420
22. The GSW Gauge Theory of Electroweak Interactions
and this was followed by nine similar events from D0 (Abachi et al. 1995a).
By February 1995 both groups had amassed more data and the discovery was
announced (Abe et al. 1995b, Abachi et al. 1995b). The 2010 experimental
value for m t is 173.1±1.3 GeV (Nakamura et al. 2010) as compared to the value
predicted by fits to the electroweak data of 173.2±1.3 GeV. This represents an
extraordinary triumph for both theory and experiment. It is surely remarkable
how the quantum fluctuations of a yet-to-be-detected new particle could pin
down its mass so precisely. It seems hard to deny that Nature has indeed made
use of the subtle intricacies of a spontaneously broken non-Abelian gauge
theory.
One feature of the ‘real’ top events in particularly noteworthy. Unlike the
mass of the other quarks, m t is greater than M W , and this means that it can
decay to b + W via real W emission:
t → W
+ + b.
(22.201)
In contrast, the b quark itself decays by the usual virtual W processes. Now we
have seen that the virtual process is supressed by ∼ 1/M
2 if the energy release
W
(as in the case of b-decay) is well below M W . But the real process (22.201)
suffers no such suppression and proceeds very much faster. In fact (problem
22.10) the top quark lifetime from (22.201) is estimated to be ∼ 4 × 10
−25
s! This is quite similar to the lifetime of the W
+ itself, via W
+
→ e
+ ν e for
example. Consider now the production of a t ¯ t pair in the collision between
two partons. As the t and ¯ t separate, the strong interactions which should
eventually ‘hadronize’ them will not play a role until they are ∼ 1 fm apart.
But if they are travelling close to the speed of light, they can only travel some
10
−16 m before decaying. Thus t’s tend to decay before they experience the
confining QCD interactions, a point we also made in section 1.2. Instead, the
hadronization is associated with the b quark, which has a more typical weak
lifetime (∼ 1.5 × 10
−12 s). By the same token, this fast decay of the t quark
means that there will be no detectable t ¯ t ‘toponium’, bound by QCD.
With the t quark safely real, the Higgs boson was the one remaining missing particle in the Standard Model complement, and its discovery was of the
utmost importance. We end this book with a brief review of Higgs physics
and the experiments leading to the probable discovery of this long-awaited
particle in 2012.
22.8 The Higgs sector
It is worth noting that an essential feature of the type of theory which has
been described in this note is the prediction of incomplete multiplets of
scalar and vector bosons.
—P W Higgs (1964)
22. The GSW Gauge Theory of Electroweak Interactions
and this was followed by nine similar events from D0 (Abachi et al. 1995a).
By February 1995 both groups had amassed more data and the discovery was
announced (Abe et al. 1995b, Abachi et al. 1995b). The 2010 experimental
value for m t is 173.1±1.3 GeV (Nakamura et al. 2010) as compared to the value
predicted by fits to the electroweak data of 173.2±1.3 GeV. This represents an
extraordinary triumph for both theory and experiment. It is surely remarkable
how the quantum fluctuations of a yet-to-be-detected new particle could pin
down its mass so precisely. It seems hard to deny that Nature has indeed made
use of the subtle intricacies of a spontaneously broken non-Abelian gauge
theory.
One feature of the ‘real’ top events in particularly noteworthy. Unlike the
mass of the other quarks, m t is greater than M W , and this means that it can
decay to b + W via real W emission:
t → W
+ + b.
(22.201)
In contrast, the b quark itself decays by the usual virtual W processes. Now we
have seen that the virtual process is supressed by ∼ 1/M
2 if the energy release
W
(as in the case of b-decay) is well below M W . But the real process (22.201)
suffers no such suppression and proceeds very much faster. In fact (problem
22.10) the top quark lifetime from (22.201) is estimated to be ∼ 4 × 10
−25
s! This is quite similar to the lifetime of the W
+ itself, via W
+
→ e
+ ν e for
example. Consider now the production of a t ¯ t pair in the collision between
two partons. As the t and ¯ t separate, the strong interactions which should
eventually ‘hadronize’ them will not play a role until they are ∼ 1 fm apart.
But if they are travelling close to the speed of light, they can only travel some
10
−16 m before decaying. Thus t’s tend to decay before they experience the
confining QCD interactions, a point we also made in section 1.2. Instead, the
hadronization is associated with the b quark, which has a more typical weak
lifetime (∼ 1.5 × 10
−12 s). By the same token, this fast decay of the t quark
means that there will be no detectable t ¯ t ‘toponium’, bound by QCD.
With the t quark safely real, the Higgs boson was the one remaining missing particle in the Standard Model complement, and its discovery was of the
utmost importance. We end this book with a brief review of Higgs physics
and the experiments leading to the probable discovery of this long-awaited
particle in 2012.
22.8 The Higgs sector
It is worth noting that an essential feature of the type of theory which has
been described in this note is the prediction of incomplete multiplets of
scalar and vector bosons.
—P W Higgs (1964)