Measurements of jet quenching using the ATLAS detector

Martin SpoustaCharles University in Prague

High-pT physics at LHC 09February 4th 2009

Measurements of jet quenching using the ATLAS detector

Martin Spoustafor the ALTAS collaboration

2 Martin SpoustaCharles University in Prague


Outline

• Motivation • Jet reconstruction strategy• Tools to study jet quenching

– fragmentation function and jT distribution– jet shapes– di-jet correlations – -jet correlations– jet RAA

• Summary



Motivation

• Jet (leading particle) quenching well visible at RHIC experiments

• LHC will deliver Pb+Pb collisions at s = 5.5 TeV/n

• Models and predictions for the LHC energy exist, details of QCD energy loss mechanisms not well understood

• Jets copiously produced at the LHC energy possibility to study details of energy loss mechanism in QGP medium

• 20 millions jets with ET>50 GeV per month of Pb+Pb running at the nominal luminosity

+ + +...



Why ATLAS ?

tracking in 2T solenoid – fragmentation studies

jet reconstruction using calorimeter,full azimuth, 10 units of pseudorapidity

first layer of LAr EM calorimeter excellent for photon identification,other layers also well segmented



Jet reconstruction strategy

Cone jet reconstruction:

• regions of interest found (seed regions) – fast sliding window algorithm used

• background computed

• excluding the seed-regions

• vs. , vs. layer

• background subtracted

• standard p+p jet finding algorithm used (seeded iterative R=0.4 cone algorithm)

An alternative: kT-algorithm based reconstruction strategy – also studied

PYTHIA dijets embedded to the unquenched HIJING events

one event before the background subtraction

one event after the background subtraction



• Tracking efficiency ~ 70% for the most central collisions, below 10 GeV pT independent

• Track to calorimeter matching used to identify tracks from a jet

• Identified tracks for jT and z determination

Fragmentation function and jT

z

jT

CaloTowers

Tracks




• jT and z computed for tracks above 2 GeV• Background distributions of jT and z are computed using

tracks that match with HIJING particles, these distributions are subtracted, correction for the jet position resolution is applied

○ - truth● - reconstructed(jet ET 70-140 GeV)

… we can well reproduce jT distribution and fragmentation function

z

jT

○ - truth● - reconstructed(jet ET 70-140 GeV)

jT (GeV)




• Both fragmentation function and jT distribution are expected to be modified due to the gluon radiation inside the medium (QCD analog of LMP effect)

• Need to study the response of the detector to a possible modification – PYQUEN (see http://lokhtin.web.cern.ch/lokhtin/pyquen/)

Salgado, Wiedemann – jT also modified N.Armesto, et al. – modification of fragmentation functions




from PYQUEN – generator level

Large jT suppressed gluons radiated from large angles

○ - quenched● - non-quenched(simulated events)

○ - quenched● - non-quenched(simulated events)

• Result at the generator level • PYQUEN settings: default setting for quenching, b=0, pT,min(jet)=70 GeV, Pb+Pb,

5.5 TeV/n

jT (GeV)

Low z enhanced, higher z suppressed leading particle suppressed, redistribution of

energy out of a jet core

jT (GeV)




from PYQUEN – reconstructed

Large jT suppressed gluons radiated from large angles

○ - quenched● - non-quenched(reconstructed events)

○ - quenched● - non-quenched(reconstructed events)

• Result at the after the reconstruction (but not embedded to the heavy ion event) – effect still well visible

jT (GeV)

Low z enhanced, higher z suppressed leading particle suppressed, redistribution of

energy out of a jet core

jT (GeV)



Jet shapes

• Measure the energy flow inside the jet at the calorimeter level

• Well defined in QCD, measured at Tevatron

... integral jet shape

... differential jet shape

jet axis



• Truth jet shapes are much more narrow than calorimeter jet shapes resulting from the calorimeter segmentation not from the background

• We are able to reconstruct jet shapes in the most central collisions with good accuracy, a small difference at low r is due to position resolution and it should be possible to correct it using jets reconstructed with smaller cone size

Jet shapes

○ - Truth jets (p+p) ● - Reconstructed jets (p+p)

○ - Jets in peripheral collisions● - Jets in the most central collisions



... jet shapes expected to be modified

... PYQUEN to study the detector response

Jet shapes

I.Vitev, et al. Salgado, Wiedemann enhancement namely

at the jet periphery

no so sizable difference



• Result at the generator level

• Almost factor of two difference in the jet core between quenched/unquenched simulations

• Differential jet shape can show better the flow of the energy – energy is redistributed out of center of the jet

Jet shapes from PYQUEN – generator level

○ - non-quenched● - quenched(simulated events)○ - non-quenched

● - quenched(simulated events)



• Results after the reconstruction (but not embedded to the heavy ion event)

• Jet quenching effect still well visible

Jet shapes from PYQUEN – reconstructed

○ - non-quenched● - quenched(reconstructed events)

… if the quenching is of that order we should be able to measure it




Di-jet correlations

• Large acceptance large rates, important for low x-region

• Similar variables studied already at RHIC with hadrons

• 60% probability for detecting an associated jet (ETB>70 GeV, ET

A>100 GeV)

… conditional yield of detecting an associated jet (B) to a leading jet (A) as a function of their relative azimuth || and pOUT = ET

B sinR



Di-jet correlations from PYQUEN

• No dramatic difference between PYTHIA and PYQUEN

• PYQUEN radiative energy loss modifies rather the structure inside the jet, the radiation doesn’t extend much outside of the jet cone

… conditional yield of detecting an associated jet (B) to a leading jet (A) as a function of their relative azimuth || and pOUT = ET

B sinR





• Direct photons form hard processes (qgq, qqg)

x large background from 0, decays

• Benefits from excellent longitudinal segmentation (0.003 for the first sampling of EMCAL) – it is possible to separate direct photons from the background using:

– shower shape cuts (factor of 3-5 rejection)

– applying a set of isolation criteria (factor of 10 rejection)

– combination of both

-jet correlations, direct isolation



single single

samein Pb+Pb

(dN/d = 2700)

samein Pb+Pb

(dN/d = 2700)

• Excellent – 0 separation• The occupancy from the underlying event is very low due to the segmentation of

the strips

-jet correlations, direct isolation



• We make precise measurements of -jet correlations• doesn’t interact strongly with the medium can be used to measure original

energy and direction of a jet interacting with the medium• Can help jet analysis at low ET – can be used for the fake rejection

-jet correlations

Medium

jet

Vacuum jet



Jet RAA

Jet RAA is sensitive to the collisional energy loss (i.e. multiple elastic scattering). Any energy loss not recoverable by the jet reconstruction leads to the softening of the spectrum.

I.P. Lokhtin, et al.

Reconstructed to input spectra without fake jet rejection. Required correction (<20% for ET,jet>80 GeV) much lower then the predicted suppression.



RAA and jet shapes have sensitivity to different energy loss mechanisms

jet shape

jet shape

jet energy spectrum

jet energy spectrum

PYQUEN with only radiative

energy loss

PYQUEN with only collisional

energy loss

○ - non-quenched● - quenched

○ - non-quenched● - quenched x - non-quenched

x - quenched

x - non-quenchedx - quenched



Summary

• Algorithms for the jet reconstruction in heavy ion collisions are part of the official software of the ATLAS experiment

• We have implemented two independent jet finding algorithms (iterative cone and fast kT algorithms)

• Variety of different measurements possible using ATLAS:– fragmentation function – jT distribution– differential jet shape– di-jet and -jet correlations– jet RAA

... these measurements should distinguish among different energy loss mechanisms

• ATLAS will make a significant impact on understanding of parton energy loss with the ability to reconstruct full jets and their properties



Backup slides

Backup slides



Jet energy resolution

• Cone algorithm R=0.4, 5 GeV seed, reconstructed jets matched to truth jets using R=0.5 cut

• Jet energy scale within 5% above 50 GeV, default p+p calibration used

• Jet energy resolution bellow 25% for 70 GeV jets in the most central collisions (dN/d~2700 – unquenched HIJING)

Backup slides



Jet energy resolution, position resolution

• Jet energy resolution as a function ofpseudorapidity

• Forward jets reconstructed with comparable resolution as in midrapidity

• Jet position resolution in • Improves with increasing jet energy

• Jet position resolution in is comparable

Backup slides



Efficiency and fake-rate

• Efficiency is almost centrality independent – easier interpretation of jet properties vs. centrality

• Above 70 GeV the efficiency is above 90%

• Above 70 GeV practically no fake jets (after the fake rejection)

Backup slides



Jet reconstruction strategy II.

kT jet reconstruction:• run fast kT algorithm• separate jet from the background• subtract background from jets • calibrate jet energy

kT algorithm

ij

… reconstructs jets backwards along fragmentation chain



Comparison between kT and cone algorithm

• kT algorithm has better energy resolution and efficiency at low ET

• kT algorithm still under the study – running kT algorithm before the subtraction delivers smaller jets (energy at a jet periphery truncated)

Backup slides



Backup slides: jet position resolution

Backup slides


High-pT physics at LHC 09February 4th 2009 Backup slides

Backup slides: tracking performance



Shape of the jet from tracking

Backup slides

Backup slides: jet’s tracks



Background distributions subtracted but the correction on the jet position resolution not applied => visible underestimation at small jT (z is not affected much by the jet position resolution). Discrepancy can be removed using jet position determined by the cone reconstruction with smaller R.

○ - truth● - reconstructed

○ - truth● - reconstructed

jet ET 70-140 GeV jet ET 70-140 GeV

Backup slides

Backup slides: Sensitivity to the jet position

Measurements of jet quenching using the ATLAS detector

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