EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

1. Direct photons at RHIC (and other issues…) or a quest for the forest G. David, BNL Even good ideas can get too much ingrained in our thinking EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

2. Real photons are EM probes, too  …sharing many advantages with dileptons – like virtually infinite mean free path after created Less (read: no) handle on when during the system evolution they were produced (as opposed to dileptons, the “mass dimension”) But produced at much higher rates (high pT reach) , and the mechanisms are (somewhat) better understood (qualifiers, qualifiers everywhere…!) Irreplaceable in proving that in-medium energy loss studies make sense (and measuring things like gluon PDFs) Revealing some real puzzles on thermalization and collective expansion Apologies in advance: I’ll spend about a third of this talk on an issue which at first glance is completely unrelated – but it is in fact a burning issue where we need all the help the community can give (and not even completely unrelated to photons) EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

3. High pT photons in p+p  pQCD testbench PRD 86, 072008 (2012) Good agreement with pQCD slight preference for pT/2 scale Two methods Isolated/all direct photons:  small contribution from fragmentation EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

4. High pT photons in p+p  pQCD with flying colors PRD 86, 072008 (2012) Simply put: one of the most glorious and beautiful plots Picture-perfect* agreement between theory and data, over many orders of magnitude in collision energy (and it even includes PHENIX low pT) Just one outlyer (E706) – maybe understood (but that’s a different talk) Universal n=4.5 exponent in coll. energy dependence  LO dominates, (would give n=4), PDF stable, … (*) It isn’t, but with the current experimental errors it would be arrogant to complain  Here we certainly seem to see both the trees and the forest EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

5. jet fragment photon annihilation compton scattering jet v2 > 0 Bremsstrahlung (energy loss) v2 < 0 If photons in p+p are understood, try heavy ions (“yesterday’s discovery is today’s calibration”) Or as the old Viennese saying goes: “Warum denn einfach, wenn es auch kompliziert geht?” …and this is only the high pT, i.e. the “easy” part… EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

6. Blessing or curse? -- my 1994 cartoon, updated “Historians”, but very-very hard to read! hard scatt jet Brems. parton-medium interaction jet-thermal sQGP hadron gas g*  e+e- virtuality log t 1 107 0.5 10 (fm/c) 1 By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c2) Mass (GeV/c2) EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

7. Blessing or curse? -- what does measured “T” mean? arXiv:1304.7030 The temperature-history: interpretation of experimentally fitted T is not trivial (depends on model). May be OK as lower limit. Dielectrons (mass-dependent T) to the rescue… EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

8. The Ianus-faced photons in heavy ion collisions The most direct observables from the medium itself The cleanest probes of pQCD, IS: they couldn’t care less about the medium PRL 104, 132301 (2010) PRL 109, 152302 (2012) arXiv:1212.3995 EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

9. The low pT (“thermal”) region – from p+p to A+A arXiv:1208.1234 No excess in p+p, apparently no excess in d+Au, substantial excess in Au+Au in the pT region where thermal radiation would be expected Note: lack of “thermal” radiation in d+Au  isn’t this evidence against collectivity (in the hydro “flow” sense)? EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

10. Jet quenching Relies on “binary scaling” and experimental handle on collision geometry, which in turn is “proven” by direct photons Direct photon RAA proves that binary scaling makes sense! The most quoted single result RHIC paper EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

11. The high pT (“pQCD”) region – from p+p to A+A Scaling is not perfect – partially explained by isospin effect arXiv:1208.1234 PRL 109, 152302 (2012) EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

12. A big relief: Ncoll scaling makes sense (at high pT) PRL 109, 152302 (2012) The basic tenets behind all “Eloss”, “jet quenching” and “tomography” - hard probes are produced before any medium, collectivity emerges - for hard probes A+A is an incoherent superposition of p+p collisions - the proportionality (Ncoll) can be derived from simple geometry and s (analytic or MC Glauber) Since photons (almost) don’t interact with the medium, they should be uneffected as they apparently are Small perturbations (like isospin effect) possible, but the fundamental picture seems to hold EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

13. Additional evidence If the basic tenets hold, “flow” of high pT photons should be about zero (fragmentation, jet-medium photons may modulate the picture) And indeed, they are: PRL 109, 122302 (2012) The sources are predominantly jets. RP measured “close” to the jet: bias RP measured “far” from the jet EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

14. Direct photons at low pT – rates only Shown in a zillion different versions, same conclusion: direct photonspectra alone, while important, not sufficient constraint EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

15. Direct photon flow at low pT – is it real? PRL 109, 122302 (2012) Initially treated with a liberal dose of scepticism, but finally got accepted for publication (around the same time when ALICE made the similar observation in Pb+Pb) QM’12, arXiv:1212.3995 EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

16. Direct photon flow at low pT – confirmation Before QM’11, when we released the photon flow paper, PHENIX was also worried, since the analysis is tricky. So we didn’t release anything until we had - for internal consumption only back then – a completely independent confirmation: External conversions: low rates, but excellent resolution, good particle ID. While challenging, all difficulties are – so to speak – “orthogonal” to the other method; really independent confirmation EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

17. Direct photon flow – where does it come from? PRC 79, 021901 (2009) The easiest way to get high rates is high (early) temperatures  but no flow there yet, just acceleration The easiest way to get high flow is late (long acceleration), just before kinetic freeze-out but lower (thermal) rates Having both high rates and high flow is something like “having your cake and eating it, too”, tantalizing theorists for years now. The mantra: you have to explain yield and flowsimultaneously! EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

18. Direct photon flow – play with aT (fireball acceleration) If true, “QGP window” is essentially gone (QGP is not the dominant source at any pT), and the large apparent temperature is mostly of hadronic (+ blue shift) origin. Van Hees, Gale, Rapp PRC 84, 054906 (2011) EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

19. Direct photon flow – play with time F.-M. Liu Early hydro initial time, QGP forms considerably later (0.6 f/c vs QGP formation times up to 2.1 f/c)  early emission (no flow part) was overestimated arXiv:1212.6587 Q: what is the emission between thydro and tQGP? Apparently unanswered (looks a bit like a “fiat” type theory so far  where’s the forest? EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

20. Direct photon flow – play with magnetic field PRL 110, 192301 (2013) Only a few words, since the author is going to speak tomorrow Origin (at least in part) of the large photon flow could be the strong magnetic field? I loved this paper, because it explicitely told what could disprove the theory! A simple exercise is here: (it could use smaller error-bars…) Also, v4 is in the works, coming soon EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

21. Direct photon flow - PHSD “Large direct photon v2 … attributed to intermediate hadronic scattering channels and resonance decays not subtracted from the data” arXiv:1304.7030 Runs counter “conventional” wisdom (which gets most of the flow from the hadronic phase). Interesting, if it holds up. Centrality dependence? Also, claiming a good description of the rate may be a stretch. (And yes, omega is subtracted.) EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

22. Orwell’s geometry: all collisions are NOT created equal arXiv:1304.3410 Ratios of identified hadron spectra in peripheral Au+Au and central d+Au Both Npart and Ncoll virtually identical (eccentricity of course is not) The ratios are constant (up to the highest pT) but not one! (0.65) Isn’t the Glauber counting too simplistic? Is a “collision” in Au+Au the same thing as in d+Au (or p+Au)? Of course it isn’t… EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

23. Now some results to lose sleep over PHENIX preliminary, QM’12 2008 (high) statistics d+Au data, nuclear modification factors vs centrality Is it possible that p0, h production at high pT in peripherals is enhanced??? EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

24. Jets, p0, h – central to peripheral For p0, h this is true pT, for jets it is total jet energy. There is no unambiguous transformation, but 1./0.7 is a reasonable compromise, and would put the points on top of each other. Important: RCP is independent of any p+p reference! The only “external” quantity here is the Ncoll value attributed to the individual centrality classes Note that RCP drops sharply, indicating major shape change from peripheral to central EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

25. Centrality: thinking out loud The theorist tends to think in terms of impact parameter (b), or Npart, Ncoll, TAB, e, … none of which is directly accessible in the experiment The experimenter is concerned whether a/ the event is taken at all (trigger bias/efficiency) b/ there are some global observables that can be tied to the theorists’ quantities and while they are correlated to those quantities, they are as uncorrelated as possible to the specific features of the event (like presence of jets, flow, etc.) Assuming such observable(s) exist, a model is agreed upon that makes the translation between experimental observables and theoretical quantities Since you want to avoid introducing biases as much as possible, the model is tuned with a large number of (more or less) average events, in regions preferably “far” from the regions with the “specific features” studied (like a large h gap) The correlation between the global observable and the theoretical quantity is typically wide: events on the average will be properly classified – but not necessarily individually. EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

26. The verifiable case: p+p Triggering and event characterization: looking for activity (e.g. charged particle production Nch, transverse energy ET) preferably close to the beam and far from the region of interest (mid-rapidity) Typical Nch dist. close to the beam for average p+p Now study those distributions as a function of the activity observed at h~0 “Activity” here is the highest pT for any particle seen around h~0; could be jet energy, etc. Can be done both in simulation and in data! Mean and RMS of the Nch dist. vs max pT in the center Trigger efficiency vs max pT in the center Note the characteristic rise initially (well-known: higher activity when hard scattering occurs) However, at higher pT they start to drop slowly. They have to, at least asymptotically, for simple kinematic reasons. Of course other mechanisms can deplete forward activity way before kinematics does! EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

27. Glauber-model and centrality in p+A, d+A, … Straight path, independent collisions with the same probability (cross section)  Ncoll, Npart Folding with the average response observed in p+p can tie Ncoll, Npart to observed Nchstatistically Weather or not fluctuations are taken into account is irrelevant here For instance: Charge distribution in BBC (South, gold going direction) 40-60% 60-88% 20-40% 0-20% Experimentally defined centrality classes Ncoll distribution for each class from the model Based on average responses, does not take into account possible special features of rare events (like high pT particle or jet in the central region) EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

28. Will this always work without further corrections? Not necessarily. For instance, as we have seen for p+p, the trigger efficiency decreases with increasing energy in the center. Since the trigger requires coincidence on both sides and in pA, dA on one side there are at most two nucleons, a similar drop in efficiency is expected. This is well known and usually taken into account. Centrality is usually defined in the direction where the large ion goes. Assume the projectile makes N collisions, one of them with very high pT. Then the expected multiplicity forward is only (N-1) times the average plus one reduced response  the multiplicity observed by the experimenter (forward) is smaller than it would be for an event that is identical except that no high pT is present If centrality is defined with fixed multiplicity thresholds based on the average events but applied to the rare, special ones, those rare events may be (mistakenly) classified as lower centrality (lower average Ncoll) than they really are. At higher pT this effect typically shifts to lower multiplicity (i.e. lower centrality) classes events that Ncoll-wise – i.e. from the point of view of how probable a rare, hard collision is – would belong in a higher centrality class. EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

29. Illustration: shift between multiplicity classes / 1 Here is your average, higher centrality event Observed fwd. mult But now a very hard scattering happened (one in a million!), with reduced fwd. response, therefore… Percieved b, Ncoll True b, Ncoll True b, Ncoll Expected fwd. mult Expected fwd. mult …this is how you classify the event… …and when you calculate RAA, the denumerator (Ncoll * spp) will be smaller than it should be  RAA increases (There can be other, even more serious effects, as we’ll theorize later) EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

30. Illustration: shift between multiplicity classes / 2 This is where it is actually found Lost! Trig. ineff. This is where the event should be Charge distribution in BBC (South, gold going direction) 40-60% 60-88% 20-40% This is where it is actually found This is where the event should be 0-20% If (experimental) centrality is determined with fixed (forward) multiplicity thresholds, irrespective of what happened at h~0, events may end up in the wrong centrality class – and attributed an incorrect <Ncoll> EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

31. More exotic possibilities Indavertent confusion from the dual use of Ncoll (???) We use it both to estimate the average soft response by folding the p+p distribution (which assumes that the likes of Ncoll average p+p collisions in fact do happen in the event) but then we also use Ncoll to estimate how much an extremely rare p+p process (hard scattering) is enhanced in p/d+A, where it is still very-very rare (<<1/event) But in those very rare instances when hard scattering did in fact happen, will the d/p nucleon for the rest of its path interact with the remaining A nucleons aa the original, intact nucleon (i.e. with the same spp a la Glauber?) If not, what will happen? Will it keep interacting, but with reduced cross-section (like spp)? Will it be completely out of the pool (no more soft production whatsoever?) Something in between? If so, what? Wounded or amputated nucleon? EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

32. Can this be tested? Reduced/vanishing cross-section in a different context: Papp, Levai, Barnafoldi, Zhang, Fai -- nucl-th/0203075 High pT biases Renk, arXiv:1212.0646 Would comparison to LHC help? If (with similar centrality determination) LHC would see no effect in our pT range, but similar effect at higher pT, the “kinematic” effect (depletion of available energy forward) could be the culprit (or dominant) If LHC would see a similar effect already in our pT range, the “dynamic” effect (reduced or vanishing cross section) could be the dominant contributor EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

33. Just to avoid confusion / misinterpretation The Glauber-model is adequate and working for what is was originally meant (soft physics, average events and / or very large systems) The fact that the presence of a high pT particle biases distributions far away in rapidity, is not only a kinematic triviality, but also proven by data In A+A getting one nucleon “out of the pool” barely changes the global event (not even in peripheral) However, in d+A (or even worse, in p+A) once a hard collision happened, one nucleon (or the nucleon!) of the projectile may be “out of the pool”,  the global event changes drastically. Applying the same centrality classification as for the average event may be misleading! This is a very serious problem since we know little, if anything about what does a nucleon do after making a hard collision – while currently we treat this case as if nothing happened, kept interacting like an intact one… EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL

34. Summary Direct photons are a perfect tool to understand various phases of the collision – if we only knew how to interpret them At high pT the appear to “behave” – reasonably well described from p+p to A+A  increased experimental precision may lead to disentangling finer effects (like positive/negative flow for isolated/non-isolated photons). No imminent panic here, just years of work ahead However, the way we characterize event geometry, seems to fail in extreme cases, like very asymmetric systems, large pT “one in a million” type events Time to re-think how we use the Glauber-model? Unexpected photon v2, long-range jet correlations in d+Au, rapidly rising RAA, … Nature punishes us if we get complacent , nevertheless Don’t cut it: put in proper perspective! Even good ideas can get too much ingrained in our thinking EM probes… ECT*, Trento, May 20-24, 2013 -- G. David, BNL