5.6 Light Resonances Coupling to Quarks or Gluons
191
software and analysis tools. A dijet search performed on these scouting events with
18.8 fb
−1 of 8 TeV data [1132] results in sensitivities for resonance masses in the
range of 500–700 GeV better by a factor of two and more than obtained from prescaled
triggers [1121], where only a fraction of events is recorded at low p T . Analyses based
on data scouting by ATLAS [1133] and CMS [1128] using 13 TeV data could improve
the bounds from the 8 TeV analysis in the mass region 450 GeV to 1.5 TeV by another
factor of two. A recent analysis by CMS based on the same 13 TeV scouting data
with 18.3 fb
−1 , could extend the mass range down to 350 GeV, by reconstructing the
resonance mass using three jets instead of two [1134].
The region below 350 GeV in resonance mass can be probed in signatures where
the Z
recoils against a high- p T photon. The photon produced by ISR reduces
the acceptance, but presents a way to trigger these events and reduce the multijet
background. A search by ATLAS based on 79.8 fb
−1 of 13 TeV data [1135] uses
events recorded by a single-photon trigger with p T > 140 GeV, or a trigger requiring
a photon with p T > 75–85 GeV, depending on the year of data acquisition, and
two small-R jets with p T > 50 GeV. The single photon trigger allows for a lower
jet p T threshold, such that invariant dijet masses down to 169 GeV can be probed.
Constraints on resonance masses in the range from 225 to 1100 GeV are placed.
Instead of a photon, the recoiling system can be W decaying leptonically, resulting in a
lepton+dijet signature. This signature probes also models with more than the minimal
particle content [1026], for example where a heavy resonance X decays to a W and a
Z
boson [1136, 1137]. Recently, ATLAS has performed a search for dijet resonances
recoiling against a charged lepton using 139 fb
−1 of 13 TeV data [1138]. Events are
required to have one isolated lepton with p T > 60 GeV and two small-R jets with
p T > 20 GeV. Events with dijet mass larger than 220 GeV are selected. Below this
value the dijet mass distribution is not monotonically falling due to a kinematic bias
from the lepton p T selection, such that the parametric background fit employed in this
search fails. Upper limits on the signal cross section times acceptance and branching
ratio are reported ranging from 100 to 0.1 fb for a Z
mass between 0.25 and 6 TeV.
A dark matter mediator between 0.25 and 1.2 TeV is excluded for g SM = 0.25 and
g χ = 1.
Resonance masses lower than about 250 GeV can not be probed in traditional dijet
resonance searches at the LHC, but jet substructure techniques enable the exploration
of Z
masses down to about 10 GeV. For low mass resonances, the recoiling ISR
particle results in a boosted Z
, which can be reconstructed by a single large-R
jet [1139, 1140]. The first search of this kind has been performed by CMS on
2.6 fb
−1 of 13 TeV data [1141], where a high- p T ISR jet provides enough energy to
satisfy the trigger requirements. The ISR jet recoils against another high- p T signal
jet, such that the analysis strategy is very similar to the measurement of H +jet
production in the H → bb channel (see Sect. 4.2.2). The highest- p T large-R jet in
the event is assumed to originate from the boosted Z
decay and is analysed further.
The jet is corrected for pileup effects with PUPPI and the soft drop mass with
z cut = 0.1 and β = 0 is chosen as sensitive observable to distinguish signal from
background events. The signal is enriched using a selection on τ 21 , transformed
using the DDT technique to τ
DDT
21
= τ 21 + 0.063ρ
, where ρ
= ln[m
2
SD /( p T μ)] and
191
software and analysis tools. A dijet search performed on these scouting events with
18.8 fb
−1 of 8 TeV data [1132] results in sensitivities for resonance masses in the
range of 500–700 GeV better by a factor of two and more than obtained from prescaled
triggers [1121], where only a fraction of events is recorded at low p T . Analyses based
on data scouting by ATLAS [1133] and CMS [1128] using 13 TeV data could improve
the bounds from the 8 TeV analysis in the mass region 450 GeV to 1.5 TeV by another
factor of two. A recent analysis by CMS based on the same 13 TeV scouting data
with 18.3 fb
−1 , could extend the mass range down to 350 GeV, by reconstructing the
resonance mass using three jets instead of two [1134].
The region below 350 GeV in resonance mass can be probed in signatures where
the Z
recoils against a high- p T photon. The photon produced by ISR reduces
the acceptance, but presents a way to trigger these events and reduce the multijet
background. A search by ATLAS based on 79.8 fb
−1 of 13 TeV data [1135] uses
events recorded by a single-photon trigger with p T > 140 GeV, or a trigger requiring
a photon with p T > 75–85 GeV, depending on the year of data acquisition, and
two small-R jets with p T > 50 GeV. The single photon trigger allows for a lower
jet p T threshold, such that invariant dijet masses down to 169 GeV can be probed.
Constraints on resonance masses in the range from 225 to 1100 GeV are placed.
Instead of a photon, the recoiling system can be W decaying leptonically, resulting in a
lepton+dijet signature. This signature probes also models with more than the minimal
particle content [1026], for example where a heavy resonance X decays to a W and a
Z
boson [1136, 1137]. Recently, ATLAS has performed a search for dijet resonances
recoiling against a charged lepton using 139 fb
−1 of 13 TeV data [1138]. Events are
required to have one isolated lepton with p T > 60 GeV and two small-R jets with
p T > 20 GeV. Events with dijet mass larger than 220 GeV are selected. Below this
value the dijet mass distribution is not monotonically falling due to a kinematic bias
from the lepton p T selection, such that the parametric background fit employed in this
search fails. Upper limits on the signal cross section times acceptance and branching
ratio are reported ranging from 100 to 0.1 fb for a Z
mass between 0.25 and 6 TeV.
A dark matter mediator between 0.25 and 1.2 TeV is excluded for g SM = 0.25 and
g χ = 1.
Resonance masses lower than about 250 GeV can not be probed in traditional dijet
resonance searches at the LHC, but jet substructure techniques enable the exploration
of Z
masses down to about 10 GeV. For low mass resonances, the recoiling ISR
particle results in a boosted Z
, which can be reconstructed by a single large-R
jet [1139, 1140]. The first search of this kind has been performed by CMS on
2.6 fb
−1 of 13 TeV data [1141], where a high- p T ISR jet provides enough energy to
satisfy the trigger requirements. The ISR jet recoils against another high- p T signal
jet, such that the analysis strategy is very similar to the measurement of H +jet
production in the H → bb channel (see Sect. 4.2.2). The highest- p T large-R jet in
the event is assumed to originate from the boosted Z
decay and is analysed further.
The jet is corrected for pileup effects with PUPPI and the soft drop mass with
z cut = 0.1 and β = 0 is chosen as sensitive observable to distinguish signal from
background events. The signal is enriched using a selection on τ 21 , transformed
using the DDT technique to τ
DDT
21
= τ 21 + 0.063ρ
, where ρ
= ln[m
2
SD /( p T μ)] and