A recent ATLAS paper, with major contributions from the Michigan ATLAS group, has been selected for the 2025 Physical Review Letters Collection of the Year. The paper, “Evidence for Longitudinally Polarized W Bosons in the Electroweak Production of Same-Sign W Boson Pairs in Association with Two Jets in pp Collisions at sqrt{s}=13 TeV with the ATLAS Detector,” was one of only 63 papers highlighted from about 2,500 Letters published in Physical Review Letters in 2025.

The result focuses on Vector Boson Scattering (VBS), a rare process that is important for testing the electroweak symmetry breaking mechanism and the role of the Higgs boson in the Standard Model. Without the Higgs boson—or if its couplings to vector bosons deviate from their Standard Model predictions—the probability of VBS interactions would grow uncontrollably at high energies, violating a foundational principle of quantum mechanics known as unitarity. Precise measurements of VBS at CERN’s Large Hadron Collider (LHC) are therefore essential for testing the Standard Model and probing possible signs of new physics. Experimentally, these measurements are extremely challenging because the signal occurs only at the femtobarn level. Even though proton beams collide 40 million times per second—and the LHC delivered 16 years of proton-proton collision data—only a few hundred VBS events are expected in the full dataset.

Professor Junjie Zhu, together with Professor Bing Zhou, former postdoc Jake Searcy, and graduate student Lulu Liu, initiated the first same-sign WW VBS study back in 2012, shortly after the discovery of a Higgs-like boson by ATLAS and CMS. The work led to the first evidence for electroweak same-sign WW production, published in Physical Review Letters as a PRL Editor’s Suggestion in 2014. Liu’s thesis documented this analysis and is available at the University of Michigan’s Deep Blue Documents. This result also provided the first evidence for any VBS process at the LHC. Zhu and Zhou continued this program with Searcy, graduate student Wenhao Xu, and collaborators from other institutions, leading to the first observation of this rare process in 2019.

Building on this foundation, Professor Zhu and postdoctoral fellow Tairan Xu, together with collaborators from other ATLAS groups, analyzed the full Run 2 dataset collected by the ATLAS detector from 2015 to 2018. Their analysis focused on the polarization of the two W bosons, a property describing the orientation of a particle’s spin relative to its direction of motion. For massive particles such as the W boson, the longitudinal polarization state is critical because it is directly linked to electroweak symmetry breaking and the Higgs mechanism. However, events in which both W bosons are longitudinally polarized account for only about 10% of the already rare VBS signal, making the measurement especially challenging. To isolate these signal events from the dominant backgrounds in which both bosons are transversely polarized, the team developed sophisticated multivariate techniques, as highlighted in the American Physical Society Viewpoints article "Probing the Higgs Mechanism with Particle Collisions and AI." This analysis found the first evidence for electroweak production of same-sign W bosons in which at least one W boson is longitudinally polarized, achieving an observed significance of 3.3 standard deviations. The study also set the most stringent constraint to date on the production of two longitudinally polarized W bosons, establishing a 95 percent confidence level upper limit of 0.45 femtobarns on the longitudinal-longitudinal production cross section. Xu served as one of the two ATLAS paper editors for this measurement.

This recognition reflects more than a decade of work by the Michigan ATLAS team and our collaborators. Each advance—from the first evidence of the same-sign WW VBS process in 2014 to this new polarization measurement in 2025—has brought the team closer to directly testing the electroweak symmetry-breaking mechanism.

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Professor Junjie Zhu
Professor Bing Zhou
Former Postdoctoral Fellow Jake Searcy
Former Graduate Student Lulu Liu
Postdoctoral Fellow Tairan Xu

2025 Physical Review Letters Collection of the Year