152
5 Direct Searches for New Physics
The lower mass limits of about 900 GeV set the target for searches at 13 TeV,
where the focus is on VLQs with masses of 1 TeV and above. An early CMS analysis
of 13 TeV data, corresponding to 2.3 fb
−1 , obtains mass limits for X between 990 and
1020 GeV, depending on its chirality [872]. The limits are based on a combination
from same-sign dilepton and +jets final states. An update of this analysis by CMS,
using 35.9 fb
−1 of data, improved these bounds to 1.3 TeV [873]. In this analysis,
the use of jet substructure methods results in a significant improvement compared to
the analysis with 2.3 fb
−1 . In the +jets channel, t tagging based on the soft drop jet
mass and τ 32 is employed. Large-R jets failing the top tagging requirement are tested
for a W tag using the pruned jet mass and τ 21 . While in the analysis with 2.3 fb
−1
of data the dilepton channel has better sensitivity than the +jets channel, additional
categories in the +jets channel with one W and one t-tagged jet result in better
sensitivity of the +jets channel in the newer analysis. A similar result is obtained
by ATLAS, where the multi-lepton analysis [874] achieves better sensitivity for X
masses below 1 TeV, and an analysis in the +jets channel [875] achieves higher
sensitivity above 1 TeV thanks to the usage of W tagging based on the trimmed
jet mass and D 2 . ATLAS obtains a mass limit of 1.35 TeV for B(X → W t) = 1,
compatible with the limit by CMS.
The presence of multiple weak vector bosons in the decay cascades of pairproduced T or B VLQs leads to a large branching fraction into +jets final states.
This is exploited in a number of analyses by ATLAS and CMS, where the presence of
an identified lepton with high p T suppresses SM multijet production. Jet substructure
techniques are used to suppress backgrounds from t, tt and V +jet production. Due to
the hadronic activity from the large number of jets produced, usually mini-isolation
or a two-dimensional lepton isolation criterion is used for the lepton reconstruction,
similar as in searches for tb resonances. The first search at 13 TeV has been performed by CMS on 2.3 fb
−1 of data [876]. This analysis is optimised for T → Ht
and T → W b decays, while being inclusive about the decay of the other VLQ in
the event. Two distinct channels in the +jets final state are considered, based on the
presence of a H - or W -tagged large-R jet, where the H jet identification considers
either one or two b-tagged subjets. The analysis has been updated using 35.9 fb
−1 ,
also adding final states with three leptons [860]. The highest sensitivity is obtained
by a combination with the search in the same-sign dilepton final state [873]. The
results are inclusive in the T T and B B decay channels and sensitivity for all branching fraction combinations is achieved. Masses of T and B quarks below 1140 and
910 GeV are excluded at 95% CL, respectively, for any combination of Bs.
ATLAS has taken a different approach and has performed four dedicated analyses
in the +jets final state, each targeting a specific VLQ decay: W b + X [877], W t +
X [875], Z (νν)t + X [878] and Ht + X [879]. In the W b and W t analyses, W jets are
identified with the trimmed jet mass and D 2 . The W jet, lepton, p
miss
T
and additional
jets are used to reconstruct m VLQ , where jets are assigned to the leptonic and hadronic
W candidates. The permutation which minimises the mass difference of the two
reconstructed VLQ candidates is used. This reconstruction works well for the targeted
decay, where it produces distinct peaks for signal events. For decays where this
algorithm fails to reconstruct the correct VLQ mass, it nonetheless provides sufficient
5 Direct Searches for New Physics
The lower mass limits of about 900 GeV set the target for searches at 13 TeV,
where the focus is on VLQs with masses of 1 TeV and above. An early CMS analysis
of 13 TeV data, corresponding to 2.3 fb
−1 , obtains mass limits for X between 990 and
1020 GeV, depending on its chirality [872]. The limits are based on a combination
from same-sign dilepton and +jets final states. An update of this analysis by CMS,
using 35.9 fb
−1 of data, improved these bounds to 1.3 TeV [873]. In this analysis,
the use of jet substructure methods results in a significant improvement compared to
the analysis with 2.3 fb
−1 . In the +jets channel, t tagging based on the soft drop jet
mass and τ 32 is employed. Large-R jets failing the top tagging requirement are tested
for a W tag using the pruned jet mass and τ 21 . While in the analysis with 2.3 fb
−1
of data the dilepton channel has better sensitivity than the +jets channel, additional
categories in the +jets channel with one W and one t-tagged jet result in better
sensitivity of the +jets channel in the newer analysis. A similar result is obtained
by ATLAS, where the multi-lepton analysis [874] achieves better sensitivity for X
masses below 1 TeV, and an analysis in the +jets channel [875] achieves higher
sensitivity above 1 TeV thanks to the usage of W tagging based on the trimmed
jet mass and D 2 . ATLAS obtains a mass limit of 1.35 TeV for B(X → W t) = 1,
compatible with the limit by CMS.
The presence of multiple weak vector bosons in the decay cascades of pairproduced T or B VLQs leads to a large branching fraction into +jets final states.
This is exploited in a number of analyses by ATLAS and CMS, where the presence of
an identified lepton with high p T suppresses SM multijet production. Jet substructure
techniques are used to suppress backgrounds from t, tt and V +jet production. Due to
the hadronic activity from the large number of jets produced, usually mini-isolation
or a two-dimensional lepton isolation criterion is used for the lepton reconstruction,
similar as in searches for tb resonances. The first search at 13 TeV has been performed by CMS on 2.3 fb
−1 of data [876]. This analysis is optimised for T → Ht
and T → W b decays, while being inclusive about the decay of the other VLQ in
the event. Two distinct channels in the +jets final state are considered, based on the
presence of a H - or W -tagged large-R jet, where the H jet identification considers
either one or two b-tagged subjets. The analysis has been updated using 35.9 fb
−1 ,
also adding final states with three leptons [860]. The highest sensitivity is obtained
by a combination with the search in the same-sign dilepton final state [873]. The
results are inclusive in the T T and B B decay channels and sensitivity for all branching fraction combinations is achieved. Masses of T and B quarks below 1140 and
910 GeV are excluded at 95% CL, respectively, for any combination of Bs.
ATLAS has taken a different approach and has performed four dedicated analyses
in the +jets final state, each targeting a specific VLQ decay: W b + X [877], W t +
X [875], Z (νν)t + X [878] and Ht + X [879]. In the W b and W t analyses, W jets are
identified with the trimmed jet mass and D 2 . The W jet, lepton, p
miss
T
and additional
jets are used to reconstruct m VLQ , where jets are assigned to the leptonic and hadronic
W candidates. The permutation which minimises the mass difference of the two
reconstructed VLQ candidates is used. This reconstruction works well for the targeted
decay, where it produces distinct peaks for signal events. For decays where this
algorithm fails to reconstruct the correct VLQ mass, it nonetheless provides sufficient