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Wolf G. HolzmannWolf G. Holzmann

2323rdrd Winter Workshop In Nuclear Dynamics Winter Workshop In Nuclear DynamicsBig Sky, Montana, February 11-17, 2007Big Sky, Montana, February 11-17, 2007

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★★ ★★★

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M. Baker, R. Debbe, A. Moraes, R. Nouicer, P. Steinberg, H. Takai, F. Videbaek, S. WhiteBrookhaven National Laboratory, USA

J. Dolejsi, M. SpoustaCharles University, Prague

A. Angerami, B. Cole, N. Grau, W. Holzmann, M. LelchoukColumbia Unversity, Nevis Laboratories, USA

A. Olszewski, B. Toczek, A. Trzupek, B. Wosiek, K. WozniakIFJ PAN, Krakow, Poland

L. RosseletUniversity of Geneva, Switzerland

J. Hill, A. Lebedev, M. RosatiIowa State University, USA

G. Atoian, V. Issakov, H. Kasper, A. Poblaguev, M. ZellerYale University, USA

A. DenisovIHEP, Russia

P. Chung, J. Jia, R. Lacey, N N.. AjitanandChemistry Department, Stony Brook University, USA

V. PozdnyakovJINR, Dubna, Russia

S. TimoshenkoMePHI, Moscow, Russia

ATLAS HI Working Group

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Heavy Ion Physics at the LHC

Phase Diagram for Nuclear Matter Phase Diagram for Nuclear Matter

Pb+Pb collisions at the LHCPb+Pb collisions at the LHCwill produce partonic matterwill produce partonic matterat unprecedented T and at unprecedented T and

Will allow for detailed study Will allow for detailed study and characterization of thisand characterization of thishigh energy density partonic high energy density partonic matter. Study evolution from matter. Study evolution from RHIC -> LHC energies.RHIC -> LHC energies.

ATLAS will target a comprehensive set of key ATLAS will target a comprehensive set of key observables (see Nathan Grau’s ATLAS overview talk)observables (see Nathan Grau’s ATLAS overview talk)

Here, I will exclusively focus on jet tomography.Here, I will exclusively focus on jet tomography.

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Jets as a tomographic probe of the medium

coneRFragmentation:

hadron

parton

pz

p≡

Jets in h+h Jets in h+h collisionscollisions

coneRFragmentation:

hadron

parton

pz

p≡

Jets in HI Jets in HI collisionscollisions

Gyulassy et al., nucl-th/0302077

Jet modification sensitive to gluon densities, path length, …Jet modification sensitive to gluon densities, path length, ….Jets as Tomographic Probes of the Medium!

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Jet tomography at RHIC

STAR, PRL 93 (2004) 252301

Jets studied statistically via singles yields and correlations…Jets studied statistically via singles yields and correlations…

Qualitatively successful, but quantitative interpretation difficult…Qualitatively successful, but quantitative interpretation difficult…

interm. pT interm. pT correlationscorrelations

high pT correlationshigh pT correlations

RRAAAA

-h correlations-h correlations

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Jet tomography at RHIC

Plus no real fragment. function measurements, etc…Plus no real fragment. function measurements, etc…

Correlation studiesCorrelation studiescomplicated by triggercomplicated by triggerbias effects?bias effects?

-h correlations suffer-h correlations sufferfrom statisticsfrom statistics

RAA not really constraining RAA not really constraining E-loss models?E-loss models? T. Renk, hep-ph/0607166

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Jet tomography at LHC

Can (and will) do RHIC type studies with better statisticsCan (and will) do RHIC type studies with better statistics

Can (and will) do high pCan (and will) do high pTT jet reconstruction jet reconstruction

(event-by-event jet tomography, frag. functions, jet structure…)(event-by-event jet tomography, frag. functions, jet structure…)

How can jet studies at the LHC improve on the situation?How can jet studies at the LHC improve on the situation?

Truly high pTruly high pTT jets will be jets will be

produced copiously in produced copiously in Pb+Pb collisions at the LHCPb+Pb collisions at the LHC

Why would you want to do this with ATLAS?Why would you want to do this with ATLAS?

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ATLAS Calorimetery Hadronic Barrel

Hadronic EndCap

EM EndCap

EM Barrel

Forward

Finely segmented calorimeter coverage over full rangeand large range

The ATLAS Calorimeter

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(Di)jets from PYTHIA (Di)jets from PYTHIA in Calorimter Towersin Calorimter Towers

embedded in embedded in HIJING eventHIJING event

Measuring Jets in The ATLAS Calorimeter

Energetic jets clearly visible over the heavy ion backgroundLarge coverage is important

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Jet

Background

Jet

Background

All too wide for single photons

x = 0.0028 x 0.1

– Segmentation of first EM sampling layer so fine that heavy ion background is ~ negligible (unique at LHC)

– Fine -> rejection of neutral hadron decays

– Clean 1st sampling-> prompt isolation

Taking a closer look

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Two Approaches to Jet Reconstruction in ATLAS

A) Seeded Cone Algorithm

Original cellsOriginal cells Cloned cellsCloned cells

Original towersOriginal towers

Subtracted cellsSubtracted cells

New towersNew towers

Reconstructed jetsReconstructed jets

Layer-by-layersubtraction(exclude seeds)

Currently also lookingCurrently also lookingat methods to improveat methods to improvealgorithm: seed selection,algorithm: seed selection,background subtraction, …background subtraction, …

First approach: First approach: use standard p+p cone algorithm use standard p+p cone algorithm with background subtractionwith background subtraction

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Jet Energy Resolution with Seeded Cone Algorithm

Study of different event samples embeddedinto central Pb+Pb HIJING (b=0-2 fm)

Results obtained from standard p+p Results obtained from standard p+p cone algorithm w/ backgr.- subtractioncone algorithm w/ backgr.- subtractionSome recalibration still needed.Some recalibration still needed.

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Can we control the flowing background?

Presampler Layer 1 Layer 2 Layer 3

Yes! Can measure dN/dϕ in different layers

(and sections) of calorimeters e.g. EM Barrel

η

ϕ ϕ ϕ ϕ

ϕ ϕ ϕ ϕ

η η η

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B) KT Algorithm clusters particles close in phase-space:

dij = min(k2ti,k2

tj)R2 , where R2=(i-j)2+(i-j)2

Kt algorithm purposefully mimics a walk backwardsalong the fragmentation

chain

for all possible combinations: O(N3)

Cacciari et al: “Fast” Kt optimization to O(NlogN)

Two Approaches to Jet Reconstruction in ATLAS

diB = k2ti

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How fast is fast?

“Fast” Kt algorithm outperforms cone algorithm,Becomes feasible in heavy ion environment!

M. Cacciari et al, hep-ph/0512210

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Real Jets appearas narrow towers

“Fake” Jetsappear flat and broad

UseUse jet topology to discriminate between jets and backgroundjet topology to discriminate between jets and background!!

“Fast” Kt Finder: Discriminating Jets and Background

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3 4

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1

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Initial look seems promising.Initial look seems promising.Other variables can also be Other variables can also be constructed. constructed.

E T,max = maximum ET in calo cell

<E T > = average ET in calo cell

Discriminating Jets and Background: A First Look

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PYTHIA + jet (75 GeV) superimposed on b=4 fm HIJING Pb+Pb event, full GEANT

Jet

+Jet in ATLAS

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PYTHIA + jet (75 GeV) superimposed on b=4 fm HIJING Pb+Pb event, full GEANT

Background subtracted

Jet

+Jet in ATLAS

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EM

Layer

1 E

T

(GeV

)

Isolated photon gives clean signal in EM first sampling layer

Even in central Pb+Pb !

One (of 64) rows in barrel EM calorimeter 1st sampling layer

Δη×Δϕ = 0.003x0.1

+Jet in ATLAS

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+Jet in ATLAS

Direct triggered angularcorrelationsenergy calibrated: - jet studies - mach cone studies

Photon bremsstrahlung injet cone?

Many interesting possibilities: let your imagination run wild :-)

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Summary and Outlook

Jet modification studies at the LHC hold much potential forquantitative tomography of the partonic medium

ATLAS is uniquely positioned to perform key jet measurements well

Lots of ground work on jet reconstruction in heavy ion environment(seeded cone algorithm, fast Kt algorithm, different backgroundsubtraction schemes, etc…) being done in ATLAS

Studies shown only an “amuse gueule” expect much more, soon

New collaborators are welcome!

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Backup Slides

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Jet Position Resolution with Seeded Cone Algorithm

Results obtained from standard p+p Results obtained from standard p+p cone algorithm w/ backgr.- subtractioncone algorithm w/ backgr.- subtractionSome recalibration still needed.Some recalibration still needed.

Resolutions in and for <ET>~50 GeV

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Jet

Background

Jet

Background

All too wide for single photons

x = 0.0028 x 0.1

– Segmentation of first EM sampling layer so fine that heavy ion background is ~ negligible

– Fine -> rejection of neutral hadron decays

– Clean 1st sampling-> prompt isolation

The ATLAS Calorimeter

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The ATLAS Calorimeter

Δη×Δϕ in LAr Barrel:Layer 1: 0.003x0.1Layer 2: 0.025x0.025Layer 3: 0.05x0.025

Finely segmented calorimeter coverage over full rangeand large range

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Infrared and collinear safe

Exceptionally suited to study jet sub-structure: - modification of jet topology in Pb+Pb - hard radiation within the jet

New ways to distinguish jets and background

Systematic cross-check to cone algorithm

Advantages of “Fast” Kt Algorithm