34
1. The Particles and Forces of the Standard Model
Higgs mass can be obtained directly, through searching for its production and
subsequent decay; non-observation will lead to a lower bound for m H . There
are also indirect constraints, coming from fits to precision measurements of
electroweak observables. The latter are sensitive to higher order corrections
which involve the Higgs boson as a virtual particle; these depend logarithmically on the unknown parameter m H and give upper bounds on m H , assuming,
of course, that the SM is correct.
A lower bound
m H > 114.4 GeV (95% C.L.)
(1.34)
was set at LEP (LEP 2003) by combining data on direct searches. Combining
this with a global fit to precision electroweak data, an upper bound
m H < 186 GeV (95% C.L.)
(1.35)
was obtained (Nakamura et al. 2010).
By early 2012, the combined results of the CDF and D0 experiments at
the Tevatron, and the ATLAS and CMS experiments at the LHC, excluded an
m H value in the interval (approximately) 130 GeV to 600 GeV, at 95 % C.L.
Finally, in July 2012 the ATLAS (Aad et al. 2012) and CMS (Chatrchyan et
al. 2012) collaborations announced the discovery, with a significance of 5σ,
of a neutral boson with a mass in the range 125–126 GeV, its production and
decay rates being broadly compatible with the predictions for the SM Higgs
boson. The existence of the measured decay to two photons implies that the
particle is a boson with spin different from 1 (Landau 1948, Yang 1950), but
spin-0 has not yet been confirmed. Nevertheless, it is probable that this is the
(or perhaps a) Higgs boson. Its long-anticipated discovery opens a new era
in particle physics: the experimental exploration of the symmetry-breaking
sector of the SM.
1.5 Summary
The Standard Model provides a relatively simple picture of quarks and leptons
and their non-gravitational interactions. The quark colour triplets are the
basic source particles of the gluon fields in QCD, and they bind together to
make hadrons. The weak interactions involve quark and lepton doublets – for
instance the quark doublet (u, d) and the lepton doublet (ν e , e
− ) of the first
generation. These are sources for the W
± and Z
0 fields. Charged fermions
(quarks and leptons) are sources for the photon field. All the mediating force
quanta have spin-1. The weak and strong force fields are generalizations of
electromagnetism; all three are examples of gauge theories, but realized in
subtly different ways.
1. The Particles and Forces of the Standard Model
Higgs mass can be obtained directly, through searching for its production and
subsequent decay; non-observation will lead to a lower bound for m H . There
are also indirect constraints, coming from fits to precision measurements of
electroweak observables. The latter are sensitive to higher order corrections
which involve the Higgs boson as a virtual particle; these depend logarithmically on the unknown parameter m H and give upper bounds on m H , assuming,
of course, that the SM is correct.
A lower bound
m H > 114.4 GeV (95% C.L.)
(1.34)
was set at LEP (LEP 2003) by combining data on direct searches. Combining
this with a global fit to precision electroweak data, an upper bound
m H < 186 GeV (95% C.L.)
(1.35)
was obtained (Nakamura et al. 2010).
By early 2012, the combined results of the CDF and D0 experiments at
the Tevatron, and the ATLAS and CMS experiments at the LHC, excluded an
m H value in the interval (approximately) 130 GeV to 600 GeV, at 95 % C.L.
Finally, in July 2012 the ATLAS (Aad et al. 2012) and CMS (Chatrchyan et
al. 2012) collaborations announced the discovery, with a significance of 5σ,
of a neutral boson with a mass in the range 125–126 GeV, its production and
decay rates being broadly compatible with the predictions for the SM Higgs
boson. The existence of the measured decay to two photons implies that the
particle is a boson with spin different from 1 (Landau 1948, Yang 1950), but
spin-0 has not yet been confirmed. Nevertheless, it is probable that this is the
(or perhaps a) Higgs boson. Its long-anticipated discovery opens a new era
in particle physics: the experimental exploration of the symmetry-breaking
sector of the SM.
1.5 Summary
The Standard Model provides a relatively simple picture of quarks and leptons
and their non-gravitational interactions. The quark colour triplets are the
basic source particles of the gluon fields in QCD, and they bind together to
make hadrons. The weak interactions involve quark and lepton doublets – for
instance the quark doublet (u, d) and the lepton doublet (ν e , e
− ) of the first
generation. These are sources for the W
± and Z
0 fields. Charged fermions
(quarks and leptons) are sources for the photon field. All the mediating force
quanta have spin-1. The weak and strong force fields are generalizations of
electromagnetism; all three are examples of gauge theories, but realized in
subtly different ways.