phys. dept. seminar, jan. 15, 2008 – g. david, bnl

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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

Electromagnetic Probes at RHIC-II

G. DavidPhysics Department

BNLJan. 15, 2008

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Roots: Nov. 2004 Physics Working Group organized to explore physics possibilities offered by RHIC-II (actually: what is RHIC-II?)

Trigger: arXiv: nucl-ex/0611009 (RHIC-II electromagnetic probes working group write-up, soon to be published, and Rachid read it… )

Easy: preaching to the choir (hopefully )

Hard: timing is awkward (QM’08 is around the corner but several new results cannot be shown now…)

The story line: - same facility, same experiment(s) reduced systematics - exploration phase precision, precision, precision - rare probes, differential quantities - role of electromagnetic probes in mapping the phase diagram - the dual use of luminosity: high statistics and/or species/energy scan

This talk

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log t

pT

1 10 107

(GeV)

(fm/c)

hadrondecays

hadron gas

sQGP

hard scatt

jet Brems.

All we can see is the projection to this axis

with the dashed sources as background

dominated up to medium pT by this

Penetrating probes emitted at all stages then surviving unscathed ( e << s)“Historians” of the heavy ion collision: encode all sub-processes at all timesBut for the very same reason their message is hard to decipher!

Cartoon only: sources of , mean pT vs time (dashed: hadrons)

jet-thermal

jet fragmentation

Overlapping signals from different sources, separated in time

The promise (and trap) of electromagnetic probes

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I’ll save you (most of) the pep-talk what great discoveries we made.Instead, let me state what we are missing so far ( “exploration”)This is a partial list, with strong bias towards electromagnetic probes.

We didn’t map out the phase diagram, find (or disprove the existence of) the critical point

We don’t know what makes it thermalize incredibly fast

We don’t know what happens to vector meson masses and yields (is chiral symmetry (partially) restored?)

We don’t have a statement yet on thermal photons and initial temperature

We are just getting sensitive enough to differentiate between jet energy loss models and constrain their free parameters

We couldn’t disentangle yet different sources of medium pT direct photons

We must get more precise (while staying accurate… )


From discovery to exploration

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Excess spectrum:

2 analysis (ndf = 12, stat. errors only):

2 (i.m.h.) = 9.4 (P = 66.8%)2 (d.ρ-m.) = 34.6 (P = 0.0005%)

inclusion of syst errors(MC method) yields:

P (i.m.h.) = 84.2%P (d.ρ-m.) = 10.9%

CERES Pb-Au

CERES – PB+Au, mass

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peak: R=C-1/2(L+U)

continuum: 3/2(L+U)

nontrivial changes of all three variables at dNch/dy>100 ?

NA60: centrality dependence of spectral shape

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Strong rise of Teff with dimuon mass even persists for the pure in-medium part. Sudden drop for M>1 GeV now even more sharply defined

Rise consistent with radial flow of a hadronic source (here →→)

Drop signals sudden transition to low-flow source, i.e. source of partonic origin ?(here qq→)

Disentangle pT spectra of thermal continuum (from peak), get slopes (steeper than peak)Flow is not directly measured (yet?) pT spectra do not depend on centrality, but do depend on mass

Combining M and p T of thermal dileptons to break hadron-parton duality?

NA60: the rise and fall of radial flow of thermal dimuons

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Mantra: “same experiment, same systematics”

“acceptance at y=0 unchanged, multiplicities grow very slowly”

“evolution (ratios) have smaller errors than individual points…”

H. Stocker calls it RHIC…

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Energy/species scan is on everybody’s mind

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Energy/species scan is on everybody’s mind

(M. Gazdzicky)

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The equation-of-state at high B

• collective flow of hadrons• particle production at threshold energies (open charm)

QCD critical endpoint• excitation function of event-by-event fluctuations (K/π,...)

Onset of chiral symmetry restoration at high B

• in-medium modifications of hadrons (,, e+e-(μ+μ-), D)

• mostly new measurements• CBM Physics Book (theory) in preparation

Deconfinement phase transition at high B • excitation function and flow of strangeness (K, , , , )• excitation function and flow of charm (J/ψ, ψ', D0, D, c)• charmonium suppression, sequential for J/ψ and ψ' ?

Compressed Baryonic Matter @ FAIR – high B, moderate T:

searching for the landmarks of the QCD phase diagram• first order deconfinement phase transition • chiral phase transition (high baryon densities!)• QCD critical endpoint

in A+A collisions from 2-45 AGeV starting in 2015 (CBM + HADES)

FAIR

(Claudia Hoehne)

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IMR (1.5-3.0GeV) pT integrated dileptons 1.5-3.0GeV direct photons

low mass,qT>2GeV/cdileptons

Possible sweet spots for electromagnetic probes (QGP)

(compilation by R. Rapp)

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Initial temperature, thermalization scale

Observables come from: production mechanisms folded with system evolution You have to know/assume Ti, Tc, Tf, v1, v2, EOS, … (all interdependent to some level)Case in point:

PHENIX preliminary low pT direct photon spectra reasonably described by models where i ranges 0.15-0.5 (1.3!)fm/c, and Ti ranges 300-660MeV

An interesting proposal: while both real and virtual photon spectra depend on Ti, v0, Tf, EOS (Tc dep. negligible) their ratio may depend only on Ti (?!)

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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNLAlam, Sinha, arXiv:0705.1591

The claim is quite interesting (and relatively “easy” to measure in PHENIX): Rem is sensitive only to the initial temperature, but (largely) independent of flow, Tc, i, form of the EOS, …Note that this independence doesn’t seem to have a “deep” underlying reason (or at least none the authors are aware of…)

Direct measurement of initial temperature?

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Thermal (?) photons by internal conversion

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Direct photons – high and low pT PRL 94 (2005) 232301

QM’05

QM’06

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World data vs own data

The point: Same accelerator, same experiment, similar systematic errors more precise mapping of the evolution (even if 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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PHENIX – “Isospin Effect” (?)

The isospin effect (charge 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

RAA with pQCD

RAA with p+p data

W. Vogelsang, NLO pQCD + isospin

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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)

What is 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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Remember: integrated RAA with NLO pQCD

Most central collisions

Is the high pT

suppression real?

Is it suppression at all?

Are p+p data the right thing to normalize photon RAA?

Photon RAA (200 GeV) with fit to p+p data

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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? Stay tuned!

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1

).().().( 22

2

R

bkgdvinclvRdirv

PHENIX PreliminarysNN=200GeV Au+Au

But this hinges upon a reliable measurement of R and you must go out quite far in pT

Also it cries for statistics and/or better reaction plane resolution

Photon flow (PHENIX)

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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

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Source size – photon HBT (planned)

PRL 93, 162301 (2004)

Hard scattering (2GeV)

STAR proposal

Thermal (<600MeV)

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A step forward: 0 RAA vs reaction plane

PRC 76 (2007) 034904

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

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

Does this mean the era of bulk RAA is over?

Not quite!

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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

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)

0 RAA fitted with a simple straight line

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!

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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)

arXiv 0801.1665

Theoretical uncertainties are much bigger: the ball seems to be in the theorists’ court!

A case for (much) more statistics

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Summary

RHIC-II is apparently on its way, but in today’s world we shouldn’t let our guard down

Dual use of luminosity: energy/species scan and rare probes we are well positioned to find the CEP first we are equally well positioned to nail down jet energy loss mechanisms and much, much more

Statistics not only extend range, reduce statistical errors, make rare probes accessible, but often helps reduce systematic errors as well precision

We are in the exploration phase and the name of the game is precision

But precision isn’t a goal for itself, either: the goal is to constrain (or refute) theories. Sometimes it is best done with increased precision, other times with multi-differential observables

We are in this boat together. So far cooperation of theorists and experimentalists has been stellar. Let’s keep it this way!

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What fraction of pQCD photons are from the primordial hard scattering?Testable in p+p – prompt photons are isolated (little or no energy deposit in the neighborhood)

Important validation of pQCD (and feasible in p+p)

Red curve: prompt Blue curve: fragmentation (30% of all at high pT)

Caveats: - questionable at low pT - purely theoretical (exp. cuts not applied) - real-life cuts always push the red curve higher

Direct photons: prompt vs. fragmentation

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Primary (hard scattering) photons vs. photons associated with jets (fragmentation, FS hadron decay)Calculable, and reasonable agreement with data even if the usual 0.5 cone is near PHENIX’s limit

Fraction of isolated/all photons, p+p, 200 GeVIsolation cut 0.1*E > Econe(R=0.5)

By M.Werlen,JETPHOX-.35<y<.35=pTBFG set2, CTEQ6M

By W.Vogelsang,R=0.4=pT, CTEQ6M

Clean: no additional sourcefrom jet-medium interactionBiased: finite acceptance

Isolated vs. non-isolated photons in p+p

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Both and 0 consistent with 1

Direct and 0 nuclear modification factor RdA in d+Au

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Consistent with 1 No modification within the error

This is first measurement of ‘EMC effect’ for gluons

100 xT

Nuclear Modification Factor

“EMC effect” for gluons

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phys. dept. seminar, jan. 15, 2008 – g. david, bnl

Documents

thispenetrating probes

bnlelectromagnetic probes

precision rare probes

pt of thermal dileptons

thermal photons

radial flow of thermal

phase diagram

lowflow source