Alexander Akopian - 1999


Sclaling violation in inclusive jet production


Encounters with scaling laws are an everyday experience. For example, when a mother explains to her child that the age of an eight year old dog is equivalent to that of a 60 year old man, she is explaining a scaling law: for all living species, how old one is has to do with the fraction of one's age to one's life expectancy, rather than with actual age.

Many scientific breakthroughs have come from discoveries of scaling laws, but discoveries of violations of scaling are just as important, as they often point the way to new directions.

Alexander's experiment was designed to test the theory of the ``strong force", according to which the quarks inside a proton are held together by exchanging gluons, which are the quanta of the force.The energy of a fast moving proton is shared by its quarks and gluons, the so called "partons". In a head-on collision between two protons, the partons are sometimes scattered apart at large angles and appear in the form of jets of particles. One would naively think that the distribution of the fraction of the energy of the colliding protons carried by their partons, and therefore by the particle jets, would be the same regardless of the energy of the colliding protons. However, the theory predicts that this kind of scaling should be violated: the average energy fraction carried by a partons hould decrease as the collision energy increases.This is equivalent to saying that the proton carries more partons at higher energies. The reason for this is that, according to the theory, if one tries to knock a parton out of a proton, the force becomes stronger, just like the force of a spring, until finally the "spring" breaks and, like a dividing living cell, the parton multiplies. The higher the energy, the further the multiplication is carried out, and therefore the naive scaling hypothesis should not work.

Alexander's experiment, which was carried out at the world's largest particle accelerator at Fermilab, confirmed the theoretical prediction of parton splitting, showing that the proton is much more "alive" than one might imagine.