Observation of Electroweak Production of Two Jets and a Pair of Z-Bosons by the ATLAS Collaboration.

April 13, 2023 feature

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by Ingrid Fadelli, Phys.org

The ATLAS collaboration at CERN, a large research consortium analyzing data collected by the ATLAS particle collider, has recently observed the electroweak production of two Z bosons and two jets. This crucial observation, presented in Nature Physics, could aid the understanding of standard model (SM) particle physics.

The SM of particle physics is a well-established theory describing the building blocks and fundamental forces in the universe. This model describes weak bosons, which are responsible for the so-called "weak force," as mediators of the electroweak interaction.

The scattering of massive weak bosons, like W and Z bosons, is constrained specifically to interactions, where the mediators themselves carry the charge of these interactions. This scattering, also known as vector-boson scattering (VBS), involves a type of Feynman diagrams or vertices known as quartic gauge vertices that physicists have been unable to experimentally probe through other physical processes.

"Quartic gauge vertices are a so-far unconfirmed section of the SM, which is, however, of central importance to the self-consistency of the model," said Gabriela Navarro, part of the ATLAS collaboration. "A study of these processes is an independent and crucial test of the BEH-Mechanism for breaking the electroweak symmetry in the SM (EWSB)."

From the beginning, one of the crucial goals of the ATLAS Experiment has been to search for and measure VBS processes. Run 1 of the Large Hadron Collider (LHD) experiment gathered the first evidence of these processes occurring in two W bosons with the same charge. Subsequently, VBS was also observed during interactions between W and Z bosons.

"The observation of such process in ZZ channel is very challenging due to its small cross-section and requires good modeling and control of the background processes, as well as good reconstruction and calibration of the physics objects by the ATLAS detector," Navarro explained.

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

"We report the first observation of the electroweak production of two Z bosons and two jets, the rarest process where the 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 stringent tests 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 further research. In addition to providing experimental evidence confirming the robustness the SM of particle physics, it could motivate further investigations of a mechanism called electroweak symmetry breaking (EWSB) that can 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 result in new ATLAS detector measurements. This data could soon lead to new exciting observations and research efforts.

'At a longer time scale, the high-luminosity LHC is expected to deliver 3000/fb of data, which will be 20 times of the dataset analyzed in this paper,' Navarro added. 'After the observation era, we will have more precise measurements of the VBS processes with much larger dataset, which will help to probe the EWSB with higher precision, as well as look for any possible deviation from the SM prediction. Eventually, this will allow us to check if the BEH-Mechanism is the one formulated in the SM or there might be other more structures.'

Journal information: Nature Physics

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