Chapter 5
Direct Searches for New Physics
Abstract Heavy resonances, as predicted by theories extending the SM, result in
highly energetic particles and very collimated decays. Already since the first collision
data have been studied at the LHC, jet substructure methods have been an integral
part in analyses searching for unknown effects. With higher centre-of-mass energies
and the availability of larger datasets, their relevance has multiplied. Nowadays,
jet substructure methods permeate numerous analyses aiming at very different final
states. This chapter provides an overview of searches for new physical phenomena
where substructure techniques have been indispensable and will play a major role in
the future.
Jet substructure taggers have been a critical component in searches for BSM effects
from the beginning of data analyses at the LHC, unlike SM measurements, where
jet substructure has started to play a role much later. The reason lies in the fact that
the expected cross sections of SM and BSM processes differ fundamentally. The
partonic cross sections for the production of SM particles fall as 1/ˆ s once the scale
of the process is much larger than the mass of the SM particles involved, where ˆ
s
is the partonic centre-of-mass energy. Convoluted with the steeply falling PDFs for
increasing fractional parton momentum x, this results in a very small cross section at
high p T , relative to the total production cross section. For example, the cross section
for tt production with top quark p T > 350 GeV is about 13 pb [655], compared to the
total cross section of about 830 pb at
√
s = 13 TeV [627, 672, 673]. The statistical
power of the data is largest for low- p T SM processes. When the recorded dataset
becomes larger, the statistical precision becomes sufficient to probe high- p T SM
production cross sections. Hence, measurements at high p T are usually performed
later in the lifetime of a particle collider. In contrast to this, the production of a narrow,
heavy BSM resonance with a mass m X beyond 1 TeV will result in a negligible cross
section at low p T , while the production cross section will be dominated by events
with p T ≈ m X /2, where p T denotes here the transverse momentum of the resonance
decay products. Already at
√
s = 7 and 8 TeV, cross sections larger than 1 pb for the
production of resonances with m X > 1 TeV have been predicted. Since the hadronic
channels of W , Z , H and t decays are only accessible with substructure techniques
© Springer Nature Switzerland AG 2021
R. Kogler, Advances in Jet Substructure at the LHC, Springer Tracts
in Modern Physics 284, https://doi.org/10.1007/978-3-030-72858-8_5
121
Direct Searches for New Physics
Abstract Heavy resonances, as predicted by theories extending the SM, result in
highly energetic particles and very collimated decays. Already since the first collision
data have been studied at the LHC, jet substructure methods have been an integral
part in analyses searching for unknown effects. With higher centre-of-mass energies
and the availability of larger datasets, their relevance has multiplied. Nowadays,
jet substructure methods permeate numerous analyses aiming at very different final
states. This chapter provides an overview of searches for new physical phenomena
where substructure techniques have been indispensable and will play a major role in
the future.
Jet substructure taggers have been a critical component in searches for BSM effects
from the beginning of data analyses at the LHC, unlike SM measurements, where
jet substructure has started to play a role much later. The reason lies in the fact that
the expected cross sections of SM and BSM processes differ fundamentally. The
partonic cross sections for the production of SM particles fall as 1/ˆ s once the scale
of the process is much larger than the mass of the SM particles involved, where ˆ
s
is the partonic centre-of-mass energy. Convoluted with the steeply falling PDFs for
increasing fractional parton momentum x, this results in a very small cross section at
high p T , relative to the total production cross section. For example, the cross section
for tt production with top quark p T > 350 GeV is about 13 pb [655], compared to the
total cross section of about 830 pb at
√
s = 13 TeV [627, 672, 673]. The statistical
power of the data is largest for low- p T SM processes. When the recorded dataset
becomes larger, the statistical precision becomes sufficient to probe high- p T SM
production cross sections. Hence, measurements at high p T are usually performed
later in the lifetime of a particle collider. In contrast to this, the production of a narrow,
heavy BSM resonance with a mass m X beyond 1 TeV will result in a negligible cross
section at low p T , while the production cross section will be dominated by events
with p T ≈ m X /2, where p T denotes here the transverse momentum of the resonance
decay products. Already at
√
s = 7 and 8 TeV, cross sections larger than 1 pb for the
production of resonances with m X > 1 TeV have been predicted. Since the hadronic
channels of W , Z , H and t decays are only accessible with substructure techniques
© Springer Nature Switzerland AG 2021
R. Kogler, Advances in Jet Substructure at the LHC, Springer Tracts
in Modern Physics 284, https://doi.org/10.1007/978-3-030-72858-8_5
121