694
D. Froidevaux
the comparisons in terms of expected performance have been taken and somewhat
updated from a recent review [4]. For completeness, it is important to mention
also the two more specialised and smaller experiments, ALICE [5] and LHCb [6].
In 2019, at a moment when the accelerator and experiments have just completed
very successfully the so-called run-2 with 4 years of operation at a centre-of-mass
energy of 13 TeV, and after run-1 with operation at lower energies topped with the
discovery of the Higgs boson, it is interesting to look back not only on the period of
construction and integration with its great expectations, a period which is the main
focus of this chapter, but also on almost 10 years of operation and data-taking with
its own challenges and of course with the excitement stemming from the analysis of
real data.
The prime motivation of the LHC is to elucidate the nature of electroweak
symmetry breaking, for which the Higgs mechanism is presumed to be responsible.
The experimental study of the Higgs mechanism can also shed light on the
consistency of the Standard Model at energy scales above 1 TeV. The Higgs boson is
generally expected to have a mass below about 200 GeV [7]. This expectation could
be relaxed if there are problems in the interpretation of the precision electroweak
data [8] or if there are additional contributions to the electroweak observables [9]. A
variety of models without Higgs bosons have also been proposed more recently,
together with mechanisms of partial unitarity restoration in longitudinal vector
boson scattering at the TeV scale [10]. All these possibilities may appear to be
remote, but they serve as a reminder that the existence of a light Higgs boson cannot
be taken for granted.
Theories or models beyond the Standard Model invoke additional symmetries
(supersymmetry) or new forces or constituents (strongly-broken electroweak symmetry, technicolour). It is generally hoped that discoveries at the LHC could provide
insight into a unified theory of all fundamental interactions, for example in the form
of supersymmetry or of extra dimensions, the latter requiring modification of gravity
at the TeV scale. There are therefore several compelling reasons for exploring the
TeV scale and the search for supersymmetry is perhaps the most attractive one,
particularly since preserving the naturalness of the electroweak mass scale requires
supersymmetric particles with masses below about 1 TeV.
16.1.2 The Main Initial Physics Goals of ATLAS and CMS at
the LHC
There have been many studies of the LHC discovery potential as a function of the
integrated luminosity and the ones released just before data-taking [11, 12] have
focussed on the first few years, over which about 10 fb −1 of integrated luminosity
were expected to be accumulated by each experiment.
With some optimism that the performance of the ATLAS and CMS detectors
would be understood rapidly and would be close to expectations, the expectations
D. Froidevaux
the comparisons in terms of expected performance have been taken and somewhat
updated from a recent review [4]. For completeness, it is important to mention
also the two more specialised and smaller experiments, ALICE [5] and LHCb [6].
In 2019, at a moment when the accelerator and experiments have just completed
very successfully the so-called run-2 with 4 years of operation at a centre-of-mass
energy of 13 TeV, and after run-1 with operation at lower energies topped with the
discovery of the Higgs boson, it is interesting to look back not only on the period of
construction and integration with its great expectations, a period which is the main
focus of this chapter, but also on almost 10 years of operation and data-taking with
its own challenges and of course with the excitement stemming from the analysis of
real data.
The prime motivation of the LHC is to elucidate the nature of electroweak
symmetry breaking, for which the Higgs mechanism is presumed to be responsible.
The experimental study of the Higgs mechanism can also shed light on the
consistency of the Standard Model at energy scales above 1 TeV. The Higgs boson is
generally expected to have a mass below about 200 GeV [7]. This expectation could
be relaxed if there are problems in the interpretation of the precision electroweak
data [8] or if there are additional contributions to the electroweak observables [9]. A
variety of models without Higgs bosons have also been proposed more recently,
together with mechanisms of partial unitarity restoration in longitudinal vector
boson scattering at the TeV scale [10]. All these possibilities may appear to be
remote, but they serve as a reminder that the existence of a light Higgs boson cannot
be taken for granted.
Theories or models beyond the Standard Model invoke additional symmetries
(supersymmetry) or new forces or constituents (strongly-broken electroweak symmetry, technicolour). It is generally hoped that discoveries at the LHC could provide
insight into a unified theory of all fundamental interactions, for example in the form
of supersymmetry or of extra dimensions, the latter requiring modification of gravity
at the TeV scale. There are therefore several compelling reasons for exploring the
TeV scale and the search for supersymmetry is perhaps the most attractive one,
particularly since preserving the naturalness of the electroweak mass scale requires
supersymmetric particles with masses below about 1 TeV.
16.1.2 The Main Initial Physics Goals of ATLAS and CMS at
the LHC
There have been many studies of the LHC discovery potential as a function of the
integrated luminosity and the ones released just before data-taking [11, 12] have
focussed on the first few years, over which about 10 fb −1 of integrated luminosity
were expected to be accumulated by each experiment.
With some optimism that the performance of the ATLAS and CMS detectors
would be understood rapidly and would be close to expectations, the expectations