em probes… ect*, trento, may 20-24, 2013 -- g. david, bnl 1 direct photons at rhic (and other...

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


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)


High pT photons in p+p pQCD testbenchPRD 86, 072008 (2012)

Good agreement with pQCD slight preference for pT/2 scaleTwo methodsIsolated/all direct photons: small contribution from fragmentation


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


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?”

annihilationcomptonscattering

Bremsstrahlung (energy loss)

jet

jet fragment photon

v2 > 0

v2 < 0

…and this is only the high pT, i.e. the “easy” part…


(fm/c)

log t1 10 107

hadron gas

sQGP

hard scatt

Mass(GeV/c2)

0.5

1

* e+e-

virtuality

jet Brems.

jet-thermalparton-medium interaction

By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c2)

Blessing or curse? -- my 1994 cartoon, updated

“Historians”, but very-very hard to read!


Blessing or curse? -- what does measured “T” mean?arXiv:1304.7030The 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…


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

mediumPRL 104, 132301 (2010)

arXiv:1212.3995

PRL 109, 152302 (2012)


arXiv:1208.1234

The low pT (“thermal”) region – from p+p to A+A

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)?

10

Jet quenching

The most quoted single result RHIC paper

Direct photon RAA proves that binary scaling makes sense!

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

Relies on “binary scaling” and experimental handle on collision geometry, which in turn is “proven” by direct photons


arXiv:1208.1234

The high pT (“pQCD”) region – from p+p to A+A

PRL 109, 152302 (2012)

Scaling is not perfect – partially explained by isospin effect


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 (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


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


Direct photons at low pT – rates only

Shown in a zillion different versions, same conclusion: direct photonspectra alone, while important, not sufficient constraint


Direct photon flow at low pT – is it real?

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)

PRL 109, 122302 (2012)

QM’12, arXiv:1212.3995


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


Direct photon flow – where does it come from?

The mantra: you have to explain yield and flow simultaneously!

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.


Direct photon flow – play with aT (fireball acceleration)

PRC 84, 054906 (2011)

Van Hees, Gale, Rapp

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.


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 hydro and QGP? Apparently unanswered (looks a bit like a “fiat” type theory so far where’s the forest?


Direct photon flow – play with magnetic fieldPRL 110, 192301 (2013) Only a few words, since the author is going to

speak tomorrowOrigin (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


Direct photon flow - PHSD

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.)

“Large direct photon v2 … attributed to intermediate hadronic scattering channels and resonance decays not subtracted from the data”


Orwell’s geometry:all collisions are NOT created equal

arXiv:1304.3410

Ratios of identified hadron spectra in peripheral Au+Au and central d+AuBoth 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…

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 0, production at high pT in peripherals is enhanced???


24

Note that RCP drops sharply, indicating major shape change from peripheral to central

Jets, 0, – central to peripheral

For 0, 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


25

Centrality: thinking out loud

The theorist tends to think in terms of impact parameter (b), or Npart, Ncoll, TAB, , … 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 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.


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 ~0“Activity” here is the highest pT for any particle seen around ~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 centerTrigger efficiency vs max pT

in the centerNote 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!


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 Nch statisticallyWeather or not fluctuations are taken into account is irrelevant here

For instance:

Charge distribution in BBC(South, gold going direction)

0-20%

20-4

0%

40-6

0%

60-8

8%

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)


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.


29

Illustration: shift between multiplicity classes / 1

True b, Ncoll

Expectedfwd. mult

Percieved b, Ncoll

Observedfwd. mult

Here is your average, higher centrality event

True b, Ncoll

Expectedfwd. mult

But now a very hard scattering happened (one in a million!), with reduced fwd. response, therefore…

…this is how you classify the event…

…and when you calculate RAA, the denumerator (Ncoll * pp) will be smaller than it should be RAA increases

(There can be other, even more serious effects, as we’ll theorize later)


30

Charge distribution in BBC(South, gold going direction)

0-20%

20-4

0%

40-6

0%

60-8

8%

This is where the event should be

This is where it is actually found

This is where the event should be

This is where it is actually found

Lost!Trig. ineff.

Illustration: shift between multiplicity classes / 2

If (experimental) centrality is determined with fixed (forward) multiplicity thresholds, irrespective of what happened at ~0, events may end up in the wrong centrality class – and attributed an incorrect <Ncoll>


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 pp a la Glauber?)

If not, what will happen?

Will it keep interacting, but with reduced cross-section (like p)?

Will it be completely out of the pool (no more soft production whatsoever?)

Something in between? If so, what? Wounded or amputated nucleon?


32

Can this be tested?

Papp, Levai, Barnafoldi, Zhang, Fai -- nucl-th/0203075

Reduced/vanishing cross-section in a different context:

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



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…


Summary

Don’t cut it: put in proper perspective!

Direct photons are a perfect tool to understand various phases of the collision – if we only knew how to interpret themAt 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 aheadHowever, the way we characterize event geometry, seems to fail in extreme cases, like very asymmetric systems, large pT “one in a million” type events

Unexpected photon v2,long-range jet correlations in d+Au,rapidly rising RAA, …Nature punishes us if we get complacent , nevertheless

Even good ideas can get too much ingrained in our thinking

Time to re-think how we use the Glauber-model?

em probes… ect*, trento, may 20-24, 2013 -- g. david, bnl 1 direct photons at rhic (and other...

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