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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. Real photons are EM probes, too .

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slide1

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

slide2

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

slide3

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

slide4

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

slide5

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

slide6

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

slide7

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

slide8

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

slide9

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

slide10

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

slide11

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

slide12

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

slide13

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

slide14

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

slide15

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

slide16

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

slide17

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

slide18

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

slide19

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

slide20

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

slide21

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

slide22

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

slide23

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

slide24

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

slide25

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

slide26

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

slide27

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

slide28

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

slide29

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

slide30

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

slide31

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

slide32

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

slide33

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

slide34

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