6 The Discovery of the Higgs Boson at the LHC
283
The SM Higgs boson couples to the different pairs of particles in a set proportion
i.e. for fermions (f) proportional to m f
2 , and for bosons (V) proportional m V
4 /v 2
where v is the vacuum expectation value of the scalar field (v = 246 GeV). Once
produced the Higgs boson disintegrates immediately into known SM particles. Both
the production modes and decay modes and rates are precisely predicted in the
SM. A search had to be made over a large range of masses and the many possible
decay modes with differing branching ratios as shown in Fig. 6.2 and Table 6.1. For
example, at m H = 125 GeV the SM boson is predicted to decay into pairs of photons
with branching fraction (BR) of 2.2 × 10 −3 , into Z bosons and then four electrons
or muons or two muons and two electrons with BR = 1.25 × 10 −4 , into a pair of W
bosons and then into llνν with BR ~1%, etc.
For a given Higgs boson mass hypothesis, the sensitivity of the search depends
on:
– the mass of the Higgs boson
– the Higgs boson production cross section (Fig. 6.2a and Table 6.1),
– the decay branching fraction into the selected final state (Fig. 6.2b and Table 6.1),
– the signal selection efficiency,
– the observed Higgs boson mass resolution, and
– the level of backgrounds with the same or a similar final state.
Comparisons with the expectations from the SM for the various combinations of
production and decay modes are usually cast in terms of modifiers such as signal
strength, μ, that is the ratio of the measured production × decay rate of the signal
and the SM expectation i.e. μ = σ.BR/(σ.BR) SM . Signal strength of one would be
indicative of the SM Higgs boson.
CMS and ATLAS increasingly use global event reconstruction algorithms,
labeled particle-flow reconstruction, that attempt to identify, reconstruct and provide
the measurement of the energy of particles by combining information from the inner
tracker, the calorimeters and the muon system in an optimized manner. Hadronic jets
are clustered from the reconstructed particles using the infrared- and collinear-safe
anti-k T algorithm with a distance parameter usually set at 0.4. The jet momenta
are measured by summing vectorially the momenta of all particles in the jet. Jets
originating from b-jets are identified by discriminants that include the presence of
particles originating from vertices displaced from the primary interaction vertex.
A typical b-jet efficiency of around 70% is attained for a 1% misidentification
probability for light quarks and gluons. The missing transverse momentum vector is
taken as the negative of the vector sum of the momenta of all reconstructed particles
in the event; its magnitude is labeled p T
miss .
By the end of 2012 (LHC Run 1) the total amount of data that had been examined
corresponded to an integrated luminosities of ~5 fb −1 at
√
s = 7 TeV and ~ 20
fb −1 at
√
s = 8 TeV, equating to the examination of some 2 × 10 15 proton-proton
collisions.
By the end of the Run 2 (2018) the total amount of data that had been recorded
corresponded to an integrated luminosities ~150 fb −1 at
√
s = 13 TeV, equating to
some 1.5 × 10 16 proton-proton collisions.
283
The SM Higgs boson couples to the different pairs of particles in a set proportion
i.e. for fermions (f) proportional to m f
2 , and for bosons (V) proportional m V
4 /v 2
where v is the vacuum expectation value of the scalar field (v = 246 GeV). Once
produced the Higgs boson disintegrates immediately into known SM particles. Both
the production modes and decay modes and rates are precisely predicted in the
SM. A search had to be made over a large range of masses and the many possible
decay modes with differing branching ratios as shown in Fig. 6.2 and Table 6.1. For
example, at m H = 125 GeV the SM boson is predicted to decay into pairs of photons
with branching fraction (BR) of 2.2 × 10 −3 , into Z bosons and then four electrons
or muons or two muons and two electrons with BR = 1.25 × 10 −4 , into a pair of W
bosons and then into llνν with BR ~1%, etc.
For a given Higgs boson mass hypothesis, the sensitivity of the search depends
on:
– the mass of the Higgs boson
– the Higgs boson production cross section (Fig. 6.2a and Table 6.1),
– the decay branching fraction into the selected final state (Fig. 6.2b and Table 6.1),
– the signal selection efficiency,
– the observed Higgs boson mass resolution, and
– the level of backgrounds with the same or a similar final state.
Comparisons with the expectations from the SM for the various combinations of
production and decay modes are usually cast in terms of modifiers such as signal
strength, μ, that is the ratio of the measured production × decay rate of the signal
and the SM expectation i.e. μ = σ.BR/(σ.BR) SM . Signal strength of one would be
indicative of the SM Higgs boson.
CMS and ATLAS increasingly use global event reconstruction algorithms,
labeled particle-flow reconstruction, that attempt to identify, reconstruct and provide
the measurement of the energy of particles by combining information from the inner
tracker, the calorimeters and the muon system in an optimized manner. Hadronic jets
are clustered from the reconstructed particles using the infrared- and collinear-safe
anti-k T algorithm with a distance parameter usually set at 0.4. The jet momenta
are measured by summing vectorially the momenta of all particles in the jet. Jets
originating from b-jets are identified by discriminants that include the presence of
particles originating from vertices displaced from the primary interaction vertex.
A typical b-jet efficiency of around 70% is attained for a 1% misidentification
probability for light quarks and gluons. The missing transverse momentum vector is
taken as the negative of the vector sum of the momenta of all reconstructed particles
in the event; its magnitude is labeled p T
miss .
By the end of 2012 (LHC Run 1) the total amount of data that had been examined
corresponded to an integrated luminosities of ~5 fb −1 at
√
s = 7 TeV and ~ 20
fb −1 at
√
s = 8 TeV, equating to the examination of some 2 × 10 15 proton-proton
collisions.
By the end of the Run 2 (2018) the total amount of data that had been recorded
corresponded to an integrated luminosities ~150 fb −1 at
√
s = 13 TeV, equating to
some 1.5 × 10 16 proton-proton collisions.