Direct Photon Measurements:initial conditions of heavy ion

reactions at RHIC

Alberica Toiafor the PHENIX Collaboration

Stony Brook University / CERN

IV Workshop on Particle Correlations and Femptoscopy

Krakow, September 11-14 2008

Evolution of the Universe

Nucleosynthesis builds nuclei up to HeNuclear Force…Nuclear Physics

Universe too hot for electrons to bindE-M…Atomic (Plasma) Physics

E/M Plasma

Too hot for quarks to bind!!!Too hot for quarks to bind!!!Standard Model (N/P) Physics

Quark-Gluon

Plasma??

Too hot for nuclei to bindNuclear/Particle (N/P) Physics Hadron

Gas

SolidLiquidGas

Today’s Cold UniverseGravity…Newtonian/General Gravity…Newtonian/General

RelativityRelativity

The “Little Bang” in the lab• High energy nucleus-nucleus

collisions: – fixed target (SPS: √s=20GeV)– colliders

• RHIC: √s=200GeV • LHC: √s=5.5TeV

• QGP formed in a tiny region (10-14m) for very short time (10-23s)

– Existence of a mixed phase?– Later freeze-out

• Collision dynamics: different observables sensitive to different reaction stages

• 2 counter-circulating rings, 3.8 km circumference

• Top energies (each beam):– 100 GeV/nucleon Au-Au.– 250 GeV polarized p-p.– Mixed Species (e.g. d-Au)

Probing Heavy Ion Collisionstime

hard parton scattering

AuAu

hadronization

freeze-out

formation and thermalization of quark-gluonmatter?

Space

Time

expansion

Jet cc e p K

Photons and dileptons: radiation from the media– direct probes of any collision stages (no final-state

interactions)– large emission rates in hot and dense matter – according to the VMD their production is mediated in the

hadronic phase by the light neutral vector mesons (ρ, ω, and φ) which have short life-time Changes in position and width: signals of the chiral transition?

•Direct photon sources:– Compton scattering

qg q

– Annihilationqq g

– Bremsstrahlungfrom inelastic scattering of incoming or thermalized partons

Energy density in heavy ion collisions

• T.D.Lee: “In HEP we have concentrated on experiments in which we distribute a higher and higher amount of energy into a region with smaller and smaller dimensions.” [Rev. Mod. Phys. 47 (1975) 267]

• Energy density: “Bjorken estimate” (for a longitudinally expanding plasma):

Transverse Energy

central 2%

PHENIX130 GeV

int ~ 100x nucleus

~ 10x critical

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?

Photon Emission • Quark Gluon Plasma

– De-confined phase of quarks and gluonsshould emit thermal radiation

• Direct photons are an important probe to investigate the characteristics of evolution of the matter created by heavy ion collisions.– Penetrate the strong interacting matter– Emitted from every stage of collisions

• Hard photons (High pT)– Initial hard scattering, Pre-equilibrium

• Thermal photons (Low pT)– Thermodynamic information from QGP

and hadron gas measure temperature of the matter

– Dominant source for 1<pT<3 GeV/c– Measurement is difficut since the

expected signal is only 1/10 of photons from hadron decays

T

1nphard:

/ E Tethermal:Decay photons(0→, →, …)

S.Turbide et al PRC 69 014903

S.Turbide et al PRC 69 014903

Direct photons in p+p and d+Au

• p+pTest of QCD– direct participant in partonic

interaction– Less dependent on FF than

hadron production• Reduce uncertainty on pQCD

photons in A+A

• good agreement with NLO pQCD

• Important baseline for Au+Au

• d+Au: – initial-state nuclear effects – no final-state effects (no medium

produced)– Study initial-state effects

2

Extended in RUN5 data

Blue line: Ncoll scaled p+p cross-section

Direct Photons in Au+Au

Au-Au data consistent with pQCD calculation

scaled by Ncoll

THERMAL PHOTONS?Measurement at low pT (where an excess above the know sources may hint to thermal photon production) difficult because of detector resolution

Alternative: Virtual Photons• 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-

Signal ExtractionarXiv: 0706.3034arXiv: 0802.0050 Au+A

up+p

• Real signal– di-electron continuum

• 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

Hadronic Cocktail Calculation

arXiv: 0802.0050

n0T2TT

3

3

pp)bpapexp(

A

pd

σdE

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

• Estimate hadron components using hadronic cocktail

• 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

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

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)

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

• No significant indication that this low mass enhancement contribute to m<300 MeV/c2 and pT>1 GeV/c

• We assume that excess is entirely due to internal conversion of direct

Low mass & High pT region

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.

Determination of * fraction, r

r : direct */inclusive *

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

eedirecteecocktaileedata MfrMfrMf 1

Mee (GeV/c2)

Reminder : 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 q-q, -, …?

SLOPE ANALYSIS• 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

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

Summary• We have measured e+e- pairs in p+p and Au+Au

collisions at √sNN=200 GeV– Large excess above hadronic background is observed

• For m<300MeV/c2 and 1<pT<5 GeV/c– Excess is much greater in Au+Au than in p+p

• Treating the excess as internal conversion of direct photons, the yield of direct photon is deduced.

• Direct photon yield in p+p is consistent with a NLO pQCD• Direct photon yield in Au+Au is much larger.

– Spectrum shape above TAA scaled pp is exponential, with inverse slope T=221 ±23(stat)±18(sys) MeV

• Hydrodynamical models with Tinit=300-600MeV at 0=0.6-0.15 fm/c are in qualitative agreement with the data.

• Additional excess in Au+Au at pT<1GeV/c– Inverse slope T~120 MeV Additional source of virtual g around Tcrit, responsible of most of

the inclusive dilepton yield, so far not explained by theories…

Backup

Centrality Dependency

submitted to Phys. Rev. Lett

arXiv:0706.3034

0 region:•Agreement with cocktail

LOW MASS

Low Mass:•yield increases faster than

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

Intermediate Mass:•yield increase proportional to

Ncoll charm follows binary scaling

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

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

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

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

36

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