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High p T jets: quenching, E loss , shape modification. We got some good answers. but what is the question???. Hot and dense matter in the RHIC-LHC era Tata Institute for Fundamental Research Feb. 12, 2008. G. David, BNL PHENIX Coll.

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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

We got some good answers

but what is the question???

Hot and dense matter in the RHIC-LHC era

Tata Institute for Fundamental Research

Feb. 12, 2008

G. David, BNL

PHENIX Coll.

Credits: Andrew Adare, Terry Awes, Mike McCumber, Hua Pei,

Matt Nguyen, Klaus Reygers, … The PHENIX Collaboration

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Why use high pT jets to get medium properties?

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 g-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

- …

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Trigger p0

Medium

“Conditional”

charged hadron

at high-pt

Assoc h

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)

Bulk suppression (f-integrated)

Establishing

the original

parton energy

 g-jet

  • In the medium initial geometry

  • and evolution influences DE

  • Bulk suppression w.r.t. reaction plane

  • Multiparticle correlations

…and even more

complex measurements

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

The starting point: nuclear modification factor

  • Hadrons are suppressed, direct photons are not

  • No suppression in d+Au

  • Evidence for parton energy loss

    • Static medium

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

  • 1D expansion, e.g., GLV model

This is a f-integrated, inclusive observable

(“bulk suppression”). Of course it can be

redefined into double, triple… differentials

  • RAA constrains medium properties

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Improved p+p Reference Data

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!

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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!

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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]

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Energy scan / II: centrality dependence of p0RAA in Cu+Cu

  • 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

PHENIX, arXiv:0801.4555 [nucl-ex]

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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)

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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!

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

A step forward: p0 RAA vs reaction plane

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!

Unbiased

Still hard to interpret

PRC 76 (2007) 034904

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Pathlength dependence of suppression

  • Approximate scaling in rLxyexpected for parton energy loss

  • Experimental evidence weak

  • Path length dependence of parton energy loss remains an open question

PHENIX, PRC 76, 034904

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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 (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(f) (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?

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Calibrated probe – how well calibrated?

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

  • Leading Order picture

    • (almost) exact momentum balance

    • w/ away-side jet

    • Compton dominance

  • p+p: measure gluon distribution function

  • A+A:

    • calibrated probe of energy loss

    • more sensitive probe than single particle spectra, di-hadron correlations

    • the golden channel for jet tomography?

  • the fine print

    • fragmentation photons

    • initial state effects (shadowing , kT)

    • still sensitive to geometry / space-time evolution

    • quark vs. gluon energy loss

Very low rates: aemas

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

-h correlations – fragmentation photons

~D(z)

  •  “measures” recoil parton momentum

    • Measure fragmentation function D(Z)

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

1/Ntrig dN/dDf(A. U.)

1/Ntrig dN/dDf(A. U.)

0

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

1/Ntrig dN/dDf(A. U.)

Au+Au analysis is challenging: Additional sources of uncertainty from ZYAM normalization, flow subtraction and p0 combinatorial background

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

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Dihadron correlations:

system, energy, centrality dependence

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

All cases:

● Peripheral similar to p-p

● Central shows development of “lobe”-like structure

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Two-particle correlations – head, shoulder

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

Strong dependence on associated pT

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

IAA for head and shoulder regions

IAA for head and head/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)

arXiv:0801.4545 [nucl-ex]

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Shape vs centrality (Npart)

nucl-ex/0611019

● Shape saturates above 100 Npart

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

High pTp0-h correlations – near-side, away-side widths

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.)

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

PRL 98 212301, 2007

tangential emmision

reaction

plane

punch- through

Why the discrepancy?

  • Some possibilities:

    • Theorists are overpredicting E-loss

    • High pT dijets don’t probe the medium

      • Sizable P(DE) fluctuations  we observe mainly punch-thru

      • Geometric bias  we observe primarily surface emission

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Change the surface-bias of near-side?

Trigger p0

Trigger p0

Medium

“Conditional”

charged hadron

at high-pt

Assoc h

Assoc h

Select events that have botha high-pt p0and a back-to-back hadron (back-hemisphere of p0 )

Removes some events where

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

Path lengths comparable in dense medium.

A.k.a., 2+1 correlations

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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

  • Expected in p+p! HigherQ2comes withhigher pT away-side particle.

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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?

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Direct photon – hadron Df correlations in p+p

Df [rad]

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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.

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

- 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

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

Medium response

p+p, peripheral Au+Au

central Au+Au

New:

- Near-side Modification – “Ridge”

- Away-side Modification – “Shoulder”

Typical:

- Near-side Jet

- Away-side Jet – “Head”

Near-side Ridge theories: Boosted Excess, Backsplash, Local Heating,…

Away-side Shoulder theories: Mach, Jet Survival + Recom, Scattering,…

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


Post qm feb 12 14 2008 tifr mumbai india g david bnl

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


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