Around the Laboratories The decay of a W boson into a tau lepton and its neutrino, as 'seen ' in the UA 1 detector at CERN's proton-antiproton collider. The tau is identified through its decay into a narrow 'jet' of hadrons (top), while the neutrino is picked up through 'missing energy'—an energy imbalance around the detector, showing that something has passed through otherwise undetected (bottom). CERN Tau neutrinos Physicists like to think there are just three kinds of electrically charged leptons—particles inter­ acting through the weak force of nuclear beta decay. Each of these three charged leptons—the elec­ tron, the muon and the tau, is as­ sociated with its own kind of neu­ trino (also a lepton, but devoid of electric charge). The electron and the muon have been known and loved for a long time—the electron for almost a century and muon phenomena for some fifty years (although the muon was not finally identified until the 1940s). The newcomer, the tau, turned up at the big Stan­ ford linac in the middle 1970s. A pattern of three charged lep­ tons and their corresponding neu­ trinos fits nicely into the current physics jigsaw, but it is difficult to explain why the muon is some two hundred times heavier than its elec­ tron cousin and the tau (1784 MeV) is seventeen times heavier than the muon. Putting aside the question of the seemingly unrelated charged lepton actions showed that the underlying masses, a long cherished idea is strength is indeed the same. To Enter missing energy that the weak force feels the same probe the tau sector was more for all leptons, regardless of mass difficult, but recent measurements One of the major aims of the or type—so-called 'weak univer­ of the lifetime of the tau by exper­ UA1 experiment at the CERN pro­ sality'. iments at electron-positron collid­ ton-antiproton Collider was to de­ This says that the beta disinte­ ers (PEP at Stanford, CESR at Cor­ tect the long-awaited W particle, gration of a nuclear neutron into nell and PETRA at Hamburg) gave the electrically charged carrier of a proton—releasing an electron an indication of the strength of the the weak force, identifying it by and a neutrino—should have the tau coupling, showing that it was its decays into an electron or same strength as a decay produc­ in line with that of the electron and a muon plus the appropriate ing a muon and its neutrino, and the muon. neutrino. again as the breakup of a more However the kinematic range This was accomplished in 1983 exotic particle emitting a tau and covered by these tau decays is using the 'missing energy' tech­ its neutrino. necessarily limited, and certainly nique to detect the otherwise invi­ Over the years, many compari­ the tau neutrino played no direct sible neutrinos—the energy re­ sons of electron and muon inter­ part in the proceedings. leased sideways in a proton-anti- CERN Courier, April 1987 9 proton annihilation is carefully heavier than the tau I). A few addi­ ened up from future colliding beam measured all around the detector. tional types of neutrinos cannot experiments. Any imbalance shows that some­ be excluded, but this will be tight­ thing has flown through unde­ tected. With the W in the bag, the UA1 team capitalized on the power of Second-class the missing energy method to look for other interesting physics. If the current? W had been seen through its elec­ All the measured properties of in the (isospin) space describing tron and muon decays, why not the tau particle discovered ten protons and neutrons as differ­ the tau decay as well? years ago at the Stanford Linear ent projections of a nucléon. While electrons and muons can Accelerator Center (SLAC) un­ The observed tau decay up­ be picked up and identified directly, derline its candidature as the sets G parity. One possibility is the highly unstable taus are another third generation lepton (after a so-called 'second-class cur­ matter. UA1 looked for taus the electron and the muon). rent', a kind of weak interaction through their decay into a narrow However because it is so hea­ with unusual G parity properties. 'jet' of hadrons, plus the elusive vy (1784 MeV), it can decay in These were postulated by Stev­ tau neutrino's missing energy. many different ways, and there en Weinberg almost thirty years Using the vast amount of data has been trouble in fitting to­ ago to supplement the conven­ collected during three years of gether all the tau decays. tional weak interactions seen 7 careful work (some 10 events on Using data collected over five in beta decay, etc., but conclu­ 10 tape from 10 proton-antiproton years, the HRS collaboration sive evidence for them has nev­ annihilations), 56 events were fil­ (Argonne / Indiana / Michigan er been found. What is more, tered off after painstaking elimina­ / Purdue) at the PEP electron- the elegant framework of the tion of electron and muon events positron collider at Stanford electroweak picture, unifying and of unwanted background. looked for production of the weak and electromagnetic inter­ These were scrutinized to deter­ neutral eta meson (identified actions and supremely success­ mine whether the hadron jets had through its decay into a pair of ful so far, does not have much the properties of those expected photons) in tau decays. (Etas room for these additional from taus, and a final sample of from tau decay were also found currents. 29 tau decays isolated. A kinema- through their disintegration into As the HRS physicists con­ tical analysis showed that the three pions.) clude in their paper 'it is impor­ strength of the tau decays is in­ After careful analysis of back­ tant that the results be con­ deed the same (to within a few grounds, examples are found firmed or denied by other exper­ percent) as that of electron inter­ of the decay of a (positively iments on tau decay'. actions. Comparing this with the charged) tau into a (positive) They see signs of another tau decay rate of Ws into muons seen pion, an eta and an antineutrino. decay with a neutral pion pro­ in the same detector, the tau/muon This helps to clear up the tau duced in addition to the charged strength is also equal. decay incompatibilities, but the pion and the eta. This does not As well as confirming the idea quantum numbers of this decay violate any rules, but only a of weak universality, the identifi­ are unconventional, violating a small portion of the observed cation of tau neutrinos through the pattern seen in all other weak eta signal can be ascribed to missing energy technique is the interactions. this process. This decay mode first direct evidence for these One useful notion in particle is also seen by other experi­ particles. interactions is 'G parity'— a ments studying the disintegra­ To make the picture even tidier, combination of charge conjuga­ tion of the tau at electron-posi­ the same UA1 data says that if tion (interchange of particles tron colliders. there is a fourth lepton beyond the and antiparticles) and invariance tau, it has to be heavier than 41 GeV (more than 400 times 10 CERN Courier, April 1987 .