Looking for the W-prime

The transverse mass distribution of electron + missing transverse energy for data and standard model expectation. The red region shows our expectations for a W-prime of mass equal to 800 GeV/c2.

In the Standard Model the quarks and leptons communicate with each via the exchange of particles known as bosons. The bosons of the Standard Model are the photon, the gluon and the W and Z particles. It is natural to wonder if there are additional bosons present in nature; after all there is nothing in the Standard Model that restricts the number to four. The observation of a new boson would be a major discovery and would signal, perhaps, the existence of a new force of nature. Since the discovery of the W boson in 1983, many experiments have looked for a hypothetical heavier cousin to the W, generically called the W’ (pronounced W prime).

A recent analysis at CDF searched for evidence that a W’ was produced and decayed to an electron and a neutrino using a 5.3 inverse femtobarn data sample. The Standard Model W bosons also decay to an electron and a neutrino, and the Tevatron produces them by the millions. In order to get away from these everyday bosons, we looked for candidate events that are much more energetic than expected.

In order to get a really clean sample of electrons necessary to isolate these hypothetical W’ bosons, we measure their momentum in our tracking chamber and their energy in our calorimeter. For electrons, the momentum and energy should be about the same. The neutrinos in the event stream right through the CDF detector without leaving a trace. But we can infer their energy by looking for a momentum imbalance. The figure shows a distribution that represents the combined energy of the electron and neutrino. A W’ particle would show up as an excess of events on the right hand side of this distribution. As you can see, our data is well described by the Standard Model backgrounds, so we find no evidence of the W boson’s heavier cousin. We use this lack of evidence to conclude that the mass of the W-prime boson must be larger than 1.12 TeV/c2 at the 95 percent confidence level This result essentially means that if the W’ exists, it will not be found at the Tevatron, but perhaps it will be discovered at the higher energies of the LHC.

These physicists are responsible for this analysis. From left: Dong Hee Kim, Jieun Kim and Yu Chul Yang; all from Kyungpook National University, Korea.