--- title: "The tau appeared through an electron, a muon and missing energy" description: "How could two visible particles reveal a short-lived particle that escaped direct observation?" canonical_url: "https://sandboxphysics.com/stories/particle-evidence/perl-tau-missing-energy/" language: "en" page_type: "story" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/stories/particle-evidence/perl-tau-missing-energy/" last_modified: "2026-10-24" --- [Finding a new particle](https://sandboxphysics.com/stories/particle-evidence/#discoveries) Particles · Experiments · Discovery # The tau appeared through an electron, a muon and missing energy How could two visible particles reveal a short-lived particle that escaped direct observation? Sandbox Physics · 2026-10-24 Nobel contribution: 1995-1 1. Select opposite-charge electron–muon events 2. Measure the visible energy and directions 3. Test conventional backgrounds 4. Build the tau-pair decay explanation The 1975 anomaly concerned the tau lepton. Direct observation of tau-neutrino interactions came with DONUT in 2000. ## Two tracks left energy unaccounted for In 1975, Martin Perl and colleagues reported electron–positron collisions that left an electron and a muon with opposite charges, accompanied by substantial missing energy and momentum. The events came from the SLAC–Lawrence Berkeley magnetic detector at SPEAR. They were unusually spare: no additional charged particles or photons were detected. A new heavy relative of the electron eventually explained why so much of each collision remained unseen. An electron and a muon are easy to describe as a pair but difficult to produce in that pattern by ordinary electron–positron annihilation. A muon is a heavier charged particle in the same family as the electron. Familiar annihilation reactions readily make an electron–positron pair or a muon–antimuon pair. One electron and one muon, with a large invisible remainder, required the team to examine what else might have happened. Sources: [1] ## The detector had to recognize two different kinds of particle The magnetic field measured track momentum and the sign of electric charge. Other detector layers helped distinguish particle types. Electrons deposited energy in the electromagnetic shower counters, while muons could cross enough material to reach the muon detectors. The selected tracks pointed to a collision and were separated in direction, which helped suppress unrelated particles and certain detector mistakes. Particle identification was essential because much more common reactions could imitate an electron–muon pair if one particle was misidentified. A pion might penetrate into a muon detector, or decay into a muon before reaching it. An electron could be lost or wrongly identified. The researchers estimated these possibilities using measured detector responses and control samples, then asked how many apparent mixed pairs ordinary reactions could supply. The momentum information supplied another test. In a simple two-body annihilation, the outgoing particles balance in opposite directions and carry the available energy. The selected mixed pairs were often not back-to-back, and their measured energy was well below the collision energy. Missing momentum therefore accompanied the missing energy. The original paper concluded that at least two additional particles were required in these events. Sources: [1] ## A heavy pair could decay in two different ways A consistent explanation was pair production of a new charged lepton. The electron and positron could create a positive tau and a negative tau. Each tau then decayed so rapidly that the detector saw its decay products instead of a long tau track. One tau could yield an electron and two neutrinos; the other could yield a muon and two neutrinos. The neutrinos carried away energy and momentum with very little chance of interacting in the detector. This picture explains why the visible electron and muon need not line up as a simple two-body pair. They come from separate decays, and the invisible particles share the energy in each decay. It also predicts a production threshold, distributions of visible momenta and additional decay channels. Those predictions turned a plausible explanation into a programme of measurements. The 1975 paper itself used cautious language about anomalous lepton production. The tau interpretation developed through later data, improved background studies and measurements of other decay modes. Reading the discovery as a sequence is more informative than imagining that one unusual event immediately established a complete new particle family. Sources: [1] [2] ## One interpretation had to connect several event classes A pair-decay explanation also gives a useful comparison between event classes. If both taus choose electron decays, the detector can see two electrons with invisible partners. If both choose muon decays, it can see a muon pair with missing energy. Hadronic tau decays add still other channels. The mixed electron–muon class was attractive because fewer ordinary reactions naturally supplied it. A convincing new-particle account had to connect the special class to the wider set of expected decays. Sources: [1] [2] ## The tau and the tau neutrino were separate discoveries Perl received half of the 1995 Nobel Prize for discovering the tau lepton; Frederick Reines received the other half for neutrino detection. Those contributions share a prize year but concern different experiments. The tau is charged and heavy, whereas its associated tau neutrino is neutral and interacts weakly. A decay analysis could infer invisible particles long before an experiment directly observed tau-neutrino interactions. DONUT reported that direct observation in 2000 using a neutrino beam and emulsion to identify the short tau track produced in a neutrino interaction. The distinction keeps two important achievements in their proper places. SPEAR found a new charged lepton through its decay patterns. DONUT later observed the interaction of its neutral partner. In both, a missing or very short track became meaningful because the surrounding event was measured carefully. Sources: [1] [2] [3] [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. [Perl et al., Evidence for Anomalous Lepton Production in Electron–Positron Annihilation (1975)](https://doi.org/10.1103/PhysRevLett.35.1489) Read the opposite-charge electron–muon events, missing energy and missing momentum; the first paper describes anomalous events before the full tau interpretation was established. 2. [Perl, The Tau Lepton and the Search for New Elementary Particle Physics (1998)](https://arxiv.org/abs/hep-ph/9812400) Perl’s own review separates the tau lepton, its decay channels and the then-outstanding tau-neutrino questions. 3. [DONUT Collaboration, Observation of Tau Neutrino Interactions (2000)](https://arxiv.org/abs/hep-ex/0012035) The collaboration reports four tau-neutrino candidates identified with emulsion; this is distinct from the tau-lepton discovery. ## Follow the next question [DONUT: catching a tau before it disappears](https://sandboxphysics.com/stories/neutrinos/donut-tau-neutrino/) [Water, scintillator and argon: what does each detector see?](https://sandboxphysics.com/stories/neutrinos/neutrino-detector-measurement-chains/) [Particles, detectors and the rules of matter](https://sandboxphysics.com/stories/particle-evidence/) [The lead plate that revealed a positron](https://sandboxphysics.com/stories/particle-evidence/anderson-positron-lead-plate/) [Why Yukawa expected a particle between the electron and proton](https://sandboxphysics.com/stories/particle-evidence/yukawa-meson-range/) [Finding the antiproton among thousands of pions](https://sandboxphysics.com/stories/particle-evidence/chamberlain-antiproton/) [Two experiments met at the same narrow particle peak](https://sandboxphysics.com/stories/particle-evidence/november-revolution-jpsi/)