6 The Discovery of the Higgs Boson at the LHC
265
of the unification of electromagnetism and weak interactions, now labeled the
electro-weak theory. Earlier work on a similar model had been carried out by S.
Glashow [12]. Weinberg and Salam assumed that W and Z bosons acquired mass by
interacting with the field introduced in the earlier papers [3–7].
Both Weinberg and Salam conjectured that such a model would be renormalizable i.e. calculations would give finite answers. The key prediction of their theory
was the existence of the Z 0 boson, in addition to the long-known charged W bosons.
Again not much attention was paid to these papers.
The situation changed dramatically in 1971. t’Hooft in a tour de force, using
methods developed by Veltman, outlined the proof that, indeed, the electro-weak
theory would be renormalizable [13]. The electro-weak theory started being taken
very seriously, so much so that Weinberg’s paper [10] has now become the most
cited paper in physics.
Experimentally, the 1973 discovery of weak neutral currents [14], mediated by
the Z 0 boson, provided strong evidence for the verity of the electro-weak theory.
In parallel much progress had been made in understanding the particles that were
being discovered in the 1950s and 1960s. Eventually, these were understood through
an underlying gauge field theory, where the “charge” of strong interactions was
labeled “colour”, and the interactions of coloured quarks are mediated by gluons.
The theory [15, 16] displayed two main properties: colour confinement, resulting in
the hadrons being colourless, and asymptotic freedom, leading to a steady decrease
in the strength of the interaction between quarks and gluons as the interaction energy
scale increases. The latter enabled the use of perturbation theory for calculating
strong interaction processes at high energies, which has been key to understanding
the physics at the LHC.
Further major discoveries included those of new quarks and the gluon meant that
the discovery, in 1983, of the W and Z bosons [17, 18] at CERN set the stage for the
search for the Higgs boson. The Higgs boson, that earlier had been considered to be
a minor and uninteresting feature of the spontaneous breaking mechanism, became
to assume a role of central importance as the still missing key particle of the SM.
The SM worked so well that the Higgs boson, or something else doing the same job,
more or less had to be present.
In 1984, one year after the discovery of the W and Z bosons, a workshop was held
in Lausanne where first ideas were discussed about a possible proton-proton collider
and associated experiments to make a search for such a particle. The aim was to
reuse the LEP tunnel after the end of the electron-positron programme. Amongst
the leading protagonists were the scientists from UA1 and UA2 experiments. An
exploratory machine was required to cover the wide range of mass values possible
for the SM Higgs boson, its diverse decay signatures and production mechanisms
and to discover any new high-mass particles at a centre-of-mass energy ten times
higher than previously probed. A hadron (proton-proton) collider is such a machine
as long as the proton energy is high enough and the instantaneous luminosity, L ,
measured in cm −2 s −1 , is sufficiently large. The rate of production of a given particle
is determined by L × σ where σ is the cross section of the production reaction,
measured in units of cm 2 . The most interesting and easily detectable final states at a
265
of the unification of electromagnetism and weak interactions, now labeled the
electro-weak theory. Earlier work on a similar model had been carried out by S.
Glashow [12]. Weinberg and Salam assumed that W and Z bosons acquired mass by
interacting with the field introduced in the earlier papers [3–7].
Both Weinberg and Salam conjectured that such a model would be renormalizable i.e. calculations would give finite answers. The key prediction of their theory
was the existence of the Z 0 boson, in addition to the long-known charged W bosons.
Again not much attention was paid to these papers.
The situation changed dramatically in 1971. t’Hooft in a tour de force, using
methods developed by Veltman, outlined the proof that, indeed, the electro-weak
theory would be renormalizable [13]. The electro-weak theory started being taken
very seriously, so much so that Weinberg’s paper [10] has now become the most
cited paper in physics.
Experimentally, the 1973 discovery of weak neutral currents [14], mediated by
the Z 0 boson, provided strong evidence for the verity of the electro-weak theory.
In parallel much progress had been made in understanding the particles that were
being discovered in the 1950s and 1960s. Eventually, these were understood through
an underlying gauge field theory, where the “charge” of strong interactions was
labeled “colour”, and the interactions of coloured quarks are mediated by gluons.
The theory [15, 16] displayed two main properties: colour confinement, resulting in
the hadrons being colourless, and asymptotic freedom, leading to a steady decrease
in the strength of the interaction between quarks and gluons as the interaction energy
scale increases. The latter enabled the use of perturbation theory for calculating
strong interaction processes at high energies, which has been key to understanding
the physics at the LHC.
Further major discoveries included those of new quarks and the gluon meant that
the discovery, in 1983, of the W and Z bosons [17, 18] at CERN set the stage for the
search for the Higgs boson. The Higgs boson, that earlier had been considered to be
a minor and uninteresting feature of the spontaneous breaking mechanism, became
to assume a role of central importance as the still missing key particle of the SM.
The SM worked so well that the Higgs boson, or something else doing the same job,
more or less had to be present.
In 1984, one year after the discovery of the W and Z bosons, a workshop was held
in Lausanne where first ideas were discussed about a possible proton-proton collider
and associated experiments to make a search for such a particle. The aim was to
reuse the LEP tunnel after the end of the electron-positron programme. Amongst
the leading protagonists were the scientists from UA1 and UA2 experiments. An
exploratory machine was required to cover the wide range of mass values possible
for the SM Higgs boson, its diverse decay signatures and production mechanisms
and to discover any new high-mass particles at a centre-of-mass energy ten times
higher than previously probed. A hadron (proton-proton) collider is such a machine
as long as the proton energy is high enough and the instantaneous luminosity, L ,
measured in cm −2 s −1 , is sufficiently large. The rate of production of a given particle
is determined by L × σ where σ is the cross section of the production reaction,
measured in units of cm 2 . The most interesting and easily detectable final states at a