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TitleExpected Performance of the ATLAS Experiment, Detector, Trigger and Physics
TagsPhysics
LanguageEnglish
File Size43.7 MB
Total Pages1852
Table of Contents
                            Title
Contents
Authorlist
Introduction
Performance
	Tracking
	Electrons and Photons
	Muons
	Tau Leptons
	Jets and Missing Transverse Energy
	b-Tagging
	Trigger
Physics
	Standard Model
	Top Quark
	B-Physics
	Higgs Boson
	Supersymmetry
	Exotic Processes
                        
Document Text Contents
Page 1

CERN-OPEN-2008-020
December 2008

Expected Performance of the ATLAS Experiment
Detector, Trigger and Physics

The ATLAS Collaboration

A detailed study is presented of the expected performance of the
ATLAS detector. The reconstruction of tracks, leptons, photons,
missing energy and jets is investigated, together with the performance
of b-tagging and the trigger. The physics potential for a variety of
interesting physics processes, within the Standard Model and beyond, is
examined. The study comprises a series of notes based on simulations
of the detector and physics processes, with particular emphasis given to
the data expected from the first years of operation of the LHC at CERN.

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ATLAS

R=0.40 Tower∆Cone
R=0.70 Tower∆Cone

Kt R=0.40 Tower
Kt R=0.60 Tower

Figure 2: Energy resolution for jets from light quarks (integrated over all η values), as a function
of the quark energy, for various jet algorithms (events simulated without pileup).

W-boson mass is shown in Figure 10.
To obtain mass values from the invariant mass spectra of Figure 10, the sum of a Gaussian and a

4th degree Chebychev polynomial is fitted to the distribution. The polynomial describes the background
from wrong jet combinations and from background events, whereas the mean value of the Gaussian and
its error are interpreted as the mass (mW) and its statistical error, σstat . The width of the Gaussian is
referred to as σGauss. Table 2 shows the obtained W-boson mass with the E recombination scheme for
various choices of the kT parameters.

The error resulting from 1% variation on the jet energy scale (JES) is obtained by varying the recon-
structed jet energies by ±1% followed by a linear fit across the three W-boson masses (-1%, nominal
value, +1%).

With increasing R and Dcut parameter values, the reconstructed W-boson (and also top quark) mass
rise monotonically. Higher parameter values lead to fewer but more energetic jets and thus their com-
bination has a higher invariant mass. Part of this effect could in principle be absorbed by the in-situ
calibration, but on the other hand, the event is more likely to be misreconstructed due to unwanted jet
merging, which makes the choice of the three jets maximizing the pT-sum unpredictable.

The efficiencies quoted in Table 2, defined as the number of events in the gaussian part of the mass
distribution divided by the initial number of events, which can be as high as 5.8%, drop to less than 4%
when the jets become too big.

Table 2 also shows the purity of the W-boson reconstruction, defined as the fraction of events in the
gaussian part of the distribution, in the range [−1σgaus, 1σgaus] from the mean value of the gaussian fit.
A purity around 64% can be obtained with some choices of the algorithms / parameters, for example
cone 0.4 or kT with R = 0.4, whereas other choices may lead to purities about 15% smaller.

This study indicates that, for the W-boson mass reconstruction in tt̄ events, the cone 0.4 algorithm

5

TOP – JETS FROM LIGHT QUARKS IN tt̄ EVENTS

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0.05

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ATLAS

R=0.40 Tower Quark-Jet∆Cone
R=0.70 Tower Quark-Jet∆Cone
R=0.40 Tower Monte Carlo Jet-Jet∆Cone
R=0.70 Tower Monte Carlo Jet-Jet∆Cone

Figure 3: Comparison of the jet energy resolutions with respect to the Monte Carlo hadrons
(“Monte Carlo jets”) and with respect to the initial quarks from the W-boson decay, for events
simulated without pileup.

[GeV]QuarkE
50 100 150 200 250 300 350 400

)
Q

ua
rk

/E
Je

t
)/m

ea
n(

E
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ua
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/E
Je

t
(E

σ

0.08
0.1

0.12
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0.16
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0.2

0.22
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ATLAS

R=0.40 Topo∆Cone

R=0.40 Tower∆Cone
R=0.40 Topo with PileUp∆Cone
R=0.40 Tower with PileUp∆Cone

Figure 4: Comparison of the jet energy resolutions for events simulated with and without pile-up.

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Page 1851

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