Observation of electroweak production of two jets and a Z-boson pair with the ATLAS detector at the LHC

Jing LiUmich / SJTU

ATLAS Weekly, October 1st, 2019on behalf of the VBS ZZ analysis team

Paper draft: ATL-COM-PHYS-2019-742Internal note: ATL-COM-PHYS-2018-1338Supersede: ATLAS-CONF-2019-033Target journal: Nature Physics

1

Motivation• Vector boson scattering is a key process to

probe EWSB• Sensitive to new physics (interpretations to

be down in a next paper)• Observation of Electroweak ZZjj production

• This completes observation of weakboson scattering (ssWWjj, WZjj, andZZjj), and sparks new ways to test EWSB

• Very rare process, but clean mode using Zdecays to charged leptons; exploit also 𝑍 →𝜈𝜈 decay Output of this paper:

• XS of the inclusive ZZjj with cut-based analysis• Observation of EW ZZjj with MVA method

2

Experimental signature:• Two weak bosons• Two jets in the forward and backward regions

with large eta separation

Analysis• Using ATLAS full Run2 dataset (139 fb-1)• Signal region

• two(one) opposite-sign same-flavor lepton pairs for ℓℓℓℓ𝑗𝑗(ℓℓ𝜈𝜈𝑗𝑗) channel• ℓℓℓℓ𝑗𝑗(ℓℓ𝜈𝜈𝑗𝑗) dijet selection: 𝑚)) > 300 400 𝐺𝑒𝑉, Δ𝑌)) > 2• MET-significance>12, B-jet veto to reduce ℓℓ𝜈𝜈𝑗𝑗 backgrounds

• QCD ZZjj in ℓℓℓℓ𝑗𝑗 channel is constraint using data events in QCD enriched control region (by reverting 𝑚)) and Δ𝑌)) cut).

• Multiple control regions have been defined in the ℓℓ𝜈𝜈𝑗𝑗 channel, including a 3-lepton CR (for 𝑊𝑍 background), a e𝜇 CR (for 𝑡 ̅𝑡 , 𝑊𝑊 … background)

• the fiducial cross-section of inclusive ZZjj production is measured from the SRs

• MVA discriminants are trained using sensitive kinematic distributions and fitted simultaneously in both final states to search for EWK ZZjj.

3

Results• Cross-section measurements of inclusive ZZjj processes

• EW ZZjj has been observed for the first time, using the full Run 2 data

• More detailed studies will follow up in the future

Measured fiducial 𝝈 [fb] Predicted fiducial 𝝈 [fb]

ℓℓℓℓ𝑗𝑗 1.27±0.12(stat) ±0.02(theo) ±0.07(exp) ±0.01(bkg) ±0.03(lumi) 1.14±0.04(stat) ±0.20(theo)

ℓℓ𝜈𝜈𝑗𝑗 1.22±0.30(stat) ±0.04(theo) ±0.06(exp) ±0.16(bkg) ±0.03(lumi) 1.07±0.01(stat) ±0.12(theo)

𝝁𝑬𝑾 𝝁𝑸𝑪𝑫ℓℓℓℓ𝒋𝒋 Significance

Obs.(Exp.)

ℓℓℓℓ𝑗𝑗 1.54±0.42 0.95±0.22 5.48(3.90) 𝜎

ℓℓ𝜈𝜈𝑗𝑗 0.73±0.65 -- 1.15(1.80) 𝜎

Combined 1.35±0.34 0.96±0.22 5.52(4.30) 𝜎

4