Concrete Quarks
G. Zweig, RLE at MIT November 17, 2015 email: zweig@mit.edu
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• QCD - developed in two phases:
– Discovery of quarks
– Specification of their interactions
• Arose from two very different traditions – Rutherford-Bohr
– Einstein
• Discovery of radioactivity: Henri Becquerel (1896)
Phosphorescence? Becquerel’s photographic plate fogged by exposure to radiation from uranium salts. A metal Maltese Cross placed between the plate and the uranium salts is visible.
• Rutherford at Cambridge (1899): α and β
• Rutherford & Soddy at McGill (1903):
“the spontaneous disintegration of [a] radio-element, whereby a part of the original atom was violently ejected as a radiant particle, and the remainder formed a totally new kind of atom with distinct chemical and physical character.”
Nobel prize in Chemistry (1908), Soddy (1921)
¨ Interpretation (Rutherford 1911)
¨ Impossible!
¨ Marsden (1914): Nuclei contain protons!
¨ Bohr (1912, 1914-1916): Stationary states Charge separation & Quantization
Rutherford’s group at Manchester University, 1912.
Rutherford is seated second row, center.
Back rows: (standing): C. G. Darwin, J. M. Nuttall, J. Chadwick, 2nd row: H. Geiger, E. Rutherford,
Front row: H. G. J. Moseley, E. Marsden.
¨ The nuclear force (1927)
¨ Heisenberg (1925 for atoms; 1943 & 1944 for the nucleus): Work only with observables!
New nuclear particles (π, K) discovered in 1947
eA jonrnat of experimental and theoretical physics established by B L¹.chats in 2893
SzcoND SsRias, Vot., 76, No. 12 DECEMBER 15, 1949
Are Mesons Elementary Particles' ?
K. FERMI AND C.N. YANG*
Institute forNuclear Studies, University ofChicago, Chicago, Illinois
I,'Received August 24,1949)
The hypothesis that ~-mesons may be composite particles formed by the association ofanucleon with an anti-nucleon is discussed. From an extremely crude discussion of the model itappears that such ameson would have in most respects properties similar tothose of the meson of the Yukawa theory.
I. INTRODUCTION
' N recent years several new particles have been
~- discovered which are currently assumed to be
"elementary, "that is, essentially, structureless. The probability that all such particles should be really elementary becomes less and less as their number increases.
It is by no means certain that nucleons, mesons, electrons, neutrinos are all elementary particles and it could bethat atleast some of the failures of the present theories may be due todisregarding the possibility that some of them may have acomplex structure. Unfortu- nately, we have no clue to decide whether this is true, much less to 6nd out what particles are simple and what particles are complex. In what follows we mill try to work out in some detail a special example more as an illustration of a possible program of the theory of particles, than in the hope that what we suggest may actually correspond to reality.
We propose to discuss the hypothesis that the ~- meson may not be elementary, but may be acomposite particle formed by the associations ofa nucleon and an anti-nucleon. The first assumption will be, therefore, that both an anti-proton and an anti-neutron exist, having the same relationship to the proton and the neutron, as the electron to the positron. Although this is an assumption that goes beyond what is known experimentally, we donot view itasavery revolution- ary one. We must assume, further, that between a nucleon and an anti-nucleon strong attractive forces exist, capable of binding the two particles together.
*Now at the Institute for Advanced Studv. Princeton, New Jersey.
We assume that the x-meson is apair of nucleon and anti-nucleon bound in this way. Since the mass of the x-meson is much smaller than twice the mass of a nucleon, it is necessary to assume that the binding energy is so great that its mass equivalent is equal to the diR'erence between twice the mass of the nucleon and the mass of the meson.
According to this view the positive meson would be the association ofaproton and an anti-neutron and the negative meson would be the association of an anti- proton and a neutron. As a model of a neutral meson one could take either apair ofa neutron and an anti- neutron, or of aproton and an anti-proton.
Itwould be dificult to set up a not too complicated scheme of forces between anucleon and an anti-nucleon, without about equally strong forces between two ordi- nary nucleons. These last forces, however, would be quite diferent from the ordinary nuclear forces, because they would have much greater energy and much shorter range. The reason why no experimental indication of them has been observed for ordinary nucleons may be explained by the assumption that the forces could be attractive between a nucleon and an anti-nucleon and repulsive between two ordinary nucleons. If this is the case, no bound system of two ordinary nucleons would result out of this particular type of interaction. Because of the short range very little would be noticed of such forces even in scattering phenomena.
Ordinary nuclear forces from the point of view of this theory will be discussed below.
Unfortunately we have not succeeded in working out a satisfactory relativistically invariant theory of nu- cleons among which such attractive forces act. For this reason all the conclusion that will be presented will be 1739
• M. Gell-Mann & E.P. Rosenbaum, “Elementary Particles,” Scientific American, July 1957, 72- 86: 19 in number
M. Gell-Mann & A.H. Rosenfeld, “Hyperons and Heavy Mesons,” Ann. Rev. Nucl. Sci, 1957, 407-478:
Two kinds of Elementary Particles:
Point particles Spin 1/2 leptons Particle Mass
e´ 1
µ´ 206.7
ν 0
Spin 1 photon Particle Mass
γ 0
Extended particles (strongly interacting) Spin 1/2 baryons
Multiplet Particle Mass (me)
Ξ Ξ0 ?
Ξ´1 2585 Σ
Σ´1 2341 Σ` 2325 Σ0 2324
Λ Λ 2182
N n 1838.6
p 1836.1
Spin 0 mesons
Multiplet Particle Mass π
π` 273.2 π´1 273.2 π0 264.2 K
K` 966.5 K´ 966.5 K01 965 K02 965 – No resonances mentioned!
• Caltech:
– Bob Christy ... Alvin Tollestrup
– My thesis: A test of time reversal symmetry K` Ñ π0 ` µ` ` ν.
– Mexico!
– Murray?
• Every Thursday at 1:30 PM during 1962-63
• Theoretical physics:
– Axiomatic field theory (no physics)
– Theory related to belief (Chew, June 1961):
“I believe the conventional association of fields with strongly interacting particles to be empty.
... field theory..., like an old soldier, is destined not to die but just fade away.”
– Theory related to experiment:
∗ Classification (no dynamics):
· Sakata model: Wrong baryon spectrum
· G(2) & SU(3) were in contention
∗ Dynamics (no classification): Bootstrap Fred Zacharisen (1961)
ðñ
Exchanging a ρ binds two pions into a ρ.
But cannot bootstrap the π!
• Experimental physics:
– More particles discovered since 1957:
∗ Point particles: the 4th lepton (νµ)
∗ Extended particles: the 8th spin 1/2 baryon (Ξ0), and an 8th spin 0 meson (η)
∗ Resonances: 26 meson resonances listed in the RMP, April 1963 (ρ, ω, K˚, φ, ¨ ¨ ¨ )
• One Thursday afternoon:
P.L. Connolly, et al., “Existence and Properties of the φ Meson”, Phys. Rev. Lett. 10, 371 (1963):
φ Ñ K ¯K
φ Ñ{ ρ ` π
“The observed rate [for φ Ñ ρ ` π] is lower than ... predicted values by one order of magnitude; however the above estimates are uncertain by at least this amount so that this discrepancy need not be discon- certing.”
ΓK ¯K
Γρπ „ ˆ pK ¯K pρπ
˙3 ,
“ 1{4 pexpectedq, ě 35 pobservedq.
– Feynman:
– GZ:
• Assumed hadrons have constituents a called aces:
r N0, Λ0 s & r ¯N0, ¯Λ0 s
r pp0, n0q, Λ0 s & r p ¯p0, n¯0q, ¯Λ0 s Mesons ” a¯a with ÒÓ (π, K and η) and
ÒÒ (ρ, ω, K˚ and φ).
Baryons ” aaa with ÒÒÓ (p or n),
Nonet of vector mesons represented as “deuces”
FIG. 2, CERN report TH-401, January 1964.
• A rule for decay (“Zweig’s Rule”) (in modern notation):
Meson decay: a is an ace, ¯a an antiace.
– Implies φ Ñ{ ρ ` π
• A hierarchy of mass relations:
Mass = Σ constituent masses + energies of interaction, |∆m| ą |∆E|.
– Identical binding energies:
m2pρq « m2pωq ă m2pK˚q ă m2pφq.
7502 7842 8882 10182
– EΛN¯0
0 “ ENΛ¯0
0 « 12pEΛΛ¯0
0`ENN¯0
0q, N0 “ p0, n0 : m2pφq « 2m2pK˚q ´ m2pρq.
10182 10072
Like the “constituent-quark model,”
but no potential is assumed.
• Since aaa is a baryon, B “ 13,
Q “ erIz ` B`S2 s,
r pp0, n0q, Λ0 s Ñ r p23, ´13q, ´13 s.
3 ˆ 3 ˆ 3 “ 1 ` 8 ` 8 ` 10.
Octet of baryons represented as “treys”
CERN report TH-412, February 1964
• Mass differences break SU(3) & SU(2) symmetry – SU(3) symmetry: mpp0q “ mpn0q “ mpΛ0q, – Broken SU(3): mpp0q “ mpn0q ă mpΛ0q, – Broken SU(2): mpp0q ă mpn0q.
• Interactions: Aces, not hadrons, interact.
– Strong interaction couplings: “Zweig’s rule”
– Electromagnetic and weak couplings:
γ ` a Ñ a
a Ñ a1 ` e´ ` ν when n Ñ p ` e´ ` ν, n0 Ñ p0 ` e´ ` ν The “current-quark model”
• concrete-quarks ”
current-quarks, constituent-quarks ` Zweig’s rule
Summary
• Hadrons have point constituents
• Leptons Ø Aces
• Origin of SU(3) symmetry
• Beyond SU(3) symmetry:
– Restricted representations, quantum numbers:
∗ Baryons only in 1, 8, 10, Mesons only in 1, 8, and 9.
∗ There is an ~L and an ~S, with ~J “ ~L ` ~S.
∗ L “ 0 baryons: (8, JP “ 12`) and (10, 32`q, L “ 0 mesons: p8, JP C “ 0´`q and p9, 1´´q.
∗ Higher L excitations.
∗ ~L ¨ ~S interactions.
∗ 0´´; 0`´, 1´`, ¨ ¨ ¨ forbidden for any L.
• 80 pages
• Not as easy as it looks:
• What did people think? Were aces real?
– GZ: Aces had dynamics!
– Murray Gell-Mann:
∗ “Concrete-quark model”
∗ Five years after the deep inelastic scattering experiments at SLAC (partons) “Quarks,”
Acta Physica Austriaca, Suppl. IX, 733- 761 (1972)
“In these lectures I want to speak about at least two interpretations of the concept of quarks for hadrons and, the possible relations between them.
First I want to talk about quarks as ‘constituent quarks’.
These were used especially by G. Zweig (1964) [italics added] who referred to them as aces. ...”
More precise to say:
These were introduced by G. Zweig
“The whole idea is that hadrons act as if they are made up of quarks, but the quarks do not have to be real. ...”
That’s a mischaracterization.
“There is a second use of quarks, as so-called ‘current quarks’
which is quite different from their use as constituent quarks ...
If quarks are only fictitious there are certain defects and virtues.
The main defect would be that we never experimentally dis- cover real ones and thus will never have a quarkonics indus- try. The virtue is that then there are no basic constituents for hadrons ´ hadrons act as if they were made up of quarks but no quarks exist - and, therefore, there is no reason for a distinction between the quark and bootstrap picture: they can be just two different descriptions of the same system, like wave mechanics and matrix mechanics.” [italica added]
This was Murray’s vision. Concrete quarks?
– Heisenberg (early 1970s)
“Even if quarks should be found (and I do not believe that they will be), they will not be more elementary than other particles, since a quark could be considered as consisting of two quarks and one anti-quark, and so on. I think we have learned from experiments that by getting to smaller and smaller units, we do not come to fundamental units, or indivisible units, but we do come to a point where division has no meaning. This is a result of the experiments of the last twenty years, and I am afraid that some physicists simply ignore this experimental fact.”
– Richard Feynman:
∗ Current quarks/aces?
∗ Constituent quarks/aces?
· “The correct theory should not allow you to say which particles are elementary.”
∗ Zweig’s rule?
· “ Everything that can possibly happen does ¨ ¨ ¨ ” .
∗ “Did I miss anything Zweig?”
Problems with acceptance
• Aces violated the spin-statistics theorem – Rutherford’s atom & Bohr’s orbits – Wegener’s continental drift
• Aces violated current dogma:
– Nuclear democracy
– Work with observables.
(Copernicus’s view of the solar system)
A way to judge new theories
Bayes Theorem:
P pA|Eq “ 1`λ1 , where λ ě 0, and λ “ P pE| ¯P pE|AqAqP p ¯P pAqAq. Since
P p ¯Aq « 1 and P pE|Aq « 1, λ « P pE| ¯P pAqAq.
Accept A when P pE|Aq ăă P pAq.
– Einstein tradition: P pE|Aq ąą P pAq:
– Rutherford-Bohr tradition: P pE|Aq ăă P pAq
When did acceptance come?
– Pauling – Bogolubov – Dalitz
– Feynman
– Deep inelastic scattering – ψ/J
Invention or discovery?
Invention: “a product of the imagination.”
Discovery: “the act of finding or learning some- thing for the first time.”
– Current quarks invented (Einstein)
– Constituent quarks discovered (Rutherford-Bohr) – Aces contained a bit of each
google: zweig CERN interview
Conclusion of
CERN report TH-412, February 1964
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