Observation of electroweak production of two jets in association with an isolated photon and missing transverse momentum, and search for a Higgs boson decaying into invisible particles at 13 $$\text {TeV}$$ with the ATLAS detector

Abstract

This paper presents a measurement of the electroweak production of two jets in association with a $$Z\gamma $$ Z γ pair, with the Z boson decaying into two neutrinos. It also presents a search for invisible or partially invisible decays of a Higgs boson with a mass of 125 $$\text {GeV}$$ GeV produced through vector-boson fusion with a photon in the final state. These results use data from LHC proton–proton collisions at $$\sqrt{s}$$ s = 13 $$\text {TeV}$$ TeV collected with the ATLAS detector and corresponding to an integrated luminosity of 139 $$\hbox {fb}^{-1}$$ fb - 1 . The event signature, shared by all benchmark processes considered for the measurements and searches, is characterized by a significant amount of unbalanced transverse momentum and a photon in the final state, in addition to a pair of forward jets. Electroweak $$Z\gamma $$ Z γ production in association with two jets is observed in this final state with a significance of 5.2 (5.1 expected) standard deviations. The measured fiducial cross-section for this process is $$1.31\pm 0.29$$ 1.31 ± 0.29 fb. An observed (expected) upper limit of 0.37 ($$0.34^{+0.15}_{-0.10}$$ 0 . 34 - 0.10 + 0.15 ) at 95% confidence level is set on the branching ratio of a 125 $$\text {GeV}$$ GeV Higgs boson to invisible particles, assuming the Standard Model production cross-section. The signature is also interpreted in the context of decays of a Higgs boson into a photon and a dark photon. An observed (expected) 95% CL upper limit on the branching ratio for this decay is set at 0.018 ($$0.017^{+0.007}_{-0.005}$$ 0 . 017 - 0.005 + 0.007 ), assuming the Standard Model production cross-section for a 125 $$\text {GeV}$$ GeV Higgs boson.

Measurements of multi-boson production including vector-boson scattering at ATLAS

“…This work reports the observation of electroweak production of 𝑍𝛾 + 2jets process, which probes the 𝑍𝛾 version of VBS. Two decay channels of 𝑍 → ℓℓ and 𝑍 → 𝜈𝜈 are studied separately [4,5]. The final state of 𝑍𝛾 𝑗 𝑗 has contributions from electroweak-induced processes (Process (a), (b) and (c) in Figure 1) as well as QCD-induced processes (Process (d) and (e) in Figure 1).…”

“…The fiducial production cross-section is measured to be 4.49 ± 0.58 fb. For 𝑍 → 𝜈𝜈 channel, a similar centrality is defined to condense the electroweak events [5] and the post-fit plot of 𝑚 𝑗 𝑗 is (c) in Figure 2. The reported observation significance of electroweak production is 5.2𝜎 (exp.…”

Study of corrections for anomalous coupling limits due to the possible background BSM contributions

“…Toy model in this work doesn't include backgrounds which are usually estimated using data-driven techniques (e.g. backgrounds from particle misidentification like e → γ, j → γ misidentification or fake missing transverse energy from jets energy mismeasurement), since they are not dominant ones [10,17,18], especially in the high-energy regions, where the EFT sensitivity is the highest.…”

“…VBS Zγ and W γ productions in pp collisions require two associated hadronic jets at the tree level. Z(ν ν)γjj and W ( ν)γjj are rare processes and were observed [10,11] only recently in the analyses of collision data, collected during full Run II of the LHC. This article uses these processes as example signals for taking into account corrections to the limits on Wilson coefficients and to show the possible gain from this kind of correction.…”

Measurements of quartic coupling and vector boson scattering in ATLAS

“…The EW 𝑝 𝑝 → 𝑍𝛾 𝑗 𝑗 process was previously observed by the CMS experiment in the 𝑍 → ℓ − ℓ + decay channel [11]. The ATLAS experiment has studied this process in the 𝑍 → 𝜈 ν decay channel [10] exploiting high signal event yield in the final state with low values of photon transverse energy 𝐸 𝛾 T < 110 GeV, that provided the opportunity to make an observation but had negligible sensitivity for the BSM searches. The reported ATLAS analysis [7] is aimed at the anomalous QGC measurement and studies the events with a photon with 𝐸 𝛾 T > 150 GeV, high 𝐸 miss T and at least two hadronic jets.…”