physics-subatomic-particles

was 285 MeV . In fact, due to Fermi motion in the target, a beam of only about 180 MeVneeds to be directed at a stationary target to produce pions . In 1948 the firs tartificial pions were obtained by M .Cardner and J .Lattes using the new 184" cyclotronat Berkeley . The momenta of the resultant pions, muons, and protons were analysedin a magnetic field, and their range was measured using a stack of nuclear emulsions .The charged pion was found to have a mass of about 139 .6 MeV, and the muon of abou t105 .6 MeV . The muon mass was then found even more precisely by measurement of th efrequencies of x-rays produced by muonic atoms, and this yielded a mass of about105 .66 MeV for the muon .It was found that the energy of the muon produced in pion decay was always 4 .1 MeV ,indicating that there was only one other particle involved in the decay. Accordingto the law of charge conservation, this particle must have a neutral charge, and ,knowing the mass of the pion, it was estimated to have zero mass . There are only twoparticles which have zero mass known at present : the photon and the neutrino (se echapter 4) . If the particle were a photon, it should have been able to produc evirtual electron-positron pairs, but O'Cealleigh showed in 1950 that it did not, andso the particle was inferred to be a neutrino . The lifetime of the charged pion wa sestimated by stopping a pion in a scintillator, and then measuring the time until theflash from the decay muon was observed . This yielded a value of 2 .56 x lU s s . By asimilar method, the lifetime of the muon was found to be about 2 .203 x l0 6 s . Theelectron in muon decay was found to have a whole range of kinetic energies, and so i twas inferred that at least two other particles must take part in the decay, and mus tboth be massless . Conservation laws indicate that these two particles must be a muneutrinoand an anti-s-neutrino .But the charge independence of the force also demanded a neutral pion . It wasconsistent with the conservation laws for this particle to decay into two gamma rays ,thus indicating a short lifetime in the order of 10 ''s . In 1949 various physicistssuggested that neutral pions might be responsible for the soft component of cosmi cradiation . In 1950 Bjorkland, Crandell, Moyer, and York showed that it was impossibl eto attribute the gamma rays in cosmic radiation to any form of nuclear excitation .In the same year Carlson, Hooper, and king measured the angles and energies of photon sdetected in emulsions at a height of 2.1 km, and found these to be consistent withproduction from a neutral pion of mass about 150 MeT . Their experiments also put th eupper limit of 5 x 1T'4 s on the lifetime of the neutral pion . It was thought best t ofind the more accurate mass of the z ° by finding the mass difference between itselfand then By considering the reactions n~p s n,T and n"p ---b n y , the mas sdifference mn-- m,,t .was found to be about 5 .4 MeV . The first accurate measurements of'n° lifetime were made by studying the decay 'n°--> y e* e; and measuring distances ina track-forming detector . However, the extreme smallness of the times concerned meantthat this method was not very accurate . The best estimate to date for the n° lifetimewas obtained in 1965 by Bellettini et al., making use of the Primakoff effect . Thi sis the photoproduction of a r‘' by the encounter of a gamma ray with a virtual gamm aray in the Coulomb field of a nucleus, which should take the same time as the decayof a Il e into two gamma rays . Using this method, the best estimate obtained for thert° lifetime was 7 .3 x 10`" s .