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Dileptons and Photons Huan Z Huang Department of Physics and Astronomy University of California, Los Angeles Department of Engineering Physics Tsinghua University

Dileptons and Photons

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Dileptons and Photons. Huan Z Huang Department of Physics and Astronomy University of California, Los Angeles Department of Engineering Physics Tsinghua University. Collision Dynamics. Photon Sources in Quark-Gluon Plasma. “Naïve” Leading Order Processes: q + q (g) → g (q) + γ. q. q. - PowerPoint PPT Presentation

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Page 1: Dileptons and Photons

Dileptons and Photons

Huan Z HuangDepartment of Physics and AstronomyUniversity of California, Los Angeles

Department of Engineering PhysicsTsinghua University

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Collision Dynamics

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Photon Sources in Quark-Gluon Plasma

“Naïve” Leading Order Processes: q + q (g) → g (q) + γ






Ts 0/q-2230 4



[Kapusta etal ’91, Baier etal ’92]

But: other contributions to OO(αs)

collinear enhanced Dg=(t-mD2)-1 ~ 1/αs

[Aurenche etal ’00, Arnold,Moore+Yaffe ’01]

Bremsstrahlung Pair-ann.+scatt. + ladder resummation (LPM)



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Photon Sources in Hadron Gas





• Photon-producing reactions:

mostly at dominant (q0>0.5GeV) gauge invariance! q0<0.5GeV a1-strength problematic

Hadron Form Factor Important for photon yield

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More Meson Gas Sources

(i) Strangeness Contributions: SU(3)F MYM

(iii) Higher ResonancesAx-Vec: a1,h1→, Vec: ,’,’’→ other: (1300)→ f1→ , K1→K K*→K a2(1320)→


γK* K

~25% of→

~40% of→

(ii) t-Channel γ

G large!

potentially important …[Turbide,Gale +RR ’04]

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Baryonic Contributions

• use in-medium–spectral funct:

• constrained by nucl. -absorption:


mIm med 02










absA 0


0 4em

N → N,

N →



[Urban,Buballa,RR+Wambach ’98]

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HG Emission Rates: Summary


[Turbide,RR+Gale ’04]

• t-channel (very) important at high energy

• form factor suppression (2-4)

• strangeness significant

• baryons at low energy

Though EM is well understood, photon production in collisions complicated dynamics !!

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Initial hard production: pp → γX

scaling with xT=2pT /√s , + power-law fit [Srivastava ’01]

PQCD photons

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Naively Thermal Photons T

“Freeze-Out” Hadron Gas QGP ?!

Au + Au

The higher temperature, the more thermal radiation !

But the relation between initial T and photons is non-trivial !

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Predictions for Central Au+Au Collisions at 200 GeV

‘pre-equilibrium’ contribution from parton cascading major contributionQGP thermal radiation 1-2 GeV (maybe)

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Direct in d+Au

• p+p and d+Au spectra compared to NLO pQCD

• ratio to NLO pQCD• consistent with 1• No indication for

nuclear effects



No surprises !

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Direct Photons Surely There!

0 suppression



Nbinary Scaling

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Direct Photons

• Direct photon spectra over centralities

• Systematic Error: ~15-20%

• Clearly seen that we measured photons over the order of 1027!– See the scale please..

• Again, Thickness-scaled NLO QCD calculation describes all the spectra very well– From Central to Peripheral– No exception within current


• Yellow bands show uncertainty on NLO pQCD calculation and thickness function

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Results (RAA)• Photon RAA is consistent with unity over all the centrality.

– the yield follows thickness-scaled hard scattering– p-p reference from NLO pQCD Calculation 0 RAA decreases to ~0.2 at Npart=320

• Dotted line shows uncertainty of thickness function– Error bars show total error (systematics + statistical) except

thickness function error– Yellow shows uncertainty on pQCD calculation



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Comparison with calculations• Any of pQCD calculations

describe data well– Adding kT broadening

makes factor of ~2 difference• Around same factor as

E706– Calculation suggests that

slopes of the spectra at RHIC and E706 are same

• Jet Photon included calculation (Fries et al., PRL 90, 132301 (2003)) is also shown– Fits very well above 4GeV!– Assuming existence of hot

dense medium• Prompt partons scatter with

thermal partons– The line approaches to

simple pQCD calculation in high pT

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Any Hope for QGP Radiation?

• Most realistic calculation– Including all the contributions

PHENIX may be able to see QGP contribution in 1-3GeV/c


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Quenching = Jet-Plasma interaction. Does this have an EM signature?

qg q

The plasma mediates a jet-photon conversion

Fries, Mueller & Srivastava, PRL 90, 132301 (2003)

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Comparison between model and exp

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Lattice QCD

Chiral CondensatePolyakov Loop

• Coincident transitions: deconfinement and chiral symmetry restoration it is seen to hold also vs quark mass

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Chiral Symmetry

How Chiral Symmetry has manifested innucleus-nucleus collisions?

We must measure vector mesons in both hadronic and leptonic decay channels!

electron PID TOF upgradeHFT – reduce conversion BK

K-pi PID TOF upgradeand ’ EMC + high statistics

Baryonic resonances ()....)

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Vector Meson Mass and Chiral Symmetry

mesons: No significant width change or mass shift has beenobserved. Measurement of both K+K- and e+e channels.

mesons: Some kind of mass shift has been observed in STAR, butthe interpretation of the shift is not clear! Measurement of mesons in e+e channel is needed!

Measurement of invariant mass and the residual distributionafter combinatorial background subtraction

--- and mesons--- the nature of mesons (q-qbar or four-quark)

Electron measurement will be possible with the TPC and TOF-- remove photon conversion and Dalitz background !

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What is expected (dileptons)• Low masses receive

significant contribution from radiative decays

• High masses dominated by DY

• Intermediate mass region interesting from QGP perspective, (Shuryak (78), Shor (89))

• Photons: similar story, but featureless spectra

• Experiments: DLS, Helios, TAPS, NA38, -50, WA98, CERES, PHENIX, HADES, NA60


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Low Masses:Vector Meson Spectral Densities:Hot Meson Gas

The spectral density is flattened

and broadened

Rapp, Gale (99)

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Very Difficult to Measure Di-leptons

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NA60 Comparison of data to RW, BR and NA60 Comparison of data to RW, BR and Vacuum Vacuum (Broadening vs Shift) (Broadening vs Shift)

pT dependence

Sanja Damjanovic

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Quark-Gluon Fluid

Chiral Properties at Tc

-- quasi-particles-- mass shift-- width broadening

Dilepton Measurement-- in the low mass region 0.5-2 GeV/c2

-- very difficult

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pTassociated varies pT

trigger varies


R p





R p





R p





R p




Two-Particle Correlations in d+Au

Background-subtracted correlations between a high-pT trigger charged particle and an associated charged particle

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Photon-hadron correlations


R p





R p




+jet correlation in Au+Au in run4?

More accurate determination of initial Et

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Results for p-p

• NLO-pQCD calculation– CTEQ6M PDF.

– Gluon Compton scattering

+ fragmentation photon

– Set Renormalization scale

and factorization scale pT/2,pT,2pT

• Systematic Error: – 20(high pT)-45(low pT)%

The theory calculation shows a good agreement with our result.

(Subtraction)Bands represent systematic errors.

Errors on the backgrounds result in enlarged errors on the signal,especially at low-pT region.

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dE/dx at high pT (62.4 GeV)

rigidity (charge*dE/dx) [keV/cm]


2/ndf = 1.5

pT > 3 GeV/cPion-protonSeparation !

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Jet Photon overwhelms QGP?• Break-up of Fries prediction

• Jet Photons overwhelms all the other contributions below 7GeV/c

• Jet production rate calculated by LO pQCD with K factor compensation of 2.5

• pQCD photon calculation from LO with no K factor

• Fitting too good!– In Peripheral, the calculation

should fit the data as well• RAA and spectra themselves tell

you what happens– Calculation is assuming

existence of hot dense medium, which is not the case in peripheral!