-Kaviranjana Antony

The current state-of-the-art model that encompasses matter, energy, and the way they interact is called the Standard Model of particle physics and it comprises seventeen elementary particles. Thirteen of these are matter particles, constituting the material world around us while the rest mediate force interactions between the matter particles.

The Standard Model is regarded as one of the most notable achievements of mankind in the late 20th century, as it has confirmed many significant results such as the Higgs boson as well as acting as the premise for constructing better theories to overcome the limitations of the Standard Model.

At the quantum level, matter particles interact by exchanging bosons. Essentially, bosons function as force carriers. The W boson is an elementary particle that is responsible (along with the Z boson) for one of the four fundamental forces- the weak nuclear force, which governs radioactive decay in atoms.

This means that the W boson has the ability to change protons into neutrons and vice versa. In fact, the weak nuclear force is better understood by the electroweak theory which unifies both electromagnetism and the weak force, defining them as two different manifestations of the same force.

There have been 10 attempts to measure the mass of the W boson so far, and no two of them are the same. The most recent attempt to measure the mass of the W boson was in April 2022 by the CDF experiment at Fermilab, and the results astounded the entire scientific community. The mass of the W boson turned out to be 80.4335 ± 0.009 GeV, a number that is inconsistent with Standard Model predictions as well as previous measurements. In particle physics, any new measurement over 5 standard deviations is considered to be a new discovery.

This measurement is 7 standard deviations from the predicted mass of the W boson, 80.4357 ± 0.006 GeV, thus giving rise to numerous explanations and predictions of “new physics”, which could possibly account for this discrepancy in mass. The predicted mass of the W boson is obtained through an elegant formula Mw/Mz=cos x , where Mz is the mass of the Z boson, Mw is the mass of the W boson and x is the Weinberg angle. When the most precise measurement of the W boson till date violates the prediction from the electroweak theory, the implications are huge.

There are three possibilities:

1.Flaws in the mathematical predictions

2.Error in the measurement

3.New physics that is affecting the mass which is unaccounted for.

The first scenario is unlikely, as the sophisticated maths that is behind the measurement of the W boson is corroborated to such a high degree that it is nearly perfect. Flaws in the experimental technique or analysis are another likelihood, as instrumentation will only get better over time, enabling measurements that have greater levels of precision. Of course, the most intriguing contender is the third one; the existence of “new physics” that is currently missing from the Standard Model, resulting in a flux of hypothesised particles and forces from theorists that could account for this variation. The Standard Model is the best tool we have at our disposal to explain phenomena at the most fundamental level, however it is still far from complete. There are numerous exceptions and many occurrences that it fails to explain.

As of now, the Standard Model is regarded as a stepping stone towards developing a theory of everything, a theory that unifies all of physics into a single framework. For now though, the next step in this journey is to evaluate the experimental analysis behind the CDF experiment, verify the findings with the LHC, and hopefully get closer to straightening out the riddles posed by the universe.

References:

Cover image credits: Lucas Taylor / CERN - http://cdsweb.cern.ch/record/628469

https://www.science.org/doi/10.1126/science.abk1781

https://www.symmetrymagazine.org/article/whats-up-with-the-w-boson-mass

https://science.thewire.in/the-sciences/w-boson-mass-anomaly-tevatron-new-physics-standard-model/

https://home.cern/science/physics/w-boson-sunshine-and-stardust

https://astronomy.com/news/2022/04/trillions-of-collisions-show-the-w-boson-is-more-massive-than-expected