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Discoverers of the Neutrino and Tau Recognised RESEARCH I NEWS Discoverers of the Neutrino the turn of two of the six leptons of today's and Tau Recognised particle physics, the electron-neutrino ve and the T lepton, tau, to acquire 'Nobelity'. Particles that Acq ui red 'hlobei ity' K V L Sarma Frederick Reines (University of California, Irvine), now 77,with Clyde L Cowan Jr. (who The 1995 Nobel Prize in Physics is shared by died in 1974) reported the first direct evidence the American physicists Frederick J Reines of the existence of the 'neutrino' in 1956. The and Martin L Perl for their pioneering experi­ neutrino is a tiny neutral particle hypoth­ mental contributions to 'lepton' physics. Lep­ esized by Wolfgang Pauli Jr. in 1930. Twenty tons, such as the electron, do not feel the five years later and with the invention of the nuclear force but only the electromagnetic nuclear reactor it was possible to verify its and the weak beta decay force. This year it is existence. This year's Nobel award in Physics is in recognition belong. A summary of the discoveries made in the of two landmark experiments in elementary par­ world of leptons is given in the table below. The ticle physics. One provided the confirmation of the third generation has now started getting Nobel neutrino, as envisaged by Pauli more than two prizes. It should be noted that v T still needs to be decades earlier. The second discovered the heavy identified experimentally land hence the question charged lepton "t which heralded the third, and marks in the last row of the table!. For this one has perhaps the last, generation of the ult!mate con­ to demonstrate that "t'S are produced directly in stituents of matter to which the top and bottom collisions of v T with nuclear targets. Generation Lepton Discoverer(s {year}} Nobe/ Prize {year} electron e J J Thomson (1897) J J Thomson (1906) electron-neutrino ve C L Cowan eta/ (1956) F J Reines (1995) 2 muon Il J C Street, E C Stevenson C D Anderson, S H Neddermeyer 11936) muon-neutrino v 11 G Danby eta/ 11962) M Schwartz, L M Lederman J Steinberger 11988) 3 tau "t M L Perl eta/11975) M L Perl (1995) tau-neutrino v T ? ? -------------VVWV\/ ·-----------1-07 RESONANCE I February 1996 RESEARCH I NEWS ------ - -----~--- - - - . ------- --- - ---- - ---------~-- Martin Perl (Stanford University, Stanford), not fully recognize the implications of the aged 68, assisted by a 35-member team, dis­ neutrino, particularly in regard to its pen­ covered the 'tau lepton' in 1975. This particle etrating power. Initially he thought he had carries one unit of electric charge and weighs done a'frightful thing' as the neutrino was approximately twice as much as a proton. It expected to have penetrating power similar to, has a very short life, ofonly a fraction of a pico­ or about 10 times larger than, a gamma ray. second (millionth of a millionth of a second). However in 1934 Hans Bethe and Rudolf Thus tau has the distinction of being 'the Peierls argued that the neutrino had to be heaviest' and 'the shortest-lived' lepton ob­ even more elusive as its interaction mean free served. It is interesting that while Reines's neu­ path had to be astronomical in magnitude. trino results were in some sense expected, Perl's tau discovery came as a complete surprise. Detection of the Reactor Neutrino Brief History of the Neutrino The first neutrino reaction to be observed (as is customary, here we are using the word Neutrinos, they are very small. 'neutrino' in its generic sense although, strictly They have no charge and have no mass speaking, what Reines and his group detected A nd do not interact at all. were signals from the electron-antineutrino The earth is just a silly ball v) was the reaction. To them, through which they simply pass, e Like dustmaids down a drafty hall Or photons through a sheet of glass. driven by the antineutrinos from the nuclear (John Updike) reactor. This reaction is essentially the reverse The 'birth' of neutrino can be traced to a letter of the neutron decay of 4 December 1930 written by Pauli from Zurich. He mentioned that he had hit upon the neutrino as a "desperate remedy" to save, The underlying idea was to look for a pair of among other things, the principle of energy scintillator pulses; the first (prompt) pulse conservation in beta decay. The particle was due to positron annihilation and the second to possess no electric charge but carried the (delayed) one due to capture of the 'moder­ missing energy and momentum and escaped ated' neutron. The experiment was performed the detecting equipment. The famous beta around 1955-56. Projectiles came from the decay theory of Fermi appeared in 1934, reactor located at the Savannah River Plant, in wherein Fermi named the Pauli particle 'the South Carolina State, USA, and targets were neutrino', meaning the little neutral one. the protons in a solution of water mixed with cadmium chloride (Cd is a good absorber of It is interesting that even the great Pauli did thermal neutrons). The experimental appara- -- ---- ----- ------- - -------~-----~---~----~-------~-~ 108 RESONANCE I February 1996 RESEARCH I NEWS N(UTRINO / / / / / / / / / / / / "0 CAP TURE IJo/ CAOIolIVW AT TER "OO(RATIOH ® llQUIO 'CII'lTILlATIOH OtHCTO" Figure 1 Schematic diagram of the neufrlno among other things, measuring energies of deledor of the Reines-Cowan experiment the pulses, their time-delays, etc. The ob­ served signal did vary with the reactor power tus was a sandwich of target tanks (containing output, and hence with the neutrino flux. It a solution of water and CdClz) and liquid gave an average rate of 2.88 ± 0.22 countslhour, scintillation detectors. consistent with the value that is expected from the inverse beta reaction. In this way the neu­ An event meant the detection of two prompt trino was experimentally verified. coincidences (see Figure 1): the first one was between the two photons (each having 0.511 We know very little in regard to the electron­ MeV energy) of the positron annihilation, and neutrino (and even less about other neutri­ the second coincidence was due LO the capture nos). As for its rest mass, dataon the end-point of the neutron by cadmium giving a few pho­ of the beta electron spectra only show that it tons. The second pulse occurred after several cannot exceed a few eV. An important feature micro-seconds of the positron flash, because of ve is that it is left-handed: the spin of the the neutron took that much time to degrade neutrino, which has a magnitude of half a its energy to the level of thermal energies in natural unit, is observed to be aligned opposite the target water. The experiment involved, to the momentum direction. This intrinsic ~------~. -------- RESONANCE I February 1996 109 RESEARCH , NEWS property of the neutrino was discovered in 1958 charged particles each having a rest mass of in an exceedingly clever experiment performed about 2 GeV could easily be produced at SPEAR. by Goldhaber, Grodzins and Sunyar. Anomalous Events: The 'anomalous' events Experimental data using the ve's are not ex­ reported by Perl and collaborators corre­ sponded to the following reactions tensive because it is difficult to obtain ve beams. The sun is a good source of v but the e e- + e+ -> e- + ~ + + i.p., solar neutrino fluxes are not well understood and constitute one of the challenging prob­ -> e+ + W + i.p., lems of current research. Recently the first where 'i.p.' denotes invisible particles which experiment using a man-made source of pure ve left no trace in the detector. The ingenuity of was performed. It made use of an intense source the experimenters consisted in establishing of reactor-produced 51Cr (which emits ve by that the oppositely-charged e~ pair was the electron capture) to calibrate the GALLEX so­ result of separate decays of two new particles lar neutrino detector located in Italy. which were oppositely-charged and short­ Discovery of the Tau Lepton lived. At the energy Eern = 4.8GeV there were 24 events (13 e+J..c and 11 e-J..l+). Tau is a heavier electron. It entered the scene unexpectedly in 1975. While the results of The occurrence of anomalous events as a func­ tion of E exhibited an increase around 4 Reines needed the construction ofpower reac­ em tors, the discovery of the tau lepton needed GeV, indicating a threshold for producing high energy electron-positron colliders. Tau them. The events were interpreted as proceed­ lepton is the third kind of charged lepton that ing from the reaction exists in nature, the other two being the elec­ tron and the muon. (The Greek letter 1 is the first in the word triton, meaning third). where the taus decayed immediately into lighter leptons. A new conservation law called The experiment was performed at the elec­ the ~tau-lepton number' conservation is as­ tron-positron collidercalled SPEAR (Stanford sumed. According to this, in the 't - decay, a Positron Electron Accelerator Ring), where neutrino called the tau-neutrino (v t) is always beams of e- and e+ were accelerated simulta­ emitted; in the 1+ decay a tau-antineutrino neously in opposite directions in a ring and (v) is emitted. Thus the anomalous event made to intersect. As the total center-of-mass with, say, e-Il + was the result of decays energy (the sum of beam energies) 1 - - > e- +-v e + V r 1+ -> J..l + v V E em =Ee - + E e+ = 2E e- , + Il + t was tunable in the range 3-8 GeV, a pair of wherein the two neutrinos and the two anti- --------------------0~J~~~------------------- 110 RESONANCE I February 1996 RESEARCH I NEWS the Beijing Electron Positron Collider (BEPC) 1995 Nobel Laureates in Physics = in China, give mT 1777 MeV, with an error which is less than 1 MeV.
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