From: "JACK L. URETSKY (C) 1996; HEP DIV., ARGONNE NATIONAL LAB, ARGONNE, IL 60439" <JLU@hep.anl.gov>
Date: Sat, 15 Nov 1997 11:25:03 -0600
Hi Ludwik-
You ask:
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Here is a question I did not know how to answer. What is the directionality
of the so-called "weak" force, is it attractive, repulsive or both? I do
know gravitational and "strong" forces are always attractive and that the
electromagnetic forces can be either attractive and repulsive.
Ludwik Kowalski
P.S.
By "strong" I mean forces between nucleons (ignoring their inner structure).
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After reading the first nine answers to your question, I'm moved to
comment.
The word force, in elementary particle physics, seems to have taken
on a meaning that is a bit removed from its conventional use in elementary
mecchanics (i.e., Newton's Laws). That's because in particle physics we
nearly always start from a Lagrangean. The ususal Lagrangean separates
nicely into two parts. One part looks like a kinetic energy term
(relativized), the other part contains the interactions among the
particles and would be a potential energy term in ordinary mechanics.
It is the latter term that is usually referred to when one speaks
of "force". It gives rise, in perturbation theory, to expressions that can
be described (in Feynman diagram language) in terms of "exchanges" of quanta.
When the "strength" of the interaction term is small (QED, high
momentum transfer QCD, for example) the lowest order perturbation term
often corresponds to a coulomb or Yukawa potential. In this sense it can
be characterized as a "force" having a definite sign. This kind of
inerpretation was used by the respondent who spoke of "pion exchange"
forces, or vector particle eexchange forces.
Another way of characterizing the sign of forces, not dependent
on perturbation theory, is to look directly at scattering amplitudes and
phase shifts. The sign of the force giving rise to scattering can be
deduced if the nuclear part of the amplitude interferes with a coulomb
part. The sign of the interference term then characterizes the "direction"
of the strong force.
Another way is to look at low energy scattering. The behavior of
the lowest partial-wave phase shift reflects the sign of the force, as
explained in ancient editions of Blatt & Weiskopff.
Regards,
Jack
p.s. The exchange of a Z-particle interferes with the exchange of a
photon, giving rise to observable effects in atoms.