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An introduction to the rapidly developing subject of diffractive processes con-
taining jets is given, with emphasis on an s-channel picture of the dynamics.
Invited Lectures presented at the
SLAC Summer Institute on Particle Physics:
Spin Structure in High Energy Processes
Stanford, California - July 26-August 6, 1993
* Work supported by the Department of Energy, contract DE-AC03-76SF00515.
J. D. BJORKEN
Stanford Linear Accelerator Center
Stanford University. Stanford, California 94309
1 Introduction; What is Diffraction?
This talk is intended as a re-introduction and generalization of simple and
ancient ideas on diffraction for a generation of physicists trained mainly for the
study of hard collision processes.
Diffraction is shadow physics; hence it is most important when opaque objects
collide. Elastic scattering of hadrons at high energies is the most immediate
example. But the subject is much more subtle. Inelastic diffraction exists as well,
as anticipated long ago by Good and Walker,' as a consequence of the composite
nature of hadrons. If, in a peripheral collision of hadrons, part of the wave-function
of the projectile is attenuated more than the rest, then the internal wave function
of the outgoing projectile no longer is the ground-state eigenfunction. Therefore,
excitations will exist, even in "shadow" processes.
Because there are many kinds of diffractive processes, and because there is not
a uniform terminology on what is meant by the word "diffractive," it is appropriate
to start with definitions of what at least I mean by it:
A process is diffractive if and only if there is a large rapidity gap in the
final-state phase space which is not exponentially suppressed.
Some elaboration of this definition is clearly needed. The final-state phase space
variables implied in the definition are the lego-variables: (pseudo)-rapidity rl,
azimuthal angle 0, and transverse momentum pg. "Large" will mean much greater
than 2, at least 4 to 6 units of rapidity. "Non-exponentially suppressed" means
that the probability of finding the gap in the final state is not a strong function
of gap width, when the remaining contents of the lego plot are held fixed. Let us
elaborate more on this last point:
In general, when a rapidity gap exists, the frame of reference can be chosen so
that p; = 0 is in the middle of the gap. Then all collision products are divided into
either left-movers or right-movers; all of whose production angles are very small.
There are no "wee" hadrons produced. So the above statement "non-exponentially
suppressed" means that in the diffraction process
a+b-+A-+ B, (1)
Here’s what’s next.
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Bjorken, J.D. Hard diffraction, article, March 1, 1994; Menlo Park, California. (digital.library.unt.edu/ark:/67531/metadc625669/m1/3/: accessed January 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.