2
1 Introduction
precision measurements of fundamental parameters of the theory. Taken together,
this has resulted in SM consistency tests with remarkable precision, confirming the
validity of the theory up to scales probed by the LHC. There is an ever increasing
degree of scientific confidence in the SM.
While the SM is exceptionally successful, no explanation exists for a variety of
fundamental questions. These include the unlikeness of the Higgs boson mass from
its naive quantum-mechanical expectation, the nature of dark matter, the masses of
neutrinos, the number of fermion generations, to name a few. Especially intriguing
is the question of the nature of the Higgs boson and the dynamics of the underlying
scalar field, responsible for the spontaneous electroweak (EW) symmetry breaking.
Central to this mechanism is the top quark, the SM particle with the largest coupling
to the Higgs field. With its centre-of-mass energy
√
s = 13 TeV, the LHC is the ideal
machine to study these questions and probe the SM at the EW scale v 246 GeV
and above.
Broadly, there exist three different approaches to probe for effects from unknown
physics: direct searches, precision measurements and measurements of very rare
processes. At the LHC, direct searches have been focussing on finding traces of
particles predicted by beyond the standard model (BSM) theories, often with very
specific signatures. So far, these direct searches have not found any evidence for
BSM effects, even though hundreds of analyses have been carried out [11]. Precision
measurements allow for the possibility to find small deviations from SM expectations, giving indirect hints of BSM effects. Important milestones of the experimental
programme at the LHC are precision measurements of Higgs boson properties, such
as its mass, its coupling to vector bosons and fermions, and its parity. The programme
also includes measurements of the W boson mass, the top quark mass, the effective
weak mixing angle and cross sections of W , Z and jet production. Taken together,
these measurements have the potential to reveal unknown effects through comparisons with precision calculations and global fits of SM parameters [12–16]. The last
possibility, the observation of very rare processes, allows to probe for BSM physics
through quantum corrections entering the calculations of rare decays or production
mechanisms. Examples are B
0
(s) → μ
+
μ
− [17, 18], four-top [19] or di-Higgs [20]
production at the LHC.
With the end of the data acquisition period from the end of 2015–2018, called
Run 2, LHC analyses are being completed using the full 13 TeV data corresponding
to about 140 fb
−1 . It becomes apparent that none of the three approaches has resulted
in evidence for unknown effects. In this sense, particle physics at the LHC is at a
crossroads. The next data acquisition period of the LHC experiments, Run 3 , is
planned to start in 2022 and continue until the end of 2025. The total amount of
data collected by the end of 2024 will correspond to about 300 fb
−1 . However, the
sensitivity of direct searches for BSM effects improves only little when doubling the
amount of data. In terms of discovery significance, the mass reach improves only by
10–20% for analyses dominated by the statistical precision of the data and even less
for analyses dominated by systematic uncertainties. Precision measurements face the
1 Introduction
precision measurements of fundamental parameters of the theory. Taken together,
this has resulted in SM consistency tests with remarkable precision, confirming the
validity of the theory up to scales probed by the LHC. There is an ever increasing
degree of scientific confidence in the SM.
While the SM is exceptionally successful, no explanation exists for a variety of
fundamental questions. These include the unlikeness of the Higgs boson mass from
its naive quantum-mechanical expectation, the nature of dark matter, the masses of
neutrinos, the number of fermion generations, to name a few. Especially intriguing
is the question of the nature of the Higgs boson and the dynamics of the underlying
scalar field, responsible for the spontaneous electroweak (EW) symmetry breaking.
Central to this mechanism is the top quark, the SM particle with the largest coupling
to the Higgs field. With its centre-of-mass energy
√
s = 13 TeV, the LHC is the ideal
machine to study these questions and probe the SM at the EW scale v 246 GeV
and above.
Broadly, there exist three different approaches to probe for effects from unknown
physics: direct searches, precision measurements and measurements of very rare
processes. At the LHC, direct searches have been focussing on finding traces of
particles predicted by beyond the standard model (BSM) theories, often with very
specific signatures. So far, these direct searches have not found any evidence for
BSM effects, even though hundreds of analyses have been carried out [11]. Precision
measurements allow for the possibility to find small deviations from SM expectations, giving indirect hints of BSM effects. Important milestones of the experimental
programme at the LHC are precision measurements of Higgs boson properties, such
as its mass, its coupling to vector bosons and fermions, and its parity. The programme
also includes measurements of the W boson mass, the top quark mass, the effective
weak mixing angle and cross sections of W , Z and jet production. Taken together,
these measurements have the potential to reveal unknown effects through comparisons with precision calculations and global fits of SM parameters [12–16]. The last
possibility, the observation of very rare processes, allows to probe for BSM physics
through quantum corrections entering the calculations of rare decays or production
mechanisms. Examples are B
0
(s) → μ
+
μ
− [17, 18], four-top [19] or di-Higgs [20]
production at the LHC.
With the end of the data acquisition period from the end of 2015–2018, called
Run 2, LHC analyses are being completed using the full 13 TeV data corresponding
to about 140 fb
−1 . It becomes apparent that none of the three approaches has resulted
in evidence for unknown effects. In this sense, particle physics at the LHC is at a
crossroads. The next data acquisition period of the LHC experiments, Run 3 , is
planned to start in 2022 and continue until the end of 2025. The total amount of
data collected by the end of 2024 will correspond to about 300 fb
−1 . However, the
sensitivity of direct searches for BSM effects improves only little when doubling the
amount of data. In terms of discovery significance, the mass reach improves only by
10–20% for analyses dominated by the statistical precision of the data and even less
for analyses dominated by systematic uncertainties. Precision measurements face the