Dilepton Radiation Measured in PHENIX probing the Strongly Interacting Matter

Created at RHIC

Y. Akiba (RIKEN Nishina Center)

for PHENIX Collaboration

Quark Matter 2009

Knoxville, TN, USA

April 2nd, 2009

Electromagentic probes (photon and lepton pairs)

• Photons and lepton pairs are cleanest probes of the dense matter formed at RHIC

• These probes have little interaction with the matter so they carry information deep inside of the matter– Temperature?– Hadrons inside the matter?– Matter properties?

e+

e-

What we can learn from lepton pair emission

Emission rate of dilepton per volume

Boltzmann factortemperature

EM correlatorMedium property

ee decay

Hadronic contributionVector Meson Dominance

qq annihilation

Medium modification of mesonChiral restoration

From emission rate of dilepton, the medium effect on the EM correlator as well as temperature of the medium can be decoded.

e.g. Rapp, Wambach Adv.Nucl.Phys 25 (2000)

q

q

Thermal radiation frompartonic phase (QGP)

Relation between dilepton and virtual photon

Emission rate of dilepton per volume

Emission rate of (virtual) photon per volume

Relation between them

Virtual photon emission rate can be determined from dilepton emission rate

For M0, n* n(real); real photon emission rate can also be determined

M ×dNee/dM gives virtual photon yield

Dilepton virtual photon

Prob. *l+l-

This relation holds for the yield after space-time integral

Theory prediction of dilepton emission

Vaccuum EM correlatorHadronic Many Body theoryDropping Mass Scenarioq+q ee (HTL improved)(q+gq+qee not shown)

Theory calculation by Ralf RappdMdydpp

dN

tt

ee at y=0, pt=1.025 GeV/c

Usually the dilepton emission is measured and compared as dN/dptdM

The mass spectrum at low pT is distorted by the virtual photonee decay factor 1/M, which causes a steep rise near M=0

qq annihilaiton contribution is negligible in the low mass region due to the M2 factor of the EM correlator

In the caluculation, partonic photon emission processq+gq+qe+e- is not included

1/M*ee

qq*e+e-

≈(M2e-E/T)×1/M

Virtual photon emission rate

dydpp

dN

dMdydpp

dNM

tttt

ee* at y=0, pt=1.025 GeV/c

dydpp

dN

tt

Vaccuum EM correlatorHadronic Many Body theoryDropping Mass Scenarioq+q annihilaiton (HTL improved)

The same calculation, but shown as the virtual photon emission rate.

The steep raise at M=0 is gone, and the virtual photon emission rate is more directly related to the underlying EM correlator.

When extrapolated to M=0, the real photon emission rate is determined.

q+gq+* is not shown; it should be similar size as HMBT at this pT

Real photon yield Turbide, Rapp, Gale PRC69,014903(2004)

q+g q+* ?

qq*≈M2e-E/T

Electron pair measurement in PHENIX

• 2 central arms: electrons, photons, hadrons– charmonium J/, ’ -> e+e-

– vector meson r, w, -> e+e- – high pT po, p+, p-

– direct photons– open charm – hadron physics

Au-Au & p-p spin

PC1

PC3

DC

magnetic field &tracking detectors

e+e

designed to measure rare probes: + high rate capability & granularity+ good mass resolution and particle ID- limited acceptance

p+p results

• 2.25pb-1 of triggered p+p data• Data absolutely normalized

• Excellent agreement with Cocktail• Filtered in PHENIX acceptance

Light hadron contributions subtracted

Heavy Quark Cross Sections:• Charm: integration after cocktail

subtraction σcc = 544 ± 39 ±142 ± 200 μb

(stat) (sys) (model)

• Simultaneous fit of charm and bottom:

– σcc = 518 ± 47 ± 135 ± 190 μb (stat) (sys) (model)

– σbb = 3.9 ± 2.4 +3/-2 μb

• Charm cross section from single electron measurement:

– σcc = 567 ± 57 ± 193 μb

Published in PLB670,313(2009)

8

Charm and bottom cross sections

CHARM BOTTOM

Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end)

Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end)

First measurements of bottom cross section at RHIC energies! 9

Di-electron:PLB670,313(2009)e-h corr: arXiv:0903.4851

pp and AuAu normalized to p0 Dalitz region (~ same # of particles)

p+p: agree with the expected background from hadron decays

Au+Au: large Enhancement in 0.15-0.75 GeV/c2

p+p NORMALIZED TO mee<100 MeV

PLB670,313 (2009) arXiv:0706.3034

PHENIX low mass dielectrons

AuAu

pp

low mass

intermediate mass

J/

’

Mass Spectra: pT dependency

• Study pT dependency of the low mass enhancement in Au+Au

• p+p in agreement with cocktail• Au+Au low mass enhancement concentrated at low pT

0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c

0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c

11

Excess of virtual photon

The excess of electron pairs over the cocktail is almost constant level at high pT.

The excess is converted to the excess of virtual photon by divided by 1/M shape coming from the virtual photon decay.

The distribution is ~flat over half GeV/c2

No indication of strong modification of EM correlator at this high pT region(presumably the virtual photon emission is dominated by hadronic scattering process like ++* or q+gq+*

Extrapolation to M=0 should give the real photon emission rate. Talk by Y. Yamaguchi (4C-3)

(Data-cocktail)× Mee

Exc

ess

*M (

A.U

).

Au+Au 200 GeV Vaccuum HMBT @ pt=1.025 GeV/c Drop mass qqdMdpp

dNM

tt

ee

What is the Source of the huge excess?

A very large excess at low pT and low mass in Au+AuThe shape of the excess seems to be incompatible with aconstant virtual photon emission rate.

Large enhancement of EM correlator at low mass, low pT?

Dilepton Spectra

p+p Au+Au

• p+p– Agreement with cocktail

• Au+Au– pT<1GeV/c: large excess for 0.3<M<1 GeV

– Low temperature component with strong modification of EM correlator?

0.3<Mee<1GeV

Summary

• EM probes (photons and dileptons) are ideal “penedtrating probes” of dense partonic matter created at RHIC

• Double differential measurement of dilepton emission rates can provide– Temperature of the matter– Medium modification of EM spectral function

• PHENIX measured dilepton continuum in p+p and Au+Au– pp

good agreement with cocktail

heavy flavor xsection is deduced– Au+Au

Much larger enhancement, strong dependence on pT

hint of modification of the spectral function?• Higher statistics data with Hadron Blind Detector (HBD) allows more

precise measurements ( HBD poster by J.M.Durham (#908))• Photon emission can be deduced from dilepton Y.Yamaguchi (4C-3)