Recent Electroweak Results from ATLAS - Bing Li Shandong University QCD21, (Online) Montpellier - FR
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Recent Electroweak Results from ATLAS
Bing Li
On behalf of the ATLAS Collaboration
Shandong University
QCD21, (Online) Montpellier - FR
5-9 July 2021
Single and Di-boson Production @LHC Rel.
rate
ß Measurement of single and diboson productions at LHC is
important to test the validity of the Standard Model (SM) at 1
TeV scale
Þ Many precision differential measurements
Þ VBS processes with relative lower cross-section, being key
process to probe the mechanism of electroweak symmetry 10-1
breaking (EWSB)
ß Important test of perturbative QCD
ß Involve with Triple or Quartic Gauge Couplings (T/QGCs) 10-2
Þ To look for vector boson self-couplings, only charged couplings
allowed in SM
Þ Probe new physics through deviations from SM couplings
ß EFT interpretation
10-3
ß A way to search for high mass resonance decaying to VV
final state
July 9, 2021 Bing Li 2
Current Measurements at ATLAS
Results covered today
ü Inclusive four-lepton: arXiv:2103.01918,
accepted by JHEP
ü Photon-induced WW: Phys. Lett. B 816
(2021) 136190
ü WW + jet: JHEP 06 (2021) 003
ü EW Z + jj: Eur. Phys. J. C 81 (2021) 163
ü EW ZZ + jj: arXiv:2004.10612
ü HàWW* and WW EFT combination:
ATL-PHYS-PUB-2021-010
July 9, 2021 Bing Li 3
Inclusive 4l Measurements arXiv:2103.01918,
accepted by JHEP
ü Vey rich physics information can be
obtained from the four-lepton channel, given
the very clean and fully reconstructable final
states
ü Very clear structures in the four-lepton mass
distributions at different mass regions
ü Z region, Higgs region, on-shell ZZ region
and off-shell ZZ region
On-shell ZZ On-shell ZZ Z region Higgs region
region from qq region from gg
July 9, 2021 Bing Li 4
Integrated Cross-section Measurements ü Four leptons selected with mll > 5 GeV for SFOS pairs Uncertainty breakdown ü Then split to different four-lepton mass regions based on dominant physics processes In the Zà4l region, the branching ratio is extracted to be Most precise to date and compatible with SM prediction and previous measurements July 9, 2021 Bing Li 5
Differential Measurements and EFT
ß Differential cross sections measured vs. several
variables, in four different m4l region Limits set using most sensitive observable
for each EFT coefficient
Þ m12, m34, pT12, pT34, cosθ12 …
July 9, 2021 Bing Li 6
Example for BSM Interpretation
Most sensitive
observables in each
parameter space
ü Minimal assumptions are made on the theory for this measurement
ü Unfolding is very independent of the signal MC and biases from injecting alternative signals have been
evaluated
ü An example for BSM interpretation with a model in which the global baryon-number-minus-lepton-number
(B−L) symmetry is treated as a local gauge symmetry and spontaneously broken (JHEP 08 (2018) 181)
July 9, 2021 Bing Li 7
Phys. Lett. B 816 (2021) 136190
Observation of Photon-induced WW Production
ß Production through trilinear and quartic gauge-boson interactions
Þ Only gauge-boson couplings at LO à direct test of the SM SU(2)xU(1)
Þ Sensitive to anomalous gauge-boson interactions
ß For the pp(γγ) → p(∗)WWp(∗)
Þ The final state protons stay either intact or fragments after emitting a photon (elastic, single-dissociative
and double-dissociative WW production)
Þ Rather clean events (two charged leptons and no additional charged-particle activity) for elastic case
Þ In the dissociative cases, remaining charged particles often fall into the forward region beyond tracking
detector
ß Signal selections
Þ eµ pairs with different charges, no tracks near leptons vertex, pT(ll) > 30 GeV (MET proxy)
ß Major challenges
Þ Modeling of number of charged particles from underlying events for background process
Þ Corrected with data measurements of DYàll production
July 9, 2021 Bing Li 8
Cross-section Measurements
Integrated event yields from different
regions are used in profile likelihood
fit to get the signal events
Signal strength:
Events categorized into 4 regions based on ntrk and pT(eμ) Significance: 8.4 σ
SR: ntrk = 0 and pT(eμ) > 30 GeV Measured fiducial cross-section:
Other 3 CRs to constrain dedicated backgrounds
July 9, 2021 Bing Li 9
WW (àeνµν) + jets Production
JHEP 06 (2021) 003
ß First differential measurements in this final state at the LHC
ß Test of perturbative QCD and EW theory
ß Dominant background from top processes
Þ Estimated with data-driven method
Þ Factor of 5 uncertainty reduction w.r.t. MC based
July 9, 2021 Bing Li 10Integrated Crosse Section Measurements ß Measured cross section, 10% uncertainty! : July 9, 2021 Bing Li 11
Differential Cross Section Measurement
Very good agreement between measurements and SM predictions Looking into high pT region (leading jet pT >
200 GeV)
Relative contributions from linear terms
become larger
NLO EW corrections expected to be non-
negligible
Fit on meμ
Confidence intervals for cW
July 9, 2021 Bing Li 12EW Zjj Production
Eur. Phys. J. C 81 (2021) 163
ß Signal
Þ EW production of Z(àee/µµ) + 2 jets in back and forward regions
Þ No central jets between the two tag jets
ß Major background
Þ Strong production of Zjj
Þ Poor theoretical modeling especially in the high mjj region
Þ Several control regions defined to constrain the strong production
Þ Different MC generators used for comparison
July 9, 2021 Bing Li 13Differential Cross-section Measurements EW Zjj Inclusive Zjj Herwig7+VBFNLO agrees best with data July 9, 2021 Bing Li 14
EFT Interpretations
Impact on the EW Zjj differential XS
from different operators
Two CP-even and two CP-odd operators tested.
Differential cross section vs. Δφjj used to set limit
July 9, 2021 Bing Li 15Observation of EW ZZjj arXiv:2004.10612
ß Two Z bosons decaying leptonically
Þ ZZ à llll (4e, 4µ, 2e2µ)
Þ ZZ à llνν (2e2ν, 2µ2v)
ß Two jets in the back and forward regions
ß Clear experimental signatures
ß Large modeling uncertainties of the dijets
July 9, 2021 Bing Li 16Kinematics and Cross-section Measurements
lllljj channel llννjj channel
Inclusive (EW+QCD) cross-section of ZZjj
ü Statistical dominant
ü Precision in lllljj channel measurement better than theory prediction
July 9, 2021 Bing Li 17Extract the EW Component
ß BDT used to separate EW from other processes
ß Large systematic uncertainties due to the modeling of QCD ZZjj
Þ In addition to the standard QCD scale, PDF, parton shower uncertainties, different generators (MG vs.
Sherpa) have been compared at truth level
Þ Dedicated QCD ZZjj control region (reverting mjj or dYjj cut) used in lllljj channel to help constrain this
process With a statistical fit over the BDT output distributions
llννjj channel
lllljj channel
Leads to the first observation of the EW ZZjj production
at the LHC
Measured cross-section of the EW ZZjj: 0.82 +/- 0.21 fb
July 9, 2021 Bing Li 18HàWW* and WW EFT combination ATL-PHYS-PUB-2021-010
ß Both measurements are sensitive to a large
number of dimension-six SMEFT operators
ß Major chanllenges:
Þ Event overlap between two analyses. SM WW being
background for HàWW*
Þ Systematic correlation
Þ Different analysis techniques
ß Similar chanllenges expected for future EFT
combinations
Þ Instructional test case towards future combinations of
EFT-sensitive measurements at the LHC
July 9, 2021 Bing Li 19Summary
ß Recent ATLAS results on diboson and VBF/VBS with Run 2
data
Þ Precision measurements to test the SM at TeV scale
Þ Starting to be able to look for rare SM processes not observed before
Þ Looking for new physics through deviations from SM predictions
ß More results can be found at
https://twiki.cern.ch/twiki/bin/view/AtlasPublic/Publications
July 9, 2021 Bing Li 20backup July 9, 2021 Bing Li 21
yyàWW: Major Challenges
ß Tracks from pile-up collisions
Þ Will lower the selection efficiency
ß Modeling issues of number of charged particles
from underlying events
Þ Issue for background modeling
Þ Assuming similar underlying event for quark-induced
production of different colourless final states if the
transverse momenta of the final states are comparable
(Eur. Phys. J. C 74 (2014) 3195)
Þ Data measurements of DYàll production are used to
correct this distribution
July 9, 2021 Bing Li 22EW Zjj: QCD Background Constrain in Control Regions
ß Dedicated control
regions used to
constrain the QCD
Zjj modeling, for
both normalization
and shape
ß SR and CRs defined
using number of
gap jets, and
centrality of the Z
boson
ß SR and 3 CRs fitted
simultaneously
when extracting the
EW component
July 9, 2021 Bing Li 23You can also read
