114
4 Standard Model Measurements
which is fixed by a selection based on p T of the associated V when combining the
two results. The final observed upper limit at 95% CL on the SM signal strength
modifier for VH , H → cc is 70, with an expected limit of 37
+16
−11 . The sensitivity
exceeds previous expectations and can further be improved by analyses of the full
available LHC data.
4.2.3 Top Quark Production
Measurements of top quark production provide important information on the dynamics of the heaviest particle in the SM. Through its mass, the top quark couples strongly
to the H boson, resulting in its special role in EW symmetry breaking. Its large mass
also implies that top quark production is a difficult multi-scale problem, where
m t can be of the order of top quark p T , and hence can not be neglected. Precise
predictions therefore require the inclusion of higher order terms and resummation
corrections [619–623]. Differential measurements of top quark production allow for
precise tests of QCD with heavy quarks, provide important input for the determination of PDFs [624, 625] and can be used to extract fundamental theory parameters,
such as α S and m t [626, 627]. The reach in p T of these measurements is limited
by the onset of collimated decays, where selection efficiencies of standard reconstruction techniques start to decrease. In order to access the high- p T regime, which
provides also highest sensitivity to BSM effects [628–633], jet substructure techniques are indispensable. In addition, for certain BSM models interference effects
with SM production are important and require precise differential measurements in
order to resolve the dip-peak structure caused by the presence of new scalar particles
[634–637].
A first differential measurement of the tt production cross section using top tagging has been performed on 8 TeV data by ATLAS [638]. The measurement is carried
out in the +jets channel. The high p T of the two top quarks implies a back-to-back
topology of the collimated top quark decays. This results in standard lepton selection
criteria to fail, because the lepton may get reconstructed inside the b jet from the
t → bW → bν decay. Since the collimation, and thus the overlap, is expected to
increase with increasing p T , the efficiency for selecting leptons from the W decay
decreases when requiring isolated leptons. In ATLAS and CMS, lepton isolation
is defined by the relative p T of particles in a fixed cone around the lepton fourvector [407, 639–643]. Usually, a selection based on this measure of isolation efficiently rejects leptons from weak decays within jets, at high selection efficiencies
for leptons from W and Z decays. In semi-leptonic decays of boosted top quarks
this isolation fails because of the b jet constituents falling inside the lepton isolation
cone. In order to overcome this, a variable called mini-isolation [644] is used, which
introduces a shrinking isolation cone size, proportional to p
−1
T . In high- p T tt production, mini-isolation helps to retain high lepton reconstruction efficiency, while
sufficiently suppressing backgrounds from QCD multijet production. It should also
be noted that once a lepton has been reconstructed within a jet, an overlap removal
4 Standard Model Measurements
which is fixed by a selection based on p T of the associated V when combining the
two results. The final observed upper limit at 95% CL on the SM signal strength
modifier for VH , H → cc is 70, with an expected limit of 37
+16
−11 . The sensitivity
exceeds previous expectations and can further be improved by analyses of the full
available LHC data.
4.2.3 Top Quark Production
Measurements of top quark production provide important information on the dynamics of the heaviest particle in the SM. Through its mass, the top quark couples strongly
to the H boson, resulting in its special role in EW symmetry breaking. Its large mass
also implies that top quark production is a difficult multi-scale problem, where
m t can be of the order of top quark p T , and hence can not be neglected. Precise
predictions therefore require the inclusion of higher order terms and resummation
corrections [619–623]. Differential measurements of top quark production allow for
precise tests of QCD with heavy quarks, provide important input for the determination of PDFs [624, 625] and can be used to extract fundamental theory parameters,
such as α S and m t [626, 627]. The reach in p T of these measurements is limited
by the onset of collimated decays, where selection efficiencies of standard reconstruction techniques start to decrease. In order to access the high- p T regime, which
provides also highest sensitivity to BSM effects [628–633], jet substructure techniques are indispensable. In addition, for certain BSM models interference effects
with SM production are important and require precise differential measurements in
order to resolve the dip-peak structure caused by the presence of new scalar particles
[634–637].
A first differential measurement of the tt production cross section using top tagging has been performed on 8 TeV data by ATLAS [638]. The measurement is carried
out in the +jets channel. The high p T of the two top quarks implies a back-to-back
topology of the collimated top quark decays. This results in standard lepton selection
criteria to fail, because the lepton may get reconstructed inside the b jet from the
t → bW → bν decay. Since the collimation, and thus the overlap, is expected to
increase with increasing p T , the efficiency for selecting leptons from the W decay
decreases when requiring isolated leptons. In ATLAS and CMS, lepton isolation
is defined by the relative p T of particles in a fixed cone around the lepton fourvector [407, 639–643]. Usually, a selection based on this measure of isolation efficiently rejects leptons from weak decays within jets, at high selection efficiencies
for leptons from W and Z decays. In semi-leptonic decays of boosted top quarks
this isolation fails because of the b jet constituents falling inside the lepton isolation
cone. In order to overcome this, a variable called mini-isolation [644] is used, which
introduces a shrinking isolation cone size, proportional to p
−1
T . In high- p T tt production, mini-isolation helps to retain high lepton reconstruction efficiency, while
sufficiently suppressing backgrounds from QCD multijet production. It should also
be noted that once a lepton has been reconstructed within a jet, an overlap removal