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Pion and Photon Production in Heavy Ion Collisions We got some good answers, but what is the question? G. David, BNL High pT Physics at LHC – Tokaj, Hungary March 17, 2008 High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
What I’m trying to convince you (or just call it simply “outline”) p0 nuclear modification factors: from qualitative to (painfully) quantitative - decreasing errors, systematic species/energy scan - measuring the reference in the same experiment is crucial - is (f-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 High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
The starting point: nuclear modification factor hadrons vs photons, 200 GeV Au+Au • Hadrons are suppressed, direct photons are not • No suppression in d+Au • Evidence for parton energy loss • Static medium • 1D expansion, e.g., GLV model Run 2: (PRL 94, 232301 (2005)). This is a f-integrated, inclusive observable (“bulk suppression”). Of course it can be redefined into double, triple… differentials • RAA constrains medium properties
Reference data are crucial RAA relates A+A yields to p+p yields. Where does the reference come from? 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!
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) p0 RAA, 62GeV Au+Au: p0 points are the same, but the reference changed from fit to world data to our own p+p measurement Newp0 RAA, 62GeV Au+Au compared to suppression in 200GeV Au+Au If the new result survives, the physics message changes quite a bit!
pT- and centrality dependence:New p0 RAA in Au+Au and Cu+Cu at sNN = 200 GeV Cu+Cu, 200 GeV, 0-10% Cu+Cu, 200 GeV, 60-94% Spectra are similar at all centralities and p+p RAA shapes similar (~constant) integration makes sense
Npart dependence of p0 RAA in Au+Au at sNN = 200 GeV Parton energy loss models suggest: PHENIX, arXiv:0801.4020 [nucl-ex] Relation to RAA: Fit Npart dependence of RAA with: Centrality Dependence of RAA consistent with parton energy loss There is no end in sight: U+U will show even more suppression
System size dependence:Npart dependence of p0 RAA in Au+Au and Cu+Cu Npart scaling of RAA expected at the same sNN Indeed observed: RAA in Au+Au and Cu+Cu similar at same Npart
Energy scan / I:pT dependence of p0RAA in central Cu+Cu • 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 PHENIX, arXiv:0801.4555 [nucl-ex]
Energy scan / II: centrality dependence of p0RAA in Cu+Cu • 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 PHENIX, arXiv:0801.4555 [nucl-ex]
Sloss: a measure of the fractional parton energy loss DE/E Centrality dependence, all energies 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 Energy dependence, same Npart • 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 PHENIX preliminary Klaus Reygers, QM’08
Suppression: comparison of particle species:p0, h, f mesons and direct g in Au+Au at 200 GeV Same suppression pattern for p0 and h: Consistent with parton energy loss and fragmentation in the vacuum Larger RAA for f (and likely also w)
Getting quantitative: statistical analysis Final results (Run-4) on p0 RAA (PHENIX) Does this bulk (f-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! arXiv 0801.1665
Quantitative constraints on opacity (PQM) Experimental uncertainties only! PQM predictions (one specific implementation) for various <q> (red curve: best fit) 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. arXiv 0801.1665
Quantitative constraints on gluon density (GLV) Experimental uncertainties only! GLV predictions for various dNg/dy (red curve: best fit) GLV: <L>, opacity exp., Bj. exp. medium, radiative only, IS mult. scat., mod. PDF. arXiv 0801.1665
Quantitative constraints on gluon density (WHDG) Experimental uncertainties only! WHDG predictions for various dNg/dy (red curve: best fit) WHDG: <L>, opacity exp., Bj. exp. medium, radiative and collisional, no IS mult. scat., no mod. PDF. arXiv 0801.1665
p0 RAA fitted with a simple straight line Slope consistent with zero: m = 0.0017 +/-0.0035 (+/- 0.0070) c/GeV (1 and 2s) 1, 2, 3s uncertainty contours With present experimental uncertainties the statement that single high pTp0 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 arXiv 0801.1665
A case for higher statistics Higher statistics helps improve on systematic errors as well! Five highest points contribute 70% of the total c2. If the fits are limited to 5-10GeV/c, p-values increase to 55% (PQM), 36% (GLV) 17% (WHDG), 75% (linear fit) Theoretical uncertainties are much larger!
A step forward: p0 RAA vs reaction plane (a better handle on geometry) Double-differential RAA reveals strong pTand 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! Less biased (?) Still hard to interpret PRC 76 (2007) 034904
Pathlength dependence of suppression • Approximate scaling in rLxyexpected for parton energy loss • Experimental evidence weak • Path length dependence of parton energy loss remains an open question PHENIX, PRC 76, (2007) 034904 Density time path length averaged over jet productions points in transverse (x,y) plane
An argument for tomography (dihadron correlations) “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 c2 reaches its minimum at the same value nontrivial! Quenching description is sane PRL 98 (2007) 212301 High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Let’s see: if with the available dataset… …I can measure RAA with this precision and IAA with this, which one tells me more? … and then I still didn’t speak about the errors on the theoretical curves! High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Not so fast: RAA est mort – vive l’RAA “Theory shoot-out” at HP2006: - confronting Eloss models (mostly with PHENIX preliminary p0 RAA data) f-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(f), 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?
High pT direct photon RAA – 200GeV Au+Au New data and different p+p reference Is the high pT suppression real? Is it suppression at all? Are p+p data the right thing to normalize photon RAA? High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Evolution of the p+p reference -- calculation vs data 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) 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) High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
RAA with pQCD RAA with p+p data 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 F. Arleo, JHEP09 (2006) O15 W. Vogelsang, NLO pQCD + isospin High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
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? High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Isospin effect – xT scaling 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! High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
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 High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Near-side g-h correlations – p+p, 200 GeV X10-3 • 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 g and hadron - decay g correlations • Decay corr’s are made by tagging p0 and h by invariant mass • Must know tagging efficiency and false tagging rate precisely dominant source of systematic uncertainty h-inclusive g X10-3 h-decay g X10-6 2.5 < pT,g < 3.5 10-6 h-direct g (M. Nguyen, QM’08, Ali Hanks) High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Fragmentation photons – p+p, 200 GeV First measurement of its kind at RHIC Constrain 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% 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 (M. Nguyen, QM’08, Ali Hanks) High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Direct photons at low pT – p+p, Au+Au, 200 GeV “Internal conversion” method, now applied to p+p High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Photon flow -- Au+Au, 200 GeV These are still Run-4 preliminary data No 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 High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Isolated photons – p+p, 200 GeV PRL 98 (2007) 012002 (PHENIX) Dr=0.5 cone, Econe < 0.1 Eg p0 tagging Open circles: isolated g from p0 / all g from p0 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 High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
jet fragment photon v2 > 0 annihilation compton scattering jet v2 > 0 Medium induced (inc.energy loss) v2 < 0 The promise of flow and isolation – sources at mid-pT Fragmentation: non-isolated Bremsstrahlung: non-isolated Jet-photon conversion: isolated “Primordial”: isolated Note: assuming no energy loss fragmentation g is isotropic jet-g conversion dominates v2 v2<0 1/ Get the NN part (including isospin effect) 2/ Get the jet-conversion (jet-th) part from isolated, v2<0 3/ Get the fragmentation from non-isolated, v2>0 4/ … TALL ORDER, TO SAY THE LEAST So if something like this were the truth, in principle you could try to disentangle the components like this: High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
Summary 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? Constraints on free parameters in theories: experimental and theoretical uncertainties combined are what determines the (discriminative) power of a measurement 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 High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL
High pT Physics at LHC, March 16-19, 2008, Tokaj, Hungary -- G. David, BNL