Dileptons @RHIC a clock and a thermometer

of relativistic heavy ion collisions

Alberica Toiafor the PHENIX Collaboration

Stony Brook University / CERN

International School of Nuclear Physics“Heavy ion collisions from the Coulomb barrier to the Quark-Gluon Plasma”

Erice, September 16-24 2008

Dileptons: radiation from the medium

(KEK E235)

CERES

DLS

NA60

HADES

CBM PHENIX

10 158 [A GeV]

17 [GeV]√sNN200

// // //

// // //

ALICE

[A TeV]

time

hard parton scattering

AuAu

hadronization

freeze-out

formation and thermalization of quark-gluonmatter?

Space

Time

expansion

Jet cc e p K

• direct probes• large emission rate in hot and dense matter• light vector mesons: signals of chiral transition?

Measurement at RHIC:• different energy• different initial conditions• different hydro evolution• hard processes have larger cross sections• collider geometry

Dilepton Signal ExtractionarXiv: 0706.3034

Au+Au

• Single electron – Track reconstruction– Electron Identification (RICH

+ EMCal)• Background sources

– Combinatorial background– Material conversion pairs– Additional correlated

background• Visible in p+p collisions• Cross pairs from decays with

4 electrons in the final state• Pairs in same jet or back-to-

back jet

• Real signal– di-electron continuum

p+parXiv: 0802.0050

Hadronic Cocktail Calculation

arXiv: 0802.0050

n0T2TT

3

3

pp)bpapexp(

A

pd

σdE

• Remaining pairs after background subtraction– Real signal + Hadron decay components

• Parameterized PHENIX 0 data with assumption of 0 = (++-)/2

Other mesons are fit with mT scaling of π0 parameterization pT→√(pT

2+mmeson2-mπ

2)

fit the normalization constant All mesons mT scale!!!

• Hadronic cocktail was well tuned to individually measured yield of mesons in PHENIX for both p+p and Au+Au collisions.

• Mass distributions from hadron decays are simulated by Monte Carlo.– 0, , ’, , , , J/, ’

• Effects on real data are implemented.– PHENIX acceptance, detector effect,

efficiencies …

Cocktail ComparisonarXiv: 0802.0050 arXiv: 0706.3034Au+A

up+p

• p+p– Excellent agreement with cocktail

• Au+Au– Large enhancement in low mass region

– Integrated yield in 150MeV < mee < 750MeV• data/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model)

Intermediate Mass:•yield increase proportional to

Ncoll charm follows binary scaling

Centrality Dependency

submitted to Phys. Rev. Lett

arXiv:0706.3034

LOW MASS

INTERMEDIATE MASS

0 region:•Agreement with cocktail

Low Mass:•yield increases faster than

proportional to Npart enhancement from binary annihilation (ππ or qq) ?

Source of the excess?

• Freeze-out Cocktail + “random” charm + spectral function

Low mass• M>0.4GeV/c2:

some calculations OK• M<0.4GeV/c2:

data still above theory calculations

Intermediate mass• Random charm +

thermal partonic may work

0 < pT < 8 GeV/c

0 < pT < 0.7 GeV/c

0.7 < pT < 1.5 GeV/c

1.5 < pT < 8 GeV/c

PHENIX Preliminary

pT-Sliced Mass Spectra

○ Au+Au● p+p

Normalized by the yield in mee < 100MeV

• The low mass enhancement decreases with higher pT

• Different shape at low and high pT

low mass enhancement does not contribute to m<300 MeV/c2 and pT>1 GeV/c?

p+p Au+Au (MB) 1 < pT < 2 GeV/c2 < pT < 3 GeV/c3 < pT < 4 GeV/c4 < pT < 5 GeV/c

• p+p– Good agreement between

real and cocktail

– Small excess at higher pT

• Au+Au– Good agreement in Mee <

50MeV/c2

– Enhancement is clearly seen above 100MeV/c2.

Low mass & High pT region

Sources of e+e- pairsDalitz:0e+e-e+e-0e+e-e+e-

Direct:e+e-e+e-e+e-J/e+e-’e+e-

Heavy flavor:cce+e- +Xbbe+e- +X

Drell-Yan:qqe+e-

cocktail

Gluon Compton

q

g q

e+

e-

Rising at low mass (~1/m):due to Dalitz decays of baryonic/mesonic resonancesa1 a1 e+e-N* N N* N* Ne+e-where the underlying process is:

Quarks and gluons can be “free” (in QGP) or “hidden” in hadrons

Theoretical predictions:e+e- threshold at 2m~300MeV

qqbare+e- “threshold-like”mq < 150 MeV?Much smaller than cocktail

CocktailHadron GasQGP (qq*e+e-)

Mass (GeV/c2)

Direct Photons Measurement• Any source of real can emit * with very low mass.• If the Q2 (=m2) of virtual photon is sufficiently small, the source strength should be the

same• The ratio of real photon and quasi-real photon can be calculated by QED Real photon yield can be measured from virtual photon yield, which is observed as

low mass e+e- pairs

SMM

m

M

m

dNdM

Nd

eeee

e

ee

e

ee

121

41

3

22

2

2

22

inclusive

direct

inclusive

direct

Kroll-Wada formula

S : Process dependent factor• Case of Hadrons

•

• Obviously S = 0 at Mee > Mhadron

• Case of *• If pT

2>>Mee2

• Possible to separate hadron decay components from real signal in the proper mass window.

3

2

222 1

hadron

eeee M

MMFS

1S

q

g q

e+

e-

Determination of * fraction, r

r : direct */inclusive *

Direct */inclusive * is determined by fitting the following function for each pT bin.

eedirecteecocktaileedata MfrMfrMf 1Reminder : fdirect is given by Kroll-Wada formula with S = 1.

• Fit in 80-300MeV/c2 gives– Assuming direct * mass

shape• 2/NDF=11.6/10

– Assuming shape instead of direct * shape

• 2/NDF=21.1/10• Twice as much as

measured yield

• Assumption of direct * is favorable.

direct */inclusive *

μ = 0.5pT

μ = 1.0pT

μ = 2.0pT

μ = 0.5pT

μ = 1.0pT

μ = 2.0pTBase line Curves : NLO pQCD calculations with different theoretical scales done by W. Vogelsang.

ThadronT

NLOT

NLO dpddpddpd //// p+p• Consistent with NLO pQCD

– better agreement with small µ

Au+Au• Clear enhancement above NLO pQCD

p+p Au+Au

Direct Photon Spectra

inclusiveinclusive

directdirect

inclusive

direct

inclusive

direct

The virtual direct photon fraction is converted to the direct photon yield.

• p+p– First measurement in 1-4GeV/c– Consistent with NLO pQCD and

with EmCal method– Serves as a crucial reference

• Au+Au– Above binary scaled NLO pQCD– Excess comes from thermal

photons?NLO pQCD (W. Vogelsang)

Fit to pp

exp + TAA scaled pp

nTppAAT bpATTpA )/1()/exp( 2

scaled ppexponential

1st measurement of Thermal Radiation• Au+Au = pQCD + exp.

T = 221 23 (stat) 18 (sys)

• Initial temperatures and times from theoretical model fits to data:

– 0.15 fm/c, 590 MeV (d’Enterria et al.)– 0.17 fm/c, 580 MeV (Rasanen et al.)– 0.2 fm/c, 450-660 MeV (Srivastava et al.)– 0.33 fm/c, 370 MeV (Turbide et al.)– 0.6 fm/c, 370 MeV (Liu et al.)– 0.5 fm/c, 300 MeV (Alam et al.)

From data: Tini > 220 MeV > TC From models: Tini = 300 to 600 MeV = 0.15 to 0.5 fm/c

D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006)

Dilepton Spectrap+p Au+Au

• p+p– Agreement with cocktail

• Au+Au– pT>1GeV/c: small excess internal conversion of direct photons

– pT<1GeV/c: large excess other source???

Summary

p+pLOW MASS:• Excellent agreement with hadronic

decay cocktail

INTERMEDIATE MASS:• Extract charm and bottom cross sections• σcc = 544 ± 39 (stat) ± 142 (syst) ± 200

(model) μb• σbb= 3.9 ± 2.4 (stat) +3/-2 (syst) μb

DIRECT PHOTONS• p+p in agreement with pQCD

Au+AuLOW MASS:• Enhancement above the cocktail expectations:

3.4±0.2(stat.) ±1.3(syst.)±0.7(model)• Centrality dependency: increase faster than

Npart

• Enhancement concentrated at low pT

INTERMEDIATE MASS:• Coincidence agreement with PYTHIA• Room for thermal radiation?

DIRECT PHOTONS:• Dielectron mass shape for pT > 1 GeV and mee

< 300MeV consistent with internal conversions of virtual photons

• Au+Au above pQCD• excess with inv. slope: Teff = 221 ± 23 (stat) ±

18 (syst)• well described by hydrodynamical models with

initial coonditions of Tinit=300–600 MeV at τ0 = 0.15–0.6 fm/c

• First dielectron continuum measurement at RHIC

– Despite of low signal/BG– Thanks to high statistics– Very good understanding of background

normalization

•HBD upgrade will reduce background great improvement of systematic and statistical uncertainty (LMR)

•Silicon Vertex detector will distinguish charm from prompt contribution (IMR)

17

Backup

The matter is so dense that it melts(?) J/ (and regenerates it ?)

sQGP @ RHIC

The matter is so opaque that even a 20 GeV 0 is stopped

The matter is so strongly coupled that even heavy quarks flow

The matter is so dense that it modifies the shape of jets

PHENIX preliminary

strongly interacting Quark-Gluon Plasma (sQGP) in HI collisions at RHIC

The matter is so dense that even heavy quarks are stopped

What does it emit?What is the temperature?

Not a new idea…

• The idea of measuring direct photon via low mass lepton pair is not new one. It is as old as the concept of direct photon.

• This method is first tried at CERN ISR in search for direct photon in p+p at 55GeV. They look for e+e- pairs for 200<m<500 MeV, and they set one of the most stringent limit on direct photon production at low pT

• Later, UA1 measured low mass muon pairs and deduced the direct photon cross section.

J.H.Cobb et al., PL 78B, 519 (1978)

Credit to try it in PHENIXgoes to Y.Akiba

Previous measurements

The enhancement is concentrated at low pT

CERES measured an excess of dielectron pairs, confirmed by NA60, rising faster than linear with centrality attributed to in-medium modification of the spectral function from annihilation.

CERES

CERES

NA60

NA60

Understanding the pT dependency

• Comparison with cocktail

• Single exponential fit:– Low-pT: 0<mT<1 GeV– High-pT: 1<mT<2 GeV

• 2-components fits– 2exponentials– mT-scaling of 0 +

exponential• Low pT:

– inverse slope of ~ 120MeV

– accounts for most of the yield

Extract 2 components2 EXPONENTIALS HAGEDORN + EXPONENTIAL

•We fit the sum of 2 exponentials (a*exponential1 + b*exponential2) •We fit Hagedorn to Mee<100MeV (0-dominated)

•Then we fit (a*mT-scaling + exponential) to the other mass bins•Because of their different curvature, mT-scaling and the exponential account for more or less of the yield in the low-pT region.

Yields and Slopes

•Intermediate pT: inverse slope increase with mass, consistent with radial flow•Low pT:

•inverse slope of ~ 120MeV •accounts for most of the yield

SLOPES YIELDS

Total yield (DATA)

2expo fitmT-scaling +expo

fit

Low-pT yield

Theory Comparison II• Freeze-out Cocktail + “random”

charm + spectral functionLow mass• M>0.4GeV/c2:

some calculations OK• M<0.4GeV/c2:

not reproducedIntermediate mass• Random charm + thermal partonic

may work

Low-pT slope not reproduced

Gluon Compton

q

g q

e+

e-q

q

HADRONIC

PARTONIC

- annihilationq-q annihilation

R.Rapp + H.vanHeesK.Dusling + I.ZahedE.Bratkovskaja + W.Cassing

Theory Comparison II

Calculations fromR.Rapp & H.vanHeesK.Dusling & I.ZahedE.Bratovskaja & W.Cassing (in 4)

Questions

1. Enhancement at M<2M

If pions are massless can annihilation produce ee with M<300MeV?

2. Enhancement at low pT, with T~120 MeV and now flow

Is the same low-pT enhancement seen at SPS never reproduced by theory?

Different initial temperatureDifferent system evolutionDo we miss something in the system

evolution which may have different relevance at SPS and at RHIC?

RHIC SPS

PHENIX (Pioneering High Energy Nuclear Interaction eXperiment)

• 2 central arms: electrons, photons, hadrons– charmonium J/, ’ ee

– vector meson ee – high pT

– direct photons– open charm – hadron physics

• 2 muon arms: muons– “onium” J/, ’, – vector meson – open charm

• combined central and muon arms: charm production DD e

• global detectors forward energy and multiplicity– event characterization

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

Photon conversion rejectionConversion removed with orientation angle of the pair in the magnetic field

Photon conversion

e+e- at r≠0 have m≠0(artifact of PHENIX tracking: no tracking before the field)• effect low mass region• have to be removed

Beampipe

MVD support structures

r ~ mee

InclusiveRemoved by phiV cutAfter phiV cut

z

y

x e+e-

BConversion pair

z

y

xe+

e-

B

Dalitz decay

Photon conversion cutNo cutM<30 MeV & V<0.25 &

M<600 MeV & V<0.04 M<600 MeV & V<0.06 M<600 MeV & V<0.08 M<600 MeV & V<0.10 M<600 MeV & V<0.12 M<600 MeV & V<0.14 M<600 MeV & V<0.20 M<600 MeV & V<0.40

Physical backgroundSemi-correlated Background

π0

π0

e+

e-

e+

e-

γ

γ

π0

e-

γ

e+

X

• 0*

e+e- e+e-

• “jets”

z

y

x e+ e-

B

Conversion pairz

y

x e+

e-

B

Dalitz decay

Photon conversion

e+e- at r≠0 have m≠0(artifact of PHENIX tracking)Conversion removed with orientation angle of the pair in the magnetic field

Background is charge-independentCalculate the shape with MCNormalize to the like-sign spectra Good description of the data

arXiv: 0802.0050

Combinatorial BackgroundPHENIX 2 arm spectrometer acceptance:

dNlike/dm ≠ dNunlike/dm different shape need event mixing(like/unlike differences preserved)Use Like sign as a cross check for the shape and to determine normalization Small signal in like sign at low massN++ and N–- estimated from the mixed events like sign B++ and B-- normalized at high mass (> 700 MeV)Normalization: 2√N++ N-- Uncertainty due to statistics of N++ and N--: 0.12%Correction for asymmetry of pair cut K=k+-/√k++ k-- = 1.004Systematic error (conservative): 0.2%

TOTAL SYSTEMATIC ERROR = 0.25%

LIKE SIGN SPECTRA

Use same event topology (centrality, vertex, reaction plane)Remove every unphysical correlation

Comparison of BG subtraction Methods

33

to determine syst. uncertainty:spread of two extreme cases (blue & orange): 5-10%

Monte Carlo methodLike sign method (with some variations)give consistent results over the full invariant mass range

Acceptancech

arg

e/p

T

0

0

z vertex

• Define acceptance filter (from real data)• Correct only for efficiency IN the acceptance• “Correct” theory predictions IN the acceptance

Single electron pT > 200 MeVPair mT > 400 MeVNot an analysis cut, but a constrain from the magnetic field

mass

pT

Cross check Converter MethodWe know precise radiation length (X0) of each detector material

The photonic electron yield can be measured by increase of additional material (photon converter was installed)The non-photonic electron yield

does not increasePhotonic single electron: x 2.3Inclusive single electron :x 1.6Combinatorial pairs :x 2.5

Photon Converter (Brass: 1.7% X0)

Ne Electron yield

Material amounts:

0

0.4% 1.7%

Dalitz : 0.8% X0 equivalent radiation length

0

With converter

W/O converter

0.8%

Non-photonic

Photonic

converter