110
4 Standard Model Measurements
overlapping distributions from VH and V V production, the small branching fractions of leptonic decays, which result in low event counts once a selection is made
to suppress the main backgrounds, and systematic uncertainties related to b tagging
and the jet reconstruction. Here, HF refers to heavy flavour and subsumes bb, bc,
b+light and cc production. While VH production with to H → bb has been observed
in resolved final states with a significance higher than five standard deviations by
ATLAS [35] and CMS [36] using data from 2011–2017, corresponding to about
100 fb
−1 per experiment, a first measurement using boosted H tagging has achieved
a significance of 2.1 standard deviations using 137 fb
−1 of data [610]. The measurement profits from a number of recent developments, most notably in subjet b tagging,
where VR track-jets [187] are used to adjust the jet distance parameter dynamically
to the decay kinematics [514, 515]. It is instructive to compare the result of the measurement to the phenomenological study from 2008 [40], as shown in Fig. 4.9. The
jet mass distributions have been obtained in final states with one lepton and missing
transverse momentum, consistent with the decay of a W boson in WH production.
There are differences in the selected phase space, where e.g. the phenomenological study uses CA jets with R = 1.2 and p T > 200 GeV and ATLAS uses anti-k T
jets with R = 1.0 and 250 < p T < 400 GeV, but the distributions can be compared
qualitatively. In the study from 2008, the tt and V V backgrounds have been underestimated, while the signal efficiency is too optimistic. However, the predicted shape
of the W +jets background is very similar to the one observed in the ATLAS measurement, which is remarkable. On the other hand, the shape of the tt background is
very different, where its contribution below the W Z and H jet mass regions has been
underestimated. Similar observations can be made when comparing the Z → and
Z → νν channels. While it took a long time to obtain sufficient sensitivity in boosted
VH production, this channel will play an important role in future studies, especially
for differential cross section measurements.
3
An important observation is that measurements of high- p T H production through
the gluon-gluon fusion (ggF) process provide sensitivity to the top quark Yukawa
coupling. In ggF production, the H p T is induced by the radiation of a hard gluon
or quark, which resolves the loop induced contributions and allows the study of
anomalous production mechanisms [611]. For a long time, the H → bb channel has
been considered impossible for ggF production due to the high background from
QCD multijet production. However, with recent developments in jet substructure
tagging, a first analysis in this channel has resulted in an observed significance of 1.5
standard deviations (with 0.7 expected) for p T > 450 GeV [612], using 35.9 fb
−1
of 13 TeV data. Similar to the VH analysis, the distribution in the jet mass, with
soft drop grooming in this case, is used to search for a signal. The main challenge
is the suppression of the QCD multijet background, while retaining the ability to
predict its shape reliably. Anti-k T R = 0.8 jets are selected based on the double-b
tagger discriminant, which is by construction uncorrelated to m jet . However, any
further suppression of one-prong jets with jet substructure methods would result in a
3 An area where H tagging has been indispensable for some time already are searches for heavy
resonances decaying to VH or HH, which are described in Sect. 5.
4 Standard Model Measurements
overlapping distributions from VH and V V production, the small branching fractions of leptonic decays, which result in low event counts once a selection is made
to suppress the main backgrounds, and systematic uncertainties related to b tagging
and the jet reconstruction. Here, HF refers to heavy flavour and subsumes bb, bc,
b+light and cc production. While VH production with to H → bb has been observed
in resolved final states with a significance higher than five standard deviations by
ATLAS [35] and CMS [36] using data from 2011–2017, corresponding to about
100 fb
−1 per experiment, a first measurement using boosted H tagging has achieved
a significance of 2.1 standard deviations using 137 fb
−1 of data [610]. The measurement profits from a number of recent developments, most notably in subjet b tagging,
where VR track-jets [187] are used to adjust the jet distance parameter dynamically
to the decay kinematics [514, 515]. It is instructive to compare the result of the measurement to the phenomenological study from 2008 [40], as shown in Fig. 4.9. The
jet mass distributions have been obtained in final states with one lepton and missing
transverse momentum, consistent with the decay of a W boson in WH production.
There are differences in the selected phase space, where e.g. the phenomenological study uses CA jets with R = 1.2 and p T > 200 GeV and ATLAS uses anti-k T
jets with R = 1.0 and 250 < p T < 400 GeV, but the distributions can be compared
qualitatively. In the study from 2008, the tt and V V backgrounds have been underestimated, while the signal efficiency is too optimistic. However, the predicted shape
of the W +jets background is very similar to the one observed in the ATLAS measurement, which is remarkable. On the other hand, the shape of the tt background is
very different, where its contribution below the W Z and H jet mass regions has been
underestimated. Similar observations can be made when comparing the Z → and
Z → νν channels. While it took a long time to obtain sufficient sensitivity in boosted
VH production, this channel will play an important role in future studies, especially
for differential cross section measurements.
3
An important observation is that measurements of high- p T H production through
the gluon-gluon fusion (ggF) process provide sensitivity to the top quark Yukawa
coupling. In ggF production, the H p T is induced by the radiation of a hard gluon
or quark, which resolves the loop induced contributions and allows the study of
anomalous production mechanisms [611]. For a long time, the H → bb channel has
been considered impossible for ggF production due to the high background from
QCD multijet production. However, with recent developments in jet substructure
tagging, a first analysis in this channel has resulted in an observed significance of 1.5
standard deviations (with 0.7 expected) for p T > 450 GeV [612], using 35.9 fb
−1
of 13 TeV data. Similar to the VH analysis, the distribution in the jet mass, with
soft drop grooming in this case, is used to search for a signal. The main challenge
is the suppression of the QCD multijet background, while retaining the ability to
predict its shape reliably. Anti-k T R = 0.8 jets are selected based on the double-b
tagger discriminant, which is by construction uncorrelated to m jet . However, any
further suppression of one-prong jets with jet substructure methods would result in a
3 An area where H tagging has been indispensable for some time already are searches for heavy
resonances decaying to VH or HH, which are described in Sect. 5.