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!?
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
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