Higgs boson in decays to two W bosons using the ATLAS detector
Dominik Duda, on behalf of the ATLAS collaboration Max-Planck-Institut fuer Physik, Germany
E-mail: dominik.duda@cern.ch
This proceedings presents the latest measurements of the Higgs boson production cross sections times branching ratio in the H → WW ∗ → `ν`ν decay mode for the three most dominant pro- duction processes of the Higgs boson: gluon-gluon fusion, vector boson fusion and the associ- ated production of a vector boson and a Higgs boson. The measurements are performed based on proton-proton collision data collected at 13 TeV by the ATLAS detector. The cross section times branching ratios of the gluon-gluon fusion and vector boson fusion production modes are measured in a dataset corresponding to an integrated luminosity of 36.1 fb −1 and are found to be 12.6 +1.3 −1.2 (stat.) +1.9 −1.8 (sys.)pb and 0.5 +0.24 −0.23 (stat.) ± 0.18(sys.)pb , respectively. The cross sec- tion times branching ratio of the associated production of a vector boson and a Higgs boson is measured in a dataset corresponding to an integrated luminosity of 5.8 fb −1 and is found to be 0.9 +1.1 −0.9 (stat.) +0.7 −0.8 (sys.)pb. For all three production modes, the measured cross section times branching ratio are compatible to the SM predictions within one standard deviation.
XXXIX International Conference on High Energy Physics -ICHEP2018, 4.-11. July 2018
Seoul,South Korea
1. Introduction
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The discovery of the Higgs boson in 2012 was the greatest success of the LHC experiments so
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far. Measurements of its properties are consistent with the Standard Model (SM) predictions. The
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Higgs boson mass was measured by ATLAS and CMS to be m H = 125.09 ± 0.21(stat) ± 0.11(syst),
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while the Spin and CP state of the Higgs boson, determined by probing angular distribustion of its
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decay products, hints very strongly to a Spin CP state of 0 + , while alternative models are rejected
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with a CL of more than 99.9%. Still, measurements of the Higgs boson properties remain to be of
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great interest as they are very sensitive to the presence of new phyiscs.
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The H → WW ∗ → `ν`ν decay channel has the second largest branching fraction and a rel-
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atively clear signature in the detector. Thus it allows for precise measurements of Higgs boson
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properties.
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Due to the spin-0 nature of the Higgs boson and the V-A structure of the subsequent W boson
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decays, this decay channel is characterised by two oppositely charged leptons with a small opening
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angle and thus a relatively small invariant mass m `` . The presence of at least two neutrinos in the
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targeted final state prevents the full reconstruction of the four-vector of the Higgs boson and thus
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its invariant mass. However, the transverse mass m T of the Higgs boson can be calculated via
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m T = q
(E `` + E T miss ) 2 − |p T,`` + E T miss | 2 (1.1) with E `` =
q
|p T,`` | 2 + m 2 `` as well as E T miss and p T,`` , which are the missing transverse momentum
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and the combined dilepton four-vector in the transverse plane, respectively. The m T distribution
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has a kinematic upper bound at the Higgs boson mass, which makes it ideal to distinguish between
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Higgs boson production modes and the non-resonant WW and top-quark production.
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In this proceedings, the latest measurements of the Higgs boson production cross sections in
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the H → WW ∗ → `ν`ν decay mode are presented for the three most dominant production modes
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of the Higgs boson: gluon-gluon fusion, ggF (86%), vector boson fusion, V BF , (7%) and the
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associated production of a vector boson and a Higgs boson (5%). Exemplary Feynman diagrams
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for these processes are presented in Figure 1.
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g g
t h
q' q
W/Z
W/Z
h
q' q
q q
W/Z
h W/Z
Figure 1: Exemplary Feynman diagrams of the ggF (left), V BF (middle) and V H (right) production modes of the Higgs boson at tree-level.
2. Measurement of the ggF and V BF production cross sections
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The latest measurements of the ggF and V BF Higgs boson production cross-sections are preformed
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in the H → WW ∗ → `ν`ν decay mode using a dataset that has been recorded by the ATLAS detector
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[1] in the 2015 and 2016 runs of the LHC. This dataset was taken during proton-proton collisions
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at a centre-of-mass energy of 13 TeV and corresponds to an integrated luminosity of 36.1 fb −1 .
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Selected H → WW ∗ → `ν`ν candidate events are classified according to their multiplicity of
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jets 1 N jets with a transverse momentum p T larger than 30 GeV and with an absolute value of the
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pseudo-rapidity |η| smaller than 4.5. Event categories with N jets = 0 and N jets = 1 are defined in
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order to target the ggF production mode, while a N jets ≥ 2 category is used for measurements in
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the VBF production mode. Topology specific variables such as the invariant mass and the rapidity
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gap of the dijet system (m j j and ∆Y j j ) are used as inputs for a boosted decision tree (BDT), which
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allows to further enhance the VBF signal with respect to the sum of backgrounds.
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Dedicated control regions are defined in order to constrain the normalisations of the most dom-
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inant background processes (WW , t¯ t +W t and Zγ ∗ + jets), while smaller backgrounds such as W Z,
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ZZ, W γ or Zγ are entirely taken from the simulation. In order to estimate the contributions from
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background processes containing mis-identified leptons, a control sample is defined using events
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with one well-defined lepton and one lepton failing nominal object definitions requirements but
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passing looser requirements (referred to as anti-identified). The contribution of the mis-identified
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lepton background to the signal region is estimated by scaling the control sample via p T and η
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dependent extrapolation factors, which are defined as the ratio of the well-defined leptons to anti-
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identified leptons.
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The cross sections for the ggF and V BF production modes are obtained by a simultanious fit to
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the various control and signal regions maximising a likelihood function. The fit is performed on the
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transverse mass m T for the N jets = 0 and N jets = 1 categories, while in the VBF category the score
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of the boosted decision tree is used. Data-to-simulation comparison of both these distributions
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are depicted in Figure 2. The ggF production mode is profiled while the significance of the V BF
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production mode is determined, and vice-versa.
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Systematic uncertainties enter the likelihood function as nuisance parameters. The most dom-
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inant uncertainties are related to the modelling of the various background processes ( ± 8% in ggF
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and ±21% VBF), the data statistics (±8% in ggF and ±46% in VBF), the statistics of the simulated
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event samples (±5% in ggF and ±23% VBF), as well as the flavour-tagging efficiency (±5% in
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ggF and ±6% VBF). The cross section times branching ratios for the ggF and V BF production
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modes are found to be:
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σ ggF × B H→WW
∗= 12.6 +1.3 −1.2 (stat .) +1.9 −1.8 (sys.) pb
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and
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σ V BF × B H→WW
∗= 0.5 +0.24 −0.23 (stat.) ± 0.18(sys.) pb
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respectively, while the predicted values are 10.4 ± 0.6 pb (for ggF) and 0.81 ± 0.02 pb (for VBF).
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Thus the signal strength parameters, defined as the ratio of the measured to predicted production
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cross section, are µ ggF = 1.21 +0.12 −0.11 (stat.) +0.18 −0.17 (syst.) and µ V BF = 0.62 +0.30 −0.28 (stat .) ± 0.22(syst.).
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Thus the measurements are consistent with the SM prediction within one standard deviation. The
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measurement of the ggF production mode cross section is dominated by systematic uncertainties,
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1
Jets correspond to hadronic particle showers that are reconstructed from topological clusters of energy deposits in
the calorimeter system using the anti-k
Talgorithm with a distance parameter of R = 0.4.
whereas the V BF production cross section is still dominated by statistical uncertainties. The ob-
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served (expected) significances are 6.3 (5.2) standard deviations for the ggF production mode and
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1.9 (2.7) standard deviations for the VBF production mode. The two dimensional 68% and 95%
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CL contours of σ ggF × B H→WW
∗and σ V BF × B H→WW
∗are presented in Figure 2 together with the
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Standard Modell predictions.
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0 200 400 600 800 1000 1200 1400 1600 1800 2000
Events / 10 GeV
Data Uncertainty
WW VV Mis-Id
γ* Z/
/Wt t t Higgs
ATLAS Preliminary
≤ 1 Njet ν, µ ν
→e
→WW*
H = 13 TeV, 36.1 fb-1 s
50 100 150 200 250
[GeV]
mT 0
100 200 300
Data-Bkg.
BDT score
0 0.5 1 1.5 2 2.5 3
Events
1 10 102
103
104
105
Data HVBF
HggF tt/Wt
WW Z/γ*
Mis-Id VV
Uncertainty HVBF×30
ATLAS Preliminary
= 13 TeV, 36.1 fb-1 s
2 VBF
≥ N jet
ν, µ ν
→e
→WW*
H
5 0 5 10 15 20 25
ggF
B
H WW*[pb]
0.5 0.0 0.5 1.0 1.5 2.0
VBF
B
HWW*[p b] ATLAS Preliminary
ps=13 TeV, 36.1 fb
1 68% CL95% CL Best fit SM
Figure 2: Post-fit distribution of the transverse mass for the ggF analyis (top left) and the BDT response for the VBF analysis (top right), as well as the 68% and 95% confidence level two-dimensional likelihood contours of the σ ggF × B H→WW
∗vs. σ V BF × B H→WW
∗compared to the SM prediction (bottom) [2].
3. Measurement of the W H production cross sections
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The latest measurements of the Higgs boson production cross section via associated WH production
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is based on a part of the 2015 and 2016 proton-proton collision runs of the LHC and corresponds
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to an integrated luminosity of 5.8 fb −1 recorded by the ATLAS detector at √
s = 13 TeV.
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For this measurement, candidate events with exactly three charged leptons are considered and
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further classified into two orthogonal categories according to the lepton pairing. One of these
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categories targets events with at least one same-flavour opposite charge pair and the other category
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exclusively contains events with no same-flavour opposite charge pair. The first region is dominated
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by processes containing Z-bosons, while the second one is Z-boson depleted. The normalisation
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of the most dominant backgrounds (W Z/Wγ ∗ , Zγ, Z+jets and t¯ t/W t/t¯ t +V ) is obtained from the
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data using dedicated control regions.
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The cross section times branching ratio for the associated production of a Higgs boson and
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a W boson is determined via a simultaneous fit to all signal and control regions using simply the
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event rates in all considered categories. Systematic uncertainties enter again as nuisance parameters
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in the likelihood function. The dominant uncertainties are related to the statistics of the data set
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(∼ 120%) and the simulated event samples (∼ 70%), pile-up activity (∼ 24%), Jet energy resolution
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( ∼ 23%), the modelling of the most dominant background processes (e.g. ∼ 20% on the modelling
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of the t t) production. The production cross section times branching ratio was found to be ¯
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σ W H × B H→WW
∗= 0.9 +1.1 −0.9 (stat.) +0.7 −0.8 (sys.) pb ,
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while the predicted production cross section is 0.293 ±0.007 pb. Hence, the signal strength param-
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eter is µ W H = 3.2 +3.7 −3.2 (stat.) +2.3 −2.7 (syst.). The observed (expected) significance is 0.77σ (0.24σ),
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thus an upper limit is set to the cross section times branching ratio of the W H production mode,
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which is found to be 3.3 pb at the 95% confidence level. The post-fit distribution of the event yields
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in the various signal and control regions are presented in Figure 3.
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Fit regions
CRa CRb
e-fake CRc
-fake
µCRc CRd CRe
Z-dominated SR
Z-depleted SR
Events / bin
0 100 200 300 400 500 600 700 800
data SM (sys)
VV Z+jets
VVV Top
Other Higgs WH
ATLAS Preliminary
= 13 TeV, 5.81 fb-1
s
ν νl νl
→l
*
→WWW WH
5 10 15 20
Rescaled y-axis for SRs
Figure 3: Fit regions used in the W H cross section measurement. The signal and background predictions are normalised to the results of the likelihood fit. The signal and control region categories are displayed with different y-axis. The hatched band (denoted as "SM (sys)") includes MC statistical, experimental, and theory systematic uncertainties associated with the prediction of the signal and background processes [3].
4. Conclusion
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Latest measurements of the cross section times branching ratios for Higgs bosons decaying via
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H → WW ∗ have been presented in the ggF and VBF production modes for the data collected by
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ATLAS in 2015 & 2016 and in the W H production mode for a partial 2015+2016 dataset. For all
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three production modes, the measured cross sections are compatible to the SM predictions within
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one standard deviation.
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References
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[2] ATLAS Collaboration, "Measurements of gluon-gluon fusion and vector-boson fusion Higgs boson
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production cross-sections in the H → WW ∗ → eν µ ν decay channel in pp collisions at √
s = 13 TeV
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with the ATLAS detector", 2018, arXiv:1808.09054.
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[3] ATLAS Collaboration, " Measurements of the Higgs boson production cross section via Vector Boson
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Fusion and associated W H production in the WW ∗ → `ν`ν decay mode with the ATLAS detector at
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√
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