The atlas enters under the hood of the Higgs mechanism

Detection of longitudinally polarized W-boson production in large hadron colliders is an important step in understanding how primitive electrosymmetry has broken, producing the mass of the basic particles.

In 2012, the discovery of Higgs Boson in collaboration with Atlas and CMS at CERN opened a new window into the innermost function of the universe. It revealed the existence of a mysterious and ancient field where basic particles interact to acquire a very important mass.

This process is governed by a delicate mechanism called ElectroWeak symmetry fracture, which was first proposed in 1964, but is one of the least understood phenomena of standard models of particle physics. To investigate this important mechanism in the evolution of the universe, physicists need a very large dataset of high-energy particle collisions.

Last week, at the Rencontres de Moriond Conference, the Atlas collaboration brought physicists a step closer to understanding the nature of Electroweak’s symmetry-breaking mechanisms. Using the complete proton-proton collision dataset of LHC Run 2 collected at 13 TEV energy between 2015 and 2018, the team presented the first evidence of a critical process involving W Boson, one of the mediators of weak forces.

This paper is published on the ARXIV preprint server.

In the standard model of particle physics, electromagnetic and weak interaction are two aspects of the same coin, unified with electro-woolk interaction. It is believed that when the universe was extremely hot, the electro-wooek interaction spread right after the Big Bang. However, the symmetry between the two interactions has somehow broken because carriers of weak interactions, W and Z bosons, has been observed to be large. On the other hand, photons that mediate electromagnetic interactions do not have mass.

This symmetry breakdown is achieved in the standard model through the Brout-Englert-Higgs (Beh) mechanism. The discovery of Higgs Boson provided the first experimental confirmation of this mechanism. The next step is to measure the properties of the new particles. In particular, how strongly it interacts with other basic particles. These measurements are currently underway and are intended to confirm that the mass of the basic material particles is the result of interaction with the Beh field.

However, the Behed mechanism also makes other predictions. In particular, two processes need to be measured to ensure that the mechanism is indeed as standard models predict. It is the interaction between the longitudinally polarized W or Z boson interaction and the Higgs boson interaction itself.

Higgs’ self-interaction studies are expected to be the earliest possible with high-luminosity LHC to begin manipulation in 2030, but future corridors need to be fixed in detail, but initial studies of longitudinally polarized gauge bone scattering should be possible sooner.

In the case of particles, polarization refers to the way that its spin is directed towards space. The vertically polarized particles have spins perpendicular to the direction of momentum. This is possible only with massive particles. The presence of longitudinally polarized W and Z bosons (WL and ZL) is a direct result of the BEAD mechanism, and the way these states interact with each other is a very sensitive test of how ElectroWeak symmetry breaks.

By studying this interaction, physicists can identify whether symmetry breakdown is achieved through a minimal bare mechanism, or whether new physics beyond the standard model are involved. The new Atlas results provide a first glimpse into this elusive process.

The WL–WL interaction can be investigated experimentally in proton-proton collisions by studying a process called Vector–Boson scattering (VBS). The VBS process can be visualized as a quark of each proton, where the W bosons and their two W bosons interact and release two W bosons that produce a pair of W or Z bosons. VBS can be identified by looking for collisions containing two boson decay products, forming two particles that travel in opposite directions in two quarks involved in the interaction.

The new ATLAS analysis targets collisions in which two W-bosons decay into electrons or moons and their respective neutrinos. Both leptons must be the same charge to suppress background from processes that mainly involve top-grade pair production. Thus, the experimental signature is a pair of the same charging leptons (electrons, muon-muon or electron-muon), two particles “jets” with opposite directions generated by the attenuation of Quarks, and a lack of energy from the undetectable neutrinos.

Once a candidate for the VBS process is selected, the polarization of the W-boson must be determined. This is extremely challenging and can only be done by in-depth analysis of the correlation between the reconstructed electrons and the muon orientation and the properties of other particles produced in the interaction.

Dedicated neural networks have been trained to distinguish between horizontal and longitudinal polarization, allowing the final results to be extracted. Evidence with statistical significance of 3.3 sigma that at least one of the two W-bosons is longitudinally polarized.

“This measurement is a milestone in the study of core physics values ​​via polarized boson interactions in the vector-boson scattering process,” says Yusheng Wu of Atlas Standard Model Group Converer. “We mark the path to the final study of longitudinally polarized boson scattering using LHC RUN-3 and HL-LHC data.”