by single electron measurements.
A prediction
R. Rapp, nucl-ex 0204003
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 18
Loss-mass pairs• Famous CERES results
– enhancement in the pair yield
• A probe of Chiral symmetry restoration and/or medium modification
• Only hadronic decays so far at RHIC–
• STAR (Phys. Rev. Lett. 92, 092301)
– K+K- • PHENIX (nucl-ex/0410012)• STAR (nucl-ex/0406003)
• Lepton-pair measurement is demanding– should provide information on space-
time evolution at RHIC, which might be very different from SPS
– mass shift vs broadening
R. Rapp (Nucl. Phys A661(1999) 238c
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 19
Combinatorials is overwhelming
• Combinatorial background is determined with ~1% accuracy in Run3 and Run2 using a mixed event method
• Higher statistics from Run4 data should help, but it may not be enough for low-mass vector mesons
Real and Mixed e+e- Distribution
Run2 AuAuMinimum Bias
0
Real - Mixed with systematic errors
1 2 3 [GeV/c2] 0 1 2 3 [GeV/c2]
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 20
How to measure low-mass pairs• At PHENIX, R&D efforts to develop
a Dalitz rejecter, in order to manage combinatorial backgrounds
• HBD (hadron Blind Detector) is a strong candidate– UV photon detector
• with CsI cathode• CF4 gas radiator• Ne(Cherenkov) > Ne(ionization)
S/B ~ 1/500
“combinatorial pairs”
total background
Irreducible charm backgroundall signal
charm signal
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 21
Summary and Outlook• Direct photons
– thermal photons may not be abundant
• Single electron measurement in Au+Au collisions– dominant contributions from charm and bottom– yield at mid-rapidity scales with Ncoll
no significant effect from initial state
– possible finite v2 and high pT suppression• flow and stopping effect -- to be determined
• J/ result for HI collisions at RHIC– inconclusive result from Year-2 RUN– good statistics in Year-4 RUN stay tuned
• Thermal pairs and low-mass vector mesons– combinatorials, charms, ...– HBD is to buster combinatorials
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 22
Heavy flavor production• Charm (or bottom) production
– Leading order at low x = ’’gluon fusion’’
A good probe of; • Initial state (probably OK)
– Parton distribution functions– kT broadening
• Early stage of evolution (?)– Parton energy loss?– Thermalization & Flow? – Thermal production?
• How to measure?– exclusive– leptonic decay measure electrons
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 23
The result implies;
• no significant initial state effect in the mid-rapidity at RHIC energy– gluon shadowing & anti-shadowing are not large
• no anomalous enhancement due to thermal production
• No energy loss?• No thermalization? next slide
Pb / p
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 24
Is Heavy quarks immune to Hot matter?
• The answer is “not definite”• Electron pT distribution is not
sensitive to modification of pT distribution of the parent, due to decay kinematics
• Need to extend to higher pT, in order to see “flow’’ or “energy loss’’ effects
Year-4 RUN
– A caveat
D from PYTHIA
D from Hydro
B from PYTHIA
B from Hydro
e from PYTHIA
e from Hydro
S. Batsouli et al.: Phys.Lett. B557 (2003) 26
2/10/2005 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki 25
Limitation of inclusive e measurement
• High pT region in e– c decay b decay– not high pT/m for parent c and
b
• Exclusive measurement– D0 K+ + - ... bySTAR– OK in p+p & d+Au– combinatorial will be a killer in
Au+Au
need vertex determination
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