Electroweak BuZZ

The production of two Z bosons in a single collision is among the rarest phenomena expected to occur at the Tevatron. It is about 20,000 times rarer than finding a single W boson. Exploiting the entire Run II DZero data set netted just 10 events of this kind.

The weak nuclear force is the weakest interaction that physicists at the Tevatron have successfully studied. Like all subatomic forces, we can understand it as being caused by the exchange of force carrying particles, specifically the W and Z bosons. These bosons were discovered in 1983, netting a quick Nobel Prize the following year.

The W and Z bosons are physical particles that are seen as a consequence of the unification of the weak and electromagnetic forces. The unification theory initially predicts that these force carriers are massless, but become massive by interacting with the hypothetical Higgs boson. So the study of the W and Z bosons is tied together with the Higgs search.

During the course of the following 25 years, physicists observed other interesting types of events containing W and Z bosons, each rarer than the ones before. In 2008, the DZero experiment observed the production of pairs of Z bosons. This phenomena is very rare; it’s about 20,000 times harder to produce pairs of Z bosons than it is to produce a single W boson.

Since the discovery, DZero has accumulated almost four times as much data and extended the analysis. To give a sense of the rarity of the phenomenon and difficulty of analysis, when the entire data set we’ve collected over the last eight years was exploited, we observed a grand total of ten particle collisions in which two Z bosons were created.

Ten is a small number, but it’s enough to start studying things in detail, including such things as the energy and direction of the Z bosons and their decay products. We even evaluated the data to see if there was evidence for a new type of particle produced that decayed into Z bosons.

The data was consistent with the Standard Model, which is bad news for people searching for new physical phenomena, but crucial input for our Higgs boson searches. You see, if the Higgs boson is heavy, it will decay into two Z bosons.

— Don Lincoln

Ryan Hooper is affiliated with Brown University and Bradley University and performed this analysis.
The muon detector operation team ensured that the DZero collaboration’s muon data was of high quality. Since Z bosons can decay into pairs of muons, this group directly contributed to this analysis. From left: Al Ito, Fermilab; ,Igor Churin, JINR; Penny Kasper, Fritz Bartlett, Yuri Yatsunenko, and Norik Khalatyan, Fermilab; Igor Vasilyev, IHEP; Amitabha Das, University of Arizona; Lev Uvarov, PNPI; Vladimir Goryachev, ITEP; Sergey Kulikov, IHEP; Ivan Kiselevich, ITEP; (Inset: Valeri Tokmenin, JINR)