Direct photons and Jet correlation in HI

Direct photons and Jet correlation in HI

DongJo Kim Feb 15 2007 Munster DongJo Kim Feb 15 2007 Munster

Integrated IAA(0.4<xE<1.0) similar with RAA!

• more suppression, as peripheral central • similar to 0 RAA values!

Promising solution: direct -jets

q g

Calibrated probe of the QGP – at LO, ET, = Pre-quenched ET,jet

No Surface Bias – clearer picture of jet fragmentation modification Hard process – pQCD calcs agree well with data

has no E-loss in medium!

Size of medium

But D(z) is effectively softened in opposing jet

How to construct direct -h yield i. Construct inclusive -h yield

ii. Construct decay -h yield via:– Pair by pair weighted

summation method i. convolutes all 0-h pair

contributions from higher pT

ii. Weight reflects the probability from kinematics for a 0 at given pT to decay into a photon in a given pT range

iii. Subtraction via:

)1

(/11

1hdechinchdir Y

RY

RY −−− −

−=

Assume two photon sources, ( = # photons in data sample):

directdecayincl +=

pT=5-7 GeV/c

pT=9-12 GeV/c

decaydirect

decay direct

Direct photons:

LO (compton, annihilation,…)

NLO (bremsstrahlung, fragmentation)

Hadronic decay photons:

2

2

each’decay

0

Measuring direct -jet yields (I)

Assume two photon sources, ( = # photons in data sample):

Let Y = per-trigger conditional jet pair yield (1/Ntrig)dNpairs/d():

Write in terms of R

decaydirectincl +=

decayincl

decaydir

incl

dirincl YYY

+=

decay

dir

decay

incl

simdecay

measincl

R

+=≅≡ )/()/(

)1(

1)

11(

−−

=→+−=

RYYR

YYR

YR

Y decayincldirdecaydirincl

Measuring direct -jet yields (I)

direct-h± in PYTHIA*

• (LO) prompt-h±

• (NLO) frag-h±

pT =7-9 GeV/c, pT

h =2-5 GeV/c 200GeV

Direct -h:

On near side, PYTHIA says only NLO s contribute.

*For amusement only. Not for serious quantitative predictions

Direct -h yield: Au+Au

• Near side yields consistent with 0 or very small from fragmentation photons

• Away side yields also small, suppressed!?

Comparison btw Au+Au and p+p

• Any clue on away side modification in central AuAu

Away side Integrated yield

• p+p shows a systematic trend of having higher yields in the away side than Au+Au!

We expect that the proportion of direct photons is enhanced as the size of the medium increases, since R is sensitive to suppression.

If two different systems (e.g. Central Cu+Cu and mid-central Au+Au) suppress 0s and s by the same amount, then they should have comparable R values.

Component 2 of 3: “double ratio” R

Component 2 of 3: “double ratio” R

4444 84444 76%200%5030 −− ≈ partpart NCuCuNAuAu

Double ratio R measured in AuAu, but not yet in CuCu…

Use <Npart> scaling to map AuAu R CuCu.

We will refer to the quantity “(direct/decay)Au+Au interp.”

Full test of method in PYTHIA

Black: correlations with “true” direct photon triggers (ID’ed in event record)

Blue: direct photon correlations produced by subtraction method

The bias toward a low jet yield is worse for lower pT photon triggers, where R is smaller.

If one increases R by ~10%, the “true” correlation is roughly recovered. We use this 10% to gauge the systematic error from the subtraction method.

Magenta: direct g-jet per-trigger yield, subtraction method as above, but with R scaled up by 10%

R = 1.90

R = 1.1*1.90 *Please note: This bias is due to the method, not uncertainty in R!

-h+/- != decay-h+/- (can’t neglect ,…)

We are currently working hard to reduce this systematic error.

Dir

ect -

h p

airs

/tri

gg

er

Preliminary Cu+Cu Results: direct -h+/-

per-trigger jet pair yields: systematic from R

systematic from subtraction method

Cu+Cudirect-h+/- vs. -h+/- jet yields

per-trigger jet pair yields: systematic from R

systematic from subtraction method

Direct photons in 200GeV Au+Au

Blue line: Ncoll scaled p+p cross-section• 900M events

• Reached up to 18GeV/c

• Qualitatively well described by NLO pQCD calculations

Direct photon RAA in 200GeV Au+Au

• Used p+p data is the denominator– NLO pQCD as

denominator is shown as well for a reference

• For pT<10GeV, RAA is consistent with Ncoll scaled p+p reference.

• RAA seems to decrease at very high-pT

• Difference of NLO pQCD calcuation and p+p data affects quite a bit.

RAA with pQCD

RAA with p+p data

Comparison with some models

• Turbide et al. (Phys. Rev. C72 (2005) 014906 + Private communication.)– AMY formalism for jet-

quenching effect for fragmentation photons.

– Systematically data points are below theoretical prediction.

• F. Arleo (JHEP 0609 (2006) 015)– High-pT suppression due to

isospin effect, in addition to jet-quenching and shadowing.

• BDMPS for jet-quenching.– Medium induced jet-photon is

not taken into account.– The suppression of very high-

pT photon is well reproduced.??

What is expected from structure function?

100 xT

Au+Au minimum bias

Structure function ratios drops by ~20% from x=0.1 to 0.2?

Eskola,Kolhinen,Ruuskanen Nucl. Phys. B535(1998)351

What is expected from isospin effect?

• Werner Vogelsang provided direct photon cross-section in p+p, p+n and n+n at 200 and 62.4GeV

• Minimum bias Au+Au can be calculated by:

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

pnpp22

collAA σσσσ ++×=

What is RAA for pure hard scattering

• Consistent with F. Arleo’s work.

• Werner’s comment: The isospin effect has to be bigger than for 0, because for 0 the qg channel is proportional to g(x1)*(u+d+ubar+dbar)(x2), among other things, which is flavor and isospin "blind"…

• 62GeV Direct photon would be a good measure of the effect

– The effect could be seen in lower pT region, where the analysis is rather established

• Interplay of jet-photon conversion and isospin effect in 4-6GeV/c region would be, though

One message: RAA<1 for direct photons does not necessarily mean that our messageof “0 suppression at high pT” changes

Direct photon v2 ~what we would see?~• pT>3GeV/c

– N-N and jet fragmentation (v2>0)– Jet-photon conversion, in-medium

bremsstrahlung (v2<0)– Higher the pT gets, lower the v2

become.

• pT<4GeV/c– Thermal photon v2 based on

hydro calculation (roughly 20-30% centrality)

• Quark flow ~ photon flow– dilepton v2 is also predicted, and

is in the same order

annihilationcomptonscattering

Bremsstrahlung (energy loss)

jet

jet fragment photon

v2 > 0

v2 < 0 Chatterjee et al. PRL 96, 202302 (2006)

Turbide, et al., PRL96, 032303(2006)

Direct photon v2 in Au+Au collisions• Hadron decay photon subtracted

from inclusive photon v2.

• Reached up to ~8GeV/c (~4GeV/c in Run2)

• Tends to be positive?

PHENIX PreliminarysNN=200GeV Au+Au

PHENIX PreliminarysNN=200GeV Au+Au

Finalizing systematic errors..

1

).().().( 22

2 −−×

=R

bkgdvinclvRdirv