A remeasurement of an old experiment may have given us a big clue to some big unanswered questions in physics. Fermilab’s Collider Detector (CDF), a particle accelerator experiment that ran until 2011, recently caused a stir by remeasuring the mass of a particle known as the ‘W boson’.
Each of the four fundamental forces (strong force, weak force, electromagnetism and gravity) has associated particles that ‘carry’ the force: for example, the photon – a particle of light – is a carrier of the electromagnetic force. The W boson is one of the carriers of the weak force.
It’s unusual for an experiment that stopped collecting data more than a decade ago to arouse so much interest. The reasons are subtle but compelling. To see why, let’s take a step back and see where our knowledge of the fundamental forces and constituents of matter – expressed in the so-called ‘Standard Model’ – currently stands. The Standard Model describes the strong, weak, and electromagnetic forces, and all known elementary particles.
The theory explains the mass of W (and all other fundamental particles), and also predicted the existence of the Higgs boson, which was then discovered at CERN in 2012. This ‘completes’ the Standard Model, but leaves several questions unanswered. For example, how does gravity (a glaring omission from the model!) fit together?
Why, according to astrophysical observations, is there so much ‘dark matter’ in the Universe, and what is it? Why is there so much more matter than antimatter? The Standard Model is clearly not the whole story, and many extensions of it have been postulated.
The Standard Model is a subtle framework, however. In the subatomic world of quantum mechanics, particles influence each other even when there is not enough energy to exist. They can traverse small loops, forming and annihilating before being directly observed. We call them ‘virtual’ particles, but their influence is very real and measurable. One thing they do is influence the masses of particles, with the consequence that while the Standard Model does not predict the absolute values of the masses of particles, it predicts – very accurately – some relationships between them.
Back to mass W then. It can be measured directly, which is what CDF has done (and more on that soon). But it can also be calculated using all the other measurements we’ve made, combined with the relationships between the masses in the Standard Model.
The directly measured value must agree with the calculated value, otherwise something is wrong. Excitingly, there may be new virtual particles beyond the Standard Model participating in these loops. Interest has increased because the new CDF mass measurement does not agree with the calculated mass W.
If you were to do this CDF measurement a million million times, you would only expect such a large discrepancy, if the Standard Model is correct. As always, there are some reasons for caution. The CDF measured the W bosons produced in high-energy collisions between protons and antiprotons.
The measurement took more than a decade because it is very difficult to be so precise. When a W is produced, it decays instantly, and one of the things it produces is a neutrino, which the CDF cannot detect. The information about the neutrino (and therefore about the mass W) is calculated by assuming that it must balance out everything else produced in the collision.
This means that many different sources of uncertainty can have a significant influence, such as the distribution of particles within the proton, extraneous background particles, and of course the precise geometry and accuracy of the detector itself.
Even so, errors can never be completely ruled out, and the new measurement is, in fact, somewhat misaligned with other measurements, even those made earlier by the CDF. Now that the result is out there, it will receive a level of scrutiny that few other measurements get, and other experiments, especially those at CERN, will be scrambling to match its accuracy and confirm or disprove the discrepancy.
That said, this is a very strong indication that answers to some of the big questions left open by the Standard Model may soon be within reach, as the Large Hadron Collider begins its third runtime and will increase the accuracy with which can probe the energy frontier.
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