ATLAS Recent observation and measurements of vector-boson fusion and scattering with - VBF VBS
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Recent observation and measurements of
vector-boson fusion and scattering with
ATLAS
VBF VBS
July 26, 2021
Dag Gillberg, Carleton University
On behalf of the ATLAS Collaboration
European Physics Society conference on high energy physics, 2021
The VBF and VBS processes
• Vector boson fusion and scattering are characterized by a triple or quartic gauge vertex
• Probes gauge bosons self-interaction and electroweak symmetry breaking
• Although very challenging and vast data required, now benchmark SM measurements
• Sensitive to many new physics scenarios; crucial input to for EFT studies
• Tests anomalous triple and quartic gauge couplings (aTGC and aQGC)
• EFT: Probe new physics at scales beyond direct reach of LHC
V V V
TGC V
Higgs can be
QGC
V H exchanged
V
VBF Higgs not part of
V this talk. See talks in the
Higgs sessions.
V V
2
The VBF and VBS signatures V
• VBF and VBS cannot be probed in isolation jet 1
V
• The standard approach is to study EW Vjj and EW VVjj
production, to which the VBF and VBS diagrams contribute z
EW Vjj VBF + non-VBF + … dijet system
Side view
( pz, pT )
• Rare processes but distinct topology: jet 2
Angular view
ϕ ( y, ϕ )
1. Large rapidity gap between jets Δyjj
2. Large dijet invariant mass mjj
3. Little hadronic activity (few extra jets), jet 1
gap
especially in rapidity gap between two leading jets (Njets ) V
4. Low pT (or lack) of third jet / low pT of (V)Vjj system y
5. Boost (rapidity) of (di)boson and dijet system similar * jet 2
V dijet system (jj)
6. pT of (di)boson and dijet system similar + back-to-back in ϕ
“Gap region"
* Centrality: ξ ≈ (yV(V) − yjj) / Δyjj 3
Δyjj
The VBF and VBS signatures V
μ+
• VBF and VBS cannot be probed in isolation jet 1
V
• The standard approach is to study EW Vjj and EW VVjj
production, to which the VBF and VBS
p diagrams contribute z
p
μ+ e−
EW Vjj VBF γ +W non-VBF ν + … dijet system
Side view
− ( pz, pT )
γ
• Rare processes but distinct W
topology: e jet 2
Angular view
p rapidity gap between jets Δyjj ν ϕ ( y, ϕ )
1. Large p
2. Large dijet invariant mass mjj
3. Little hadronic activity (few extra jets), jet 1
gap
especially in rapidity gap between two leading jets (Njets ) V
4. Low pT (or lack) of third jet / low pT of (V)Vjj system y
•5. New
Boostway to probe
(rapidity) VBS atand
of (di)boson LHC
dijet(2021)
system similar * V dijet system (jj)
jet 2
6. •pTPhoton induced
of (di)boson VVsystem
and dijet similar + γγ
production: → WW in ϕ
back-to-back
• No forward jets; Very clean, but very rare Details in Savannah
“Gap region"
Clawson’s talk, Thursday
* Centrality: ξ ≈ (yV(V) − yjj) / Δyjj 3
Δyjj
VBF and VBS measurements at ATLAS
• Electroweak Vjj and
VVjj and γγ → WW
production are all
very rare
• Rate relative to inel.
pp → X
−12
σZjj /σinel ≈ 10
−14
σZZjj /σinel ≈ 10
• Challenging analysis:
Small signal swamped
by backgrounds; often
poorly modelled
• 5σ observation for
WWjj, WZjj, ZZjj etc.
4
VBF and VBS measurements at ATLAS
• Summary of ATLAS
measurements targeting VBF
and VBS
• The following slides presents
a selected subset of four
recent Run-2 results:
• EW Zjj EPJC 81 (2021) 163
• EW Z(→ℓℓ)γjj ATLAS-CONF-2021-038
• EW Z(→νν)γjj CERN-EP-2021-137
• EW ZZjj arXiv:2004.10612
References to all these results can be found
in the backup slides + slide on γγ → WW
observation, PLB 816 (2021) 136190 5
CERN-EP-2020-045, arXiv:2006.15458 , EPJC 81 (2021) 163
Measurement of EW Zjj (1/3)
• EW Zjj was first observed by ATLAS using Run-1 data
JHEP 04 (2014) 031
• With the full Run-2 dataset, differential cross sections
are measured for four characteristic observables
• Dijet mass mjj and rapidity separation Δyjj
• Signed azimuthal dijet separation Δϕjj and pT,ℓℓ
j1
• Z → ee and Z → μμ data, pT > 85 GeV, mjj > 1000 GeV
• Main challenge: separate strong Zjj and EW Zjj
• Strong Zjj poorly modelled in VBF topology region
• EW Zjj enhanced signal region using VBF topology cuts
• Control regions used to constrain strong Zjj prediction
• Likelihood fit measures EW Zjj bin-by-bin
6
CERN-EP-2020-045, arXiv:2006.15458 , EPJC 81 (2021) 163
Measurement of EW Zjj (2/3)
Fit
EW Zjj measured in 5 mjj bins
Here: 20 bins, 5 POIs (EW Zjj bin yields)
12 free parameter that constrain strong Zjj Analogously in 9 Δyjj, 10 pT,ℓℓ and 12 Δϕjj bins
Main uncertainties:
• Data statistics
• Strong generator choice (switching between 3)
• Jet systematics (JES, JER)
7
CERN-EP-2020-045, arXiv:2006.15458 , EPJC 81 (2021) 163
Measurement of EW Zjj (3/3)
• Measured event yields are corrected to particle level (Iterative Bayesian unfolding)
• Measurements compared to various MC prediction
Impact from BSM modifications on the
➡ Guidance on generator choice; refinement of parameter settings measured EW Zjj differential cross sections
• Differential cross sections are used to set limits on BSM models
using an EFT framework
• Δϕjj is CP-odd and very sensitive to certain Wilson coefficients (cW)
STDM-2017-27, arXiv:2006.15458 , EPJC 81 (2021) 163
Δϕjj = ϕj1 − ϕj2, where yj1 > yj2 8
STDM-2017-19, arXiv:2004.10612
Submitted to Nature Physics
Observation of EW ZZjj
• EW ZZjj: very rare; unique sensitivity to non-SM quartic 4-Z coupling EWSB
• Main challenge: separate from strong ZZjj production
• Analysis:
• Decay channels: ZZjj → 4ℓjj and ZZjj → ℓℓννjj
• Preselection: typical ZZ (4ℓ, ℓℓνν) topology,
b-jet veto (reduce t t̄ ),
loose VBF topo. selection (mjj, Δyjj)
• Following preselection, BDTs are used to
separate EW ZZjj from backgrounds
• EW and strong Zjj measurements:
• μEW = 1.35 ± 0.34
μstrong = 0.96 ± 0.22
• EW ZZjj significance: 5.5σ (4.3σ expected)
• EW+strong fiducial cross sections measured
for 4ℓjj and ℓℓννjj separately
9ATLAS-CONF-2021-038
Observation of EW Z(ℓℓ)γjj
• EW Zγjj is measured in the eeγjj and μμγjj channels
• Analysis targets VBF topology + Z → ℓℓ + γ:
j1 j2 γ
pT > 50 GeV, pT > 50 GeV, pT > 25 GeV, mjj > 150 GeV, Δyjj > 1
mℓℓ > 40 GeV, and mγℓℓ + mℓℓ > 2 mZ (veto Z → γℓℓ)
• Main backgrounds: Strong Zγjj, Z+jets with fake γ, tt̄γ
• Key observable: Zγ centrality ζℓℓγ; SR: ζℓℓγ < 0.4
• Fit performed to mjj spectrum
• Observation of EW Z(ℓℓ)γjj with significance
well above 5σ (∼10σ)
EW Zγjj
• Fiducial cross section is measured:
d ± pred
σEW Zγ = 4.49 0.58 fb, σEW Zγ = 4.73 ± 0.27 fb Strong Zγjj Strong Zγjj
• The strong+EW Zγjj cross section is measured
+1.4
to be: 20.6−1.2 fb (predicted: 20.4−2.0 fb)
+2.6
10
fiCERN-EP-2021-137, EXOT-2021-17
Observation of EW Z(νν)γjj
• First observation of EW Z(νν)γjj
miss
• Strategy: target VBF topology+ET :
j1 j2
pT > 60 GeV, pT> 50 GeV, Δϕjj < 2.5,
Nγ = 1, Nℓ = 0, mjj > 250 GeV, γ between jets
• Multiple control regions to constrain
backgrounds
• EW Z(νν)γjj signal established with 5.2σ
(5.1σ) observed (expected) significance
• Measurements:
EW μZγ = 1.03 ± 0.25
σ d = 1.31 ± 0.29 fb
• In addition to EW Zγjj measurements, also sets
limits on invisible / partially inv. decays of a
Higgs boson (VBF H→invisible)
fi
11V V V
TGC
V Summary QGC
V V V
• Precision measurements of processes that probes VBF and VBS provides:
• Important test of EWSB
• Sensitivity to searches for new phenomena (BSM)
• Crucial input to EFT fits
• Typically, the associated cross sections are very small →
small signal swamped by large, challenging backgrounds
• Precision analyses only recently possible due to large dataset required
• Most measurements are statistics limited
• 5σ observation established for major VBF and VBS sensitive processes & channels
• Focus shifting to precision differential cross section measurements
• Exciting times ahead with the larger datasets of Run 3 and beyond
12Backup slides
CERN-EP-2020-165, arXiv:2010.04019 , PLB 816 (2021) 136190
See Savannah Clawson’s
Observation of γγ → WW
talk on photon fusion,
Thursday
p
p
• Photon-photon scattering μ+
γ W ν
• Incoming protons intact or
fragment outside acceptance γ W e−
• WW → eνμν channel very clean; p
p
ν
opposite charged ℓ, no other tracks
± ∓
• Analysis selects ℓ ℓ events, fulfilling:
ℓ1 ℓ2 eμ
pT > 27 GeV, pT > 20 GeV, mℓℓ > 30 GeV, SR: pT > 30 GeV
• Count tracks within 1 mm of ℓℓ primary vertex
• Expect ntrk = 0 additional tracks from γγ → WW
• Backgrounds constrained using CRs:
eμ
pT < 30 GeV and ntrk ≥ 1
• Background-only hypothesis rejected
with 8.4σ significance
• σ d = 3.13 ± 0.31 (stat) ± 0.28 (syst) ± 1 mm
14
fi
1415
EW Zjj/VVjj measurement in global BSM fits
arXiv:2012.02779, JHEP 04 (2021) 279
• The ATLAS EW Zjj measurements helps
constrain CW in particular
16Zjj: Generators, cutflow, fid. def
17Modelling issues for strong backgrounds
Sherpa and (LO) MG5+Py8 over predict the strong Zjj contribution after VBF topology selection
18Likelihood fit for EW Zjj signal extraction
20 bins, 5 POIs, 12 free parameter that constrain strong Zjj (5+5+2)
Fit
19EW Zjj uncertainties
20Zjj: EFT fit and HEP-data
Links to:
HepData entry with
all 20 unfolded
measurements +
statistical cross
correlation
Associated
Rivet routine
21EW ZZjj measurement details
Results:
22EW ZZjj candidate
23https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2021-17/
Observation of EW Z(νν)γjj
24Single and double boson production
Electroweak Ratio
Strong production production (VBF)
H H 13 : 1
50 pb 3.8 pb
Z Z 2700 : 1
23 pb
60 nb
W W 2800 : 1
190 nb 68 pb
@ 13 TeV
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