• upgrade
• Higgs
• ATLAS
• tracker

This thesis is focused on the assessment of the performances of the tracking system (ITk)
which will replace the current tracker of the ATLAS experiment during the high-luminosity
upgrade of the Large Hadron Collider (LHC), scheduled for 2026.

LHC has been working at a center-of-mass energy of 7 - 8 TeV during Run-1 and at 13 TeV during Run-2, which started last year. It is scheduled to be upgraded in two different stages: the first one will begin after the end of Run-2 After the Long Shutdown 2, scheduled
for 2019-2020, LHC will work at an energy of 14 TeV, reaching its maximum design value; the
second one will start at the end of Run 3 during the Long Shutdown 3 in 2024-2026. The
latter stage will lead to what is dubbed as High-Luminosity LHC (HL-LHC) during which the
instantaneous luminosity will reach the maximum value of 7.5×1034 cm^−2s^−1, corresponding
to approximately 3000 fb−1 over ten years of data taking.

The increase of the instantaneous luminosity will cause the average number of proton-
proton interactions per bunch crossing (“pile-up” events) to reach the value 〈μ〉 = 200, which
is about 10 times the current one (〈μ〉 = 23). By the end of Run-3, ATLAS detector will be using
15-20 years old components. In particular, the pixel and strip sensors of the current Inner
Detector will have reached the end of their lifetime, while the Transition Radiation Tracker will
not be able to withstand the harsh conditions imposed by HL-LHC. Thus, the Inner Detector
must be completely replaced by the Inner Tracker (ITk), which will have to guarantee the same
or an improved performance during that phase.

Due to the large number of pile-up events, the time required by the Monte Carlo simulations
is extremely high, making it difficult to produce samples with large statistics. In this thesis,
a fast simulation method that significantly decreases the time required by the simulation
while allowing to simulate the effect of the high number of pile-up events on the tracking and
physics performances was developed. The idea is to limit the simulation to specific regions
of interest, excluding a considerable part of the generated event. While the idea was inspired
from previous works, we extended the use of this technique to samples containing physics
processes.

At the moment, the ATLAS Collaboration is working on the design of a robust ITk layout
(with extended angular coverage with respect to the current Inner Detector) which will be able
to perform adequately in the high-luminosity conditions. A fast simulation method is particu-
larly useful as we want to quickly compare, in a realistic scenario, different detector designs
and layouts. In this thesis, the performances of three ITk layouts are compared both from the
point of view of the track reconstruction capabilities, which have been explored by considering
single particle samples with either charged pions or muons, and from the point of view of the
physics performances, in the H → ZZ* → 4μ channel. In fact, as Higgs physics is of the utmost
importance, it is useful to check our capability to perform well in the high pile-up environment.

Results obtained in this thesis showed that the layouts proposed are robust, as no significant
dependence of the performance on the pile-up scenario (up to 〈μ〉 = 200) was observed. Also,
the three layouts respect the requirements fixed by the Collaboration on the expected track
parameters resolution in the central region. Our work shows also that, while no clear choice
among the different layouts can be drawn at this stage, the extension of the angular coverage
up to |η| = 4.0 will provide the Higgs studies with better statistics. A signal-to-background
ratio of at least 7 can be obtained in the H → Z Z ∗ → 4μ channel alone, with an estimated
experimental ∆μ/μ of about 2.5% (before inclusion of luminosity uncertainty) for the various
layouts, which is very close to the requirement of the Collaboration.