The Dijet Mass Spectrum and the Search for the Smallest Objects Known

August 25 1998

What are Quarks and how do you look at them?

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.

Physicists working at the world's highest energy particle accelerator, the Tevatron collider at Fermilab (near Chicago), have investigated the collisions of protons with their antimatter counterparts anti-protons, at a combined energy of almost two trillion electron volts (2 TeV). (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 anti-proton; 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. Production of these jets is one of the signs that the quarks inside the proton and anti-proton are hitting each other hard.

What is the Dijet Mass?

In most of the proton anti-proton collisions two jets are produced. By measuring the energy of each of the jets, physicists can calculate their mass (using Einstein's E=mc²), this is called the dijet mass. The Tevatron accelerator is capable of producing jets with a dijet mass of up to 1.2 TeV (1.2 trillion electron volts), which is seven times heavier than the "top" quark (and six times heavier than a gold atom)!

The standard model predicts the number of times we expect to see a proton anti-proton collision produce two jets with a given dijet mass. If more events than predicted are found with a very large dijet mass (greater than approximately 0.6 TeV) this could be interpreted as evidence that quarks are not the smallest objects possible and that they may be made up of something even smaller.

New Results from DØ

A new measurement of these dijet mass events has been made by the DØ ("D-zero") detector group. DØ is an international collaboration of about four hundred physicists who designed and constructed a detector to study these high energy proton-anti-proton collisions. In a paper recently submitted to Physical Review Letters, the DØ physicists describe how they selected collisions where the highest energy jets were produced. The dijet masses of the jets from each collision were found to have a distribution that is similar to that predicted by the standard model.

A plot of the measured distribution (solid circles) of the dijet masses compared with
the standard model prediction (red line, produced using a program called JETRAD).

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 (an improvement of 20% over the previous DØ result).

Previous Results from DØ

Previously DØ had studied the angles at which such jets are emitted from the collision (in a paper published in Physical Review Letters 80, 666 (1998)). If quarks had smaller particles inside them, the angles of the jets predicted by the standard model would be different from 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.

For further information contact Dr. Iain Bertram, Northwestern University, email:bertram@fnal.gov

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