5.3 Vector-Like Quarks
157
duction are large. In order to improve this situation, a dedicated analysis targeting
the W bW b and ZbZb final states is performed in the same publication, achieving
expected mass exclusion limits of 1070 GeV for B(T → W b) = 1 and 1130 GeV
for B(B → Zb) = 1, complementing the inclusive analysis with BEST.
Naturally, the best overall sensitivity to VLQs is obtained by combining dedicated
analyses targeting different VLQ decays. Similar as in combinations of searches for
diboson resonances, the feasibility of a combination relies on exclusive signal, control
and validation regions. While it would be possible to include correlations between
regions, the exact implementation and calculation of these correlations is technically very involved. This needs coordination and planning, already in the design
stage of all analyses entering the combination. ATLAS has performed such a combination, including seven searches for the pair production of VLQs, all based on
13 TeV data corresponding to 36.1 fb
−1 [888]. The analyses included are the four
analyses in the +jets final state W b + X [877], W t + X [875], Z (νν)t + X [878]
and Ht + X [879], the dilepton analysis Z ((()t/b + X [883], the multi-lepton analysis [874], and the all-hadronic analysis [885]. The analyses are either complementary
by the final state, or complentarity is ensured by the use of consistent definitions of
physics objects and jet substructure taggers. Only small adjustments had to be made
to individual analyses, the largest one in the multi-lepton analysis, where events are
removed with more than three leptons or events with a lepton pair having an invariant
mass compatible with a Z boson decay. The loss in sensitivity by the individual analysis is compensated by the inclusion of the results from the Z ((()t/b + X search. The
results of the combination are shown in Fig. 5.12, which also displays the expected
95% upper cross section limits of the individual analyses. The results are shown for
a T and B in their electroweak singlet representations with branching fractions of
approximately B(T → W b) = 0.5, B(T → Ht) = 0.25 and B(T → Zt) = 0.25,
and B(B → W t) = 0.5, B(B → Hb) = 0.25 and B(B → Zb) = 0.25. The best
sensitivity on T T production is achieved by the Ht + X search, but the W b + X
and Z ((()t/b + X significantly contribute to the overall result, too. The combination improves the individual limits by up to a factor of 1.7, resulting in the
best constraints on T T pair production to date. For B B production, the best cross
section limits for m VLQ > 900 GeV are obtained from the W t + X search, while for
m VLQ < 900 GeV the Z ((()t/b + X and the multilepton analyses are more sensitive. The ultimate strength of a combination of VLQ searches is that sensitivity to
all combinations of branching fractions is achieved, such that mass limits independent of the exact VLQ representation can be obtained. Under the assumption that
there are no invisible decays, i.e. B(T → W b) + B(T → Ht) + B(T → Zt) = 1
and B(B → W t) + B(B → Hb) + B(B → Zb) = 1, mass limits above 1 TeV are
obtained for any combination of branching fractions, as shown in Fig. 5.13. In particular, the observed lower mass limits for T are 1.31 TeV with 1.22 TeV expected, and
1.03 TeV for B with 0.98 TeV expected. At the time of writing, these limits constitute
the strongest model-independent constraints on VLQs and it is conceivable that it will
take a long time until these can be improved considerably. The weakest mass limits
for T are observed for the singlet configuration with slightly lowered B(T → Ht),
where all decay channels contribute, but the dominant decays are T → W b and
157
duction are large. In order to improve this situation, a dedicated analysis targeting
the W bW b and ZbZb final states is performed in the same publication, achieving
expected mass exclusion limits of 1070 GeV for B(T → W b) = 1 and 1130 GeV
for B(B → Zb) = 1, complementing the inclusive analysis with BEST.
Naturally, the best overall sensitivity to VLQs is obtained by combining dedicated
analyses targeting different VLQ decays. Similar as in combinations of searches for
diboson resonances, the feasibility of a combination relies on exclusive signal, control
and validation regions. While it would be possible to include correlations between
regions, the exact implementation and calculation of these correlations is technically very involved. This needs coordination and planning, already in the design
stage of all analyses entering the combination. ATLAS has performed such a combination, including seven searches for the pair production of VLQs, all based on
13 TeV data corresponding to 36.1 fb
−1 [888]. The analyses included are the four
analyses in the +jets final state W b + X [877], W t + X [875], Z (νν)t + X [878]
and Ht + X [879], the dilepton analysis Z ((()t/b + X [883], the multi-lepton analysis [874], and the all-hadronic analysis [885]. The analyses are either complementary
by the final state, or complentarity is ensured by the use of consistent definitions of
physics objects and jet substructure taggers. Only small adjustments had to be made
to individual analyses, the largest one in the multi-lepton analysis, where events are
removed with more than three leptons or events with a lepton pair having an invariant
mass compatible with a Z boson decay. The loss in sensitivity by the individual analysis is compensated by the inclusion of the results from the Z ((()t/b + X search. The
results of the combination are shown in Fig. 5.12, which also displays the expected
95% upper cross section limits of the individual analyses. The results are shown for
a T and B in their electroweak singlet representations with branching fractions of
approximately B(T → W b) = 0.5, B(T → Ht) = 0.25 and B(T → Zt) = 0.25,
and B(B → W t) = 0.5, B(B → Hb) = 0.25 and B(B → Zb) = 0.25. The best
sensitivity on T T production is achieved by the Ht + X search, but the W b + X
and Z ((()t/b + X significantly contribute to the overall result, too. The combination improves the individual limits by up to a factor of 1.7, resulting in the
best constraints on T T pair production to date. For B B production, the best cross
section limits for m VLQ > 900 GeV are obtained from the W t + X search, while for
m VLQ < 900 GeV the Z ((()t/b + X and the multilepton analyses are more sensitive. The ultimate strength of a combination of VLQ searches is that sensitivity to
all combinations of branching fractions is achieved, such that mass limits independent of the exact VLQ representation can be obtained. Under the assumption that
there are no invisible decays, i.e. B(T → W b) + B(T → Ht) + B(T → Zt) = 1
and B(B → W t) + B(B → Hb) + B(B → Zb) = 1, mass limits above 1 TeV are
obtained for any combination of branching fractions, as shown in Fig. 5.13. In particular, the observed lower mass limits for T are 1.31 TeV with 1.22 TeV expected, and
1.03 TeV for B with 0.98 TeV expected. At the time of writing, these limits constitute
the strongest model-independent constraints on VLQs and it is conceivable that it will
take a long time until these can be improved considerably. The weakest mass limits
for T are observed for the singlet configuration with slightly lowered B(T → Ht),
where all decay channels contribute, but the dominant decays are T → W b and