6 The Discovery of the Higgs Boson at the LHC
301
6.7.4.2 Compatibility of the Observed State with the SM Higgs Boson
Hypothesis: Signal Strength
In the SM, the Higgs boson is a fundamental scalar particle with spin-parity
J P = 0 + , and couples to fundamental fermions as m f
2 /v 2 and to fundamental
bosons as m V
4 /v 2 where v = 246 GeV. Several individual tests of compatibility
with expectations from the SM have been discussed above.
Here we discuss the signal strength, μ, as determined by the combination of
results from all channels. ATLAS and CMS combined their data from Run 1 [37]
from the analysis of five production processes, namely gluon fusion, vector boson
fusion, and associated production with a W or a Z boson or a pair of top quarks, and
of the five decay modes H → γγ, ZZ ( ∗ ) , WW ( ∗ ) , bb, and ττ. All results are reported
assuming a value of 125.09 GeV for the Higgs boson mass. The Higgs boson
production and decay rates measured by the two experiments are combined within
the context of three generic parameterisations: two based on cross sections and
branching fractions, and one on ratios of coupling modifiers. Several interpretations
of the measurements with more model-dependent parameterisations are also given.
The combined signal yield relative to the SM prediction is measured to be
1.09 ± 0.11. The error is broken down as ±0.07 (statistical), ±0.04 (experimental
systematic), ±0.03 (theoretical on background) and ± 0.07 (theoretical on signal).
The most recent measured values of μ, using the above-mentioned channels,
are:
• ATLAS: μ = 1.13 +0.09 –0.08 , using 79.8 fb −1 at
√
s = 13 TeV [55],
• CMS: μ = 1.17 ± 0.1, using 35.9 fb −1 at
√
s = 13 TeV [56].
The error in the measurement from ATLAS has the following breakdown: ±0.05
(statistical), ±0.05 (experimental systematic), ±0.03 (theoretical on background)
and (+0.05, −0.04) (theoretical on signal). This should give a flavor of the
possibilities for extrapolation into the future.
6.7.4.3 Compatibility of the Observed State with the SM Higgs Boson
Hypothesis: Couplings
The 25 products, μ i × μ f , where i (f) is the production (decay) index can also
be considered as free parameters. This can be viewed as the measurements of
cross sections times branching fractions, sigma×BR, by production mechanism and
decay mode. The results from the ATLAS and CMS have been combined and are
illustrated in Fig. 6.22 from the Particle Data Group [54] showing compatibility with
the SM.
Figure 6.23a, from the ATLAS experiment [55], illustrates the dependence of
the Higgs boson couplings on mass of the decay particles (μ, τ, b-quark, W, Z
and t-quark). The plot is made for reduced coupling strength modifiers, κ, using
κ F (m F /v) for fermions and (
√
κ V )(m V /v) for vector bosons, where v = 246.22 GeV.
The couplings modifiers κ F and κ V are measured assuming no BSM contributions
301
6.7.4.2 Compatibility of the Observed State with the SM Higgs Boson
Hypothesis: Signal Strength
In the SM, the Higgs boson is a fundamental scalar particle with spin-parity
J P = 0 + , and couples to fundamental fermions as m f
2 /v 2 and to fundamental
bosons as m V
4 /v 2 where v = 246 GeV. Several individual tests of compatibility
with expectations from the SM have been discussed above.
Here we discuss the signal strength, μ, as determined by the combination of
results from all channels. ATLAS and CMS combined their data from Run 1 [37]
from the analysis of five production processes, namely gluon fusion, vector boson
fusion, and associated production with a W or a Z boson or a pair of top quarks, and
of the five decay modes H → γγ, ZZ ( ∗ ) , WW ( ∗ ) , bb, and ττ. All results are reported
assuming a value of 125.09 GeV for the Higgs boson mass. The Higgs boson
production and decay rates measured by the two experiments are combined within
the context of three generic parameterisations: two based on cross sections and
branching fractions, and one on ratios of coupling modifiers. Several interpretations
of the measurements with more model-dependent parameterisations are also given.
The combined signal yield relative to the SM prediction is measured to be
1.09 ± 0.11. The error is broken down as ±0.07 (statistical), ±0.04 (experimental
systematic), ±0.03 (theoretical on background) and ± 0.07 (theoretical on signal).
The most recent measured values of μ, using the above-mentioned channels,
are:
• ATLAS: μ = 1.13 +0.09 –0.08 , using 79.8 fb −1 at
√
s = 13 TeV [55],
• CMS: μ = 1.17 ± 0.1, using 35.9 fb −1 at
√
s = 13 TeV [56].
The error in the measurement from ATLAS has the following breakdown: ±0.05
(statistical), ±0.05 (experimental systematic), ±0.03 (theoretical on background)
and (+0.05, −0.04) (theoretical on signal). This should give a flavor of the
possibilities for extrapolation into the future.
6.7.4.3 Compatibility of the Observed State with the SM Higgs Boson
Hypothesis: Couplings
The 25 products, μ i × μ f , where i (f) is the production (decay) index can also
be considered as free parameters. This can be viewed as the measurements of
cross sections times branching fractions, sigma×BR, by production mechanism and
decay mode. The results from the ATLAS and CMS have been combined and are
illustrated in Fig. 6.22 from the Particle Data Group [54] showing compatibility with
the SM.
Figure 6.23a, from the ATLAS experiment [55], illustrates the dependence of
the Higgs boson couplings on mass of the decay particles (μ, τ, b-quark, W, Z
and t-quark). The plot is made for reduced coupling strength modifiers, κ, using
κ F (m F /v) for fermions and (
√
κ V )(m V /v) for vector bosons, where v = 246.22 GeV.
The couplings modifiers κ F and κ V are measured assuming no BSM contributions