## A more precise study of the strong force The Earth is a sphere with a diameter of approximately 8,000 miles. However, if you measure precisely enough, you find that the Earth is slightly pear shaped. The exaggerated figure conveys the basic idea, although the real variation is only about 0.1 percent, which is why the Earth looks spherical to the naked eye. This week’s article describes a similar concept in a DZero measurement.

Suppose you needed to know the exact distance between Fermilab and CERN. To first approximation, the Earth is a sphere, so you can take the laboratories’ latitudes and longitudes and work out the distance using high school geometry.

However, if you measure precisely enough, you find that the Earth isn’t perfectly spherical. It is slightly pear shaped and it has mountains. A more precise measurement must take into consideration these finer points.

Similar to calculating distance based on the generally spherical shape of the Earth, we can use simplified physics equations to predict the kinds of collisions we should see at the Tevatron. It turns out that we don’t know how to exactly solve the equations that describe the behavior of the strong force, which governs the behavior of matter in the center of atoms. By using simplified equations, we can predict what happens when the Tevatron beams collide and two quarks or gluons are knocked out of the interaction. However, these approximate equations are unable to give any information about what happens when three particles are created in the collision. This takes place only about 10 percent as often as the more common production of two particles. To calculate this phenomenon of creating three particles, physicists must use an improved approximation, which is analogous to taking into account the small pear-shaped corrections to the shape of the Earth. For four particles, a better approximation still (e.g. mountains) must be used. Each successive approximation is closer to using the full (and currently unsolvable) equations.

In this result, DZero physicists studied collisions in which three jets were produced. When a quark or gluon is created in a collision, it turns into a jet of particles with essentially the same energy and direction of the parent quark or gluon. Because three jets are produced in the above scenario, this collision requires equations that are a more accurate approximation to calculate what is going on. This analysis is an extension of an earlier result. This new measurement gives us a more precise glimpse of the inner workings of the strong force.

-Don Lincoln These physicists performed this analysis. Understanding how much beam has been delivered to the experiment is a crucial component of many analyses. These physicists are responsible for the day to day operations of the equipment that makes this measurement possible.