1 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL Electromagnetic Probes at RHIC-II G. David Physics Department BNL Jan. 15, 2008
Jan 29, 2016
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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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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… )
Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
From discovery to exploration
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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
Energy/species scan is on everybody’s mind
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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
Energy/species scan is on everybody’s mind
(M. Gazdzicky)
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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
Thermal (?) photons by internal conversion
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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
Direct photons – high and low pT PRL 94 (2005) 232301
QM’05
QM’06
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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
So: is it real? Stay tuned!
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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
Source size – photon HBT (planned)
PRL 93, 162301 (2004)
Hard scattering (2GeV)
STAR proposal
Thermal (<600MeV)
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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
Quantitative constraints on gluon density (GLV)
Experimental uncertainties only!
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
Quantitative constraints on gluon density (WHDG)
Experimental uncertainties only!
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
Both and 0 consistent with 1
Direct and 0 nuclear modification factor RdA in d+Au
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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL
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