Subverting the subatomic telephone game

In the telephone game a message is garbled by changes in how it is repeated. Today’s article describes how we defeat a similar subatomic process using the world’s largest collection of top quarks.

When I was young, a bunch of us would play the telephone game. In this game, a person would whisper a message to someone. That person would whisper it to someone else and, after a series of whispers, we’d compare what was the last person heard to what the first person said. Typically the initial and final phrases would be quite different.

In today’s article, I report the outcome of a similar problem in the subatomic world and how DZero tried to cleverly overcome it. In particle collisions, such as those that occur at the Tevatron, theorists can calculate what goes on as the quarks and gluons (collectively called partons) inside the beam interact. However, as they exit the collision, the partons might undergo a dozen or more interactions before they reach the detector. These interactions can obscure the information about the original collision so much that experimenters are unable to determine the conditions of the partons immediately after the collision.

One such measurement might occur when two quarks scatter in the collision. Quarks have a property called spin, and we can picture each quark as a little spinning top. Quantum chromodynamics, the theory describing the behavior of the strong force, makes firm predictions about how the spins of the two quarks are related. If you measure the spin of one quark, for example, you have knowledge of the spin of the other one.

Most quarks emit gluons as they exit the collision. The problem is that each time the quark emits particles, its nature is altered. DZero scientists needed a way to peer at the quark before the interactions occurred.

It turns out that top quarks decay in less than a trillionth of a trillionth of a second and long before they undergo any other interactions. Top quarks decay 100 percent of the time into a bottom quark and a W boson. The W boson can decay into electrons or muons (collectively called charged leptons) and an associated neutrino. Because the nature of decays involving W bosons, the charged leptons carry a lot of the spin information of the parent top quarks.

DZero physicists isolated a sample of over 400 events in which a pair of top quarks was created and decayed in the desired way. We were able to compare the trajectories of the charged leptons and ascertain how the spin of the top quarks was related. The measurement was consistent with predictions and shows how far we have come since the top quark’s discovery more than 15 years ago.

— Don Lincoln

These physicists performed the data analysis leading to this measurement.
These Fermilab physicists are responsible for the smooth operations of the DZero detector for the purpose of this analysis.