The Discovery of Quarks

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Just twenty years ago physicists were beginning to realize that the. protons and neutrons at the heart of the atomic nucleus are not elementary. particles after all Instead they appeared to be composed of curious pointlike. objects called quarks a name borrowed from a line in James Joyce s novel. Finne uns Wake First proposed in 1964 by Gell Mann 1 and Zweig 2 these. p fticles had to have electrical charges equal to l 3 or 2 3 that of an electron. or proton Extensive searches for particles with such fractional charge were. made during the rest of the decade in ordinary matter in cosmic rays and at. high energy accelerators all without success 3 But surprise results from a. series of electron scattering experiments performed from 1967 through 1973. by a collaboration of scientists from the Massachusetts Institute of Technology. MIT and the Stanford Linear Accelerator Center SLAC began to give direct. evidence for the existence of quarks as real physical entities 4 For their. crucial contributions as leaders of these experiments which fundamentally. altered physicists conception of matter Jerome Friedman and Henry Kendall. of MIT and Richard Taylor of SLAC were awarded the 1990 Nobel prize in. The Prediction of Quarks, By the beginning of the 196Os physicists had shown that protons and. neutrons known collectively as nucleons had a finite size of about lo l3. cm as indicated by elastic electron nucleon scattering experiments of. Hofsiadter and his Stanford coworkers 5 but the great majority considered. these particles to be soft objects with only a diffuse internal structure Along. with pions kaons and a host of other hadrons particles that feel the effects. of the strong nuclear force they were thought by many to be all equally. fundamental composed of one another in what had been dubbed the. bootstrap model of strongly interacting particles 6 Theories that tried to. explain the growing variety of hadrons as combinations of a small set of. fundamental entities were a definite minority until the MIT SLAC. experiments occurred, In 1961 Gell Mann and Ne eman introduced a scheme known as SU3. symmetry 7 that allowed them to impose a measure of order on the. burgeoning zoo of hadrons In this scheme particles with the same spin are. grouped together as if they are just the various distinct states of one and the. same entity similar to the way the proton and neutron can be regarded as. merely two different states of the nucleon Particles with spin 0 like the pions. and kaons form a group of eight mesons called an octet as do another. group with spin l that is whose internal angular momentum is equal to. Plan s constant h divided by 27 the proton and neutron are the lightest. members an octet of baryons with spin l 2 and there is a group of ten spin. 3 2 baryons known as a decimet In effect Gell Mann and Ne eman did for. physics what Mendeleev had done for chemistry invent a periodic table of. the hadrons Using this approach they even predicted new particles that were. later discovered with appropriate properties buttressing the faith of the. physics community in SU3 symmetry as a correct representation of physical. In seeking a deeper explanation for the regularities of the SU3. classification scheme Gell Mann and Zweig invented quarks 1 2 In this. approach there are three fundamental quarks dubbed up or u down or d. and strange or s and their antiparticles the antiquarks Mesons are built. from a quark plus an antiquark while baryons are composed of three quarks. The proton is a combination of two up quarks plus a down quark written. uud for example while the neutron is made of an up quark plus two downs. udd By assigning a charge to the up quark of 2 3e where e is the charge. on the electron and 1 3e to the other two the charges on all the known. mesons and baryons came out correctly But the idea of fractional charges was. fairly repulsive to physicists of the day in his original paper Gell Mann even. wrote that a search for stable quarks of charge l 3 or 2 3 at the highest. energy accelerators would help to reassure us of the nonexistence of real. quarks After several years of fruitless searches 3 most particle physicists. agreed that although quarks might be useful mathematical constructs they. had no innate physical reality as objects of experience. The First MIT SLAC Experiments, The first electron proton scattering experiment at SLAC in which. electrons with energies up to 20 GeV 1 GeV equals 1 billion electron volts. recoiled elastically from the proton that is without breaking it up gave no. evidence for quark substructure 8 The cross section or probability for this. process continued to plummet approximately as the 12th power of the. invariant momentum transfer from electron to proton much as had been. observed earlier in the decade at lower energies This behavior was generally. interpreted as evidence for a soft proton lacking any core it was commonly. thought that the existence of such a core would have slowed the rate at which. the cross section decreased, In the next experiment performed in late 1967 by the MIT SLAC. collaboration electrons rebounded inelastically from protons 9 the energy. imparted to the proton either kicked it into a higher energy excited state such. as one of the spin 3 2 baryons or shattered it entirely In the latter occurrence. known as deep inelastic scattering the electron rebounded with much less. energy Theoretical analyses of deep inelastic electron proton scattering made. that year by Bjorken 10 suggested that this process might indicate whether. there were any constituents inside the proton but his ideas were not well. received initially by the particle physics community. Inelastic electron scattering was measured with three spectrometers. Fig 1 in SLAC End Station A that were built largely under Taylor s direction. A beam of electrons with energy E passed through a liquid hydrogen and. later also a deuterium target Fig 2 Electrons that rebounded at a preselected. angle 8 into the acceptance of the spectrometer were momentum analyzed. those with a scattered energy that fell into a range of about f2 around a. central value E were directed onto a group of particle detectors that. distinguished electrons from a background consisting mostly of pions For. each given set of values E and 0 measurements were made at a series of. scattered energies ranging from elastic electron proton scattering at the. highest E down to deep inelastic scattering at a few GeV. In the first inelastic experiment which took place in the autumn of. 1967 the 20 GeV spectrometer was used to measure electrons that rebounded. from protons at an angle of 6 degrees The raw counting rates were much. higher than had been expected in the deep inelastic region where the electron. imparts most of its energy to the proton but there was considerable. d sagreement among the MIT and SLAC physicists as to the proper. interpretation of this effect Electrons can radiate photons profusely as they. recoil from a nucleus or pass through matter in this case the surrounding. hydrogen and target walls such an effect which can lower their energy. substantially has to be removed from the raw data before one can assess the. underlying physics These radiative corrections were very time consuming. and fraught with uncertainties they involved measuring cross sections over. a large range of E and E for a each value of 0 After the experimental run was. over a computer program 22 was used to deconvolute these data and obtain. corrected cross sections at the same kinematics as measured. When the radiative corrections were completed in the spring of 1968 it. became clear that the high counting rates in the deep inelastic region were not. due to radiative effects A plot of the cross section o versus the invariant. momentum transfer to the proton Q 2 2EE Z cos 0 showed that the. probability of deep inelastic scattering decreased much more slowly with Q2. also written 42 than that for elastic scattering Fig 3 A way to interpret this. unexpected behavior was that the electrons were hitting some kind of hard. core inside the target protons In hindsight such an observation paralleled. the discovery of the atomic nucleus by Ernest Rutherford 12 in which the. probability of large angle alpha particle scattering from gold atoms was found. to b e far larger th a n h a d b e e n a n ticipated b a s e d o n J J T h o m s o n s p l u m. pudding m o d e l o f th e a to m A t th e tim e h o w e v e r th e r e w e r e a fe w o th e r. possible interpretations o f th e inelastic electron scattering d a ta 13 th a t h a d to. b e e x c l u d e d b e fo r e o n e c o u l d c o n c l u d e th a t th e M IT S L A C g r o u p h a d fo u n d. e v i d e n c e for constituents inside th e proton. S c a l i n g a n d th e P a r to n M o d e l, In April 1 9 6 8 a t th e s u g g e s tio n o f B jorken K e n d a l l plotted th e q u a n tity.
v W 2 versus th e variable v Q 2 w h e r e v E E is th e e n e r g y lost b y electrons. in th e act o f scattering a n d W 2 is k n o w n a s a structure fu n c tio n o f th e. proton In th e first B o r n a p p r o x i m a tio n wherein a single virtual p h o to n. m e d i a te s th e electromagnetic interaction b e t w e e n th e electron a n d proton. Fig 2 th e r e a r e two s u c h structure fu n c tio n s W I a n d W 2 th e y c o n ta i n all. th e information th a t c a n b e o b ta i n e d a b o u t th e proton from unpolarized. electron scattering a n d a r e related to th e cross section o by. 2 2 W b Q 2 s i n 2 sJ,o E E 0 W v Q cos 1, Clearly W 2 d o m i n a te s th e cross section a t small angles while W 1 d e te r m i n e s. large angle scattering Making two extreme a s s u m p tio n s a b o u t th e ratio. W 7 W I K e n d a l l extracted values o f W 2 from th e 6 cross section d a ta. o b ta i n i n g a g r a p h like Fig 5 A s B jorken predicted 14 th e d a ta a p p e a r e d to. scale that is th e y fell a l o n g a single curve F 2 v W 2 th a t is a fu n c tio n of. only th e ratio v Q 2 a n d n o t v a n d Q 2 i n d e p e n d e n tly despite th e fact th a t th e. cross sections h a d b e e n m e a s u r e d a t several different energies. The physical significance of this curve and the scaling behavior became. clearer in August 1968 when Feynman interpreted them in terms of a model. in which protons were composed of generic pointlike constituents he called. partons In this model 15 scaling arose naturally because high energy. electrons rebounded elastically from charged pointlike partons he recognized. that the universal function F2 was the momentum distribution of the. partons weighted by the squares of their charges when plotted versus a. variable x Q2 2Mv where M is the mass of the proton Note that x is. actually the inverse of Bjorken s variable it represents the fraction of the. proton momentum carried by the struck parton when viewed in what. Feynman called the infinite momentum frame essentially the frame in. which the electron is at rest and the proton is speeding toward it. In his model Feynman did not advocate any specific quantum. numbers for the partons they could have whatever charges spins and other. properties were consistent with the MIT SLAC data Based on these ideas. other physicists soon formulated more specific parton models in which the. partons were interpreted as quarks 16 or as bare pointlike nucleons and. mesons 27 The spin of the partons could be determined 18 from the. behavior of the quantity X oL oT the ratio of the proton s tendencies to. absorb virtual photons that are polarized longitudinally that is along their. direction of motion or transversely R is related to WI and W2 according to. Competing theories 19 20 could also account for the observed scaling. behavior without invoking proton constituents thus further more detailed. measurements of deep inelas tic scattering were necessary before any firm. conclus ions could be drawn about what was happening ins ide the proton. Indeed such measurements were already well under way at SLAC by. th end of the year In August the MIT SLAC physic is ts obtained cross. sections at 10 using the 20 GeV spectrometer and that autumn they. employed the 8 GeV spectrometer for measurements at 18 26 and 34 In. addition to determining inelas tic elec tron proton cross sections over a much. wider k inematic range these experiments allowed the group to extract both. s tructure functions and hence R at selec ted k inematic points where data was. available for several angles After radiative corrections had been applied. l m inary data for these angles were presented at the Liverpool Conference. 22 in the summer of 1969 It became obvious there that the measured values. of R were small consis tent with the charged partons being spin l 2 partic les. and completely at odds with models based on vector meson dominance 19. which required R to be large and proportional to Q2. The data from the 6 and 10 inelas tic elec tron proton scattering. experiments were published in two papers 22 that rank among the most. highly c ited in partic le physic s for the year 1969 the 18 26 and 34 data were. published a few years later 23 but were widely available well before that. Graphs of VW and 2MW 1 versus w 1 x Fig 6 showed that both s tructure. functions scaled within the accuracy of the data consis tent with expectations. The Discovery of Quarks Michael Riordan Electrons can radiate photons profusely as they recoil from a nucleus or pass through matter in this

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