6 The Discovery of the Higgs Boson at the LHC
303
fermions, and κ V = v m
2∈
V /M 1+2∈ for vector bosons. The SM expectation, κ i = 1,
is recovered when (M, ε) = (v, 0).
Both Fig. 6.23a and b demonstrate good compatibility with the SM within the
errors on the measurements.
6.7.4.4 Compatibility of the Observed State with the SM Higgs Boson
Hypothesis: Quantum Numbers
Ascertaining the quantum numbers of the Higgs boson is essential to the understanding of its nature and its coupling properties. According to the Landau-Yang theorem
the observations made in the diphoton channel excludes the spin-1 hypothesis and
restricts possibilities for the boson to have spin-0 or -2. The diphoton decay mode
also implies that the boson has charge conjugation has C-even.
To identify the spin-parity of the Higgs boson the production and decay processes
are examined in several analyses. The angular distributions of the decay particles
can be used to test various spin hypotheses.
Much can be gleaned from the decay mode H → ZZ ( ∗ ) → 4 l, where the full
final state is reconstructed, including the angular variables sensitive to the spinparity, along with a very favourable signal/background ratio. CMS has used the
information from the five angles (see Fig. 6.24a) and the two dilepton pair masses
combined to form a discriminant based on the 0 + nature of the Higgs boson [57].
ATLAS has also tested various J P hypotheses, and in particular 0 + and 0 − .
In all scenarios investigated by both CMS [57] and ATLAS [58] experiments, the
data are compatible with the 0 + hypothesis, excluding a pseudoscalar nature at CLS
levels of 99.8% and 98.0%, respectively. The expected distribution in ATLAS of the
test statistic for the SM hypothesis (in blue) and several alternative spin and parity
hypotheses is compared in Fig. 6.24b. The combination of the three decay processes
allows the exclusion of all considered non-SM hypotheses at amore than 99.9% CL
in favour of the SM spin-0 hypothesis.
6.8 Conclusions and Outlook
In July 2012 the ATLAS and CMS experiments announced the discovery of a Higgs
boson, confirming the conjecture put forward in the 1960s. Further results from the
two experiments show that, within the current measurement precision, the Higgs
boson has the properties predicted by the SM. However, several theories of physics
beyond the SM (BSM) predict the existence of more than one Higgs boson, and
one of these would only be subtly different from that predicted in the SM one, with
signal strengths differing by between 0.5–5%, depending on the model in question,
indicative of the required level of sensitivity to distinguish it from a SM Higgs
boson.
303
fermions, and κ V = v m
2∈
V /M 1+2∈ for vector bosons. The SM expectation, κ i = 1,
is recovered when (M, ε) = (v, 0).
Both Fig. 6.23a and b demonstrate good compatibility with the SM within the
errors on the measurements.
6.7.4.4 Compatibility of the Observed State with the SM Higgs Boson
Hypothesis: Quantum Numbers
Ascertaining the quantum numbers of the Higgs boson is essential to the understanding of its nature and its coupling properties. According to the Landau-Yang theorem
the observations made in the diphoton channel excludes the spin-1 hypothesis and
restricts possibilities for the boson to have spin-0 or -2. The diphoton decay mode
also implies that the boson has charge conjugation has C-even.
To identify the spin-parity of the Higgs boson the production and decay processes
are examined in several analyses. The angular distributions of the decay particles
can be used to test various spin hypotheses.
Much can be gleaned from the decay mode H → ZZ ( ∗ ) → 4 l, where the full
final state is reconstructed, including the angular variables sensitive to the spinparity, along with a very favourable signal/background ratio. CMS has used the
information from the five angles (see Fig. 6.24a) and the two dilepton pair masses
combined to form a discriminant based on the 0 + nature of the Higgs boson [57].
ATLAS has also tested various J P hypotheses, and in particular 0 + and 0 − .
In all scenarios investigated by both CMS [57] and ATLAS [58] experiments, the
data are compatible with the 0 + hypothesis, excluding a pseudoscalar nature at CLS
levels of 99.8% and 98.0%, respectively. The expected distribution in ATLAS of the
test statistic for the SM hypothesis (in blue) and several alternative spin and parity
hypotheses is compared in Fig. 6.24b. The combination of the three decay processes
allows the exclusion of all considered non-SM hypotheses at amore than 99.9% CL
in favour of the SM spin-0 hypothesis.
6.8 Conclusions and Outlook
In July 2012 the ATLAS and CMS experiments announced the discovery of a Higgs
boson, confirming the conjecture put forward in the 1960s. Further results from the
two experiments show that, within the current measurement precision, the Higgs
boson has the properties predicted by the SM. However, several theories of physics
beyond the SM (BSM) predict the existence of more than one Higgs boson, and
one of these would only be subtly different from that predicted in the SM one, with
signal strengths differing by between 0.5–5%, depending on the model in question,
indicative of the required level of sensitivity to distinguish it from a SM Higgs
boson.