Heavy Ion Jet Physics with ATLAS
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Heavy Ion Jet Physics with ATLAS. Wolf G. Holzmann. 23 rd Winter Workshop In Nuclear Dynamics Big Sky, Montana, February 11-17, 2007. M. Baker, R. Debbe, A. Moraes, R. Nouicer, P. Steinberg, H. Takai, F. Videbaek, S. White Brookhaven National Laboratory, USA.

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Heavy Ion Jet Physics with ATLAS

Wolf G. Holzmann

23rd Winter Workshop In Nuclear Dynamics

Big Sky, Montana, February 11-17, 2007


Atlas hi working group

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

L. RosseletUniversity of Geneva, Switzerland

A. DenisovIHEP, Russia

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

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

V. PozdnyakovJINR, Dubna, Russia

S. TimoshenkoMePHI, Moscow, Russia

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

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

ATLAS HI Working Group


Heavy Ion Physics at the LHC Takai, F. Videbaek, S. White

Phase Diagram for Nuclear Matter

Pb+Pb collisions at the LHC

will produce partonic matter

at unprecedented T and 

Will allow for detailed study and characterization of this

high energy density partonic matter. Study evolution from RHIC -> LHC energies.

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

Here, I will exclusively focus on jet tomography.


Jets as a tomographic probe of the medium Takai, F. Videbaek, S. White

Gyulassy et al., nucl-th/0302077

Jets in h+h

collisions

Jets in HI

collisions

Fragmentation:

Fragmentation:

Jet modification sensitive to gluon densities, path length, ….

Jets as Tomographic Probes of the Medium!


Jet tomography at RHIC Takai, F. Videbaek, S. White

STAR, PRL 93 (2004) 252301

Jets studied statistically via singles yields and correlations…

RAA

interm. pT

correlations

-h correlations

high pT correlations

Qualitatively successful, but quantitative interpretation difficult…


Jet tomography at RHIC Takai, F. Videbaek, S. White

RAA not really constraining E-loss models?

T. Renk, hep-ph/0607166

Correlation studies

complicated by trigger

bias effects?

-h correlations suffer

from statistics

Plus no real fragment. function measurements, etc…


Jet tomography at LHC Takai, F. Videbaek, S. White

Truly high pT jets will be

produced copiously in

Pb+Pb collisions at the LHC

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

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

Can (and will) do high pT jet reconstruction

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

Why would you want to do this with ATLAS?


Atlas calorimetery

The ATLAS Calorimeter Takai, F. Videbaek, S. White

ATLAS Calorimetery

Hadronic Barrel

EM Barrel

Forward

Finely segmented

calorimeter coverage

over full range

and large  range

EM EndCap

Hadronic EndCap


Measuring Jets in The ATLAS Calorimeter Takai, F. Videbaek, S. White

(Di)jets from PYTHIA

in Calorimter Towers

embedded in

HIJING event

Energetic jets clearly visible over the heavy ion background

Large  coverage is important


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

Jet

Jet

All too wide for single photons

Background

Back

ground

 x  = 0.0028 x 0.1


Two Approaches to Jet Reconstruction in ATLAS ion background is ~ negligible (unique at LHC)

First approach:

use standard p+p cone algorithm

with background subtraction

A) Seeded Cone Algorithm

Original cells

Cloned cells

Original towers

Layer-by-layer

subtraction

(exclude seeds)

Subtracted cells

Currently also looking

at methods to improve

algorithm: seed selection,

background subtraction, …

New towers

Reconstructed jets


Jet Energy Resolution with Seeded Cone Algorithm ion background is ~ negligible (unique at LHC)

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

Results obtained from standard p+p

cone algorithm w/ backgr.- subtraction

Some recalibration still needed.


Can we control the flowing background? ion background is ~ negligible (unique at LHC)

Yes! Can measure dN/dϕ in different layers

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

η

η

η

η

Layer 1

Layer 2

Layer 3

Presampler

ϕ

ϕ

ϕ

ϕ

ϕ

ϕ

ϕ

ϕ


Two Approaches to Jet Reconstruction in ATLAS ion background is ~ negligible (unique at LHC)

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

dij = min(k2ti,k2tj)R2 , where R2=(i-j)2+(i-j)2

diB = k2ti

Kt algorithm purposefully mimics a walk backwards

along the fragmentation chain

for all possible combinations: O(N3)

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


How fast is fast? ion background is ~ negligible (unique at LHC)

M. Cacciari et al, hep-ph/0512210

“Fast” Kt algorithm outperforms cone algorithm,

Becomes feasible in heavy ion environment!


“Fast” Kt Finder: Discriminating Jets and Background ion background is ~ negligible (unique at LHC)

Real Jets appear

as narrow towers

“Fake” Jets

appear flat and broad

Usejet topology to discriminate between jets and background!


Discriminating Jets and Background: A First Look ion background is ~ negligible (unique at LHC)

E T,max = maximum ET in calo cell

<E T > = average ET in calo cell

1

2

3

4

1

3

4

2

Initial look seems promising.

Other variables can also be

constructed.


ion background is ~ negligible (unique at LHC)

+Jet in ATLAS

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

Jet


Background subtracted ion background is ~ negligible (unique at LHC)

+Jet in ATLAS

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

Jet


ion background is ~ negligible (unique at LHC)+Jet in ATLAS

Δη×Δϕ = 0.003x0.1

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

EM Layer 1 ET (GeV)

Isolated photon gives clean signal in EM first sampling layer

Even in central Pb+Pb !


Photon bremsstrahlung in ion background is ~ negligible (unique at LHC)

jet cone?

Direct  triggered angular

correlations

energy calibrated:

- jet studies

- mach cone studies

+Jet in ATLAS

Many interesting possibilities:

let your imagination run wild :-)


Summary and Outlook ion background is ~ negligible (unique at LHC)

Jet modification studies at the LHC hold much potential for

quantitative 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 background

subtraction schemes, etc…) being done in ATLAS

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

New collaborators are welcome!


Backup Slides ion background is ~ negligible (unique at LHC)


Jet Position Resolution with Seeded Cone Algorithm ion background is ~ negligible (unique at LHC)

Resolutions in f and h for <ET>~50 GeV

Results obtained from standard p+p

cone algorithm w/ backgr.- subtraction

Some recalibration still needed.


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

Jet

Jet

All too wide for single photons

Background

Back

ground

 x  = 0.0028 x 0.1


The ATLAS Calorimeter ion background is ~ negligible

Δη×Δϕ in LAr Barrel:

Layer 1: 0.003x0.1Layer 2: 0.025x0.025

Layer 3: 0.05x0.025

Finely segmented calorimeter coverage over full range

and large  range


Advantages of “Fast” Kt Algorithm ion background is ~ negligible

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


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