188
5 Direct Searches for New Physics
improve the performance at low p T . It is not clear what the additional information
carried by f rec is, compared to the other inputs τ 32 and the ECF ratios. However, f rec
carries some additional discrimination power since adding more ECF ratios and N -
subjettiness observables could not compensate for dropping f rec from the BDT [526].
Note that this t tagger is special in the sense that all substructure observables except
for f rec are groomed using soft drop, which improves the performance at low p T .
For the calculation of f rec , mass drop and filtering are used. The gain from the BDT
compared to τ 32 alone is a reduction of the background efficiency from 6.9 to 4.7%
for a constant signal efficiency of 50%. Two signal regions are defined by a selection
based on large p
miss
T
and two intervals of the BDT discriminator, resulting in a loose
and a tight t-tagged jet category. The main backgrounds are obtained from data in
control regions and extrapolated into the signal region with transfer functions. A statistical interpretation of the measured distribution in p
miss
T
results in excluded masses
between 0.2 and 1.75 TeV for flavour-changing vector and axial-vector mediators
and m χ 100 GeV, g SM = 0.25 and g χ = 1. This exclusion is higher by 820 GeV
compared to the 8 TeV result [1054], which is a remarkable result. In ATLAS, the
mono-t signature is analysed together with the single production of a VLQ T in the
T → Zt channel, with Z → ν ¯
ν, resulting in the same signature. The analysis, based
on 36.1 fb
−1 , considers J + p
miss
T
and +jets final states [908]. The dark matter signal
region is similar to the VLQ signal region, except for the absence of a forward jet
(see Sect. 5.3.2). The sensitivity is comparable with the CMS analysis, with excluded
vector mediator masses below 2 TeV for m χ = 1 GeV, g SM = 0.5 and g χ = 1.
Searches for tt + χ ¯
χ so far discussed have neglected the possibility of dark matter
production in association with a single top quark, where the interaction is mediated
by a scalar particle [1087–1089]. The same interaction giving rise to the tt + χ ¯
χ
signature leads to diagrams involving the t-channel exchange of a W boson, resulting in t +χ ¯
χ in association with a b quark and a forward jet, as well as dark matter
produced in association with t W . These are different than mono-t signatures, where
a vector or pseudo-vector interaction gives rise to t + p
miss
T
final states without additional particles. Since tb j + χ ¯
χ and t W + χ ¯
χ can be kinematically favoured over
tt + χ ¯
χ, the inclusion of the corresponding final states in dark matter searches is
expected to improve the sensitivity to scalar mediators with couplings to top quarks
by 30% up to a factor of two [1087]. This has been exploited in recent searches by
ATLAS [1090] and CMS [1091], with 139 and 36 fb
−1 , respectively. While ATLAS
analyses +jets and dilepton final states, the CMS analysis is optimised for +jets
and jets+ p
miss
T
final states. The CMS analysis results in lower mass limits on scalar
and pseudoscalar mediators of 290 and 300 GeV, respectively, thereby improving the
previous limits [1084] based on the same amount of data by 130 GeV. The ATLAS
analysis interprets the results in the context of a two-Higgs-doublet model [1088],
such that the results can not be compared directly. In this model, masses of the pseudoscalar mediator can be excluded below 200 GeV for charged Higgs boson masses
m H ± in the range 0.4–1.4 TeV and up to 330 GeV for m H ± = 900 GeV. These searches
are sensitive to single VLQ production in the T → t Z channel with Z → ν ¯
ν, but
these interpretations have not been done so far.
5 Direct Searches for New Physics
improve the performance at low p T . It is not clear what the additional information
carried by f rec is, compared to the other inputs τ 32 and the ECF ratios. However, f rec
carries some additional discrimination power since adding more ECF ratios and N -
subjettiness observables could not compensate for dropping f rec from the BDT [526].
Note that this t tagger is special in the sense that all substructure observables except
for f rec are groomed using soft drop, which improves the performance at low p T .
For the calculation of f rec , mass drop and filtering are used. The gain from the BDT
compared to τ 32 alone is a reduction of the background efficiency from 6.9 to 4.7%
for a constant signal efficiency of 50%. Two signal regions are defined by a selection
based on large p
miss
T
and two intervals of the BDT discriminator, resulting in a loose
and a tight t-tagged jet category. The main backgrounds are obtained from data in
control regions and extrapolated into the signal region with transfer functions. A statistical interpretation of the measured distribution in p
miss
T
results in excluded masses
between 0.2 and 1.75 TeV for flavour-changing vector and axial-vector mediators
and m χ 100 GeV, g SM = 0.25 and g χ = 1. This exclusion is higher by 820 GeV
compared to the 8 TeV result [1054], which is a remarkable result. In ATLAS, the
mono-t signature is analysed together with the single production of a VLQ T in the
T → Zt channel, with Z → ν ¯
ν, resulting in the same signature. The analysis, based
on 36.1 fb
−1 , considers J + p
miss
T
and +jets final states [908]. The dark matter signal
region is similar to the VLQ signal region, except for the absence of a forward jet
(see Sect. 5.3.2). The sensitivity is comparable with the CMS analysis, with excluded
vector mediator masses below 2 TeV for m χ = 1 GeV, g SM = 0.5 and g χ = 1.
Searches for tt + χ ¯
χ so far discussed have neglected the possibility of dark matter
production in association with a single top quark, where the interaction is mediated
by a scalar particle [1087–1089]. The same interaction giving rise to the tt + χ ¯
χ
signature leads to diagrams involving the t-channel exchange of a W boson, resulting in t +χ ¯
χ in association with a b quark and a forward jet, as well as dark matter
produced in association with t W . These are different than mono-t signatures, where
a vector or pseudo-vector interaction gives rise to t + p
miss
T
final states without additional particles. Since tb j + χ ¯
χ and t W + χ ¯
χ can be kinematically favoured over
tt + χ ¯
χ, the inclusion of the corresponding final states in dark matter searches is
expected to improve the sensitivity to scalar mediators with couplings to top quarks
by 30% up to a factor of two [1087]. This has been exploited in recent searches by
ATLAS [1090] and CMS [1091], with 139 and 36 fb
−1 , respectively. While ATLAS
analyses +jets and dilepton final states, the CMS analysis is optimised for +jets
and jets+ p
miss
T
final states. The CMS analysis results in lower mass limits on scalar
and pseudoscalar mediators of 290 and 300 GeV, respectively, thereby improving the
previous limits [1084] based on the same amount of data by 130 GeV. The ATLAS
analysis interprets the results in the context of a two-Higgs-doublet model [1088],
such that the results can not be compared directly. In this model, masses of the pseudoscalar mediator can be excluded below 200 GeV for charged Higgs boson masses
m H ± in the range 0.4–1.4 TeV and up to 330 GeV for m H ± = 900 GeV. These searches
are sensitive to single VLQ production in the T → t Z channel with Z → ν ¯
ν, but
these interpretations have not been done so far.