--- title: "W and Z particles emerged from the collider debris" description: "How did UA1 and UA2 recognize heavy force carriers that decayed before leaving a track?" canonical_url: "https://sandboxphysics.com/stories/particle-evidence/ua1-ua2-w-z/" language: "en" page_type: "story" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/stories/particle-evidence/ua1-ua2-w-z/" last_modified: "2026-10-24" --- [Testing the rules of matter](https://sandboxphysics.com/stories/particle-evidence/#symmetries) Particles · Experiments · Discovery # W and Z particles emerged from the collider debris How did UA1 and UA2 recognize heavy force carriers that decayed before leaving a track? Sandbox Physics · 2026-10-24 Nobel contribution: 1984-1 1. Collide protons and antiprotons 2. Identify energetic leptons 3. Measure recoil and missing transverse energy 4. Compare W and Z decay signatures The W signature includes an invisible neutrino; a Z decaying to two charged leptons can be reconstructed from both visible daughters. ## The collider needed a beam of antimatter In 1983, the UA1 and UA2 experiments at CERN found the heavy W and Z particles predicted by electroweak theory. The carriers decayed far too quickly to fly through a detector as ordinary tracks. Their energetic daughters and the momentum they carried supplied the evidence. The discovery joined a new collision machine with detectors able to recognize a few distinctive events among abundant hadronic debris. Carlo Rubbia promoted the proton–antiproton collider programme, and Simon van der Meer’s stochastic cooling made sufficiently concentrated antiproton beams practical. Cooling here means reducing the spread of particle motion in a stored beam. Measurements of deviations and corrective electromagnetic signals gradually tightened the beam. This accelerator achievement was essential to supplying enough collisions for the search. Sources: [1] [2] ## A W left one visible lepton and an invisible partner A useful W decay produces an electron and a neutrino. The electron leaves a charged track and an electromagnetic energy deposit. The neutrino usually leaves the detector without an interaction. Looking only for an energetic electron would admit many ordinary backgrounds, so the experiments also checked that the event contained a matching imbalance of momentum across the beam. The incoming proton and antiproton travel along the beam direction. Their constituent quarks share the beam momentum unpredictably, so the longitudinal momentum of an individual collision is not known exactly. Across the beam, the initial total momentum is approximately zero. Add the measured transverse momenta of the visible products and any leftover imbalance points toward invisible momentum or an imperfect measurement. The W candidates combined an isolated energetic electron with missing transverse energy on the opposite side. Isolation means the electron stood apart from a jet of nearby hadrons. A jet can imitate an electron-like deposit, and poorly measured energy can imitate a neutrino. Requiring the two signatures together, and checking how ordinary events populated them, made the candidates persuasive. The nearly complete coverage of a collider detector was therefore part of the physics. Energy lost through an uninstrumented opening or a poorly calibrated calorimeter region can create a false transverse imbalance. The experiments checked the electron’s track against its energy deposit and examined other activity in the event. Requiring an isolated electron and a substantial opposing imbalance selected a geometrical pattern expected from a W decay, while ordinary jet events provided ways to study the background. Sources: [1] [2] ## A Z could be rebuilt from two visible daughters The Z supplied a cleaner complementary channel. When it decays into an electron and a positron, or a muon and an antimuon, both daughters can be measured. Their energies and momenta determine an invariant mass. Pairs from a common Z gather near one mass even when the parent is moving in the laboratory. In 1983 the early Z papers placed that mass in the neighbourhood of ninety to ninety-five billion electronvolts divided by the speed of light squared, with much larger uncertainty than modern measurements. The UA1 discovery paper’s title refers to a mass around ninety-five. That early estimate belongs with the early data; today’s precise number came from later experiments and calibration. The W and Z channels answered different experimental questions. The W used one charged lepton and an invisible partner, so its mass information came through transverse kinematics. The Z allowed a direct two-lepton mass reconstruction. Agreement with the electroweak pattern across both kinds of event was much stronger than a single impressive picture. Sources: [3] [1] [4] ## Two experiments and a new machine completed the search UA1 and UA2 had different detector designs and independent analyses. Their agreement helped show that the signals survived a change of apparatus. Both collaborations contributed to the discovery, supported by the accelerator teams that produced and stored the antiprotons. Rubbia and van der Meer received the 1984 Nobel Prize for their decisive contributions to the project. The observations connected the short-range weak interaction to real heavy carriers. They also established a way of searching that remains familiar: identify a clean decay signature, measure everything else needed to interpret it, and test the ordinary processes that can imitate it. A force carrier that vanishes almost immediately can still leave an unmistakable pattern among its daughters. Sources: [1] [2] [3] Continue with the physics ## Try the related models ### [Reconstructing collision events](https://sandboxphysics.com/labs/particle-collider/) Compare visible tracks, energy deposits and missing transverse momentum. A parametric event generator illustrates detector reconstruction with simulated events. ## Sources and reading notes 1. [UA1 Collaboration, Isolated Electrons with Associated Missing Energy (1983)](https://doi.org/10.1016/0370-2693(83%2991177-2) The original W search combines an energetic isolated electron with missing transverse energy. 2. [UA2 Collaboration, Single Isolated Electrons with Missing Transverse Energy (1983)](https://doi.org/10.1016/0370-2693(83%2991605-2) UA2 supplies independent detector evidence for W production in proton–antiproton collisions. 3. [UA1 Collaboration, Lepton Pairs of Invariant Mass around 95 GeV (1983)](https://doi.org/10.1016/0370-2693(83%2990188-0) The discovery paper reconstructs Z candidates from lepton pairs. Its early mass estimate should be kept separate from later precision values. 4. [ALEPH, DELPHI, L3, OPAL, SLD and working groups, Precision Electroweak Measurements on the Z Resonance (2006)](https://arxiv.org/abs/hep-ex/0509008) The combined measurements test electroweak quantum corrections with the Z mass, width and angular asymmetries. ## Follow the next question [How electromagnetism and weak decay came to share one theory](https://sandboxphysics.com/stories/particle-evidence/electroweak-unification/) [LEP: counting particles that leave no trace](https://sandboxphysics.com/stories/neutrinos/lep-light-neutrinos/) [Particles, detectors and the rules of matter](https://sandboxphysics.com/stories/particle-evidence/) [What symmetry tells us about a particle](https://sandboxphysics.com/stories/particle-evidence/wigner-symmetry-principles/) [Wu’s cobalt experiment showed a preference for one direction](https://sandboxphysics.com/stories/particle-evidence/wu-parity-experiment/) [The rare kaon decay that broke CP symmetry](https://sandboxphysics.com/stories/particle-evidence/cronin-fitch-kaon-cp/) [How quarks brought order to the particle families](https://sandboxphysics.com/stories/particle-evidence/gell-mann-quark-classification/) [Electrons found compact constituents inside the proton](https://sandboxphysics.com/stories/particle-evidence/slac-deep-inelastic-scattering/) [Why quarks look almost free in a hard collision](https://sandboxphysics.com/stories/particle-evidence/qcd-asymptotic-freedom/) [How electromagnetism and weak decay came to share one theory](https://sandboxphysics.com/stories/particle-evidence/electroweak-unification/) [How quantum corrections made electroweak theory precise](https://sandboxphysics.com/stories/particle-evidence/electroweak-quantum-corrections/) [A symmetric law can have an asymmetric ground state](https://sandboxphysics.com/stories/particle-evidence/nambu-spontaneous-symmetry/) [Why a third quark generation opened a way to CP violation](https://sandboxphysics.com/stories/particle-evidence/kobayashi-maskawa-generations/) [Two mass peaks brought the Higgs field within reach](https://sandboxphysics.com/stories/particle-evidence/higgs-discovery-mass/)