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5 Direct Searches for New Physics
in this regime, searches for BSM effects have been the main application of these
techniques very early in the experimental programme of the LHC.
A wealth of results has been produced in the last decade, probing a parameter
space of BSM models inaccessible without jet substructure. Because of this large
number of results, it is neither possible nor instructive to discuss all of them in detail
here. The most recent results are described below, with a focus on results obtained
with 13 TeV data. Older ones are mentioned where appropriate, but no attempt at
completeness is made. Since no discovery has been reported, the results of these
searches are typically given as exclusion limits in terms of upper cross section limits
or lower limits on the mass of the heavy resonance. No attempt is made to interpret
these results in terms of specific BSM models. Conclusions from these interpretations
depend on the BSM model chosen and on the choice of free parameters. Any study of
a complete model would require a combination of results from different final states,
which is beyond the scope of this book. Instead, possible BSM signatures will be
discussed in terms of the experimental signatures, with a focus on the experimental
techniques needed. After all, the task of the experimental collaborations is to cover
all conceivable and improbable final states, with the highest sensitivity achievable.
5.1 Diboson Resonances
The discovery of the Higgs boson marks the first discovery of a resonance decaying
to two electroweak gauge bosons, W W and Z Z. Since m H is of similar magnitude
as m W and m Z , the two gauge bosons are produced with low p T on average at
the LHC. However, BSM models with extended scalar sectors predict the presence
of additional Higgs bosons, which can be very heavy, with masses beyond 1 TeV.
In general, most extensions of the SM predict the existence of additional scalar or
vector bosons with potentially large couplings to gauge bosons. Examples are twoHiggs-doublet models [674–679], models with a additional gauge group structures
resulting from a dynamical electroweak symmetry breaking [680–686], Little Higgs
models [687–695] or models with large extra dimensions [696–703]. They all have
in common that their experimental signatures include diboson final states, with two
highly-energetic scalar or vector bosons, produced back-to-back. The existence of
a light SM Higgs boson also resulted in new search channels for BSM resonances.
Depending on the exact model and the choice of its free parameters, decays to VH or
H H can have the highest branching fractions and thus lead to the best sensitivity. In
the following, results of searches for heavy resonances decaying to V V , VH and H H
are discussed in all-hadronic and +jets channels, where jet substructure techniques
play a central role in the identification of the V and H bosons. A recent review on
searches for BSM physics in diboson final states, including leptonic channels, γ γ
and gg can be found in [704].
5 Direct Searches for New Physics
in this regime, searches for BSM effects have been the main application of these
techniques very early in the experimental programme of the LHC.
A wealth of results has been produced in the last decade, probing a parameter
space of BSM models inaccessible without jet substructure. Because of this large
number of results, it is neither possible nor instructive to discuss all of them in detail
here. The most recent results are described below, with a focus on results obtained
with 13 TeV data. Older ones are mentioned where appropriate, but no attempt at
completeness is made. Since no discovery has been reported, the results of these
searches are typically given as exclusion limits in terms of upper cross section limits
or lower limits on the mass of the heavy resonance. No attempt is made to interpret
these results in terms of specific BSM models. Conclusions from these interpretations
depend on the BSM model chosen and on the choice of free parameters. Any study of
a complete model would require a combination of results from different final states,
which is beyond the scope of this book. Instead, possible BSM signatures will be
discussed in terms of the experimental signatures, with a focus on the experimental
techniques needed. After all, the task of the experimental collaborations is to cover
all conceivable and improbable final states, with the highest sensitivity achievable.
5.1 Diboson Resonances
The discovery of the Higgs boson marks the first discovery of a resonance decaying
to two electroweak gauge bosons, W W and Z Z. Since m H is of similar magnitude
as m W and m Z , the two gauge bosons are produced with low p T on average at
the LHC. However, BSM models with extended scalar sectors predict the presence
of additional Higgs bosons, which can be very heavy, with masses beyond 1 TeV.
In general, most extensions of the SM predict the existence of additional scalar or
vector bosons with potentially large couplings to gauge bosons. Examples are twoHiggs-doublet models [674–679], models with a additional gauge group structures
resulting from a dynamical electroweak symmetry breaking [680–686], Little Higgs
models [687–695] or models with large extra dimensions [696–703]. They all have
in common that their experimental signatures include diboson final states, with two
highly-energetic scalar or vector bosons, produced back-to-back. The existence of
a light SM Higgs boson also resulted in new search channels for BSM resonances.
Depending on the exact model and the choice of its free parameters, decays to VH or
H H can have the highest branching fractions and thus lead to the best sensitivity. In
the following, results of searches for heavy resonances decaying to V V , VH and H H
are discussed in all-hadronic and +jets channels, where jet substructure techniques
play a central role in the identification of the V and H bosons. A recent review on
searches for BSM physics in diboson final states, including leptonic channels, γ γ
and gg can be found in [704].