2.2 General Considerations
15
for momenta between 200 and 2000 GeV). Similar as for W bosons, the differences
in the angular distributions between longitudinal and transverse polarisations can
lead to differences in efficiencies in jet substructure analyses, thus depending on the
production mechanism of the Z boson.
2.2.4 Higgs Boson Decay
The Higgs boson is the youngest member of the family of SM particles. It has been
predicted already in 1964 [3, 4], but its existence has been verified only in 2012
by the ATLAS and CMS Collaborations [1, 2]. It has a mass of m H = 125.09 ±
0.24 GeV [135] and is therefore the second-heaviest particle of the SM. While it
has not been measured with a precision comparable to the one achieved for the
W and Z bosons, all evidence points to it being the SM Higgs boson [7]. Its total
and partial decay widths have been calculated in the SM up to, and including fourloop massless QCD corrections and two-loop electroweak corrections, see [136–
138] for a complete discussion and references therein. Its branching fractions are
shown in Fig. 2.4. The numerical values of the branching fractions together with
their theoretical uncertainties are given in Table 2.1. The total width, assuming no
invisible decays,
4 is H = 4.10 ± 0.06 MeV, for m H = 125.09 GeV.
The fully hadronic decay of the H boson comprises of direct decays into two
quarks, decays into two gluons through loop effects, and decays into pairs of W and
Z bosons with subsequent hadronic decays. In total, the hadronic branching fraction
is B H →had = 80.25 ± 0.86%. Especially relevant for jet substructure applications
is the H → bb decay with a branching fraction of 58.09 ± 0.73%. Here, the presence of two b quarks facilitates experimental identification using the long lifetime
of B hadrons. The second largest hadronic decay is H → W W
∗
→ 4 quarks, with
a branching fraction of 9.79 ± 0.15%. This decay can in principle be distinguished
from QCD branchings due to its four-prong nature, with two quarks having an invariant mass around the W boson mass. Equivalent considerations can be made for the
H → Z Z
∗
→ 4 quarks decay, however, this decay has only a branching fraction of
1.29 ± 0.02%. The third largest branching fraction for hadronic decays originates
from the loop-induced H → gg decay, with a rate of 8.18 ± 0.42%. However, at high
boosts, this decay is nearly indistinguishable from QCD branchings and therefore
has not been specifically targeted in substructure analyses so far.
Due to the spin-0 nature of the Higgs boson, it has only a single polarisation
state and exhibits an isotropic decay in the centre-of-mass frame. Therefore different production mechanisms do not introduce an angular dependence of the decay
fermions.
4 Except for invisible decays in the SM, namely H → Z Z ∗ → ν ¯
νν ¯
ν with a branching fraction of
0.11%.
15
for momenta between 200 and 2000 GeV). Similar as for W bosons, the differences
in the angular distributions between longitudinal and transverse polarisations can
lead to differences in efficiencies in jet substructure analyses, thus depending on the
production mechanism of the Z boson.
2.2.4 Higgs Boson Decay
The Higgs boson is the youngest member of the family of SM particles. It has been
predicted already in 1964 [3, 4], but its existence has been verified only in 2012
by the ATLAS and CMS Collaborations [1, 2]. It has a mass of m H = 125.09 ±
0.24 GeV [135] and is therefore the second-heaviest particle of the SM. While it
has not been measured with a precision comparable to the one achieved for the
W and Z bosons, all evidence points to it being the SM Higgs boson [7]. Its total
and partial decay widths have been calculated in the SM up to, and including fourloop massless QCD corrections and two-loop electroweak corrections, see [136–
138] for a complete discussion and references therein. Its branching fractions are
shown in Fig. 2.4. The numerical values of the branching fractions together with
their theoretical uncertainties are given in Table 2.1. The total width, assuming no
invisible decays,
4 is H = 4.10 ± 0.06 MeV, for m H = 125.09 GeV.
The fully hadronic decay of the H boson comprises of direct decays into two
quarks, decays into two gluons through loop effects, and decays into pairs of W and
Z bosons with subsequent hadronic decays. In total, the hadronic branching fraction
is B H →had = 80.25 ± 0.86%. Especially relevant for jet substructure applications
is the H → bb decay with a branching fraction of 58.09 ± 0.73%. Here, the presence of two b quarks facilitates experimental identification using the long lifetime
of B hadrons. The second largest hadronic decay is H → W W
∗
→ 4 quarks, with
a branching fraction of 9.79 ± 0.15%. This decay can in principle be distinguished
from QCD branchings due to its four-prong nature, with two quarks having an invariant mass around the W boson mass. Equivalent considerations can be made for the
H → Z Z
∗
→ 4 quarks decay, however, this decay has only a branching fraction of
1.29 ± 0.02%. The third largest branching fraction for hadronic decays originates
from the loop-induced H → gg decay, with a rate of 8.18 ± 0.42%. However, at high
boosts, this decay is nearly indistinguishable from QCD branchings and therefore
has not been specifically targeted in substructure analyses so far.
Due to the spin-0 nature of the Higgs boson, it has only a single polarisation
state and exhibits an isotropic decay in the centre-of-mass frame. Therefore different production mechanisms do not introduce an angular dependence of the decay
fermions.
4 Except for invisible decays in the SM, namely H → Z Z ∗ → ν ¯
νν ¯
ν with a branching fraction of
0.11%.