The world's hardest proton collisions and what they tell us about nature.

February 5, 1999

Smashing particles tells us how they tick

At one time or another, most of us have had the urge to take something apart just to see what's inside and how it works.  Maybe you've looked inside a music box to find the reeds that make each note, or taken apart a mechanical pen to find the springs and locks inside, or even whacked a piñata to find the hidden treats.  This is essentially the kind of inquiry particle physicists have been up to for the past century, but rather than studying how people put things together we study how nature makes the building blocks of our whole universe. Physicists working at the world's highest energy particle accelerator, the Tevatron collider at Fermilab (near Chicago) study the results of extremely hard collisions between protons and their antimatter counterparts, antiprotons. (How much energy do these protons have? Click here to learn more.) Much like hitting a watch with a sledge hammer, sometimes the debris from these collisions can give us a lot of insight into the structure and workings of the proton and other forms of matter.

Instead of being made like solid spheres of matter, protons are mostly empty space. They are built up from much smaller particles called "quarks" which are bound together by a strong force carried by particles called "gluons". Collectively these quarks and gluons are called "partons". Physicists have developed a theory called the standard model which describes all known particles and the ways they interact with each other. In the standard model all matter is composed of either quarks (protons and neutrons for example) or leptons (electrons are a type of lepton). In this theory, the quarks and leptons are assumed to be fundamental objects, that is, there are no smaller objects inside of them. One component of the standard model is called quantum chromodynamics (QCD), that's a mouthful, but the gist of it is that QCD describes the forces by which the quarks and gluons interact. These forces act on a special property of the quarks and gluons called "color" to hold the proton together or, in our collisions, to determine how it flies apart.

Protons going to pieces

We can use QCD to describe the collisions between our protons and antiprotons. We are mainly interested in hard and fast scattering between partons. In this case, QCD describes our collisions simply as scattering between a single parton in the proton with a single parton in the antiproton. That's a much simpler picture than thinking about two bags of quarks and gluons bouncing off of each other. We'll see below that this is a good assumption if the partons hit each other hard enough. After the collision, much of the proton's energy just travels straight along with the remnants of the broken proton, but some of the energy can be carried away at large angles by the parton that feels the big impact. If the partons hit each other hard enough, a whole stream of new particles is produced along the direction that each parton scatters. We call these streams or sprays of particles "jets".

When the scattering between the partons results from harder and harder collisions, we find more and more energy in the jets. Then proton collisions start looking more and more like a simple scattering between two point-like particles. The hardest scatters almost always involve scattering between two quarks and they can produce jets which carry away a large fraction of the proton's energy. Here are a couple of examples from our data sample:

A very hard collision

This picture shows what happens when the proton and the antiproton engage in a very hard collision. The proton and the antiproton travel along the arrows. The sphere represents our particle "detector" which surrounds the point where the particles collide. The bumps on the sphere show where we see debris flying out after the collision. (In our experiment we actually measure the energy of the particles leaving the collision site.) The two big bumps show jets produced in the collision. Notice that almost all the energy we detect is contained in the jets and they are produced at nearly 90 degree angles from the beams of protons and antiprotons. The bottom picture shows a different view of the same collision where the outside of the sphere is peeled off and displayed on a flat surface. Here the two jets clearly dominate over all the other debris in the collision.

A much softer collision

This picture shows what often happens in a much softer collision. The bumps here are magnified 30 times compared with the last picture so we can see what's going into the detector more clearly. The debris here look very different and very messy compared to the harder collision. We don't see clean looking jets dominating the debris, much of the energy we do see is traveling along the direction of the broken proton and antiproton, and the rest of the energy is pretty much evenly scattered all over the place. (There is a similar collection of soft debris in the above collision, but the energy of the two jets is so large it is difficult to see underlying debris scattered around their bases.) QCD doesn't describe these soft and messy collisions very well, but it has a lot to say about hard scattering which produces energetic jets.

Using the rules of QCD to describe parton scattering along with our knowledge of the structure of the proton (this is determined by special experiments which measure the distributions of quarks and gluons in the proton, the parton distributions) physicists can make predictions about jet production in our proton-antiproton collisions. By measuring the direction and energy of jets produced in these collisions it is possible to test the accuracy of QCD and our parton distribution models.

The DØ ("D-Zero") detector group has made the world's most precise measurement of jet production. DØ physicists have recently submitted a paper to Physical Review Letters which describes how they selected collisions where the highest energy jets were produced and how these data compare to our best theoretical predictions. The rate of jet production is sensitive to both our knowledge of the proton's structure and to the details of parton scattering in QCD. Especially for large energy jets, this rate could be affected by any new physics beyond the interactions predicted in the standard model. For example, disagreement with theoretical predictions might be an indication of some substructure within the quarks, or for the creation of new, so far undiscovered, types of particles in our collisions. A likely signal for such effects would be a larger than expected rate of production of very high energy jets found at large angles from the particle beams. Our measurement of jet production rates as a function of jet energy also tests our knowledge of the proton structure (or parton distribution functions). After analyzing millions of jets from our collisions, we have found that the number of the jets produced is in good agreement with the standard model and our models of proton structure. This result agrees with D-Zero's earlier search for evidence of quark substructure using high energy jets and these data will help to further refine our knowledge of the proton's structure.

This picture shows the cross section or rate of jet production for our proton-antiproton collisions. We plot the cross section of jets produced at large angles relative to the beamline as a function of the transverse energy of the jet (the transverse energy is the amount of energy the jet carries away at 90-degrees from the beam). The rate is very strongly dependent on the jet transverse energy, for example, we only see about one jet with a transverse energy of 450 GeV (this is half the incoming proton's energy - so these jets come from partons that are hit REALLY hard) for every million jets we see with 90 GeV (a tenth of the proton's initial energy).

 
Shown along with the data is a theoretical prediction calculated using the JETRAD program. JETRAD calculates the jet cross section using the rules of QCD and a parton structure model. In this case we have used the CTEQ3M model of the parton developed by the CTEQ group.

This picture shows the fractional difference between our data and the JETRAD prediction shown in the previous plot. Within our measurement errors (shown by the solid lines) we see good agreement with the theoretical model.

These results represent the world's most precise measurement of high energy jet production. So far, QCD is doing an excellent job of describing our collisions and these new data will add to our knowledge of the proton's structure. Beginning in the summer of 2000 DØ will repeat this experiment with higher energy protons and a much larger data set. This will not only improve the quality of our measurement, but will also allow us to study jets with greater energies, to test QCD to unprecedented levels, and to continue the search for nature's secrets at the frontier of high energy physics.

For further information contact Dr. Robert Hirosky, University of Illinois, Chicago, email:hirosky@uic.edu

Some additional reading:

The Particle Adventure - a fun and fact filled tour of the world of particle physics from the Contemporary Physics Education Project.

A more technical introduction to QCD

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