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UA1 — Discovery of the W and Z Bosons

CERN proton–antiproton collider · 1983–1991

Electromagnetism is carried by a massless photon of unlimited range. The weak force, which turns one flavour of quark into another and makes the Sun burn, reaches barely a thousandth of the width of a proton. Unification said both forces come from the same origin, and that the weak force is short-ranged only because its carriers are enormously heavy — roughly ninety times the mass of a proton. Everything turned on whether those particles existed.

To make them, CERN converted its Super Proton Synchrotron into a proton–antiproton collider. UA1 was the detector built to watch. In 1983 it found both: first the W, in events where an energetic electron flew out alone and a matching amount of momentum went missing with the unseen neutrino; then the Z, cleaner still — a pair of leptons whose combined mass came out near 95 GeV, far above anything then known.

Rohlf's role was in the identification of those first events, and then in the measurements of mass and spin that followed.

The electroweak theory was confirmed, and confirmed at the mass it had predicted. Carlo Rubbia and Simon van der Meer received the Nobel Prize in Physics the following year, in 1984, for the work that made the discovery possible.

Mass and spin

A discovery announces that something exists. Establishing what it is takes the rest of the decade. UA1 measured the masses of the W and Z to steadily improving precision — the numbers that fix the electroweak mixing angle and, through radiative corrections, constrain the mass of the then-unseen top quark and Higgs boson.

The spin was harder, and getting at it took a trick.

A W decaying to an electron and a neutrino tells you its spin through the angle at which the electron comes out in the W rest frame — but to work in that frame you must know the neutrino's momentum, and the neutrino is invisible. The missing transverse energy gives you two of its three components. For the third, you can require that the electron and neutrino together have the W mass, which yields a quadratic equation and therefore two solutions for the neutrino's longitudinal momentum. A twofold ambiguity in every event would ordinarily smear the angular distribution away.

Working with Lev Okun, Rohlf found that in practice the ambiguity usually resolves itself. In a large fraction of events one of the two roots is kinematically forbidden, and in most of the rest the two roots lie so close together that the reconstructed decay angle is the same either way. The W rest frame could therefore be reconstructed event by event after all — and the resulting angular distribution showed the electron emitted preferentially in one direction, the signature of a spin-1 boson coupling only to left-handed particles. Both the spin of the W and the V−A structure of the weak interaction were confirmed at the scale of the boson itself.

Rohlf reported the measurement at the 12th International Conference on High-Energy Accelerators, held at Fermilab in August 1983.

Rutherford scattering, with quarks

Rutherford fired alpha particles at gold foil and read the structure of the atom off the angular distribution of what came back. The same argument works one level down. When two quarks scatter hard enough to make a pair of high-mass jets, the distribution of scattering angles is fixed by what is being exchanged and by what is doing the scattering.

UA1 measured it. The jet pairs came out following the Rutherford form, 1/sin⁴(θ*/2) — and that distribution follows only if three things hold at once: the force between quarks goes as 1/r², the exchanged gluon has spin 1, and the quarks are point-like. One measurement, three fundamentals of QCD.

The last of those is also a limit rather than a fact. Quarks with internal structure would show up as an excess of jet pairs at wide angle and high mass, so the measurement sets a lower bound on the energy scale of any quark substructure — the beginning of a search that has continued at every collider since.

Rohlf later repeated the measurement at the LHC, where CMS made the same angular distribution at 7 TeV — twenty-five years later and roughly twelve times the collision energy. See jet angular distributions.

Hardware

The UA1 upgrades were designed to be read out through FASTBUS, the crate standard built for the large experiments of that era. VME was the smaller, cheaper bus living alongside it, with a growing supply of processors and memory boards that FASTBUS crates could not talk to. An interface between the two let the experiment use both — FASTBUS where the detector demanded it, VME where commodity hardware would do.

It is the earliest of Rohlf's electronics papers, and the start of a line of readout and trigger hardware that runs through the Data Concentrator Card, the AMC13 and Apollo at CMS — the same problem, restated by each new generation of collider.

Beyond the discovery

UA1 ran for most of a decade afterwards, and Rohlf is an author on 58 of its papers. The collider turned out to be a general-purpose laboratory: the fragmentation of quark and gluon jets, production of beauty quarks in hadron collisions, high transverse momentum J/ψ, searches for new heavy quarks, and events with large missing transverse energy — the earliest of the searches for supersymmetry that hadron colliders have pursued ever since.

In 1991 the experiment measured B⁰–B̄⁰ mixing, connecting the collider programme back to the b quark physics Rohlf had worked on a decade earlier at CLEO.

All UA1 papers on INSPIRE

Citation counts from INSPIRE-HEP, August 2026.