74
G. Altarelli and S. Forte
Fig. 3.13 The total width of
the SM Higgs boson [64]
Mass
[GeV ]
m H
1000
100
10
1
0.1
0.01
0.001
]
V
e
G
[
)
(
H
100 130 160 200
300
500 700 1000
Fig. 3.14 The branching
ratios of the SM Higgs boson
[65]
WW
ZZ
tt
bb
gg
cc
100
200
300
500
o
i
t
a
r
g
n
i
h
c
n
a
r
B
Mass
[GeV ]
m H
0.1
0.01
1
0.05
0.02
0.2
0.5
The dominant channel for such a Higgs is H → b ¯
b. In Born approximation the
partial width into a fermion pair is given by Djouadi [64] and Haber [66]:
(H → f ¯
f ) = N C
G F
4π
√
2
m H m
2
f β
3
f
(3.115)
where β f = (1 − 4m 2
f /m 2
H ) 1/2 . The factor of β 3 appears because the fermion pair
must be in a p-state of orbital angular momentum for a Higgs with scalar coupling,
because of parity (this factor would be β for a pseudoscalar coupling). We see that
the width is suppressed by a factor m 2
f /m 2
H with respect to the natural size G F m 3
H