128
5 Direct Searches for New Physics
CMS rejects the second lepton from the calculation of the isolation variable [713].
The analysis strategies in these searches are very similar to the ones in the ν J and
νν J final states described above. Searches in this channel result in the best sensitivity
at low Z V resonance masses, but beyond 2–3 TeV the sensitivity decreases and other
channels with higher branching fractions are better.
A recent ATLAS result, using the full 13 TeV dataset with 139 fb
−1 , includes all
three dilepton+jets final states ν J , νν J and J [714]. A recurrent neural network
is used to classify events into ggF/qq and VBF topologies. A similar methodology is
employed as used in previous analyses based on 36 fb
−1 . A combined interpretation
of the results gives the best sensitivity for V V searches to date, with no signal above
the expected backgrounds. Resonance mass exclusions are obtained at 95% CL of 4.3
and 3.9 TeV for a W
and Z
in the heavy-vector triplet (HVT) model B, respectively.
Compared to previous searches at 7 and 8 TeV by ATLAS [715–717] and CMS [490,
518, 718, 719], the new results based on 13 TeV data improve the mass exclusion
limits by more than 2 TeV.
5.1.2 WH and ZH Resonances
Diboson resonances decaying to VH offer a rich phenomenology because of the
different H decay channels. In searches targeting masses above 1 TeV, the H → bb
channel offers the best sensitivity in most cases because of its large branching fraction
and thus high signal efficiency in a region where the background falls steeply. In jet
substructure analyses of VH resonances, subjet-b and double-b tagging improves
the background rejection at a given signal efficiency compared to the V taggers
employed in V V searches.
The largest branching fraction is obtained for the all-hadronic decay channels,
with B(W H → qqbb) = 39.2% and B(Z H → qqbb) = 40.6%. While the H →
W W
(∗)
→ qqqq decay is technically also part of the all-hadronic channel, it has not
been considered in analyses so far even though it has a relatively large H branching
fraction of 9.8%. The reason is the use of b tagging for the identification of boosted
H bosons, which has a small efficiency for the decay into four quarks. In analyses
targeting the H → bb decay, contributions from the four-quark decay are largely
suppressed. New strategies would need to be developed to include this decay.
ATLAS [493] and CMS [720] have analysed data corresponding to 36 fb
−1 in
search for VH resonances in the all-hadronic final state. The use of double-b tagging
is central in both analyses. ATLAS uses p T -dependent trimmed jet mass selections
for the identification of W , Z and H jets. The selections overlap, such that the
jet with the higher mass is assigned to be the H jet. The H jet has to have either
one or two ghost-associated b jets, where the latter results in better sensitivity for
resonances with masses below about 2.5 TeV. Above 2.5 TeV, H jets with one b
tag provide higher sensitivity because the fragmentation products of both b quarks
merge into a single track-jet. The V jets have to satisfy a p T -dependent D 2 selection,
designed to achieve a constant signal efficiency in V -jet p T of about 50%. CMS
5 Direct Searches for New Physics
CMS rejects the second lepton from the calculation of the isolation variable [713].
The analysis strategies in these searches are very similar to the ones in the ν J and
νν J final states described above. Searches in this channel result in the best sensitivity
at low Z V resonance masses, but beyond 2–3 TeV the sensitivity decreases and other
channels with higher branching fractions are better.
A recent ATLAS result, using the full 13 TeV dataset with 139 fb
−1 , includes all
three dilepton+jets final states ν J , νν J and J [714]. A recurrent neural network
is used to classify events into ggF/qq and VBF topologies. A similar methodology is
employed as used in previous analyses based on 36 fb
−1 . A combined interpretation
of the results gives the best sensitivity for V V searches to date, with no signal above
the expected backgrounds. Resonance mass exclusions are obtained at 95% CL of 4.3
and 3.9 TeV for a W
and Z
in the heavy-vector triplet (HVT) model B, respectively.
Compared to previous searches at 7 and 8 TeV by ATLAS [715–717] and CMS [490,
518, 718, 719], the new results based on 13 TeV data improve the mass exclusion
limits by more than 2 TeV.
5.1.2 WH and ZH Resonances
Diboson resonances decaying to VH offer a rich phenomenology because of the
different H decay channels. In searches targeting masses above 1 TeV, the H → bb
channel offers the best sensitivity in most cases because of its large branching fraction
and thus high signal efficiency in a region where the background falls steeply. In jet
substructure analyses of VH resonances, subjet-b and double-b tagging improves
the background rejection at a given signal efficiency compared to the V taggers
employed in V V searches.
The largest branching fraction is obtained for the all-hadronic decay channels,
with B(W H → qqbb) = 39.2% and B(Z H → qqbb) = 40.6%. While the H →
W W
(∗)
→ qqqq decay is technically also part of the all-hadronic channel, it has not
been considered in analyses so far even though it has a relatively large H branching
fraction of 9.8%. The reason is the use of b tagging for the identification of boosted
H bosons, which has a small efficiency for the decay into four quarks. In analyses
targeting the H → bb decay, contributions from the four-quark decay are largely
suppressed. New strategies would need to be developed to include this decay.
ATLAS [493] and CMS [720] have analysed data corresponding to 36 fb
−1 in
search for VH resonances in the all-hadronic final state. The use of double-b tagging
is central in both analyses. ATLAS uses p T -dependent trimmed jet mass selections
for the identification of W , Z and H jets. The selections overlap, such that the
jet with the higher mass is assigned to be the H jet. The H jet has to have either
one or two ghost-associated b jets, where the latter results in better sensitivity for
resonances with masses below about 2.5 TeV. Above 2.5 TeV, H jets with one b
tag provide higher sensitivity because the fragmentation products of both b quarks
merge into a single track-jet. The V jets have to satisfy a p T -dependent D 2 selection,
designed to achieve a constant signal efficiency in V -jet p T of about 50%. CMS