ATLAS Explores the Inner Workings of the Higgs Mechanism

Display of a candidate event for the production of two W+ bosons via vector-boson scattering, followed by their decay into two muons and two muon neutrinos. The muons are represented by the red lines in the inner detector and the muon spectrometer, and the two jets by the yellow cones. The direction of the missing transverse energy associated with the two neutrinos is indicated by the dashed grey line. (Credit: ATLAS/CERN)

The 2012 discovery of the Higgs boson by the ATLAS and CMS experiments at CERN marked a breakthrough in our understanding of the Universe’s fundamental structure. It confirmed the presence of an ancient, elusive field that gives elementary particles their mass through interactions governed by a subtle process known as electroweak symmetry breaking. Although theorized in 1964, this process remains one of the least comprehended elements of the Standard Model of particle physics. Unlocking its secrets requires analyzing a vast amount of high-energy collision data.

At the recent Rencontres de Moriond conference, the ATLAS collaboration presented significant progress in exploring this phenomenon. By analyzing the complete dataset from the LHC’s Run 2 (2015–2018) at 13 TeV collision energy, researchers provided the first evidence of a vital interaction involving the W boson, a key carrier of the weak nuclear force.

According to the Standard Model, the electromagnetic and weak forces are unified as a single electroweak interaction. This symmetry is believed to have existed in the extremely hot moments following the Big Bang. However, as the Universe cooled, the symmetry broke, resulting in the W and Z bosons gaining mass, while the photon remained massless. This symmetry-breaking is explained by the Brout-Englert-Higgs (BEH) mechanism. The discovery of the Higgs boson offered the first proof of this theory.

Now, the focus has shifted to examining the Higgs boson’s properties, especially how it couples with other elementary particles. Ongoing experiments aim to confirm that particle masses truly arise from their interactions with the BEH field.

The Brout-Englert-Higgs (BEH) mechanism also makes additional key predictions that need experimental verification. Two important processes must be measured to confirm whether the mechanism aligns with the Standard Model’s expectations: the interaction between longitudinally polarized W or Z bosons, and the Higgs boson’s self-interaction. While detailed studies of the Higgs self-interaction will likely only be feasible with the High-Luminosity LHC, set to start in 2030, and may require future colliders for precision, initial investigations into the scattering of longitudinally polarized gauge bosons can be conducted sooner.

Polarization refers to the orientation of a particle’s spin relative to its motion. Longitudinal polarization means the particle’s spin is aligned perpendicular to its direction of travel, which occurs only in particles that have mass. The presence of longitudinally polarized W and Z bosons (denoted WL and ZL) arises directly from the BEH mechanism.

How these bosons interact provides a highly sensitive probe of the electroweak symmetry breaking process. Studying these interactions could reveal whether the minimal BEH mechanism fully explains symmetry breaking or if new physics beyond the Standard Model is involved. The recent ATLAS results offer an initial insight into this rare phenomenon.

Experimentally, the interaction between WL bosons can be examined in proton-proton collisions through a process called vector-boson scattering (VBS). In VBS, a quark from each incoming proton emits a W boson, and these W bosons then interact, producing a pair of W or Z bosons. This process can be identified by detecting the decay products of the two bosons alongside two jets of particles created by the original quarks, which travel in opposite directions.

The latest ATLAS study focuses on collisions where two W bosons decay into either an electron or a muon along with their corresponding neutrinos. To reduce background noise, mainly from processes involving top-quark pair production, both leptons must have the same electric charge. This results in an experimental signature consisting of a pair of same-charge leptons (electron-electron, muon-muon, or electron-muon), two particle “jets” moving in opposite directions from quark decays, and missing energy due to the undetected neutrinos.

After identifying potential candidates for the vector-boson scattering (VBS) process, the next step is to determine the polarization of the W bosons, which is a highly challenging task. This requires a detailed analysis of the relationships between the directions of the detected electrons and muons and other particles produced during the interaction.

Specially trained neural networks help distinguish between transverse and longitudinal polarizations, enabling researchers to extract the key finding: evidence with a statistical significance of 3.3 sigma that at least one of the W bosons was longitudinally polarized.

“This measurement represents a major achievement in studying fundamental physics through polarized boson interactions in vector-boson scattering,” said Yusheng Wu, convener of the ATLAS Standard Model group. “It paves the way for further investigations of longitudinally polarized boson scattering using data from LHC Run-3 and the High-Luminosity LHC.”