frank wilczek 漸近自由 The other, much heavier quarks are all unstable; “asymptotic freedom"

http://www.frankwilczek.com/Wilczek_Easy_Pieces/298_QCD_Made_Simple.pdf






漸近自由[编辑]

维基百科，自由的百科全书

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在物理學中，漸近自由是某些規範場論的性質，在能量尺度變得任意大的時候，或等效地，距離尺度變得任意小（即最近距離）的時候，漸近自由會使得粒子間的相互作用變得任意地弱。
漸近自由是量子色動力學（QCD）的一項特性，QCD是描述夸克和膠子間的核相互作用，而這兩種粒子是組成核物質的基本構成部份。在高能量時，夸克與夸克之間的相互作用非常微弱，因此可以通過粒子物理學中的，深度非線性散射的截面DGLAP方程（描述QCD的演化方程），來進行微擾計算；低能量時會進行強相互作用，來防止重子（由三個夸克組成，如質子及中子）或介子（由兩個夸克組成，如π介子）分體，這些都是核物質內的複合粒子。
漸近自由的發現者為弗朗克·韋爾切克、戴維·格婁斯和休·波利策，他們在2004年因這項發現而獲得了諾貝爾物理學獎^[1]。

Quarks and gluons

One class of particles that

carry color charge are the

quarks. We know of six different

kinds, or “flavors,” of

quarks—denoted u, d, s, c, b, and t, for: up, down,

strange, charmed, bottom, and top. Of these, only u and d

quarks play a significant role in the structure of ordinary

matter. The other, much heavier quarks are all unstable.

Quarks and gluons

One class of particles that

carry color charge are the

quarks. We know of six different

kinds, or “flavors,” of

quarks—denoted u, d, s, c, b, and t, for: up, down,

strange, charmed, bottom, and top. Of these, only u and d

quarks play a significant role in the structure of ordinary

matter. The other, much heavier quarks are all unstable.

A quark of any one of the six flavors can also carry a unit

of any of the three color charges. Although the different

quark flavors all have different masses, the theory is perfectly

symmetrical with respect to the three colors. This

color symmetry is described by the Lie group SU(3).

For all their similarities, however, there are a few

crucial differences between QCD and QED. First of all,

the response of gluons to color charge, as measured by the

QCD coupling constant, is much more vigorous than the

response of photons to electric charge. Second, as shown

in the box, in addition to just responding to color charge,

gluons can also change one color charge into another. All

possible changes of this kind are allowed, and yet color

charge is conserved. So the gluons themselves must be

able to carry unbalanced color charges. For example, if

absorption of a gluon changes a blue quark into a red

quark, then the gluon itself must have carried one

The third difference between QCD and QED, which is

the most profound, follows from the second. Because gluons

respond to the presence and motion of color charge

and they carry unbalanced color charge, it follows that

gluons, quite unlike photons, respond directly to one

another. Photons, of course, are electrically neutral.

Therefore the laser sword fights you’ve seen in Star Wars

wouldn’t work.

But it’s a movie about the future, so maybe

they’re using color gluon lasers.

A remarkable feature of QCD, which we see in figure 1,

is how few adjustable parameters the theory needs. There

is just one overall coupling constant g and six quark-mass

parameters mj for the six quark flavors. As we shall see,

the coupling strength is a relative concept; and there are

many circumstances in which the mass parameters are

not significant. For example, the heavier quarks play only

a tiny role in the structure of ordinary matter. Thus QCD

approximates the theoretical ideal: From a few purely

conceptual elements, it constructs a wealth of physical

consequences that describe nature faithfully.4

Besides confinement, there is another qualitative difference

between the observed reality and the fantasy

world of quarks and gluons. This difference is quite a bit

more subtle to describe, but equally fundamental. I will

not be able to do full justice to the phenomenological arguments

here, but I can state the essence of the problem in

its final, sanitized theoretical form. The phenomenology

indicates that if QCD is to describe the world, then the u

and d quarks must have very small masses. But if these

quarks do have very small masses, then the equations of

QCD possess some additional symmetries, called chiral

symmetries (after chiros, the Greek word for hand). These

symmetries allow separate transformations among the

right-handed quarks (spinning, in relation to their

motion, like ordinary right-handed screws) and the lefthanded

quarks.

But there is no such symmetry among the observed

strongly interacting particles; they do not come in opposite-

parity pairs. So if QCD is to describe the real world,

the chiral symmetry must be spontaneously broken,

much as rotational symmetry is spontaneously broken in

a ferromagnet.