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CLEO — the b Quark, the Υ(4S) and the B Meson

Cornell Electron Storage Ring · 1980–1983

The b quark had been discovered in 1977, bound with its own antiquark into the Υ family. But to study the b quark itself you need it paired with a light quark — a B meson — and for that you need an Υ state heavy enough to fall apart into two of them.

CLEO, at the Cornell Electron Storage Ring, found it. A fourth Υ state appeared in electron–positron annihilation, broad rather than narrow — the signature of a resonance sitting just above the threshold to decay into a pair of B mesons. The Υ(4S) turned out to be a factory for them, and remains the workhorse of B physics: the later B-factories at SLAC and KEK were built to sit on this resonance.

How it was found

The Υ(4S) is broad and sits on a large continuum background, so it does not simply announce itself as a bump in the cross-section. What distinguishes it is the shape of the events. Off resonance, electron–positron annihilation gives a quark and an antiquark flying apart — two back-to-back jets. On the resonance, the energy instead goes into a pair of B mesons produced almost at rest, and their decay products spray out nearly isotropically. Resonance events are round; continuum events are thin.

Rohlf found the state in the CLEO data by applying a set of event-shape variables that had been invented only two years earlier: the rotationally invariant moments of the energy flow now known as Fox–Wolfram moments. The second moment is near zero for a spherical event and near one for a two-jet event, which turns event topology into a single number and separates resonance from continuum cleanly. They were the work of Geoffrey Fox — Rohlf's thesis advisor at Caltech — and Stephen Wolfram.

Fox–Wolfram moments became standard equipment in collider physics and are still in use at the LHC. Wolfram went on to write Mathematica — the language behind Rohlf's later textbooks.

With B mesons in hand, the next question was how the b quark decays. The Standard Model says it must change flavour through the weak interaction, and the favoured route is b → c — the b turning into a charm quark while emitting a virtual W. When that W becomes a lepton and a neutrino, the B decay produces a muon. CLEO measured exactly those final states, single muons and dimuons, giving early access to the semileptonic decay of b-flavoured hadrons and to the strength of the b → c coupling.

Rohlf's part in this was the particle identification: picking kaons and charmed mesons out of the decay products. Charm in the final state is the fingerprint of the transition, so identifying it is what establishes the decay sequence b → c rather than merely showing that b-flavoured hadrons decay.

Those couplings — the entries of the quark mixing matrix — are what later made B mesons the proving ground for CP violation and the matter–antimatter asymmetry of the universe.

Reconstructing the B meson

Inclusive measurements show that b-flavoured hadrons are being produced and are decaying. They do not show you the particle itself. For that you must pick out a single decay mode, catch every one of its daughters, and add their momenta back up to a mass peak — and B mesons decay to many particles, most modes with branching fractions of a fraction of a percent.

In 1983 CLEO did it, reconstructing exclusive B meson decays and putting the particle on the scale as a definite object with a measured mass. The analysis was led by others in the collaboration, built on the foundation laid by the Υ(4S) measurement and the early B decay work. Every subsequent measurement of B physics — lifetimes, mixing, CP violation — rests on being able to reconstruct the meson this way.

More

Rohlf is an author on 17 CLEO papers. Beyond the discovery of the Υ(4S), they include measurements of charged and neutral kaon production at the Υ(4S), the leptonic branching ratio of the Υ, the observation of Υ(3S) → π⁺π⁻Υ(1S), and the reconstruction of exclusive B meson decays.

All CLEO papers on INSPIRE

Citation counts from INSPIRE-HEP, August 2026.