6 The Discovery of the Higgs Boson at the LHC
285
led to the discovery of a Higgs boson, independently in both the ATLAS and CMS
experiments in July 2012.
In this section we shall concentrate on the region of low mass (114.4 < m H < 150
GeV) where the two channels particularly suited for unambiguous discovery are the
decays to two photons and to two Z bosons, where one or both of the Z bosons could
be virtual, subsequently decaying into four electrons, four muons or two electrons
and two muons. These two decay modes are particularly suited for discovery as
the observed mass resolution (~1% of m H ) is the best and the backgrounds are
manageable or small.
6.7.2.1 The H → γγ Decay Mode
In the H → γ γ analysis a search is made for a narrow peak in the diphoton invariant
mass distribution in the mass range 110–150 GeV, on a large irreducible background
from QCD production of two photons (via quark-antiquark annihilation and the
gluon-fusion or “box” diagrams). There is also a reducible background where one or
more of the reconstructed photon candidates originate from misidentification of jet
fragments, with the process of QCD Compton scattering dominating. The relative
fractions of these backgrounds in the selected events are illustrated in Fig. 6.9a.
The event selection requires two “isolated” photon candidates satisfying p T
and photon identification criteria. As an example, CMS applies a threshold of
p T = m γ γ / 3 (m γ γ / 4) to the leading (sub-leading) photon in p T , where m γ γ is the
diphoton invariant mass. Scaling the p T thresholds in this way avoids distortion of
the shape of the m γ γ distribution. The background is estimated from data, without
the use of MC simulation, by fitting the diphoton invariant mass distribution in a
range (100 < m γ γ < 180 GeV).
The results from the CMS experiments are shown in Fig. 6.8a [20]. A clear peak
at a diphoton mass of around 125 GeV is seen. A similar result was obtained in the
ATLAS experiment [19].
6.7.2.2 The H → ZZ ( ∗ ) → 4 l Decay Mode
In the H → ZZ ( ∗ ) → 4 l decay mode a search is made for a narrow four-charged
lepton mass peak in the presence of a small continuum background. The background
sources include an irreducible four-lepton contribution from direct ZZ ( ∗ ) production
via quark-antiquark and gluon–gluon processes. Reducible background contributions arise from Z + bb and tt production where the final states contain two isolated
leptons and two b-quark jets producing secondary leptons.
The event selection requires two pairs of same-flavour, oppositely charged
isolated leptons. Since there are differences in the reducible background rates and
mass resolutions between the sub-channels 4e, 4 μ, and 2e2μ, they are analysed
separately. Electrons are typically required to have p T > 7 GeV. The corresponding
requirements for muons are p T > 5 GeV. Both electrons and muons are required to
285
led to the discovery of a Higgs boson, independently in both the ATLAS and CMS
experiments in July 2012.
In this section we shall concentrate on the region of low mass (114.4 < m H < 150
GeV) where the two channels particularly suited for unambiguous discovery are the
decays to two photons and to two Z bosons, where one or both of the Z bosons could
be virtual, subsequently decaying into four electrons, four muons or two electrons
and two muons. These two decay modes are particularly suited for discovery as
the observed mass resolution (~1% of m H ) is the best and the backgrounds are
manageable or small.
6.7.2.1 The H → γγ Decay Mode
In the H → γ γ analysis a search is made for a narrow peak in the diphoton invariant
mass distribution in the mass range 110–150 GeV, on a large irreducible background
from QCD production of two photons (via quark-antiquark annihilation and the
gluon-fusion or “box” diagrams). There is also a reducible background where one or
more of the reconstructed photon candidates originate from misidentification of jet
fragments, with the process of QCD Compton scattering dominating. The relative
fractions of these backgrounds in the selected events are illustrated in Fig. 6.9a.
The event selection requires two “isolated” photon candidates satisfying p T
and photon identification criteria. As an example, CMS applies a threshold of
p T = m γ γ / 3 (m γ γ / 4) to the leading (sub-leading) photon in p T , where m γ γ is the
diphoton invariant mass. Scaling the p T thresholds in this way avoids distortion of
the shape of the m γ γ distribution. The background is estimated from data, without
the use of MC simulation, by fitting the diphoton invariant mass distribution in a
range (100 < m γ γ < 180 GeV).
The results from the CMS experiments are shown in Fig. 6.8a [20]. A clear peak
at a diphoton mass of around 125 GeV is seen. A similar result was obtained in the
ATLAS experiment [19].
6.7.2.2 The H → ZZ ( ∗ ) → 4 l Decay Mode
In the H → ZZ ( ∗ ) → 4 l decay mode a search is made for a narrow four-charged
lepton mass peak in the presence of a small continuum background. The background
sources include an irreducible four-lepton contribution from direct ZZ ( ∗ ) production
via quark-antiquark and gluon–gluon processes. Reducible background contributions arise from Z + bb and tt production where the final states contain two isolated
leptons and two b-quark jets producing secondary leptons.
The event selection requires two pairs of same-flavour, oppositely charged
isolated leptons. Since there are differences in the reducible background rates and
mass resolutions between the sub-channels 4e, 4 μ, and 2e2μ, they are analysed
separately. Electrons are typically required to have p T > 7 GeV. The corresponding
requirements for muons are p T > 5 GeV. Both electrons and muons are required to