130
5 Direct Searches for New Physics
DM candidate [726] are probed. The sensitivity shows a large improvement compared
to earlier analyses by CMS using 8 TeV [491] and 13 TeV [727] data.
In an analysis by CMS, also using about 36 fb
−1 , hadronic τ decays (τ h ) are
probed in the context of VH resonances [728], a strategy which has been developed
on 8 TeV data [729]. This analysis searches for resonances in bbτ τ , where the V
or H can decay either to bb or τ τ . Besides these direct decays, the analysis is also
sensitive to V H → bbV V with V V → τ τ νν decays. The analysis is further probing
resonant H H production, as explained in the following section. The central element
in this analysis is the reconstruction of a boosted V or H boson decaying to one or
two τ h candidates. Because of their close angular proximity, the two τ h decays are
reconstructed starting with a large-R CA jet with R = 0.8 and p T > 100 GeV. The
CA jet is declustered, where in each step the mass drop condition of (2.39) is tested
with y cut = 2/3. Once two subjets satisfying the mass drop condition are found,
these are used as seeds in the standard τ identification algorithm [730, 731]. The
τ h candidate with highest p T is selected with an efficiency of 50–60%. If two τ h
candidates are found, the isolation of the second τ h candidate is relaxed to achieve
a 70–80% efficiency, with a misidentification rate of less than 0.1%. The analysis
considers τ h and τ h τ h categories, where the former includes leptonic τ decays of
the sort τ
−
→
−
¯
ν ν τ . In the τ h category, the τ h decay products are removed from
the lepton isolation cones to ensure a stable reconstruction efficiency for highly
collimated decays. After the selection of an H boson candidate decaying to τ h
or τ h τ h , events are classified based on the substructure of the other high- p T large-R
jet in the event. This results in twelve categories, depending on whether the other
jet is classified as an HP or LP W , Z or H jet. The principal SM background after
this selection is V +jet production, which is estimated with the α method. The
subdominant contributions from tt and single top quark production are taken from
simulation. Due to the smaller branching fraction B(H → τ τ ), the analysis is not
as sensitive as the analysis in the ν J , J and νν J final states, but it provides
complementary information and probes VH and H H resonances simultaneously.
5.1.3 HH Resonances
Pair production of H bosons is an important process in the SM. It probes the triple-H
coupling, which is predicted to exist by the BEH mechanism with strength λ. While
the size of λ can be inferred by measurements of m H , a direct measurement represents
a stringent test of the consistency of the theory. The parameter λ determines the form
of the Higgs field potential, which generates the H self-coupling after EWSB. The
exact form of this potential can have important cosmological implications, for example on the vacuum stability [732, 733]. The production cross section of pp → H H
is predicted to be as small as 33.5
+2.4
−2.8 fb at 13 TeV [138]. To access such a small cross
section, all experimentally accessible H branching fractions need to be considered.
A recent review on the relevant signatures, as well as the theoretical and experimental status of H H production can be found in [734]. The largest contribution to the
5 Direct Searches for New Physics
DM candidate [726] are probed. The sensitivity shows a large improvement compared
to earlier analyses by CMS using 8 TeV [491] and 13 TeV [727] data.
In an analysis by CMS, also using about 36 fb
−1 , hadronic τ decays (τ h ) are
probed in the context of VH resonances [728], a strategy which has been developed
on 8 TeV data [729]. This analysis searches for resonances in bbτ τ , where the V
or H can decay either to bb or τ τ . Besides these direct decays, the analysis is also
sensitive to V H → bbV V with V V → τ τ νν decays. The analysis is further probing
resonant H H production, as explained in the following section. The central element
in this analysis is the reconstruction of a boosted V or H boson decaying to one or
two τ h candidates. Because of their close angular proximity, the two τ h decays are
reconstructed starting with a large-R CA jet with R = 0.8 and p T > 100 GeV. The
CA jet is declustered, where in each step the mass drop condition of (2.39) is tested
with y cut = 2/3. Once two subjets satisfying the mass drop condition are found,
these are used as seeds in the standard τ identification algorithm [730, 731]. The
τ h candidate with highest p T is selected with an efficiency of 50–60%. If two τ h
candidates are found, the isolation of the second τ h candidate is relaxed to achieve
a 70–80% efficiency, with a misidentification rate of less than 0.1%. The analysis
considers τ h and τ h τ h categories, where the former includes leptonic τ decays of
the sort τ
−
→
−
¯
ν ν τ . In the τ h category, the τ h decay products are removed from
the lepton isolation cones to ensure a stable reconstruction efficiency for highly
collimated decays. After the selection of an H boson candidate decaying to τ h
or τ h τ h , events are classified based on the substructure of the other high- p T large-R
jet in the event. This results in twelve categories, depending on whether the other
jet is classified as an HP or LP W , Z or H jet. The principal SM background after
this selection is V +jet production, which is estimated with the α method. The
subdominant contributions from tt and single top quark production are taken from
simulation. Due to the smaller branching fraction B(H → τ τ ), the analysis is not
as sensitive as the analysis in the ν J , J and νν J final states, but it provides
complementary information and probes VH and H H resonances simultaneously.
5.1.3 HH Resonances
Pair production of H bosons is an important process in the SM. It probes the triple-H
coupling, which is predicted to exist by the BEH mechanism with strength λ. While
the size of λ can be inferred by measurements of m H , a direct measurement represents
a stringent test of the consistency of the theory. The parameter λ determines the form
of the Higgs field potential, which generates the H self-coupling after EWSB. The
exact form of this potential can have important cosmological implications, for example on the vacuum stability [732, 733]. The production cross section of pp → H H
is predicted to be as small as 33.5
+2.4
−2.8 fb at 13 TeV [138]. To access such a small cross
section, all experimentally accessible H branching fractions need to be considered.
A recent review on the relevant signatures, as well as the theoretical and experimental status of H H production can be found in [734]. The largest contribution to the