---
title: "Two experiments met at the same narrow particle peak"
description: "Why did the J and psi observations in 1974 change particle physics so quickly?"
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language: "en"
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last_modified: "2026-10-24"
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Particles · Experiments · Discovery

# Two experiments met at the same narrow particle peak

Why did the J and psi observations in 1974 change particle physics so quickly?

Sandbox Physics · 2026-10-24

Nobel contribution: 1976-1

1. Measure electron pairs at Brookhaven
2. Scan collision energy at SPEAR
3. Find matching narrow peaks
4. Identify a charm–anticharm bound state

The two experiments used different production and measurement methods; their agreement made the new state difficult to dismiss.

## Two routes to a mass near three billion electronvolts

In November 1974, two teams found an unexpectedly sharp signal near a mass of 3.1 billion electronvolts divided by the speed of light squared. Samuel Ting’s group at Brookhaven called the particle J. Burton Richter’s group working with the SPEAR collider at SLAC called it psi. The combined name J/psi preserves the independence of the discoveries. Their result soon became the centre of the November Revolution in particle physics.

The surprise was the sharpness. Many strongly interacting particles discovered in earlier decades decayed so rapidly that their mass distributions spread across a broad interval. Here, a very narrow feature rose out of the surrounding events. The new state survived much longer than that usual strong-interaction timescale. Its mass and unusual lifetime demanded an explanation together.

Sources: [1] [2]

## Brookhaven reconstructed a parent from its daughters

At Brookhaven’s alternating-gradient synchrotron, the team sent a proton beam into a target and searched the products for electron–positron pairs. Two arms of a magnetic spectrometer measured the outgoing particles. Each arm determined charge and momentum, with additional detectors identifying electrons among much more common hadrons. The geometry and momentum resolution were designed for a precise comparison of many pairs.

The parent had already decayed before it could be photographed as a track. Its daughters still carried its energy and momentum. Combine those quantities and one obtains the invariant mass: the mass of the pair considered in its own rest frame. A moving parent can distribute very different energies between its daughters in the laboratory, yet correctly paired daughters reconstruct the same parent mass.

Most unrelated or nonresonant electron pairs form a broad background. In the Brookhaven data, a concentrated excess appeared at one mass. The width seen by the spectrometer was limited by experimental resolution, so the natural width was even smaller than the instrument could resolve. Repeating the measurement under different conditions helped distinguish a physical peak from an instrumental accident.

Sources: [1]

## SPEAR tuned the energy needed to make it directly

SPEAR used a different route. Electron and positron beams collided head-on, and the team varied their total collision energy. If that energy matched the mass energy of a particle that the beams could create directly, the production rate could rise dramatically. The detector counted hadronic events and lepton pairs as the energy moved through the interesting interval.

Near 3.1 billion electronvolts, the production rate rose and fell over an unusually small change in beam energy. This was a resonance, a pronounced enhancement when the incoming energy matches a physical state. The collider scan located the same state that Brookhaven had reconstructed from outgoing pairs. One experiment measured daughters after a target collision; the other tuned the energy of a clean initial pair.

That agreement mattered as much as either peak. The apparatus, backgrounds and production mechanisms differed. An error peculiar to one spectrometer would have had to reproduce a sharp collider-energy feature at the same mass. Independent methods gave the new particle an unusually firm experimental footing.

Sources: [2] [1]

## A narrow peak meant a long-lived state

A short-lived unstable state has a broad natural spread in reconstructed mass energy; a longer-lived state has a narrower spread. This connection links the decay rate to the resonance width. It let the teams recognize that the J/psi’s behaviour was exceptional before a stopwatch could possibly resolve such a short lifetime.

The measured width includes the instrument. Beam-energy spread broadens a collider scan, and imperfect momentum measurements broaden a reconstructed pair mass. To infer a natural width, the physical line shape must be combined with those known responses. A peak as narrow as the apparatus can measure gives an upper limit on the natural width. Thus the first observation already posed a precise theoretical puzzle: a heavy hadron with an unusually suppressed decay rate.

Sources: [1] [2]

## Charm supplied the missing interpretation

The J/psi is understood as a bound state of a charm quark and an anticharm quark, often called charmonium. Charm had theoretical motivations before the discovery. The narrow state gave physicists a new way to test that idea, followed by more charmonium levels and particles carrying open charm. Its unusually small decay width reflects which strong-decay routes are available and which are suppressed.

The 1976 Nobel Prize recognized Richter and Ting for the discovery of the new heavy particle. The underlying measurements belonged to large experimental teams. The lasting experimental lesson is visible in either paper: a mass peak becomes convincing when its construction is clear, its backgrounds are tested and an independent method reaches the same answer.

Sources: [1] [2]

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. [Aubert et al., Experimental Observation of a Heavy Particle J (1974)](https://doi.org/10.1103/PhysRevLett.33.1404)

   The Brookhaven pair spectrometer found a narrow peak in the electron–positron invariant-mass distribution.

2. [Augustin et al., Discovery of a Narrow Resonance in Electron–Positron Annihilation (1974)](https://doi.org/10.1103/PhysRevLett.33.1406)

   The independent SPEAR energy scan measured a sharp rise in production near the same mass.

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