Search for the Smallest Objects Known 

The DØ Collaboration

October 6, 1997

Physicists have a long history of uncovering layers of matter-- understanding how things are made of smaller things which in turn are made of yet smaller things. First, it was understood that everyday objects are made of molecules, then that these molecules are composed of atoms. The atoms were later discovered to consist of a nucleus orbited by electrons, and by the 1930's it was found that the nucleus is made up of particles called protons and neutrons. In the 1960's a series of experiments at the Stanford Linear Accelerator Center in California demonstrated that these protons and neutrons are themselves made out of smaller objects, which were named "quarks" (a quirky name perhaps in tune with the spirit of the sixties).

Over the last three decades physicists developed a theory which describes all matter and forces in the universe (except gravity) called the standard model. This theory describes all known particles and interactions with just a few input numbers; it says that all matter is composed of leptons (electrons are a type of lepton) and quarks. In this theory, the quarks and leptons are assumed to be fundamental objects, that is, there are no smaller objects inside of them.

Now physicists working at the world's highest energy particle accelerator, the Tevatron collider at Fermilab (near Chicago), have reported on their attempt to find smaller particles within these quarks. Of course objects this small cannot be seen, not even using sensitive instruments like an electron microscope. Instead, Fermilab's Tevatron Accelerator is used to collide protons with their antimatter counterparts antiprotons, at a combined energy of almost two trillion electron volts. (The electron volt is a measure of energy; an electrically charged particle like a proton can be given more energy, or accelerated, using an electric voltage. The energy of the Tevatron accelerator is the same as if six hundred million regular 1.5 volt batteries had been hooked together to provide this voltage --- though of course it is not really done this way). In these high energy collisions, one of the quarks inside the proton collides with one of the quarks inside the antiproton; if the collision is violent enough, a whole shower of particles will be produced coming from the energy of the collision.

Physicists use large and complex arrays of instrumentation called "detectors" to measure these particles. At these very high energies, the outgoing particles tend to be produced in collimated sprays called jets. These jets are one of the signs that the quarks inside the proton and antiproton are hitting each other hard.

The new result comes from the ("D-zero") detector group. is an international collaboration of about four hundred physicists who designed and constructed a detector to study these high energy proton-antiproton collisions. In a paper recently submitted to Physical Review Letters, the physicists describe how they selected collisions where the highest energy jets were produced. By studying the angles at which such jets are emitted from the collision, they hoped to see hints of smaller objects --substructure --inside the quarks. If quarks had smaller particles inside them, the angles of the jets predicted by the standard model would be different than those measured by the experiment. For example, there would be more very high energy jets found at large angles sideways from the collision. After analyzing millions of jets, physicists have found that the angular distribution of the jets is exactly as predicted by the standard model.

Hence, the quarks are behaving exactly like a mathematical point --something with no size at all, and not composed of any smaller building blocks. Given the precision of the measurement, this means we can be sure the quarks are smaller than one ten thousandth of a trillionth of a centimeter, or 0.000 000 000 000 000 1 cm. These new results are the world's best test of the point-like nature of the quarks and are in fact the best measurement of these smallest objects known.

For further information contact Dr. Brad Abbott, New York University,

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