A1262288130jnLKU

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Photons at RHIC intermediate and high pT
The road from the cleanest probe to the universal
probe (and the blessings and curses that come
with it)
G. David, BNL
Data mostly from PHENIX, interpretations not
necessarily endorsed by PHENIX
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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In the beginning
In ancient times (maybe even two decades ago!)
21 major roles were anticipated for photons
? thermal radiation (low pT) ? establish Ti (or
Tc) ? unambiguosly fix the jet energy scale
(high pT) ? bonus due to dominance of Compton
in LO measure polarized gluon structure functions
How times have changed! ? the best (so far
only) Ti estimate comes from virtual photons ?
a plethora of recently conjectured photon sources
make jet E-scale somewhat murkier ? but that
also means that photons are much richer in
information than anticipated, we just have
to learn to decode it
0904.2184
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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High pT photons in pp and AA what is common
and what is different?
Hadron-hadron collisions
At high pT and lowest order Compton dominates.
Heavy ion collisions add
plus Compton/annihilation with one quark from the
thermal bath (abundant source of low x!) ?
jet-photon conversion
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Outline
Direct (prompt) photons, pp, AA spectra,
RAA Baseline decomposition Correlations
photon-hadron (jet), the golden channel Some
more exotic ideas
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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The baseline direct photons in pp collisions
Run-5 (2005) data
Domain of particle physics low multiplicities,
decay photons can be tagged (rejected) with
high efficiency above 4-5 GeV/c ? direct
photons are identified Good agreement with NLO
pQCD (favors QF1/2pT fragmentation
scale) Since Compton dominates, polarized
gluon structure functions can be
measured Much needed reference to establish
what is different in nucleus-nucleus
collisions Reference from the same experiment
reduced systematic errors
Published Run-3 results PRL 98 (2007) 012002
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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The baseline direct photons, world data
Aurenche et al, PRD 73 (2006) 094207
Good description over - 9 orders of magn. in
s - 2 orders of magn. in E_cms - both pp
and ppbar (quark annihilation turned on!)
One single outlyer
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Aside a cautionary tale on the baseline same
experiment, same systematic errors
Same AuAu p0 data, different pp reference for
RAA ? the message is quite different Even
if individual measurements are not better, the
ratio (or excitation function) is more precise
if everything is done in the same experiment
Implications for RHIC low energy beam scan!
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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The big picture in AuAu we got extremely lucky
Hadron suppression is a huge discovery plus
it makes photon measurements somewhat easier
Photon RAA is essentially unity ? TAB scaling
for hadrons is meaningful
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Essentially unity
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Small deviations isospin effect (uud, udd)
200GeV
At 62GeV there is no experimental
difficulty The isospin effect should be
(almost) independent of centrality Little
overlap with CuCu, but the two are consistent
within errors
Could dd collisions help? (One could tag pp, pn,
nn collisions!)
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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The picture so far
We have a a probe that to first order barely
changes in the pQCD regime from pp to
AuAu Caveats ? so far we didnt try to
decompose pp contributors (theres life past
LO!) ? we are testing relatively large x (at
midrapidity), but gluons pile up (and
might saturate already in cold nuclear matter) at
low x! ? you can get a close-to-null result
either because there is really no change
(new physics) or because there is some rich
physics interplay of suppressing and
enhancing mechanisms
Are the same graphs relevant?
Are these really the same?
Does the medium influence this?
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Baseline decomposition fragmentation photons
(two techniques)
Traditional way isolated photons tag direct
photons (no p0, h partner) require no
significant energy nearby Fragmentation photon
the non-isolated fraction
Novel technique trigger on hadrons look
for near-side yield of photons both inclusive
and tagged (p0, h partner) The difference
fragmentation photons
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Baseline decomposition jet shape
pout component perpendicular to trigger
hadron RMS for fragmentation photons
considerably larger
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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AA decomposition (theory)
0807.4771 (Liu, Hirano, Werner, Zhu)
0904.2184 (Gale)
(LO not plotted)
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Direct photon flow and AA decomposition
(practice)
Fragmentation non-isolated Bremsstrahlung
non-isolated Jet-photon conversion
isolated Primordial isolated In principle you
can disentangle, but in practice
Hydro region
0904.2184
Needs 3 smaller errors to become decisive
pQCD region
Can be improved in the works
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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To be more specific
Thermal photon flow, centrality dependence
Flow from all processes, 20-40
0902.1303 (Liu, Hirano, Werner, Zhu)
0904.2184 (Gale)
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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The golden channel photon-hadron (jet)
correlations
0907.4816
The idea is simple fix the jet energy scale
and its estimated magnitude
The potential problem
0904.2184
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Bipartisanship
is starting with a brand-new STAR result
(0912.1871)
IAA RAA
Differences in p0-h and g-h, but similar IAA
Where is the lost energy?
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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g-h correlations in PHENIX, high pT
Slopes

• pp b 6.89 0.64
• AuAu b 9.49 1.37

Significant change in slope (due to medium). How
literally can this FF be taken?
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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g-h correlations in PHENIX, high pT (cont)
pT dep.
cent. dep.
PRC 80, 024908 (2009)
Compared to h-h, where available
zT dep.
Calculations reject fragmentation photons
(isolation cut) data do not
Is this what we are seeing? Is it compatible
with this?
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Inclusive g-h correlations, low pT
If all these correlations make sense
But 224! All right - leading hadrons are
suppressed - jets are suppressed STILL WHERE
IS MY CHANGE??? Will I ever find it (and get
it from the same model?)
Assoc. yields in bins of h
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Some exotica
(I dont want to pick a fight with theorists, so
these are called exotica, because they are
experimentally very challenging)
nucl-th/0404050
High pT photon HBT (2-3 GeV/c pair pT) ?
pre-equilibrium size and time ? rate and
resolution challenged with real photons ?
rate2 limited with gg (or needs converter
jeopardizing other measurements) ?
needs extremely strong, solid theory support to
consider
Low pT photon HBT ? g cross-section at low pT a
la WA98 ? size (and shape!) of thermal system
very different from pions!
(time-average vs freeze-out) ? RP-dependent
measurement, hard, but feasible as gg or
gg
0907.1292
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Some more exotica
leads to (Esumi, QM09)
Remember
Trigger on a (direct) photon, study the
(asymmetric) yields away side. What fraction
of energy is accounted for?
Not exotic, just not done yet photons at low x
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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Quo Vadis, Photons? (Summary)
No big surprises in pp so far (good)
decomposition of higher order effect underway
(even better) AA so far not inconsistent with
expectations but the jury is still out ?
trivial effects only? ? rich physics, several
mechanisms counteracting? ? can flow
components be disentangled? How to make full use
of the golden channel? ? Is there a way to
tag primordial photon (the real measure of jet
energy)? ? Is there a way to find the
change? Photons are overloaded with
information decoding it is probably impossible
without very active collaboration with theorists.
Well, this is why we are here, I guess.
CATHIE workshop, Dec. 16, 2009 G. David, BNL
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CATHIE workshop, Dec. 16, 2009 G. David, BNL