16 Integration of Detectors into a Large Experiment: Examples from ATLAS. . .
773
production cross-section through gluon-gluon fusion, to simple combinations of
the most sensitive channels, and finally to the reduced 7 TeV centre-of-mass
energy of the initial run-1 data. These updated expectations, leading to potential discovery with as little as 5–10 fb −1 of integrated luminosity, resulted in a
period of great excitement within the ATLAS and CMS experiments, but also
in the community at large, from summer 2011 (with 1 fb −1 collected by the
experiments) to summer 2012 when the Higgs boson was officially announced
as having been discovered by each of the two experiments. The evolution of
the Higgs-boson signal significance over this period is illustrated in Fig. 16.34.
In summer 2011, as shown in Fig. 16.34a, there were no indications of any
signal yet and the fluctuations observed as a function of mass were compatible
with background fluctuations. At the end of 2011, however, both experiments
had excluded a Standard Model Higgs-boson signal over a mass range extending from the LEP limit of 114 to 600 GeV, except for a narrow mass range
around 125 GeV in which the largest deviation from background expectations
was observed around 125 GeV and corresponded to approximately three standard
deviations in each experiment, as shown in Fig. 16.34b. Finally, Fig. 16.34c,d
shows the observed significance in summer 2012 when the discovery was claimed
and subsequently published by both experiments [42, 43] for 10 fb −1 of data at
7 and 8 TeV.
The four-lepton and diphoton channels have always been rightly considered as
the two best channels for Higgs-boson discovery, since they both provide a clear
and narrow peak for the Higgs-boson signal in the invariant mass distribution of the
final state particles on top of a continuous background. In addition the four-lepton
channel can be observed above a much smaller continuum background, consisting
predominantly of continuum ZZ∗ → 4l final states. These features can be seen in
Figs. 16.35 and 16.36 taken from the ATLAS discovery publication [42]. In contrast,
the third channel which contributed to the discovery, namely the H → W W ∗ →
lνlν channel, has a poor mass resolution because of the presence of neutrinos in the
final state, as shown in Fig. 16.37.
After the discovery, measurements of the properties of the Higgs boson were
performed in successive stages, first focusing on its spin, then on its couplings
to bosons and fermions and on possible non-SM contributions to its width. At
the end of run-1, ATLAS and CMS produced a combined paper on the Higgsboson couplings [44], leading to the conclusion that in all production modes and
decay channels which had been measured at the time, the Higgs-boson properties
were compatible with what one would expect from the SM. More recently, each
experiment has produced updated results based also on a large fraction of the
run-2 data. This is illustrated in Fig. 16.38, which is based on the most recent
run-2 ATLAS Higgs combination results [45] and shows that the strength of the
measured Higgs-boson couplings to fermions and bosons follows the expectations
from the SM, in which for example the Yukawa fermion coupling is expected to be
proportional to the fermion mass. Finally, based on the most recent results from the
combined run-1 and run-2 datasets from ATLAS and CMS [46], Table 16.17 shows
773
production cross-section through gluon-gluon fusion, to simple combinations of
the most sensitive channels, and finally to the reduced 7 TeV centre-of-mass
energy of the initial run-1 data. These updated expectations, leading to potential discovery with as little as 5–10 fb −1 of integrated luminosity, resulted in a
period of great excitement within the ATLAS and CMS experiments, but also
in the community at large, from summer 2011 (with 1 fb −1 collected by the
experiments) to summer 2012 when the Higgs boson was officially announced
as having been discovered by each of the two experiments. The evolution of
the Higgs-boson signal significance over this period is illustrated in Fig. 16.34.
In summer 2011, as shown in Fig. 16.34a, there were no indications of any
signal yet and the fluctuations observed as a function of mass were compatible
with background fluctuations. At the end of 2011, however, both experiments
had excluded a Standard Model Higgs-boson signal over a mass range extending from the LEP limit of 114 to 600 GeV, except for a narrow mass range
around 125 GeV in which the largest deviation from background expectations
was observed around 125 GeV and corresponded to approximately three standard
deviations in each experiment, as shown in Fig. 16.34b. Finally, Fig. 16.34c,d
shows the observed significance in summer 2012 when the discovery was claimed
and subsequently published by both experiments [42, 43] for 10 fb −1 of data at
7 and 8 TeV.
The four-lepton and diphoton channels have always been rightly considered as
the two best channels for Higgs-boson discovery, since they both provide a clear
and narrow peak for the Higgs-boson signal in the invariant mass distribution of the
final state particles on top of a continuous background. In addition the four-lepton
channel can be observed above a much smaller continuum background, consisting
predominantly of continuum ZZ∗ → 4l final states. These features can be seen in
Figs. 16.35 and 16.36 taken from the ATLAS discovery publication [42]. In contrast,
the third channel which contributed to the discovery, namely the H → W W ∗ →
lνlν channel, has a poor mass resolution because of the presence of neutrinos in the
final state, as shown in Fig. 16.37.
After the discovery, measurements of the properties of the Higgs boson were
performed in successive stages, first focusing on its spin, then on its couplings
to bosons and fermions and on possible non-SM contributions to its width. At
the end of run-1, ATLAS and CMS produced a combined paper on the Higgsboson couplings [44], leading to the conclusion that in all production modes and
decay channels which had been measured at the time, the Higgs-boson properties
were compatible with what one would expect from the SM. More recently, each
experiment has produced updated results based also on a large fraction of the
run-2 data. This is illustrated in Fig. 16.38, which is based on the most recent
run-2 ATLAS Higgs combination results [45] and shows that the strength of the
measured Higgs-boson couplings to fermions and bosons follows the expectations
from the SM, in which for example the Yukawa fermion coupling is expected to be
proportional to the fermion mass. Finally, based on the most recent results from the
combined run-1 and run-2 datasets from ATLAS and CMS [46], Table 16.17 shows