Key Steps Toward The Creation of QCD

Tian Yu Cao

(Boston University)

Delivered at

THE SPECIAL SESSION TO CELEBRATE THE QCD 40TH ANNIVERSARY

THE INTERNATIONAL SCHOOL OF SUBNUCLEAR PHYSICS, 24th July, 2012

When was QCD created?

1966?

The Nobel prize for QCD as a description of strong interactions might have been awarded to Sakharov, Zeldovich, and Nambu. They had it all figured out in 1966: the Balmer formula, the Bohr atom, and the Schrodinger equation of strong interactions. All subsequent developments leading to QCD were just mathematics and public relations with no new physics. (Harry J. Lipkin, 1997)

Context for the creation of QCD

• S-matrix theory• Composite model. • Current algebra

Context for the creation of QCD

A. S-matrix theory:

1. Asymptotic states

2. General principles (unitary, causality, etc.)

3. All hadrons as poles on Regge trajectories are composites

4. no elementary particles

Context for the creation of QCD

B. Composite model:

The structure of the hadron system are characterized by a glob al symmetry

Context for the creation of QCD

C. Current algebra

1.Abstracted from underlying relativistic field theory models

2. Hadronic currents are basic entities in hadronic processes.

3.Basic assumptions:

(a) Currents are representations of a SU(3) flavour symmetry group (b) Currents obey algebraic relations derived from the symmetry group, which are closed under the equal time commutation relations

(c) Current quarks are bare quanta of the quark field; not hadron’s ingredients independently existing, merely as placeholders in currents, satisfy all constraints posed by current algebra.

Key steps toward the creation of QCD

1. Current algebra (1962)2. Local current algebra sum rules (1965-66) 3. Scaling and partons (1968-69)4. Light cone current algebra (1971-72)5. A great synthesis; the first articulation of QCD

(1972)

Current algebra

A structuralist approach moving from external to internal constraints on hadrons

Local current algebra sum rules• Measurable form factors are investigated by the high energy neutrino-nucleon

inelastic reactions.• All effects from unknown intermediate states are bracketed.• The localized behaviour of hadron scattering neutrinos is brought to the

forefront, thus opens a window for peeking into the inside of hadrons.• The focus is shifted to hadronic current and its constitution which was

unknown then, a place in the hadronic current was created for current quarks to take.

• The whole project of current algebra and its sum rules was rejected by Chew because it contradicted with bootstrap philosophy against the idea of elementary substructure and the entailed notion of completeness

• Bjorken’s comment on Adler’s work (at the 1992 SLAC conference on the history of particle physics):

His work provided a most important basis for what was to follow when the ideas were applied to electroproduction.” (1997)

Scaling and partons

Scaling: Reorientation

From exploring external relations between observable quantities to the clarification of hadron’s constituents and their dynamics that underlie the phenomena.

Achievements of the scaling-parton project: 1. Consolidated: the constituent picture of the hadrons: point-like, massless quasi-free, and half-half proportion for current quarks and gluons; gluons brought back by parton model would take the central place in the subsequent development.

2. Paves the way for identifying what partons are and what a dynamic

theory should be for describing their behaviour.

Light-cone current algebra

<1> Underlying framework:

Quark-gluon field theoretical model

(Canonical current algebra : free quark model)

<2> Synthesis:

Scaling and partons with

Broken scale invariance and operator product expansion

<3> Generality: able deal with processes beyond what could be dealt with by canonical current algebra, such as

pion-two gamma decay and the ratio R.

<4> Problem: Conflict with renormalized perturbation theory.

<5> Puzzlement: the nature of its underlying model, the nature of interactions and the gluons mediating the interactions.

A Great synthesis -- QCD

Current quarks:

from Current algebra

Gluons:

from parton model

Colour:

from constituent quark model

Colour singlet restriction:

from bootstrap philosophy

A Great synthesis

Why gluons have to have colour?

To avoid

(1) asymmetry between gluons and quarks;

(2) the mixing of gluons with observable currents of baryon numbers.

A Great synthesis

QCD started with

Asymptotic freedom (1973)?

(A. Pais, 1986; A. Pickering, 1984)

<1> AF justifies QCD;

<2> QCD came earlier (1972);

<3> AF was indicisive on the nature of colour symmetry:

exact or broken?

Congratulations!

Congratulations to Murray Gell-Mann and Fritzsch Harald

for the creation of QCD based on crucial contributions from

Stephen Adler and James Bjorken!