5.3 Vector-Like Quarks
161
trimmed jet mass has to be larger than 50 GeV. This ensures that W and t jets can be
captured, and the angular distance between the lepton, small-R jets and the selected
large-R jet is used to determine if the large-R jet originates from a W or t decay.
A further reduction of backgrounds is not necessary, as the analysis requires the
presence of a forward jet with 2.5 < |η| < 4.5, which suppresses SM backgrounds
sufficiently. A different strategy is followed by an ATLAS analysis searching for the
decay T → W b in the +jets channel [906]. The analysis reconstructs the W → ν
decay with an identified lepton and p
miss
T , such that only a small-R jet from the b
quark decay is expected in the final state. In order to suppress high- p T tt background,
a veto on large-R jets with a trimmed jet mass larger than 70 GeV is implemented.
This results in an efficient rejection of tt, reducing the tt background to about 20% of
the total background in the signal region. The observed cross section limits on single
VLQ production are translated into limits on the QqV couplings, but the achieved
sensitivity from 8 TeV analyses is weaker than indirect constraints from electroweak
precision data [837].
At a centre-of-mass energy of 13 TeV, single production of VLQs gains importance because the production cross sections increase by factors of 7 for m VLQ =
1 TeV, up to 17 for m VLQ = 2 TeV compared to 8 TeV [837]. In the following, 13 TeV
searches for the single production of up-type VLQs in the channels T → Ht and
T → Zt will be discussed, where results for T → W b will be covered as well.
Down-type VLQs are searched for in the B → Hb and B → W t channels.
A first search for single VLQ production at 13 TeV has been carried out by CMS
and uses 2.3 fb
−1 of data. It targets the T → Ht channel in the +jets final state.
The analysis targets heavy VLQs with m VLQ > 1 TeV, and makes use of non-isolated
lepton identification and H tagging of large-R jets. The H tagger uses τ 21 < 0.4 and
the soft drop jet mass has to be between 90 and 160 GeV. The number of b-tagged
subjets from the soft-drop algorithm defines the signal and control regions, together
with the presence or absence of a forward jet. The T quark mass is reconstructed
by combining the H jet with the lepton, p
miss
T
and a small-R jet that matches the
kinematics of a t decay best. The distribution of SM backgrounds in the reconstructed
VLQ mass is obtained from a control region with one b-tagged subjet and without a
forward jet. The assumption of similar background distributions in events with and
without a forward jet is verified in a validation region with a forward jet, but with no btagged subjet. The analysis sets upper limits at 95% CL on the product of T production
cross section and B(T → Ht) between 0.2 and 1 pb for T masses of 1.8 TeV and
1 TeV, respectively. Note that this analysis is only sensitive to T production if also the
couplings to bW and t Z are sizeable, because the production through a Higgs boson
is strongly suppressed. The T → Ht channel can also be probed in all-hadronic final
states, enabled by the large hadronic t and H branching fractions. A recent analysis
by CMS uses 35.9 fb
−1 of 13 TeV data and combines a resolved and a boosted
selection [907]. Both selections target the pp → (bqq
)(bb)q final state, where the
first three quarks originate from a t quark decay, the bb pair may come either from
the decay of a H or Z boson, and the last quark is the scattered quark from the
electroweak production. The similarity between the H → bb and Z → bb decays
allows for an optimisation in the T → Ht and T → Zt channels within the same
161
trimmed jet mass has to be larger than 50 GeV. This ensures that W and t jets can be
captured, and the angular distance between the lepton, small-R jets and the selected
large-R jet is used to determine if the large-R jet originates from a W or t decay.
A further reduction of backgrounds is not necessary, as the analysis requires the
presence of a forward jet with 2.5 < |η| < 4.5, which suppresses SM backgrounds
sufficiently. A different strategy is followed by an ATLAS analysis searching for the
decay T → W b in the +jets channel [906]. The analysis reconstructs the W → ν
decay with an identified lepton and p
miss
T , such that only a small-R jet from the b
quark decay is expected in the final state. In order to suppress high- p T tt background,
a veto on large-R jets with a trimmed jet mass larger than 70 GeV is implemented.
This results in an efficient rejection of tt, reducing the tt background to about 20% of
the total background in the signal region. The observed cross section limits on single
VLQ production are translated into limits on the QqV couplings, but the achieved
sensitivity from 8 TeV analyses is weaker than indirect constraints from electroweak
precision data [837].
At a centre-of-mass energy of 13 TeV, single production of VLQs gains importance because the production cross sections increase by factors of 7 for m VLQ =
1 TeV, up to 17 for m VLQ = 2 TeV compared to 8 TeV [837]. In the following, 13 TeV
searches for the single production of up-type VLQs in the channels T → Ht and
T → Zt will be discussed, where results for T → W b will be covered as well.
Down-type VLQs are searched for in the B → Hb and B → W t channels.
A first search for single VLQ production at 13 TeV has been carried out by CMS
and uses 2.3 fb
−1 of data. It targets the T → Ht channel in the +jets final state.
The analysis targets heavy VLQs with m VLQ > 1 TeV, and makes use of non-isolated
lepton identification and H tagging of large-R jets. The H tagger uses τ 21 < 0.4 and
the soft drop jet mass has to be between 90 and 160 GeV. The number of b-tagged
subjets from the soft-drop algorithm defines the signal and control regions, together
with the presence or absence of a forward jet. The T quark mass is reconstructed
by combining the H jet with the lepton, p
miss
T
and a small-R jet that matches the
kinematics of a t decay best. The distribution of SM backgrounds in the reconstructed
VLQ mass is obtained from a control region with one b-tagged subjet and without a
forward jet. The assumption of similar background distributions in events with and
without a forward jet is verified in a validation region with a forward jet, but with no btagged subjet. The analysis sets upper limits at 95% CL on the product of T production
cross section and B(T → Ht) between 0.2 and 1 pb for T masses of 1.8 TeV and
1 TeV, respectively. Note that this analysis is only sensitive to T production if also the
couplings to bW and t Z are sizeable, because the production through a Higgs boson
is strongly suppressed. The T → Ht channel can also be probed in all-hadronic final
states, enabled by the large hadronic t and H branching fractions. A recent analysis
by CMS uses 35.9 fb
−1 of 13 TeV data and combines a resolved and a boosted
selection [907]. Both selections target the pp → (bqq
)(bb)q final state, where the
first three quarks originate from a t quark decay, the bb pair may come either from
the decay of a H or Z boson, and the last quark is the scattered quark from the
electroweak production. The similarity between the H → bb and Z → bb decays
allows for an optimisation in the T → Ht and T → Zt channels within the same