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, , (and measurements in ALICE

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’s sensitive only to production processes, do not interact, sensitive to:

• initial parton distributions: Intrinsic kT, kT Broadening, Shadowing, Anti-shadowing, Saturation, …• some final state effects (parton/hadron rescatterings): Thermal, Jet/Parton Radiation or fragmentation after Eloss,…

“The virtue of and Measurements”

To properly deduce the properties of the produced QCD matter we must separately distinguish initial state effects from final state effects.

Experimental virtues (calorimeter measurement):• Measure and in same detector• Identified particles to very high pT • ’s abundantly produced

’s produced in final state (fragmentation or recombination): • initial parton distributions: as for • large final state effects: Rescattering (low pT), Absorption, kT Broadening, Jet/Parton Energy Loss ,…

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Centrality Dependence of High pT Suppression

PHENIX nucl-ex/0503003

• Trend of yield with centrality (impact parameter) follows expected binary collision scaling behavior (=constant) and agrees with pQCD (=1).

• Direct are not suppressed - strong evidence that hadron suppression is due to final state, i.e. parton energy loss.

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Direct Photons and Production for Central Au+Au

PHENIX nucl-ex/0503003

• High pT yield consistent with binary scaled pQCD in contrast to factor of 5 suppression of & hadron yields.

• Direct are not suppressed - strong evidence that hadron suppression is due to final state, i.e. parton energy loss.

• Errors don’t preclude a thermal contribution at low pT

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Centrality Dependence RAA for

pp

AuAubinaryAuAuAA Yield

NYieldR

/

• suppression increases with increasing nuclear overlap volume.• suppression same as - behaviour agrees with “meson-like” expectation

PHENIX preliminaryPHENIX preliminary

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inclusive jets10 Hz @ 50 GeV

few x 104/year for ET>150 GeV

EFS = 250 GeV(PHOS 80 GeV)

Extend measurements to much higher pT

From Peter Jacobs

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Photons (all in fact): Continum Spectrum with Many Sources

Turbide, Rapp, Gale

E

Rate

Hadron Gas Thermal Tf

QGP Thermal Ti

Pre-Equilibrium

Jet Re-interaction

pQCD Prompt

Final-state photons are the sum of emissions from the entire history of a nuclear collision.

+Weak+EM decay ’s () = Bkgd

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Resolution: PHOS vs EMCal - PHOS wins

Due to much better energy resolution, lower occupancies, and lower tower thresholds PHOS will be the preferred detector for low pT photon measurements.

PHOS: 3.5%/sqrt(E)EMCal: 12%/sqrt(E) or worse?

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Tower granularity 0 separation - PHOS wins

0 opening angle shower shape discrimination

Heather Gray, LBNL/Cape Town

rejection for pT<~30 GeV/c

At high pT merging of showers of photon pairs from decays become indistinguishable from single photons - become a background.• Use EMCal vs PHOS comparisons to determine (PHOS rate limited)• Other “tricks” like tagging conversion rate• As pT increases ’s dominate

preliminary

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So…Why Large EMCal in ALICE?

• Measure neutral energy of jets– Improves jet energy resolution, ~30% -> ~15%

– PHOS is too small to contain jet

• Extend PHOS measurements ,0, and e+/- to higher pT

• Increase acceptance for (EMCal/PHOS) + jet(TPC)

• Allow back-to-back coincidences – (PHOS) + jet (TPC+EMCal)

– (EMCal) + leading 0 (PHOS)

– (EMCal/PHOS) + e+/- from D,B decay (heavy q jet) (PHOS/EMCal)

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ALICE EMCal is large enough for jet measurements

PROBLEM: Underlying event

non-jet energy:

Radius Cone Energy (GeV)

0.7 750

0.5 380

0.3 140

0.1 15Jet Energy fraction within cone radius

20 /2 3/2

TPC

PHOS

EMCal

Jet R=0.3,0.5

0.9

-0.9

0

Use “small” jet-cone, R~0.3

Figure 5.4 Energy resolution ofreconstructed jets in central PbPb HIJINGevents (open symbols) and pp (closedsymbols) for jets of 30,50, and 100 GeV/c.For PbPb collisions, an optimum coneradius of R=0.3 is indicated.

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Direct /0 Ratio

• Ratio /0 provides figure of merit for direct gamma measurement: /0 ~ 10% easy ~ 3-5% limit

• 0 suppression by factor of ~5-10 (jet quenching) would allow direct photon measurement down to a few GeV/c

• Expectations for 0 suppression at high pT at LHC?

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Empirical Investigation of Energy Loss

Observe 0 spectra in p+p have power-law form:

pTAA kpT

pp (1 SLoss)pTpp

dN AA

dpT(1 SLoss)

n 2 A NcollpTn 1

RAA (1 SLoss)n 2

SLoss 1 RAA1/(n 2)

If we interpret suppression as a pT shift from Energy Loss:

dN pp

dpTA

pTn 1

where

then

The Energy Loss, or pT Shift Sloss, can then be extracted from RAA as

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Suppression / Energy Loss vs Centrality

• Gyulassy, Vitev, Wang: • 1D expansion, simple geometric scaling

Large gluon density dNg/dy~1000, Large energy density. If behaviour extrapolates to LHC, expect even larger pT independent suppression

E N part2 / 3

LN part1/ 3

N part1/ 3

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Further Illumination with Direct Photons

• High pT direct produced in hard q-g Compton-like scatterings. Sensitive to PDFs, (especially gluons).

• In the HI case, from initial hard scattering escape. Can be used to tag recoiling jet to study jet energy loss (“Jet Quenching”), which may produce additional Brems. ’s.

Leading Particle

Direct

Hadrons

gq

Brems.

Thermal

• Additional thermal ’s produced in scatterings in QGP or hadronic phase of collision. • If ”Gluon Saturation” occurs, initial (and 0) will be suppressed.

• With parton Eloss, the fragmentation contribution could also be suppressed.

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Investigate yields vs Reaction Plane (Pathlength)

= 0°

= 90°

• Measure Reaction Plane using BBCs• Measure Yields vs. orientation w.r.t. RP • Expect largest E-loss or suppression at = 90 in “ long” direction• Interplay between flow (v2) and Supression? Both give expectation of in-plane enhancement.

Long path, large Eloss

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and inclusive v2 analysis

• show large v2 to high pT - effect of suppression? (more later)• Inclusive show similar large v2

• Use measurement as input to MC to predict decay v2 - if no direct signal, or v2(direct ) = v2(bgd ), then should have v2(bgd ) = v2(incl )

• Use Direct measurement R=Total/Decay then v2(direct ) = ( R*v2(incl ) - v2(bgd ) ) / (R-1)

• Or, assume v2(direct ) = 0 to extract R. Gives agreement with measurement of R=0 in this pT range. (Proof of principle at this point)

pT (GeV/c)

PHENIX preliminary

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ALICE studies using Measurements

• Measure direct in p+p:– Extract fragmentation contribution by analysis

with/without isolation cuts• Measure direct in A+A:

– Search for effects of gluon saturation• Thermal region (PHOS):

– Other techniques - HBT, conversions• Measure direct vs Reaction Plane in A+A:

– q+g Compton isotropic, Thermal flows, Fragmentation enhanced or suppressed with pathlength?

• Measure +jet :– The measures pT of the recoiling parton - measure

modified recoil Fragmentation Function to extract parton energy loss directly.