Di-electron Continuum at PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo Rencontres de Moriond - QCD and High Energy Interactions

Di-electron Continuum at PHENIX

Yorito Yamaguchi for the PHENIX

collaborationCNS, University of Tokyo

Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008

1/14

Quark Gluon Plasma– De-confined phase of quarks and gluons– Predicted by lattice QCD

Temperature – higher than 150-200MeV

Energy density– higher than 1GeV/fm3

– Relevance to the history of the early universe– Properties of QGP are of keen interest Strongly interacting system Perfect fluid Experimental approach– Heavy ion collisions at RHIC

√sNN = 200GeV Au+Au collisions

Introduction


2/14

T

1nphard:

/ E Tethermal:Decay photons(0→, →, …)

Photons & Di-leptons


Comptonq

g q

e+e-

Photons and di-leptons are emitted from thermally equilibrated medium.

– Penetrating the strongly interacting matter– Carrying the thermodynamic information directly

Topics on di-electron continuum in low mass region (Below 1GeV)1.Measurement of electron-pair yield in low-mass low-pT region in p+p and Au+Au

Enhancement possibly due to thermal q-qbar and - annihilations

2.Measurement of direct photon yield in p+p and Au+Au with virtual photon method

Thermal photons are primary contributor in 1-5GeV/c.

R. Rapp nucl-th/0204003

R. Rapp nucl-th/0204003

3/14

Rencontres de Moriond - QCD and High Energy Interactions -

Signal Extraction

03/14/2008

arXiv: 0706.3034

arXiv: 0802.0050

Au+Au

p+p

Real signal di-electron continuum

Background sources1. Combinatorial background2. Material conversion pairs3. 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

After subtractionAfter subtractionAfter subtractionAfter subtraction

Remaining pairs Real signal + Hadron decay components

– Estimate hadron components using hadronic cocktail– Tuned to individually measured yields of mesons at PHENIX

4/14

Cocktail Comparison


arXiv: 0802.0050 arXiv: 0706.3034Au+Au

p+p

p+p Excellent agreement with cocktail

Au+Au Large enhancement in 150MeV < mee < 750MeV Integrated yield

– Real/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model)

5/14

Centrality Dependence


Integrated yield divided by Npart/2

a. 150MeV<mee<750MeV Strong centrality

dependence→ Increases faster than Npart a. mee<100MeV (0 region) Agreement with cocktail

arXiv: 0706.3034

● Au+Au● p+p▒ Cocktail

6/14

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


Shape Difference7/14

Shape differences between p+p and Au+Au are larger at lower pT.

○ Au+Au● p+p


pT Dependence

p+p Consistent with cocktail for all mass binsAu+Au Large enhancement at low pT as presented

p+p Consistent with cocktail for all mass binsAu+Au Large enhancement at low pT Enhancement is also observed in pT above 1GeV/c in the low-mass below 300MeV → Second topic of this presentation

8/14

p+p Au+Au

Virtual Photon Measurement

Hadron mass distribution

Obviously S = 0 at Mee > Mhadron

* mass distribution– If pT2>>Mee

2

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


Any source of real can emit * with very low mass. Convert direct * fraction to real direct photon yield

Kroll-Wada formula

SdNMM

m

M

m

dM

dN

eeee

e

ee

e

ee

121

41

3

22

2

2

2

inclusive

direct

inclusive

direct

9/14

S : Process dependent factor

Invariant mass of *

3

2

222 1

hadron

eeee M

MMFS

1S

Cocktail Comparison


p+p Au+Au (MB)

PHENIX Preliminary

03/14/2008

p+p Good agreement between real and cocktail Small excess at higher pT

Au+Au Good agreement in Mee < 50MeV Enhancement is clearly seen above 100MeV.

1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV

10/14

Determination of * fraction, r


r : direct */inclusive *

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

eedirecteecocktaileedata MfrMfrMf 1

Fit in 80-300MeV gives– Assuming direct * mass shape

2/NDF=11.6/10– Assuming shape instead of direct * shape

2/NDF=21.1/10→ Assumption of direct * is favorable.

11/14

Mee (GeV/c2)

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

direct */inclusive *


μ = 0.5pT

μ = 1.0pT

μ = 2.0pT

μ = 0.5pT

μ = 1.0pT

μ = 2.0pT

03/14/2008

Curves : NLO pQCD calculations with different theoretical scales done by W. Vogelsang. Thadron

TNLO

TNLO dpddpddpd ////

p+p• Consistent with NLO pQCD

– better agreement with small µ

Au+Au• Clear enhancement above NLO pQCD

12/14

p+p Au+Au

Direct Photon Spectra


p+p First measurement in 1-3GeV/c Consistent with NLO pQCD→ Serves as a crucial reference

inclusiveinclusive

directdirect

inclusive

direct

inclusive

direct

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

Au+Au Above binary scaled NLO pQCD→ Excess from thermal photons?

13/14

Summary & Outlook


Di-electron analyses in p+p and Au+Au collisions have been done at RHIC-PHENIX. Large enhancement at low mass is observed in Au+Au collisions while the result in p+p collisions is consistent with a hadronic cocktail.

Strong centrality dependence Large enhancement at low pT

The fractions of direct * to inclusive * above pT of 1GeV/c are obtained by making a shape comparison between real pairs and a hadronic cocktail. Direct photon yield in p+p collisions is consistent with NLO pQCD.

First measurement of direct photons for 1<pT<3GeV/c in p+p collisions. The result in p+p serves as a crucial reference to Au+Au result.

Excess of direct photon yield above binary scaled NLO pQCD in Au+Au collisions is observed. The paper on direct photon measurement with virtual photon method will be submitted soon. pT region of direct photon yield in p+p collisions will be extended upward with more statistics. Same analysis will be done in d+Au collisions.

14/14

03/14/2008 Rencontres de Moriond - QCD and High Energy Interactions -

backup

15/14

PHENIX Detector


• Minimum Bias data sample (triggered by BBC) & ERT triggered data sample

• Analyzed events = 1.6B events

• BBC– z-vertex

• DC, PC1– momentum

• RICH & EMCal– Electron ID– Energy – Track Matching – Electron Trigger

16/14

Material Conversion Pair Cut


z

y

x e+e-

B

Conversion pairz

y

x e+

e-

B

Dalitz decay

The pairs from material conversion should be removed.

These pairs can be recognized by its orientation relative to the magnetic field.

r ~ mass

PHENIX Beam Pipe

MVD support structure

No 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

17/14

Additional Correlated Background


π0

π0

e+

e-

e+

e-

γ

γ

π0

e-

γ

e+

π0 γ

e+

e-

e-

e+

Jet cross pair

Dalitz + conversion cross pair

Correlated + combinatorial background is very good agreement with the real like sign mass spectrum.Systematic error due to background subtraction ≈ 2%

18/14

Combinatorial Background (Au+Au)


Normalization factor is determined by like sign pairs.N++ 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%

19/14

Hadronic Cocktail Calculation (p+p)


arXiv: 0802.0050

n0T2TT

3

3

pp)bpapexp(

A

pd

σdE

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

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

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

–

Other mesons are measured individually. Fit with mT scaling of 0 parameterization

–

2220

mmpp mesonTT Hadronic cocktail is well tuned to PHENIX data.

20/14

Theory Comparison 1


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

Low mass• M>0.4GeV/c2:

some calculations OK• M<0.4GeV/c2:

not reproduced

21/14

Theory Comparison 2


22/14

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

T0ave=360 MeV (T0

max=590 MeV)

0=0.15 fm/c

Documents

Di-electron Continuum at PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo Rencontres de Moriond - QCD and High Energy Interactions