6 The Discovery of the Higgs Boson at the LHC
297
6.7.3.6 H → μμ Decay Mode
The H → μ + μ − decay mode extends the test of the Higgs boson’s coupling to the
second generation of fermions. Several scenarios beyond the SM predict a higher
branching fraction than the one predicted in the SM (2.2 × 10 −4 at m H = 125 GeV).
The dominant and irreducible background arises from the Z/γ ∗ → μμ process
that has a rate several orders of magnitude larger than that from the SM Higgs boson
signal. However, due to the precise muon momentum measurement achieved by
ATLAS and CMS, the dimuon mass resolution is excellent (≈ 2–3%). A search
is performed for a narrow peak over a large but smoothly falling background.
For optimal search sensitivity, events are divided into several categories. Taking
advantage of the superior muon momentum measurement in the central region
events can be subdivided by the pseudorapidity of the muons, or by selections
aiming at specific production processes. A category selecting the vector boson
fusion process with its distinctive signature and relatively large cross section is
particularly useful.
ATLAS has performed this search using data corresponding to an integrated
luminosity of 36.1 fb −1 collected at
√
s = 13 TeV [49]. No significant excess is
observed above the expected background. When combined with the data taken at
√
s = 7 and 8 TeV, the observed (expected) cross-section upper limit is 2.8 (2.9)
times the SM prediction.
The search in CMS, using an integrated luminosity corresponding to 35.9 fb −1
recorded at
√
s = 13 TeV [50], and combining with data taken at
√
s = 7 and 8
TeV, yielded an observed (expected) cross-section upper limit is 2.92 (2.16) times
the Standard Model prediction.
6.7.3.7 ttbar H Production Mode
As m t > m H the Yukawa coupling of the Higgs boson to top quarks cannot be tested
directly. However, it can be measured through the measurement in the pp. → ttH
production process. The coupling of the Higgs boson to the top quark, the heaviest
particle in the SM, could be very sensitive to the effects of physics beyond the SM.
Although the pp. → ttH production process only contributes around 1% of the
total Higgs-boson production cross section, the top quarks in the final state offer
a distinctive signature and allow many Higgs-boson decay modes to be accessed.
Of these, the decay to two b-quarks, the Higgs boson decay mode with the largest
branching fraction, also is sensitive to the b-quark’s Yukawa coupling, the second
largest in the SM.
A top quark decays almost exclusively to a bottom quark and a W boson, with
the W boson subsequently decaying either to a quark and an antiquark or to a
charged lepton and its associated neutrino. The Higgs boson has a rich spectrum
of decay modes, and ttH production is studied using a wide variety of final state
event topologies, with the Higgs boson decaying into bb, WW ( ∗ ) , τ τ , γ γ , and ZZ ( ∗ )
pairs.
297
6.7.3.6 H → μμ Decay Mode
The H → μ + μ − decay mode extends the test of the Higgs boson’s coupling to the
second generation of fermions. Several scenarios beyond the SM predict a higher
branching fraction than the one predicted in the SM (2.2 × 10 −4 at m H = 125 GeV).
The dominant and irreducible background arises from the Z/γ ∗ → μμ process
that has a rate several orders of magnitude larger than that from the SM Higgs boson
signal. However, due to the precise muon momentum measurement achieved by
ATLAS and CMS, the dimuon mass resolution is excellent (≈ 2–3%). A search
is performed for a narrow peak over a large but smoothly falling background.
For optimal search sensitivity, events are divided into several categories. Taking
advantage of the superior muon momentum measurement in the central region
events can be subdivided by the pseudorapidity of the muons, or by selections
aiming at specific production processes. A category selecting the vector boson
fusion process with its distinctive signature and relatively large cross section is
particularly useful.
ATLAS has performed this search using data corresponding to an integrated
luminosity of 36.1 fb −1 collected at
√
s = 13 TeV [49]. No significant excess is
observed above the expected background. When combined with the data taken at
√
s = 7 and 8 TeV, the observed (expected) cross-section upper limit is 2.8 (2.9)
times the SM prediction.
The search in CMS, using an integrated luminosity corresponding to 35.9 fb −1
recorded at
√
s = 13 TeV [50], and combining with data taken at
√
s = 7 and 8
TeV, yielded an observed (expected) cross-section upper limit is 2.92 (2.16) times
the Standard Model prediction.
6.7.3.7 ttbar H Production Mode
As m t > m H the Yukawa coupling of the Higgs boson to top quarks cannot be tested
directly. However, it can be measured through the measurement in the pp. → ttH
production process. The coupling of the Higgs boson to the top quark, the heaviest
particle in the SM, could be very sensitive to the effects of physics beyond the SM.
Although the pp. → ttH production process only contributes around 1% of the
total Higgs-boson production cross section, the top quarks in the final state offer
a distinctive signature and allow many Higgs-boson decay modes to be accessed.
Of these, the decay to two b-quarks, the Higgs boson decay mode with the largest
branching fraction, also is sensitive to the b-quark’s Yukawa coupling, the second
largest in the SM.
A top quark decays almost exclusively to a bottom quark and a W boson, with
the W boson subsequently decaying either to a quark and an antiquark or to a
charged lepton and its associated neutrino. The Higgs boson has a rich spectrum
of decay modes, and ttH production is studied using a wide variety of final state
event topologies, with the Higgs boson decaying into bb, WW ( ∗ ) , τ τ , γ γ , and ZZ ( ∗ )
pairs.