post-qm, feb. 12-14, 2008, tifr, mumbai, india -- g. david, bnl

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Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

High pT jets: quenching, Eloss, shape modification

Hot and dense matter in the RHIC-LHC era

Tata Institute for Fundamental ResearchFeb. 12, 2008

G. David, BNL PHENIX Coll.

We got some good answers

but what is the question???

Credits: Andrew Adare, Terry Awes, Mike McCumber, Hua Pei, Matt Nguyen, Klaus Reygers, … The PHENIX Collaboration

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It all starts with this picture: - if a medium is formed (and fast, O(1)fm/c) and its size is O(10)fm/c, hard-scattered partons will travel in it before fragmenting - they will interact with the medium, and lose energy, therefore, their yield at high pT will be depleted w.r.t. p+p yields (and the loss goes somewhere!) - photons will not lose energy, so in -jet measurements they calibrate the original parton energy - such jet suppression will characterize the medium, you just have to decode it

It is as simple as that, with minor complications - hard scattering can occur anywhere, including close to the surface - PDFs may be different in protons and ions - jets are hard to reconstruct, so we often need a proxy (leading fragment) - the lost energy flows into the vast sea of other soft particles - the calibration is tainted since hard scattering is not the only source of energetic photons - …

Why use high pT jets to get medium properties?

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Problems and possible ways out

High pT partons fragment into jets, which are hard to reconstruct in HIC – have to rely on leading particle(s)

In the medium initial geometry and evolution influences EBulk suppression w.r.t. reaction planeMultiparticle correlations

Trigger 0

“Conditional”

charged hadron

at high-pt

MediumAssoc h

Establishingthe originalparton energy -jet

Bulk suppression (-integrated)

…and even morecomplex measurements

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Run 2: (PRL 94, 232301 (2005)).

The starting point: nuclear modification factor

A+B

AB inv T p+p

NNAB coll inel

d / d,

d / d

where /

TAB

N pR

T p

T N

s

s

=´

=

• Hadrons are suppressed, direct photons are not

• No suppression in d+Au

• Evidence for parton energy loss

– Static medium

2color

ˆsE C q LaD µ

– 1D expansion, e.g., GLV model

d1 1

dg

T

NEL

E A y E

• RAA constrains medium properties

This is a -integrated, inclusive observable(“bulk suppression”). Of course it can beredefined into double, triple… differentials

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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.

Improved p+p Reference Data

Mantra: same experiment, same systematics buys you more precision!

RAA relates A+A yields to p+p yields. Where does the reference come from?

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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)

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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pT- and centrality dependence:New 0 RAA in Au+Au and Cu+Cu at sNN = 200 GeV

Cu+Cu, 200 GeV, 60-94%

Cu+Cu, 200 GeV, 0-10%

Spectra are similar at all centralities and p+p RAA shapes similar (~constant) integration makes sense

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Npart dependence of 0 RAA in Au+Au at sNN = 200 GeV

fit range

pT > 5 GeV 0.58 ± 0.07

pT > 10 GeV 0.56 ± 0.10

T part

part

part

transverse area:

inital gluon density: d d

path length:

2/ 3

1/ 3

/g

A N

N y N

L N

22 2/ 3AA eff part1 1

nnR N

2

AA part1n

R N

2/ 3effeff part

d1 1

dg

T

NEL N

E A y E

Parton energy loss models suggest:

Relation to RAA:

Fit Npart dependence of RAA with:

PHENIX, arXiv:0801.4020 [nucl-ex]

Centrality Dependence of RAA consistent with parton energy lossThere is no end in sight: U+U will show even more suppression

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Npart scaling of RAA expected at the same sNN

Indeed observed: RAA in Au+Au and Cu+Cu similar at same Npart

System size dependence:Npart dependence of 0 RAA in Au+Au and Cu+Cu

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• 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

Energy scan / I: pT dependence of 0 RAA in central Cu+Cu

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• 62.4, 200 GeV:

– Npart Dependence of RAA consistent with parton energy loss

• 22.4 GeV

– Enhancement independent of centrality

– Possible explanations

• Weak centrality dependence of Cronin enhancement

• Cronin enhancement offset by parton energy loss

Energy scan / II: centrality dependence of 0 RAA in Cu+Cu

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PHENIX preliminary

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

• 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

T T: /lossS p p

AA1/( 2)1 n

lossS R

Sloss: a measure of the fractional parton energy loss E/ECentrality dependence, all energies

Energy dependence, same Npart

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Same suppression pattern for 0 and : Consistent with parton energy loss and fragmentation in the vacuum

Larger RAA for (and likely also )

Suppression: comparison of particle species:0, , Mesons and Direct in Au+Au at 200 GeV

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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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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 (Rajagopal: 4-14)

PQM: radiative loss, static medium, no IS mult. scat., no mod. PDF.


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Quantitative constraints on gluon density (GLV)


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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Quantitative constraints on gluon density (WHDG)


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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1, 2, 3 uncertainty contoursSlope consistent with zero: m = 0.0017 +/-0.0035 (+/- 0.0070) c/GeV (1 and 2)

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!


0 RAA fitted with a simple straight line

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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)

Theoretical uncertainties are much larger!

A case for higher statistics Higher statistics helps improve on systematic errors as well!

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Double-differential RAA reveals strong pT and 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!

PRC 76 (2007) 034904

A step forward: 0 RAA vs reaction plane

UnbiasedStill hard to interpret

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Pathlength dependence of suppression

Density time path length averaged over jet productions points in transverse (x,y) plane

Approximate scaling in Lxyexpected for parton energy loss

Experimental evidence weak

Path length dependence of parton energy loss remains an open question

PHENIX, PRC 76, 034904

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RAA est mort – vive l’RAA

“Theory shoot-out” at HP2006: - confronting Eloss models (mostly with PHENIX preliminary 0 RAA data) -integrated RAA doesn’t have enough discriminating power - theorist’s plea: give us double-differential quantities (control pathlength!) repeated several times at Jaipur (QM’08)

That is a very reasonable request and we are working on it

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() (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?

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The “holy grail” of jet tomography: -jet correlations

Leading Order picture(almost) exact momentum balance w/ away-side jetCompton dominance

p+p: measure gluon distribution functionA+A:

calibrated probe of energy lossmore sensitive probe than single particle spectra,

di-hadron correlationsthe golden channel for jet tomography?

the fine print

fragmentation photonsinitial state effects (shadowing , kT)still sensitive to geometry / space-time evolutionquark vs. gluon energy loss

Calibrated probe – how well calibrated?

Very low rates: ems

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“measures” recoil parton momentumMeasure fragmentation function D(Z)

~D(z)

Use near side peak to determine direct associated with hadron, i.e. fragmentation photons

2triggerT

triggerT

partnerT

Ep

ppx

-h correlations – fragmentation photons

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1/N

trig

dN

/d

(A.

U.)

1/N

trig

dN

/d

(A.

U.)

0

Au+Au analysis is challenging: Additional sources of uncertainty from ZYAM normalization, flow subtraction and 0 combinatorial background

Little or no near-side production associated with direct photon triggers

Away-side yields indicate large jet suppression in +jet channel

1/N

trig

dN

/d

(A.

U.)

Direct photon – hadron correlations in Run-7 Au+Au

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Away-side structure vs. beam species, beam energies, and centrality

All cases:● Peripheral similar to p-p

● Central shows development of “lobe”-like structure

Dihadron correlations: system, energy, centrality dependence

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IAA is defined as the modification of per-trigger yield Yjet_ind, of AA relative to p+p.

Strong dependence on associated pT

Two-particle correlations – head, shoulder

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arXiv:0801.4545 [nucl-ex]

IAA for head and head/shoulder regions

IAA for head and shoulder regions

Strong partner pT

dependence

Jet energy redistributed via medium-jet interaction: high pT

suppression, low pT

enhancement

SR more enhanced than HR

One possibility: widening of head component:

incoherent radiation, Eloss coherent radiation (Mach, Cherenkov)

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nucl-ex/0611019

● Shape saturates above 100 Npart

Shape vs centrality (Npart)

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Near side RMS Away side RMS

No significant dependence on centrality, although broadening has been predicted! (And it is in the same ballpark as p+p.)

High pT 0-h correlations – near-side, away-side widths

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PRL 98 212301, 2007

tangential emmision

punch- through

reaction

plane

Some possibilities:

Theorists are overpredicting E-loss

High pT dijets don’t probe the medium

Sizable P(E) fluctuations we observe mainly punch-thru

Geometric bias we observe primarily surface emission

Why the discrepancy?

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Trigger 0

“Conditional”

charged hadron

at high-pt

MediumAssoc h

Path lengths comparable in dense medium.A.k.a., 2+1 correlations

Removes some events where hard-scattering occurs near surface but not tangential (large difference between path lengths)

Shift distribution of hard scattering towards center of medium. Near-side parton travels through more medium

Select events that have both a high-pt 0 and a back-to-back hadron (back-hemisphere of 0 )

Change the surface-bias of near-side?

Trigger 0

Assoc h

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Per-trigger yield of p+p on near side increase with conditional particle pT.

Expected in p+p! Higher Q2 comes with higher pT away-side particle.

In Cu+Cu the yield also increases but not same slope as in p+p.

2+1 changes near-side jets of both p+p and Cu+Cu

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Centrality dependence of near-side yields

Cu+Cu yield increases from central (left) to peripheral (right) in each bin and approaches p+p (most right point in each bin)

The fact that Cu+Cu yield is reduced at central is possibly due to

1) weaker surface-bias, 2) more “+1” particles from underlying event

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Summary

First constraints on free parameters (gluon density, transport coefficient) right now limited by uncertainties in the theory

Jet tomography emerging, but be careful: exclusive processes may prefer special regions of phase space

RAA dominated by Cronin at SPS energies, suppression dominates at 62GeV (new Cu+Cu results)

First promising results on photon-jet and fragmentation photons the “wise’s stone”, but starving for statistics, challenge in Au+Au

Measuring “excitation functions” in the same experiment (energy/species scan) is extremely important

Comprehensive theoretical description is needed within one framework and theoretical uncertainties have to be estimated

We already got quite a few good answers – so, what are the right questions?

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[rad]

Direct photon – hadron correlations in p+p

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Jets are affected by medium, on both near and far side.

Medium effect on jets vary on centrality and pT.

Thus, we quantify the medium effects as the suppression of jet, using per-trigger-yield, I_AA, J_AA. This suppression shows strong indication of jet particle sources at different kinetic region.

2+1 correlation brings another method of controlling jet source via the surface-bias, especially on exploring the near side jet suppression.

PHENIX has the brand new 2007 Au+Au data and we are showing many more results in this QM08 and near future.

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- PHENIX is measuring both Ridge and Shoulder - Shoulder & Head variation consistent with contributions of both medium response and suppressed in-vacuum jet fragmentation - Ridge and Shoulder measurements consistent with medium response, inconsistent with in-vacuum jet fragmentation - Ridge & Shoulder share much of the same behavior - appear at similar pT

- similar centrality dependence - softer than p-p counterparts

- baryon-meson ratios larger than jet fragmentation - balance pT

- At low enough pT, some triggers must come from medium response

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p+p, peripheral Au+Au central Au+Au

Typical: - Near-side Jet - Away-side Jet – “Head”

New: - Near-side Modification – “Ridge” - Away-side Modification – “Shoulder”

Near-side Ridge theories: Boosted Excess, Backsplash, Local Heating,…Away-side Shoulder theories: Mach, Jet Survival + Recom, Scattering,…

Medium response

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post-qm, feb. 12-14, 2008, tifr, mumbai, india -- g. david, bnl

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