The ATLAS collaboration observes the electroweak production of two jets and a pair of Z bosons

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Display of a candidate signal event from channel lllljj, reconstructed by the ATLAS detector. The green lines show the path of two oppositely charged electrons. The red lines show the trajectory of two muons with opposite charges. The two jets in the fore and background region are represented by a yellow cone. Copyright CERN for the benefit of the ATLAS Collaboration. CC-BY-4.0 license, reused with permission.

The ATLAS Collaboration, the large research consortium involved in analyzing data collected by the ATLAS particle collider at CERN, recently observed the electroweak production of two Z bosons and two jets. This crucial observation, presented in Natural Physicscould greatly contribute to the understanding of Standard Model (SM) particle physics.

The MS of particle physics is a well-established theory describing the building blocks and fundamental forces of the universe. This model describes weak bosons (ie the bosons responsible for what is called the “weak force”) as mediators of the electroweak interaction.

The scattering of massive weak bosons, such as W and Z bosons, is specifically constrained to interactions, where the mediators themselves carry the charge of these interactions. This scattering, also known as vector-boson scattering (VBS), also involves a type of Feynman diagrams or known vertices (i.e. quartic gauge vertices) that physicists did not have until now. ‘now not been able to experimentally probe through other physical processes.

“The quartic gauge tops are a so far unconfirmed section of the SM, which is however of key importance for the self-consistency of the model,” Gabriela Navarro, part of the ATLAS collaboration, told “An example of this self-consistency is delicate cancellation of scattering amplitudes involving triple gauge tops, quartic gauge tops, and tops involving the Higgs boson. A study of these processes is an independent and crucial test of the mechanism BEH to break the electroweak symmetry in the SM (EWSB).”

From the start, one of the crucial aims of the ATLAS experiment at CERN has been to research and measure VBS processes. Run 1 of the Large Hadron Collider (LHD) experiment collected the first evidence of these processes occurring in two equally charged W bosons. Subsequently, the VBS was also observed during interactions between the W and Z bosons.

Distribution of the multivariate discriminant in the signal region lllljj. Error bands represent standard deviations and include all systematic uncertainties. The statistical uncertainties of the data are represented by error bars. ZZ (EW), ZZ (QCD) and ggZZ represent the contributions of the EW, non-gg QCD and gg QCD ZZjj processes respectively. All minor backgrounds are summed as “Others”. Copyright CERN for the benefit of the ATLAS Collaboration. CC-BY-4.0 license, reused with permission.

“Observing such a process in the ZZ channel is very difficult due to its small cross section and requires good modeling and control of background processes, as well as good reconstruction and calibration of physical objects by the ATLAS detector,” Navarro explained.

“Our recent paper completes the observation of all major channels involving massive electroweak gauge bosons and confirms the consistency of the experimental results with the mechanism predicted by the SM. It thus also marks the beginning of a new era in studies of precision of these rare processes in the electroweak sector.

As part of their recent work, the ATLAS collaboration specifically analyzed the proton-proton collisions recorded during phase 2 of the LHC particle collider, which ran from 2015 to 2018. The final state particles of this collider then interact with the ATLAS detector, leaving bursts or deposits of energy inside that can be measured and recorded. These stored energy deposits are then reconstructed into physical objects such as electrons, muons, jets, etc.

“We report the first observation of electroweak production of two Z bosons and two jets, the rarest process where scattering of two massive gauge bosons can occur,” Navarro said. “The observation as well as the consistency with SM predictions show that the SM of particle physics survives rigorous testing down to the rarest corner with a production cross section as small as 0.1 fb.”

The ATLAS collaboration’s observation of the electroweak production of two Z bosons and two jets could have important implications for future research. In addition to providing experimental evidence confirming the robustness of particle physics SM, this could motivate further investigations into a mechanism called electroweak symmetry breaking (EWSB) that may result from VBS.

Such investigations will most likely require new data and more advanced experimental techniques. The LHC started collecting new data in 2022, which will soon translate into new measurements from the ATLAS detector. This data could soon lead to exciting new observations and research efforts.

“On a longer timescale, the High-Luminosity LHC should deliver 3000/fb of data, or 20 times the dataset analyzed in this paper,” Navarro added. “After the era of observation, we will have more accurate measurements of VBS processes with a much larger data set, which will help to probe the EWSB with greater precision, as well as to search for any possible deviations with respect to the SM prediction. Eventually, this will allow us to verify whether the BEH-Mechanism is the one formulated in the SM or there could be other more structures.”

More information:
Gabriela Navarro et al, Observation of the electroweak production of two jets and a pair of Z bosons, Natural Physics (2023). DOI: 10.1038/s41567-022-01757-y.

Journal information:
Natural Physics

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