Direct photons and jet correlations in heavy ion collisions Andrew Adare University of Colorado For the PHENIX Collaboration WWND, February 2007

Direct photons and jet correlations in heavy ion

collisions

Andrew AdareUniversity of Colorado

For the PHENIX CollaborationWWND, February 2007

Andrew Adare - WWND 2007

2Outline

• Motivation: how direct photon-jet correlations could improve knowledge of energy loss over RAA

• Expectations• Analysis techniques

– correlations and per-trigger yields– calculating decay-h from 0-h– subtraction method

• cross-checks and refinements• results


3The QGP is opaque to hadrons

RHIC measurements indicate that the quark-gluon plasma

Is strongly interacting and flows like an ideal fluid, not a gas of hadrons.

Suppresses jet yields and modifies their shapes

Suppression/energy loss depends on the path length traversed. Particles produced near the surface have lower energy loss probabilities.


4Measuring suppressed yields

0, from quark and gluon jets

High-pT hadrons are suppressed by a factor of 5 relative to scaled-up p-p collisions.

But photons are unaffected!

Nuclear modification factor RAA:

Yield (nuclear collisions)

Yield (binary scaled pp collisions)

=


5Shortcomings of RAA

Measurement of RAA is impressive, and is an important step towards quantifying QGP properties.

However, single-particle suppression does not effectively constrain detailed energy-loss pictures.

T. Renk

Need a tool to measure energy loss in the medium with more discriminating power than RAA.


6Promising 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


7

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

directdecayincl

Measuring direct -jet yields (I)

pT=5-7 GeV/c

pT=9-12 GeV/c

decaydirect

decay direct

Direct photons:

-- LO pQCD (compton, annihilation,…)

-- NLO pQCD (bremsstrahlung, fragmentation)

-- Jet-thermal photons

(See Takao’s talk…)

Hadronic decay photons:

2

2

each’decay

0

Cu+Cu:


8Expectations:direct-h± in PYTHIA

High pT direct -h:

On near side, only NLO s contribute.

But at high pT, NLO contribution is small.

Expectation: The near-side peak in direct -jet correlations should be small!

In p+p, away side yield ~ half that of -h.

pT = 9-12 GeV/c, pT

h =3-5 GeV/c

• from – h+/-

• direct – h+/-

PYTHIA 6.205, 200 GeV p+p, <kT> = 2.5 GeV/c


9

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)/(

)/(0

0

Measuring direct -jet yields (II)

)1(

1)

11(

R

YYRYY

RY

RY decayincl

dirdecaydirincl

Requires 3 components


10Component 1 of 3: incl-h+/-

Angular correlation technique:

ID a high-pT “trigger” particle in an event

Measure distribution of angles between trigger and h+/- associated particles in same event

Measure for mixed events. Ratio is C():

)]2cos(21[)(

/

/

22

at

mix

real

vvJ

ddN

ddNC

correlation functions:


11Component 1 of 3: incl-h+/-

per-trigger jet pair yields:


12

We expect that the proportion of direct photons is enhanced as the size of the medium increases.

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

decay

dir

decay

incl

R

1Recall

0 RAA in Au+Au and Cu+Cu


13Component 2 of 3: “double ratio” R

%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.”

%4020%6050 partpart NCuCuNAuAu


14Component 3 of 3: decay-h+/-

correlation functions: Note: assuming decay here


15Component 3 of 3: decay-h+/-

per-trigger jet pair yields:


16

Each 0 has some probability to decay into a photon in a pT

range.

prob. to decay a 0 into a 9-12 GeV/c as a function of 0 pT.

decay-jet yields from 0 jets

Prob(), 9<pT<12

kine

mat

ical

ly

forb

idde

n

Tp

1

Tp

cGeV /91

Each measured 0 – h pair is weighted by this 0 decay probability to give the decay – h correlations.


17Weighted 0 vs. from in PYTHIA

Black: PYTHIA “true” decay -h PTY

Red: PYTHIA pair-weighted -h PTY

Trigger pT: 5-7 7-9 9-12

h p

T : 1-2 3-5

The weighting method closely reproduces the true -h+/- correlations:


18

We can test principles of the analysis method with Monte Carlo:

Does the subtraction formula work?

Calculating decay-h jet correlations from 0-h: what effects must be accounted for?

Cross-checking with simulations

)1(

R

YYRY decayincldir

energy smearing

perfect detector

EMCal energy resolution

and other decays decay angle smearing

• from – h+/-

• decay – h+/-

9-12 x 2-5


19Full 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 -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!

We are currently working hard to reduce this systematic error.

Dir

ect -

h p

airs

/tri

gg

er


20Preliminary Cu+Cu Results: direct -h+/-

per-trigger jet pair yields: systematic from R

systematic from subtraction method


21Cu+Cudirect-h+/- vs. -h+/- jet yields

per-trigger jet pair yields: systematic from R

systematic from subtraction method


22200 GeV p+p direct -jet results

Near side consistent with expectations

Away side peak visible, but systematics are probably underestimated.


23200 GeV Au+Au direct -jet results

Both near and away side yields appear consistent with 0

But again, systematic errors need improvement….


24Summary

• Correlations involving direct photons are an important probe of energy loss and fragmentation function modification in the QGP.

• Subtraction method shows promise, but current measurements don’t yet deliver good resolution.

• Expectations seem to be matched on the near side, where we see ~no signal compared to 0-h for example.

• These are early results: reduction in systematic errors still needed to make strong statements about away-side jet yields. Stay tuned!


25Backups


2602 phase space is flat

Consider sample of many 0s, all with the same pT:

pT

decay prob.density (proportional to dN/dpT

)

pT

2/pT

Normalization requirement of prob. density fn. means prob. to get a photon at pT drops like 2/pT

as pT increases.

Therefore, dN/dpT= 2/pT

In reality, dN/dEis what is truly flat, but we assume dN/dpT

is also flat at midrapidity.

*A more mathematical way to get dN/dpT

= 2/pT

is to boost isotropic decay into lab frame.


27Photons from 0s

Number of photons in bin p1<pT<p2 from the 0 bin:

Two cases:1. 0s are in same pT bin as photons

2. 0s are in higher pT bin than photons

2

1

2

1

2p

p

TT

p

p

TT

dpp

dpdp

dNN

2

1

1

12

2,12

22

21,1

122

p

p

TT

TT

p

p

TT

TT

in

ppp

ppdp

p

pppp

pdp

p

N

T


28Weighting curves for pi0 decay probability

Using analytic form:

Decay probabilities above are combined with falling pi0 spectrum to give the decay photon “spectra”.

The decay gamma-h PTY is independent of the normalization, since both the pairs and triggers are applied the same weight.

Each pi0-h pair gets weighted by these 3 curves to produce the decay gamma-h correlation for the appropriate photon pT bin.

5<pT<7

7<pT<9

9<pT<12

2

1

1

12

2,12

22

21,1

122

p

p

TT

TT

p

p

TT

TT

in

ppp

ppdp

p

pppp

pdp

p

N

T

measured pi0 spectrum, centrality 0-20%

“spectra”.

Integral = # triggers

Documents

Direct photons and jet correlations in heavy ion collisions Andrew Adare University of Colorado For the PHENIX Collaboration WWND, February 2007