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P. Jenni and T. S. Virdee
hadron collider involve charged leptons and photons and have a low σ × BR, where
BR is the branching ratio into the decay mode of interest.
A major goal of the LHC thus became the elucidation of the mechanism for
electroweak symmetry breaking. It also was clear that a search had to be made for
new physics at the TeV energy scale as the SM is logically incomplete; it does
not incorporate gravity. A promising avenue is the superstring theory, an attempt
towards a unified theory with dramatic predictions of extra space dimensions and
supersymmetry.
The LHC and its experiments were designed to find new particles, new forces
and new symmetries amongst which could be the Higgs boson(s), supersymmetric
particles, Z bosons, or evidence of extra space dimensions. An experiment that
could cover the detection of all these hypothesized but yet undiscovered particles
would provide the best opportunity to discover whatever else might be produced at
LHC energies.
In July 2012 the ATLAS and CMS collaborations discovered a Higgs boson [19,
20].
This paper is based on the previous articles [1, 21–23] written by the authors,
some with M. Della Negra, using the recently published results from the ATLAS
and CMS Collaborations on the measurements of the properties of the Higgs boson.
6.2 The SM Higgs Boson
In the early 1990’s the search for the SM Higgs boson played a pivotal role in the
design of the ATLAS and CMS experiments. The mass of the Higgs boson (m H )
is not predicted by theory, but from general considerations, m H < 1 TeV. At the
start of the LHC operation, direct searches for the Higgs boson carried out at the
LEP collider led to a lower bound of m H > 114.4 GeV at 95% CL [24], whilst
precision electroweak constraints, including LEP data, implied that m H < 152 GeV
at 95% confidence level (CL) [25]. At time of the discovery at CERN, CDF and D0
experiments operating the Tevatron proton antiproton collider, detected an excess of
events in the range 120–135 GeV [26].
It is known that quantum corrections make the mass of any fundamental scalar
particle, such as the SM Higgs boson, float up to the next highest mass scale
present in the theory, which in the absence of extensions to the SM, can be as
high as 10 15 GeV. Hence finding the scalar Higgs boson would immediately raise a
more puzzling question: Why should it have a mass in the range between 100 GeV
and 1 TeV? One appealing hypothesis, much discussed at the time, and still being
investigated, predicts a new symmetry labeled supersymmetry. For every known SM
particle there would be a partner with spin differing by half a unit; fermions would
have boson superpartners and vice versa, thus doubling the number of fundamental
particles. The contributions from the boson and fermion superpartners, and vice a
versa, with amplitudes of opposite signs, would lead to their cancellation, and allow
a low mass for the Higgs boson. In supersymmetry five Higgs bosons are predicted
P. Jenni and T. S. Virdee
hadron collider involve charged leptons and photons and have a low σ × BR, where
BR is the branching ratio into the decay mode of interest.
A major goal of the LHC thus became the elucidation of the mechanism for
electroweak symmetry breaking. It also was clear that a search had to be made for
new physics at the TeV energy scale as the SM is logically incomplete; it does
not incorporate gravity. A promising avenue is the superstring theory, an attempt
towards a unified theory with dramatic predictions of extra space dimensions and
supersymmetry.
The LHC and its experiments were designed to find new particles, new forces
and new symmetries amongst which could be the Higgs boson(s), supersymmetric
particles, Z bosons, or evidence of extra space dimensions. An experiment that
could cover the detection of all these hypothesized but yet undiscovered particles
would provide the best opportunity to discover whatever else might be produced at
LHC energies.
In July 2012 the ATLAS and CMS collaborations discovered a Higgs boson [19,
20].
This paper is based on the previous articles [1, 21–23] written by the authors,
some with M. Della Negra, using the recently published results from the ATLAS
and CMS Collaborations on the measurements of the properties of the Higgs boson.
6.2 The SM Higgs Boson
In the early 1990’s the search for the SM Higgs boson played a pivotal role in the
design of the ATLAS and CMS experiments. The mass of the Higgs boson (m H )
is not predicted by theory, but from general considerations, m H < 1 TeV. At the
start of the LHC operation, direct searches for the Higgs boson carried out at the
LEP collider led to a lower bound of m H > 114.4 GeV at 95% CL [24], whilst
precision electroweak constraints, including LEP data, implied that m H < 152 GeV
at 95% confidence level (CL) [25]. At time of the discovery at CERN, CDF and D0
experiments operating the Tevatron proton antiproton collider, detected an excess of
events in the range 120–135 GeV [26].
It is known that quantum corrections make the mass of any fundamental scalar
particle, such as the SM Higgs boson, float up to the next highest mass scale
present in the theory, which in the absence of extensions to the SM, can be as
high as 10 15 GeV. Hence finding the scalar Higgs boson would immediately raise a
more puzzling question: Why should it have a mass in the range between 100 GeV
and 1 TeV? One appealing hypothesis, much discussed at the time, and still being
investigated, predicts a new symmetry labeled supersymmetry. For every known SM
particle there would be a partner with spin differing by half a unit; fermions would
have boson superpartners and vice versa, thus doubling the number of fundamental
particles. The contributions from the boson and fermion superpartners, and vice a
versa, with amplitudes of opposite signs, would lead to their cancellation, and allow
a low mass for the Higgs boson. In supersymmetry five Higgs bosons are predicted