5.5 Dark Matter and Mono-X
181
quark, the mono-W production can become the dominant process [1038]. Mono-Z
production can probe direct couplings between the Z boson and the dark matter
particles. In addition, mono-W and Z searches are sensitive to Higgs-mediated dark
matter models through V H production with H → χ ¯
χ. Searches in the Z ((()+ p
miss
T
final states have been reported by ATLAS [1039] and CMS [1040]. The largest background for p
miss
T
> 100 GeV originates from Z Z production, which is an irreducible
background in this channel and has to be obtained from simulation. CMS has also
performed a search in the single-lepton channel, targeting W ((ν)+ p
miss
T
[1041]. This
analysis has been optimised for W
→ ν, where the transverse mass between the
lepton and p
miss
T
is the optimal observable to distinguish signal from background
events. In the search for dark matter, where the neutrino is accompanied by two dark
matter particles escaping detection, the p T of the W boson or χ ¯
χ system can not
be reconstructed, resulting in a reduced sensitivity relative to mono-jet and mono-γ
searches. The large hadronic branching fractions of the W and Z bosons are exploited
in a mono-V search by ATLAS, where jet substructure is used for the first time in a
dark matter search [1042]. Events are selected by the presence of one large-R jet with
p T > 250 GeV and a jet mass between 50 and 120 GeV after MDT grooming with
y cut = 0.16 (see Sect. 2.4.3). Two signal regions are defined by p
miss
T
> 350 GeV and
p
miss
T
> 500 GeV. The m jet distributions in these regions are analysed to search for a
resonant signal over an approximately flat background, as shown in Fig. 5.21 (left).
In the case of constructive interference, the mono-W production is enhanced by two
orders of magnitude relative to destructive interference, resulting in much stronger
bounds. For other cases, the sensitivity is comparable to the mono-jet and mono-γ
searches, and about a factor of three better than for the leptonic decay channels. A
CMS search with 8 TeV data [1043] could improve upon this result by taking into
account fully merged and resolved V decays, as well as mono-jet events which do
not fulfil the criteria of the V selection. The boosted selection requires one large-R
jet with p T > 200 GeV, pruned jet mass in the range 60–110 GeV and τ 21 < 0.5. In
the resolved selection, the dijet system of small-R jets has to have a mass between
60 and 110 GeV. In order to suppress backgrounds, a multi-variate discriminator is
built from the jet pull angle, mass drop and the q/g discriminator. Events not falling
into the boosted or resolved signal category, but with a small-R jet balanced with
p
miss
T , are sorted into the mono-jet category. A total of nine control regions, three for
each signal category, are used to determine the main backgrounds from Z (νν)+jets
and W ((ν)+jets. The control regions are obtained by selecting events with either a
photon, or one or two muons. In the control regions, no p
miss
T
selection is applied.
Instead, the p T of the photon, the single muon or the dimuon system is removed and
p
miss
T
is recalculated. This proxy for p
miss
T
is used to determine the p
miss
T
spectrum in
the signal regions. The resulting p
miss
T
distribution in the fully merged signal region
with a V -tagged jet is shown in Fig. 5.21 (right). By taking into account three signal
regions and using the full information of the p
miss
T
spectra, this analysis improves
over the previous CMS mono-jet search by 80% in sensitivity.
Searches for mono-H probe the direct H -χ interaction, because it is unlikely that
the H is radiated from an initial state quark [1044, 1045]. Searches for H decays
into invisible particles can probe the H -χ interaction as well if m H > 2m χ . The
181
quark, the mono-W production can become the dominant process [1038]. Mono-Z
production can probe direct couplings between the Z boson and the dark matter
particles. In addition, mono-W and Z searches are sensitive to Higgs-mediated dark
matter models through V H production with H → χ ¯
χ. Searches in the Z ((()+ p
miss
T
final states have been reported by ATLAS [1039] and CMS [1040]. The largest background for p
miss
T
> 100 GeV originates from Z Z production, which is an irreducible
background in this channel and has to be obtained from simulation. CMS has also
performed a search in the single-lepton channel, targeting W ((ν)+ p
miss
T
[1041]. This
analysis has been optimised for W
→ ν, where the transverse mass between the
lepton and p
miss
T
is the optimal observable to distinguish signal from background
events. In the search for dark matter, where the neutrino is accompanied by two dark
matter particles escaping detection, the p T of the W boson or χ ¯
χ system can not
be reconstructed, resulting in a reduced sensitivity relative to mono-jet and mono-γ
searches. The large hadronic branching fractions of the W and Z bosons are exploited
in a mono-V search by ATLAS, where jet substructure is used for the first time in a
dark matter search [1042]. Events are selected by the presence of one large-R jet with
p T > 250 GeV and a jet mass between 50 and 120 GeV after MDT grooming with
y cut = 0.16 (see Sect. 2.4.3). Two signal regions are defined by p
miss
T
> 350 GeV and
p
miss
T
> 500 GeV. The m jet distributions in these regions are analysed to search for a
resonant signal over an approximately flat background, as shown in Fig. 5.21 (left).
In the case of constructive interference, the mono-W production is enhanced by two
orders of magnitude relative to destructive interference, resulting in much stronger
bounds. For other cases, the sensitivity is comparable to the mono-jet and mono-γ
searches, and about a factor of three better than for the leptonic decay channels. A
CMS search with 8 TeV data [1043] could improve upon this result by taking into
account fully merged and resolved V decays, as well as mono-jet events which do
not fulfil the criteria of the V selection. The boosted selection requires one large-R
jet with p T > 200 GeV, pruned jet mass in the range 60–110 GeV and τ 21 < 0.5. In
the resolved selection, the dijet system of small-R jets has to have a mass between
60 and 110 GeV. In order to suppress backgrounds, a multi-variate discriminator is
built from the jet pull angle, mass drop and the q/g discriminator. Events not falling
into the boosted or resolved signal category, but with a small-R jet balanced with
p
miss
T , are sorted into the mono-jet category. A total of nine control regions, three for
each signal category, are used to determine the main backgrounds from Z (νν)+jets
and W ((ν)+jets. The control regions are obtained by selecting events with either a
photon, or one or two muons. In the control regions, no p
miss
T
selection is applied.
Instead, the p T of the photon, the single muon or the dimuon system is removed and
p
miss
T
is recalculated. This proxy for p
miss
T
is used to determine the p
miss
T
spectrum in
the signal regions. The resulting p
miss
T
distribution in the fully merged signal region
with a V -tagged jet is shown in Fig. 5.21 (right). By taking into account three signal
regions and using the full information of the p
miss
T
spectra, this analysis improves
over the previous CMS mono-jet search by 80% in sensitivity.
Searches for mono-H probe the direct H -χ interaction, because it is unlikely that
the H is radiated from an initial state quark [1044, 1045]. Searches for H decays
into invisible particles can probe the H -χ interaction as well if m H > 2m χ . The