Recent measurements from the ATLAS experiment of Multi-Boson production processes at the LHC

TL;DR

Four ATLAS measurements of multi-boson production using $\,\sqrt{s}=13$ TeV$ and $L=140$ fb$^{-1}$ test the non-Abelian electroweak sector and Higgs couplings. The analyses span electroweak diboson production with jets, polarization of same-sign $W$ bosons, and triboson final states $WWγ$ and $VVZ$, with results interpreted in a dimension-8 EFT framework to constrain anomalous quartic gauge couplings. All cross sections align with SM predictions within uncertainties, with significances reaching up to $7.4\sigma$ for EW diboson processes and $6.4\sigma$ for the full $VVZ$ channel. The measurements tighten bounds on new physics in gauge-boson self-interactions and demonstrate the LHC's continued sensitivity to high-dimension EFT operators.

Abstract

The high-energy proton-proton collisions at the Large Hadron Collider provide the ideal conditions to study the rare processes predicted by the Standard Model (SM) such as the production of multiple electroweak bosons. These processes involve the self-interactions of the gauge bosons through triple and quartic gauge couplings (TGCs and QGCs), in addition to interactions with the Higgs boson. Therefore, precision measurements of multi-boson final states probe the electroweak symmetry breaking mechanism and allow to search for deviations from the SM. Four recently published ATLAS results are summarised in this document. The first two results are measurements of the production of di-boson final states in association with jets: the observation of $WW/WZ/ZZ$ production in semileptonic final states is described, and a measurement of the polarisation states of same-sign $W$ pairs is also discussed. The second set of measurements are related to the triple production of electroweak bosons: the first analysis reports the observation of $WWγ$ in the leptonic final state, while the second result presents the observation of triple vector boson production where at least one of the produced bosons is a $Z$ boson. All the measurements use datasets collected at a centre-of-mass energy of 13~TeV and corresponding to an integrated luminosity of 140~fb$^{-1}$.

Figures (4)

• Figure 1: Signal stregth extracted from the combined fit, and from the independent fits using the channels with different lepton multiplicities and different jet reconstruction regimes. Both systematic and statistical uncertainties are displayed. Taken from STDM-2018-27.
• Figure 2: DNN scores of the algorithm trained to separate $W^{\pm}_{L}W^{\pm}$ from $W^{\pm}_{T}W^{\pm}_{T}$. Case (a), (b), (c) show events that have been classified by the inclusive DNN in bins with increasing background rejection power. Taken from STDM-2019-26.
• Figure 3: BDT score in the $WW\gamma$ signal region after applying all the normalisation factors extracted from the LH fit for the different processes. Taken from abcd.
• Figure 4: Scores of the BDT algorithm trained to separate $VVZ$ from the background contributions in the 4$l$ channel. (a) shows a sample where the two leptons from the $W$ decays have different flavours (DF) and same flavour (SF) for (b) and (c). The inZ (outZ) label refers to an event selection where the two $W$-leptons are required to have an invariant mass in (outside) a 20 GeV window from the $Z$ boson mass. Taken from STDM-2020-08.