Direct Photons: Flow, Thermal Yield and High p T R AA Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration

Direct Photons: Flow, Thermal Yield and High pT RAA

Takao Sakaguchi

Brookhaven National Laboratory

For the PHENIX Collaboration

Direct photons basics

Production Process– Compton and annihilation (LO,

direct)– Fragmentation (NLO)– Escape the system unscathed

Carry dynamical information of the state

Temperature, Degrees of freedom

– Immune from hadronization (fragmentation) process at leading order

– Initial state nuclear effect Cronin effect (kT broardening)

2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico2

Photon Production: Yield s

g

*ge+

e-

PHENIX detector

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-

pg

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

2012-04-103 T. Sakaguchi, WWND2012@Puerto Rico


High pT gdir in p+p – (p)QCD test NLO pQCD calculation of gdir yield is tested with p+p collisions

The calculation works very well

Aurenche et al., PRD73, 094007(2007)

Scaling in p+p direct photons Plotting cross-sections from various

experiments against xT = 2pT/√s

Hard scattering process should scale with xT

Scale yields by (√s)n

PHENIX data includes virtual photon results in low pT(1<pT<5GeV/c)

n=4.5 makes a universal line– Consistent with expectation from NLO

pQCD.– See similar analysis arXiv:1202.1762


T. Sakaguchi, WWND2012@Puerto Rico6 2012-04-10

A theory: F. Arleo (JHEP 0609 (2006) 015)• Isospin effect, in addition to jet-quenching(BDMPS) and shadowing.• Jet-photon conversion is not taken into account• Low pT region is underestimated because of lack of jet-photon conversion?

Direct photons in 200GeV Au+Au Direct photons suppressed at very high pT?

– Final result is coming

7 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico

(fm/c)

log t1 10 107

hadrondecays

sQGP

hard scatt

Rich sources of photons in QGP

jet Brems.

jet-thermalparton-medium interaction

hadron gas

Eg

Rate

Hadron Gas

sQGP

Jet-Thermal

Jet Brems.

Hard Scatt

See e.g., Turbide, Gale, Jeon and Moore, PRC 72, 014906 (2005)

Adding virtuality in photon measurement


(fm/c)

log t1 10 107

hadrondecays

hadron gas

sQGP

hard scatt

Mass(GeV/c2)

0.5

1

g* e+e-

virtuality

jet Brems.

jet-thermalparton-medium interaction

By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c2)


Comptonq g*

g q

e+

e-

Internal conv.

One parameter fit: (1-r)fc + r fd

fc: cocktail calc., fd: direct photon calc.

r*dir(m 0.15)

*inc (m 0.15)

*dir(m 0)

*inc (m 0)

dirinc

1

N

dNeedmee

23

14me

2

mee2

(12me

2

mee2

)1

meeF(mee

2 )2(1

mee2

M 2)3

Focus on the mass region where p0 contribution dies out

For M<<pT and M<300MeV/c2

– qq ->* contribution is small– Mainly from internal conversion

of photons

Can be converted to real photon yield using Kroll-Wada formula

– Known as the formula for Dalitz decay spectra

PRL104,132301(2010), arXiv:0804.4168

Low pT photons with very small mass

System size dependence of g fraction g fraction = Yielddirect / Yieldinclusive

Lines are NLO pQCD calculation with mass scales (pT=0.5, 1.0, 2.0)

Largest excess above pQCD is seen at Au+Au.– Moderately in Cu+Cu.


No excess in d+Au(no medium)

Excess also in Cu+Cu

d+Au Min. Bias

Low pT photons in Au+Au (thermal?)

11

PRL104,132301(2010), arXiv:0804.4168


Inclusive photon × gdir/ginc

Fitted the spectra with p+p fit + exponential function– Tave = 221 19stat 19syst MeV (Minimum Bias)

Nuclear effect measured in d+Au does not explain the photons in Au+Au

Au+Au

Initial kT broadening or recombination?


Recombination model claims that the Cronin effect in hadron production is built up by recombination– e.g. R. Hwa, Eur.Phys.J.C43:233(2005)– Cronin effect in direct photon production should be smaller than one in p0

Within quoted errors, the effect is same for p0 and photon production

p0 RAA in d+Au at 200GeV. PRL91, 072303 (2003)

Comparison with a model calculation

Nuclear Effect is slightly seen.


Direct photon v2


15 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico

Photon source detector ~v2~

annihilationcomptonscattering

Bremsstrahlung (energy loss)

jet

jet fragment photon

v2 > 0

v2 < 0

Depending the process of photon production, path length dependence of direct photon yield varies– v2 of the direct photons will become a source detector

– Later thermalization gives larger v2

For prompt photons: v2~0Turbide et al., PRC77, 024909 (2008)

Measuring direct photon v2


Calculation of direct photon v2

= inclusive photon v2 - background photon v2(p0, ,h etc)

PHENIX, arXiv:1105.4126

Centrality dependence of direct photon v2

Very large flow in low pT

v2 goes to 0 at high pT

– Hard scattered photons dominate


PHENIX, arXiv:1105.4126

Comparison with models. No success.. Later thermalization gives larger v2 (QGP photons)

Large photon flow is not explained by partonic flow models


Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1

thermal

diluted by prompt

Chatterjee, Srivastava PRC79, 021901 (2009)

Hydro after t0

This fits to data well, but.. Large flow can not be produced in partonic phase, but could be in

hadron gas phase

This model changed ingredients of photon spectra drastically!– We realized the importance of the data…


thermal + prim. g

van Hees, Gale, Rapp, PRC84, 054906 (2011)

Summary


Primordial hard scattering process is investigated with direct photons in p+p collisions– xT scaling parameter n=4.5 makes a universal curve

Low pT photons in Au+Au show thermal characteristics (exponential slope)

Low pT photons in d+Au exhibit initial state nuclear effect– The effect is very small.

Direct photon v2 has been measured for the first time– Powerful source detector– Unexpectedly large flow was seen. Not explainable by models except for the

one assuming long hadron gas phase.

Backup


2012-04-10T. Sakaguchi, WWND2012@Puerto Rico22

New production mechanism introduced

qg q Jet in-medium bremsstrahlung

Jet-photon conversion

Both are “thermal hard”

• Bremsstrahlung from hard scattered partons in medium (Jet in-medium bremsstrahlung)

• Compton scattering of hard scattered and thermal partons (Jet-photon conversion)

Turbide et al., PRC72, 014906 (2005)R. Fries et al., PRC72, 041902 (2005)Turbide et al., PRC77, 024909 (2008)Liu et al., arXiv:0712.3619, etc..


A plate ~After cooking up ingredients~


Is it (only) an isospin effect?

Taking for example, the isospin effect: Direct photon cross-sections for p+p, p+n and n+n are different because of different charge contents (eq

2)

Effect can be estimated from NLO pQCD calclation of p+p, p+n and n+n– In low pT, quarks are from gluon split no difference between n and p– At high pT, contribution of constituent quarks manifests

Minimum bias Au+Au can be calculated by:

)Z)-(A Z)-2Z(A (Z)(1/A /N nn2

pnpp22

collAA

(sAA/Ncoll)/spp vs pT (sAA/Ncoll)/spp vs xT

Same suppression will be seen in lower pT at sNN=62.4GeV

TS, INPC07, arXiv.org:0708.4265


Looks like there is an isospin effect (and/or PDF effect)

Question: p+p is a right reference to take?– Isospin effect is electric charge dependent, which affects to photons; 0 is

color charge dependent– Therefore, e-loss models so far are still valid

Also see: Miki, Session XV

The test: 62GeV Au+Au direct photons

~18GeV/c@200GeV

Both are reasonable!

Calculations reasonably agree with data

Factors of two to be worked on ..

Correlation between T and t0

Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c

26 T. Sakaguchi, WWND2012@Puerto Rico2012-04-10

0-20% Au+Au

PRL104,132301(2010), arXiv:0804.4168


Cent dN/dy (pT>1GeV)

Slope (MeV)

c2/DOF

0-20% 1.50±0.23±0.35

221±19±19

4.7/4

20-40% 0.65±0.08±0.15

217±18±16

5.0/3

MinBias 0.49±0.05±0.11

233±14±19

3.2/4

Inclusive photon × gdir/ginc

Fitted the spectra with p+p fit + exponential function

Barely dependent of centrality

PRL104,132301(2010), arXiv:0804.4168

Direct photons through dileptons

Reconstruct Mass and pT of e+e-– Same as real photons

– Identify conversion photons in beam pipe using their orientation w.r.t. the magnetic field

Reject them

– ge+e- at r≠0 have m≠0(artifact of PHENIX tracking: no tracking before the field)

2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 28

Dilepton analysis (I)

z

y

x e+e-

B

Conversion pair

z

y

x e+

e-

B

Dalitz decay

Comptonq g

*

g q

e+

e-

e+

e-

External conv. Internal conv.

PHENIX applied internal conversion technique– Real photons can convert to virtual photons– Inv. mass shapes for Dalitz decay of mesons are calculable

using Kroll-Wada formula

If M<<pT, the ratio of observed inv. mass to expected is proportional to direct photon excess ratio

Take ratio where p0 contribution is small S/B increases


Review low-mid pT photons

÷

÷

÷

0-30

90-140

140-200

200-300

Rdata

32

222

2

2

2

2

112

14

13

21)

M

m()m(F

m)

m

m(

m

m

dm

dN

Nee

eeeeee

e

ee

e

ee

ee

Compton

q g*

g q

e+

e-


Hard scattering gdir in Au+Au (high pT)

Blue line: Ncoll scaled p+p cross-section

Au+Au = p+p x TAB holds – pQCD factorization works

NLO pQCD works. Non-pert. QCD may work in Au+Au system


Outcome from Au+Au collisions

Comparing with various sources of electron pairs

Cocktail of the sources are calculated based on p0/h spectra measured in PHENIX

Huge excess over cocktail calculation is seen in 0.2-0.8GeV/c2

PRC81, 034911(2010), arXiv:0912.0244

Thermal radiation from QGP (1<pT<3GeV)– S/B is ~5-10%– Spectrum is exponential. One can extract temperature, dof, etc..

Hadron-gas interaction (pT<1GeV/c): () (), K* K


Difficult objects! Photons from QGP~big challenge~

)0(Im23

Mfpd

dRE em

Bem

)0(Im324

MfMpd

dRem

Bemee

fB: Bose dist. em: photon self energy

photons

dileptons

Interesting, but S/B is small

54321

S/B ratio

Reconstruct Mass and pT of e+e-– Same as real photons

– Identify conversion photons in beam pipe using and reject them

Subtract combinatorial background

Apply efficiency correction

Subtract additional correlated background:– Back-to-back jet contribution– well understood from MC

Compare with known hadronic sources


Dilepton Analysis

π0

π0

e+

e-

e+

e-

γ

γ

π0

e-

γ

e+

2N N N

( , ) 22

T T

BGFG m p FG FG

BG BG

Documents

Direct Photons: Flow, Thermal Yield and High p T R AA Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration