Forty years ago, on 25 January 1983, the UA1 collaboration and the UA2 collaboration announced in a press conference at CERN the discovery of the W boson. Both Collaborations published in the following months the results of two independent searches made on data recorded at the Super Proton Synchrotron at CERN while it was running as a proton-antiproton collider.
The charged W boson is a fundamental particle that, together with the Z boson (discovered later in 1983), is responsible for the weak force, one of four fundamental forces that govern the behaviour of matter in the Universe. Particles of matter interact by exchanging these two bosons, but only over short distances.
In October 1984, The Royal Swedish Academy of Sciences awarded Professor Carlo Rubbia and Dr. Simon Van der Meer with the Nobel Prize in Physics for their decisive contributions to the large project which led to the discovery of the field particles W and Z, mediators of weak interaction.
In the last forty years, researchers have learned a lot about the W boson and, for this reason, it has become a crucial piece of knowledge to perform new analysis and research.
We have asked the CMS physicist Guillelmo Gomez-Ceballos to tell us more about the W boson.
What is known and what is unknown about the W?
To a large extent, we know almost everything about the W boson. Nevertheless, a larger precision may lead to discrepancies not seen previously. This is like the lenses of a photo camera looking at a forest: the better the lenses are, the more details can be seen. Moreover, there are some possible decays of the W boson not being observed so far, and therefore this is something we don't know yet. These are predicted within the Standard Model, but with a probability far below our experimental reach. Finding such very rare decays would imply an indication of a disagreement with the Standard Model prediction.
Why is the W boson so important to us?
The W boson is a very important handle to go on with our physics program. Since its properties are rather well-known, these are the first measurements to perform, so that other particles may be either studied or searched afterwards. As an analogy, when looking at the landscape with a photo camera, we use W bosons to focus our subject before moving our sights around and looking for other things. The W boson is also very interesting by itself. Given the current experimental precision and the predictive power of the Standard Model, it is crucial to measure the fundamental parameters of the W boson with very large precision.
How does CMS use it to explore the properties of the Higgs boson?
The Higgs boson decays to a pair of W bosons with a probability of about 20%. Furthermore, there are some Higgs boson production modes with one or more associated W bosons, which also play an important role in the studies on the Higgs boson. Therefore, the W boson is crucially used to study the properties of the Higgs boson.
How does CMS use it to search for physics beyond the Standard Model?
The W boson is "used" to search for a large number of physics models beyond the Standard Model. First of all, many models are predicting new particles decaying to W bosons, therefore this is a direct use of W bosons. In addition, there are background events that may mimic our possible new physics particles. In some cases, we use W boson events to estimate the contribution of those Standard Model background processes.
What would the Universe look like without the W boson?
It is not easy to imagine the Universe without a W boson. If there were no W bosons, there would likely be no Z bosons either, and therefore there would not be a weak force to talk about. As an example, the nuclear reactions occurring inside the Sun every minute are possible because of the weak force. Without the W boson, and Z boson, the Sun would not be as we know it, and life on Earth would not have been possible.
On the same line, the Higgs mechanism no longer existed, and electroweak symmetry breaking never occurred either. As simple as that, if the W boson did not exist, the rest of the Standard Model would not quite work out.
Disclaimer: The views expressed in CMS blogs are personal views of the author and do not necessarily represent official views of the CMS collaboration.