136
5 Direct Searches for New Physics
Similar to V V and VH resonances, also H H resonance searches are combined
in dedicated analyses [764, 765]. These combinations include mostly searches for
resonances below masses of 1 TeV, with sensitivity to the SM H H production cross
section. The best sensitivity at low masses is achieved by the bbγ γ analyses [766,
767]. At masses above 1 TeV, the bbbb [743, 744] and bbτ τ [728] final states dominate the combined results. For spin-0 resonances decaying to H H with masses
above 2 TeV, production cross sections of 6 fb or higher can be excluded at 95% CL.
It should be noted that the bbW W
(∗) analysis, with W W
(∗)
→ qqν [748], has not
been considered in these combinations so far. Further improvements are expected
from its inclusion.
5.1.5 V γ and Hγ Resonances
A special class of diboson resonances are resonances decaying to photons, where
the decay is usually loop-mediated. A prominent example of such a diphoton decay
is H → γ γ , with a branching fraction of only 0.23%. Nevertheless, this channel
was one of the discovery channels. Even smaller is the branching fraction to Z γ
with 0.15%, which has not been observed to date [768]. In BSM models, high-mass
Z γ resonances appear as loop-mediated decays as well. Examples are compositeness models, like the little Higgs model [769], extra-dimension models [770, 771]
or extensions of the SM with an additional scalar field [772]. Some models of
technicolour predict W γ resonances in addition to Z γ , originating from a triplet
of technirhos [773]. These BSM theories can be generalised by an effective model
introducing colour-neutral spin-0 or spin-2 states, denoted by X , decaying to Z γ .
Spin-1 W
and Z
can also be introduced, with the loop-induced W
and Z
decays
to W γ and H γ [774], respectively. The branching fraction for the H γ decay is typically very small with B(Z
→ H γ ) = O(10
−5
) and smaller, but if the photon is
not a single photon but a cluster of photons from the decay of one or more highly
boosted light particles, much larger branching fractions are obtained [774]. Since
these decay cascades would lead to the same detector signature as a single photon,
searches for H γ resonances might lead to discoverable BSM effects.
Early searches for W γ and Z γ resonances have focussed on resonance masses
below 1 TeV and leptonic V decays. Examples are searches by ATLAS and CMS
using 7 and 8 TeV data [775, 776] and small samples of 13 TeV data [777]. A
search by ATLAS using 36.1 fb
−1 of 13 TeV data [778] in the Z → channel
results in upper limits on the product of branching fraction and production cross
section, σ ( pp → X )B(X → Z γ ), between 88 and 2.8 fb for masses in the range
250–2.4 TeV. The reach in resonance mass can be improved by considering Z → νν
decays, resulting in a mono-γ signature, which is a common signature in searches
for dark matter (see Sect. 5.5). An example is the interpretation of an ATLAS
search for dark matter [779] as a search for a Z γ resonance. This search extends
the reach in the Z γ resonance mass up to 5 TeV. The observed upper limits on
σ ( pp → X )B(X → Z γ ) are between 26 and 43 fb for a mass range of 2–5 TeV.
5 Direct Searches for New Physics
Similar to V V and VH resonances, also H H resonance searches are combined
in dedicated analyses [764, 765]. These combinations include mostly searches for
resonances below masses of 1 TeV, with sensitivity to the SM H H production cross
section. The best sensitivity at low masses is achieved by the bbγ γ analyses [766,
767]. At masses above 1 TeV, the bbbb [743, 744] and bbτ τ [728] final states dominate the combined results. For spin-0 resonances decaying to H H with masses
above 2 TeV, production cross sections of 6 fb or higher can be excluded at 95% CL.
It should be noted that the bbW W
(∗) analysis, with W W
(∗)
→ qqν [748], has not
been considered in these combinations so far. Further improvements are expected
from its inclusion.
5.1.5 V γ and Hγ Resonances
A special class of diboson resonances are resonances decaying to photons, where
the decay is usually loop-mediated. A prominent example of such a diphoton decay
is H → γ γ , with a branching fraction of only 0.23%. Nevertheless, this channel
was one of the discovery channels. Even smaller is the branching fraction to Z γ
with 0.15%, which has not been observed to date [768]. In BSM models, high-mass
Z γ resonances appear as loop-mediated decays as well. Examples are compositeness models, like the little Higgs model [769], extra-dimension models [770, 771]
or extensions of the SM with an additional scalar field [772]. Some models of
technicolour predict W γ resonances in addition to Z γ , originating from a triplet
of technirhos [773]. These BSM theories can be generalised by an effective model
introducing colour-neutral spin-0 or spin-2 states, denoted by X , decaying to Z γ .
Spin-1 W
and Z
can also be introduced, with the loop-induced W
and Z
decays
to W γ and H γ [774], respectively. The branching fraction for the H γ decay is typically very small with B(Z
→ H γ ) = O(10
−5
) and smaller, but if the photon is
not a single photon but a cluster of photons from the decay of one or more highly
boosted light particles, much larger branching fractions are obtained [774]. Since
these decay cascades would lead to the same detector signature as a single photon,
searches for H γ resonances might lead to discoverable BSM effects.
Early searches for W γ and Z γ resonances have focussed on resonance masses
below 1 TeV and leptonic V decays. Examples are searches by ATLAS and CMS
using 7 and 8 TeV data [775, 776] and small samples of 13 TeV data [777]. A
search by ATLAS using 36.1 fb
−1 of 13 TeV data [778] in the Z → channel
results in upper limits on the product of branching fraction and production cross
section, σ ( pp → X )B(X → Z γ ), between 88 and 2.8 fb for masses in the range
250–2.4 TeV. The reach in resonance mass can be improved by considering Z → νν
decays, resulting in a mono-γ signature, which is a common signature in searches
for dark matter (see Sect. 5.5). An example is the interpretation of an ATLAS
search for dark matter [779] as a search for a Z γ resonance. This search extends
the reach in the Z γ resonance mass up to 5 TeV. The observed upper limits on
σ ( pp → X )B(X → Z γ ) are between 26 and 43 fb for a mass range of 2–5 TeV.