2011 QCD and High Energy Interactions - Rencontres de Moriond ...

Fractional JES systematic uncertainty
0.12
0.1
0.08
0.06
0.04
0.02
0
Anti-k R=0.6,
EM+JES, 0.3 ≤ | η | < 0.8, Data 2010 + Monte Carlo QCD jets
t
ALPGEN + Herwig + Jimmy Noise Thresholds
JES calibration non-closure PYTHIA Perugia2010
Single particle (calorimeter)
Total JES uncertainty
Additional dead material
30 40
2
10
2
2×
10
ATLAS Preliminary
3
10
p
jet
T
3
2×
10
[GeV]
Fractional JES systematic uncertainty
0.25
0.2
0.15
0.1
0.05
0
Anti-k R=0.6,
EM+JES, 3.6 ≤ | η | < 4.5, Data 2010 + Monte Carlo QCD jets
t
ALPGEN + Herwig + Jimmy Noise Thresholds
JES calibration non-closure PYTHIA Perugia2010
Single particle (calorimeter) Additional dead material
Intercalibration
Total JES uncertainty
30 40 50 60 70
ATLAS Preliminary
2
10
2×
10
p [GeV]
Figure 1: Fractional jet energy scale systematic uncertainty as a function of pT for jets in the pseudorapidity
region 0.3 < |η| < 0.8 in the barrel calorimeter (left) and in the pseudorapidity region 3.6 < |η| < 4.5 in the
forward calorimeter (right).
which are cells with large signal to noise ratio. Neighbouring cells in 3D space are collected to
form the cluster in an iterative procedure based on cell energy significances and neighbouring
relations. Resulting clusters are used then as input objects for an anti-kT algorithm 8 with
distance parameters R=0.6 and R=0.4 to form jets.
The ATLAS calorimeter system is non-compensating, i.e. it generates smaller signal per unit
ofincomingenergyforhadronsthanforelectrons. Additionalimperfectionsinthereconstruction
can arise from energy deposits outside active regions of the calorimeters, shower leakage, and
threshold effects from clustering and jet algorithms.
The present calibration scheme 9 applies jet-by-jet Monte Carlo based corrections as a function
of the jet energy and pseudorapidity b to jets reconstructed at EM scale. To derive the
correction factors, truth jets are formed by running the same anti-kT algorithm on stable particles
from Monte Carlo simulation. Each correction factor is then calculated by dividing the truth
particle jet energy by the energy of the matching calorimeter jet at EM scale. This calibration
scheme restores jet energy scale (JES) within 2% for the full kinematic range and allows a direct
evaluation of the systematic uncertainty.
2.2 Jet energy scale uncertainty
The jet energy scale uncertainty is the dominant experimental uncertainty for numerous physics
results with jets in final states. The systematic uncertainty of the present calibration scheme
is determined from a combination of test-beam data, in-situ measurements in proton-proton
collisions and from systematic variations of parameters of the Monte Carlo simulations.
As a first step, the uncertainty of the response of the ATLAS calorimeter system to single
isolated hadrons is determined from the E/p ratio measured in-situ by the calorimeter and
the tracker. This data is supplemented by the uncertainty known from single pion testbeam
measurements in the well-understood barrel region |η| < 0.8. Finally the uncertainty of the
calorimeter responce to hadrons is propagated to the uncertainty of the calorimeter response to
jets using Monte Carlo and known jet composition. Additional uncertainties on the description
of the material of the ATLAS detector, of the electronic noise and uncertainties from the modeling
of the fragmentation and underlying event are estimated using Monte Carlo test samples
generated with different conditions.
Figure 1 (left) shows the final fractional jet energy scale uncertainty and its individual
b the pseudorapidity is defined in terms of the polar angle θ as η = −lntan(θ/2)
jet
T
2