Old-school electroweak

Of the three forces we understand in the subatomic realm, the electromagnetic and weak forces are said to be unified, resulting in the electroweak force.

For centuries, scientists have been showing that seemingly disparate phenomena are really one and the same. In the late 1600s, Isaac Newton showed that the force that keeps us on the ground is the same as the one that governs the motion of the heavens. We now know this as Newton’s theory of universal gravity. In the 1800s, a series of experimental and theoretical studies culminated in the blending of our understandings of electricity and magnetism into the theory of electromagnetism. The 1960s was the era in which electromagnetism and the weak nuclear force were found to be two different facets of a combined force that we now call the electroweak force. The Higgs boson was invented as a way to smooth out a few technical kinks that were encountered in the process of uniting these two theoretical models.

However, long before the recent Higgs excitement, scientists were exploring this melding of the two forces in other ways. One of the features of electromagnetism is that if you replaced left with right and right with left everywhere in the theory, you wouldn’t notice the difference. In contrast, the weak force has very different behavior. If you swapped the right and left in equations of the weak force, the new equations would predict behavior that has never been observed. Thus studying what happens when you swap left and right in electroweak studies is a way to investigate which aspect of the two forces, electromagnetism or the weak force, dominates as collision conditions change. We look at the production and subsequent decay of a particle we call a Z/γ, which is written this way to denote that in electroweak theory we cannot identify whether a particle is a Z boson or a photon. This may very well be confusing, as a Z boson is massive and a photon is massless. It seems as though it would be easy to distinguish between the two, but we are talking here about the quantum realm. In this subatomic world, the mass of a particle is often different from our simple expectations, resulting in massive photons and Z bosons with unusual masses.

CMS physicists studied events in which a Z/γ was produced and subsequently decayed into an electron or muon matter-antimatter pair. They studied the direction of the trajectory of the matter decay particle compared to the trajectory of the Z/γ to see if they tended to be in the same or opposite directions. Electroweak theory can predict this variable, and observing a difference could be the sign of new physical phenomena, including the existence of a new and heavier cousin of the Z boson. The data were in excellent agreement with the Standard Model, so there’s no new physics yet. However, as the data set increases, it will be possible to repeat the measurement with higher precision.

—Don Lincoln

These physicists contributed to this analysis.

The CMS Endcap Muon group is constructing additional cathode strip chambers at CERN. These chambers will augment the existing detector and increase its capabilities. Pictured here: P. Bulteau (Wisconsin), X. Yang (UCLA), S. Kreyer (UC-Santa Barbara), M. Weber (UCLA), E. Takasugi (UCLA), J. Beres (UCLA), C. Farrow (Wisconsin), A. Lanaro – Manager (Wisconsin).