High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL

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High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL

Pion and Photon Production in Heavy Ion Collisions

G. David, BNLHigh pT Physics at LHC – Tokaj, Hungary

March 17, 2008

We got some good answers,but what is the question?

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What I’m trying to convince you (or just call it simply “outline”)

0 nuclear modification factors: from qualitative to (painfully) quantitative - decreasing errors, systematic species/energy scan - measuring the reference in the same experiment is crucial - is (-integrated) RAA truly dead? Pro’s and con’s of “tomography” - suppression: is it actually flat in our (RHIC) range? Are we at the limit?

Photon spectra: starting to conquer the low pT region (p+p, Au+Au) - once again disagreement with ISR? - first shot at isolated / fragmentation photons - is there a path to disentangle medium pT production mechanisms is Au+Au?

Photon RAA: discovery or experimental bias? - isospin (quark charge squared) effect - 200 vs 62 GeV, updated

QM’08 finished just a month ago, but you’ll see some new results

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Run 2: (PRL 94, 232301 (2005)).

• Hadrons are suppressed, direct photons are not

• No suppression in d+Au

• Evidence for parton energy loss

– Static medium

2color

ˆsE C q LaD µ

– 1D expansion, e.g., GLV model

d1 1

dg

T

NEL

E A y E

• RAA constrains medium properties

This is a -integrated, inclusive observable(“bulk suppression”). Of course it can beredefined into double, triple… differentials

The starting point: nuclear modification factor hadrons vs photons, 200 GeV Au+Au

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p+p 62 GeV (Run 6)

J.Phys.G31:S491 (2005)

PHENIX 62 GeV p+p cross section approx. 2 times higher than ISR average.

Mantra: same experiment, same systematics buys you more precision!

RAA relates A+A yields to p+p yields. Where does the reference come from?

Reference data are crucial

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World data vs data from the same experiment

The point: Same accelerator, same experiment, similar systematic errors more precise mapping of the evolution (even if individual errors are relatively large)

0 RAA, 62GeV Au+Au: 0 points are the same, but the reference changed from fit to world data to our own p+p measurement

New0 RAA, 62GeV Au+Au compared to suppression in 200GeV Au+Au If the new result survives, the physics message changes quite a bit!

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pT- and centrality dependence:New 0 RAA in Au+Au and Cu+Cu at sNN = 200 GeV

Cu+Cu, 200 GeV, 60-94%

Cu+Cu, 200 GeV, 0-10%

Spectra are similar at all centralities and p+p RAA shapes similar (~constant) integration makes sense

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Npart dependence of 0 RAA in Au+Au at sNN = 200 GeV

fit range

pT > 5 GeV 0.58 ± 0.07

pT > 10 GeV 0.56 ± 0.10

T part

part

part

transverse area:

inital gluon density: d d

path length:

2/ 3

1/ 3

/g

A N

N y N

L N

22 2/ 3AA eff part1 1

nnR N

2

AA part1n

R N

2/ 3effeff part

d1 1

dg

T

NEL N

E A y E

Parton energy loss models suggest:

Relation to RAA:

Fit Npart dependence of RAA with:

PHENIX, arXiv:0801.4020 [nucl-ex]

Centrality Dependence of RAA consistent with parton energy lossThere is no end in sight: U+U will show even more suppression

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Npart scaling of RAA expected at the same sNN

Indeed observed: RAA in Au+Au and Cu+Cu similar at same Npart

System size dependence:Npart dependence of 0 RAA in Au+Au and Cu+Cu

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• 62.4, 200 GeV:– Suppression consistent with

parton energy loss for pT > 3 GeV/c

• 22.4 GeV:– No suppression– Enhancement consistent with

calculation that describes Cronin enhancement in p+A

• Parton energy loss starts to prevail over Cronin enhancement between 22.4 and 62.4 GeV

Energy scan / I: pT dependence of 0 RAA in central Cu+Cu

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• 62.4, 200 GeV:

– Npart Dependence of RAA consistent with parton energy loss

• 22.4 GeV

– Enhancement independent of centrality (flat!)

– Possible explanations

• Weak centrality dependence of Cronin enhancement

• Cronin enhancement offset by parton energy loss

Energy scan / II: centrality dependence of 0 RAA in Cu+Cu

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PHENIX preliminary

PHENIX preliminary

• RAA depends on energy loss and steepness of parton spectrum

• Thus, define “fractional energy loss”:

• Relation to RAA for a pion spectrum described by power law with power n

• RAA 0.5 – 0.6 in Pb+Pb at 17.3 GeV (0-1%, p+C reference, WA98)

• However, Sloss at 17.3 GeV is much smaller than at RHIC

– Au+Au, 200 GeV: Sloss = 0.2

– Pb+Pb, 17.3 GeV: Sloss = 0.05

T T: /lossS p p

AA1/( 2)1 n

lossS R

Sloss: a measure of the fractional parton energy loss E/ECentrality dependence, all energies

Energy dependence, same Npart

Klaus Reygers, QM’08

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Same suppression pattern for 0 and : Consistent with parton energy loss and fragmentation in the vacuum

Larger RAA for (and likely also )

Suppression: comparison of particle species:0, , mesons and direct in Au+Au at 200 GeV

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Getting quantitative: statistical analysis

arXiv 0801.1665

Final results (Run-4) on 0 RAA (PHENIX)

Does this bulk (-integrated) quantity really tell you something?

Would it tell you something if the errors on the last points were reduced?

Important: often increase in statistics not only reduces your statistical error, but opens up new ways to reduce systematic errors as well!

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Experimental uncertainties only!

arXiv 0801.1665

PQM predictions (one specific implementation) for various <q> (red curve: best fit)

Quantitative constraints on opacity (PQM)

Note: <q> is not cast in stone, it’s implementation dependent; theoretical uncertainties (much) bigger than experimental ones (Rajagopal: 4-14)

PQM: radiative loss, static medium, no IS mult. scat., no mod. PDF.

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Quantitative constraints on gluon density (GLV)


arXiv 0801.1665

GLV predictions for various dNg/dy (red curve: best fit)

GLV: <L>, opacity exp., Bj. exp. medium, radiative only, IS mult. scat., mod. PDF.

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Quantitative constraints on gluon density (WHDG)


WHDG predictions for various dNg/dy (red curve: best fit)

arXiv 0801.1665

WHDG: <L>, opacity exp., Bj. exp. medium, radiative and collisional, no IS mult. scat., no mod. PDF.

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1, 2, 3 uncertainty contoursSlope consistent with zero: m = 0.0017 +/-0.0035 (+/- 0.0070) c/GeV (1 and 2)

arXiv 0801.1665

With present experimental uncertainties the statement that single high pT 0 is “fragile” to opacity is not supported (more uncertainty in theories).This of course doesn’t mean that multi-differential observables should not be pursued. But they also come at a price

0 RAA fitted with a simple straight line

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Five highest points contribute 70% of the total 2.If the fits are limited to 5-10GeV/c, p-values increase to55% (PQM), 36% (GLV) 17% (WHDG), 75% (linear fit)

Theoretical uncertainties are much larger!

A case for higher statistics Higher statistics helps improve on systematic errors as well!

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Double-differential RAA reveals strong pT and reaction plane (geometry) dependence stronger constraint on energy loss models

But requires more statistics (RXPN better detector resolution is equivalent to higher statistics)

Does this mean the era of bulk RAA is over?

Not quite!

PRC 76 (2007) 034904

A step forward: 0 RAA vs reaction plane(a better handle on geometry)

Less biased (?)Still hard to interpret

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Density time path length averaged over jet productions points in transverse (x,y) plane

Approximate scaling in Lxyexpected for parton energy loss

Experimental evidence weak

Path length dependence of parton energy loss remains an open question

PHENIX, PRC 76, (2007) 034904

Pathlength dependence of suppression

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PRL 98 (2007) 212301

“dihadron pairs are found to be originate mainly from jet pairs produced close and tangential to the surface… a substantial fraction also comes from jets produced at the center with finite energy loss… more sensitive to the initial gluon density than the single hadron spectra that are dominated by surface emission… more robust as probes of the dense medium”

Both 2 reaches its minimum at the same value nontrivial! Quenching description is sane

An argument for tomography (dihadron correlations)

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Let’s see: if with the available dataset…

…I can measure RAA with this precision and IAA with this, which onetells me more?

… and then I still didn’t speak about the errors on the theoretical curves!

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Not so fast: RAA est mort – vive l’RAA “Theory shoot-out” at HP2006: - confronting Eloss models (mostly with PHENIX preliminary 0 RAA data) -integrated RAA doesn’t have enough discriminating power - theorist’s plea: give us double-differential quantities, correlations (control pathlength!) repeated several times at Jaipur (QM’08)

That is a very reasonable request and we are working on it (on the theory side: explain centrality dependence of existing data!)

But there is a catch: - at any given moment (Run-?, RHIC-II) we have some fixed amount of data - from these, RAA can be analyzed better than RAA(), jet pairs,… (stats, reaction plane syst.) - the issue is not only statistics: better statistics usually brings syst. errors down

Therefore, the question becomes quantitative: - what is the incremental gain in discriminating power on the theory side? - what is the incremental loss in precision on the experimental side? - which way to get maximum physics insight?

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High pT direct photon RAA – 200GeV Au+Au

Is the high pT

suppression real?Is it suppression at all? Are p+p data the right thing to normalize photon RAA?

New data and different p+p reference

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Photons in 200GeV p+p (Run-5)0.5pT favored, but even this misses the shape

Black circles: Run-5 data divided by an empirical fit.Blue lines: NLO pQCD (different both in magnitude and shape)

Evolution of the p+p reference -- calculation vs data

20-30% deviation (only!) would be a reason to celebrate 5-6 years ago, but now we are trying to confirm / refute additional signals at that level (like jet-photon conversion or isospin effect)


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PHENIX – “isospin effect” (?)

The isospin effect (charge square sum difference between uud and udd) SHOULD be there, but is this (and only this “trivial effect”) what we see?Or do we see in addition some genuine photon suppression?

Only “primordial” photons should be unaltered, “medium-induced” photons can be enhanced or suppressed

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

pnpp22

collAA

F. Arleo, JHEP09 (2006) O15 W. Vogelsang, NLO pQCD + isospin


RAA with pQCD

RAA with p+p data

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RAA with pQCD

RAA with p+p data

Isospin effect – 200 GeV Au+Au, Cu+Cu

Same pT reach, apparently no suppression in Cu+Cu.Why not? And if suppression is (mostly) isospin, why is it absent in peripheral Au+Au?

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Unfortunately the suppression is seen in a region where we are very sensitive to detector bias (cluster merging).Also, so far it was seen only in one of the detectors (the one more prone to merging)

xT scaling to the rescue?

The reason: certain known detector imperfections (like shower merging, nonlinearity…) are smaller! Yes, we do our best to correct for them but nothing beats not having the problem in the first place…

The catch: sources at intermediate pT (like jet conversion) that are so far of unknown magnitude, come into play, too!


Isospin effect – xT scaling

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So: is it real? Photon RAA at 62 GeV Au+Au


Watch out: here normalization for 200 GeV is data, for 62 GeV NLO pQCD

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10-

6

X10-6

X10-3

X10-3

h-inclusive

h-decay

h-direct 2.5 < pT, < 3.5

Idea: By triggering on a hadron and looking for near-side direct photon partners one can measure the fragmentation photon yield directly

Measure hadron - inclusive and hadron - decay correlations

Decay corr’s are made by tagging0 and by invariant mass

Must know tagging efficiency and false tagging rate precisely dominant source of systematic uncertainty

Near-side -h correlations – p+p, 200 GeV

(M. Nguyen, QM’08, Ali Hanks)

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Fragmentation photons – p+p, 200 GeV First measurement of its kind at RHICConstrain photon fragmentation function

Well-defined measurement, with somewhat ambiguous meaning - at 5-8 GeV direct/inclusive typically 0.15 - fragmentation/inclusive 0.1 (this result) - this would mean 2/3 of direct is fragmentation, theory says <30%

(M. Nguyen, QM’08, Ali Hanks)

But only near side is measured!

Does triggering on a hadron bias the di-jet?

- if not (both sides are similar) 2/3 is valid for the entire event, and contradicts ~30%

- if yes, because only NLO 23 processes contribute (anticorr. between frag. photons on the two sides) then 2/3 (on one side, 0 on other!) becomes 1/3 averaged to the full event

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“Internal conversion” method, now applied to p+p

Direct photons at low pT – p+p, Au+Au, 200 GeV

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Photon flow -- Au+Au, 200 GeV


These are still Run-4 preliminary dataNo discriminative power yet (although the apparent flow at centrality 40-60% so far survived intense scrutiny)

Run-7 (larger dataset, improved reaction plane) will be decisive

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PRL 98 (2007) 012002 (PHENIX)

Isolated photons – p+p, 200 GeV

r=0.5 cone, Econe < 0.1 E

0 taggingOpen circles: isolated from 0

/ all from 0

Dominated by Compton

Good agreement with NLO pQCD above 7GeV

At 3 GeV an additional 15% loss is expected from underlying event (PYTHIA)

Promising first step to extract gluon polarization

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annihilationcomptonscattering

Medium induced(inc.energy loss)

jet

jet fragment photon v2 > 0

v2 > 0

v2 < 0

Fragmentation: non-isolatedBremsstrahlung: non-isolatedJet-photon conversion: isolated“Primordial”: isolated

So if something like this were the truth, in principle you could try to disentangle the components like this:

1/ Get the NN part (including isospin effect)2/ Get the jet-conversion (jet-th) part from isolated, v2<03/ Get the fragmentation from non-isolated, v2>04/ …

TALL ORDER, TO SAY THE LEAST

Note: assuming no energy loss fragmentation is isotropic jet- conversion dominates v2 v2<0

The promise of flow and isolation – sources at mid-pT


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Summary

Constraints on free parameters in theories: experimental and theoretical uncertainties combined are what determines the (discriminative) power of a measurement

PHENIX now does system size and energy dependence of nuclear modification factors. How much can we learn from them? (From all of them, simultaneously!)

Provocatively: we got some good answers – what is the question?

Measuring the evolution / references in the same experiment, similar systematics is crucial

Direct photon suppression so far not disproven, but major open questions persist

Internal conversion photons in p+p seem to confirm (thermal?) excess in Au+Au

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High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL