Z bosons forward and backward

Because it is well known that protons and antiprotons contain quarks, we can precisely study subtle interplays of how quarks interact to form Z bosons and photons.

Today’s result involves a study of photons and Z bosons. Photons carry the electromagnetic force. Z bosons carry the weak force. For photons, the laws of physics are symmetric. This just means that if you see an electromagnetic physics process going to the right, you could just as easily see it going to the left. Z bosons do not have the same symmetry. If you see a weak force process going to the left, you’ll rarely see it going to the right. It makes you wonder how physicists were able to show in the 1960s that the electromagnetic and weak forces were two facets of the same thing.

Technically, this asymmetry is called the forward/backward asymmetry, although we have substituted the words left and right because it’s a bit easier to visualize. Which of the two forces dominates depends on the energy of the collision, which explores the interplay between the weak and electromagnetic forces. Further, the asymmetry is different for up and down quarks.

The LEP experiments at CERN dominated the study of Z bosons during the 1990s and many of those results are still the best ever. These experiments studied a large sample of events in which electrons and their antiparticles, positrons, annihilated to make Z bosons. The Z bosons decayed into a pair of quarks and antiquarks. The experiments recorded the direction in which the quark was produced (left vs. right). Quarks aren’t observed directly, but detectors can see jets of particles that have about the same energy as the quarks. Historically, LEP experiments are often the definitive word on Z boson studies but it’s hard figure out from a jet whether the original quark was of the up or down variety.

The DZero experiment exploited a property of nature called time reversal symmetry that says if A can make B, then B can make A. Unlike the LEP experiments in which electrons and positrons collide to make quarks, at the Tevatron, quarks and antiquarks collide to make electrons and positrons. In this case, careful note was made of how often the electrons flew to the left compared to the right. According to the Standard Model, these two directions don’t occur with equal probability, although whether right or left was preferred depends on the violence of the collision.

The measurement was in good agreement with the Standard Model and was even more precise than the corresponding LEP measurement. This is a very difficult standard to have achieved and, with even more data to analyze, DZero’s measurement will only get more precise.

— Don Lincoln

These physicists performed this analysis.

For particle physics experiments, reliable computer operations are the cornerstone on which all other achievements are based. These physicists are responsible for DZero computer operations.