A decade after it was shut down, results analyzed from the Collider Detector at Fermilab (CDF) have measured the mass of the W-Boson, an elementary particle, to be slightly heavier than that predicted by the so-called Standard Model of particle physics. The results have been calculated after analyzing the results from CDF for ten years, and it is the most precise measurement of the W-Boson’s mass ever made.
The Standard model is our model of reality at its most fundamental levels. It describes the elementary particles, Fermions and Bosons, of different types similar to the W-Boson, and the forces which govern interactions between them. Out of the four fundamental forces that govern all reality, the Standard Model describes three: electromagnetism, the strong nuclear force which keeps nuclei together, and the weak nuclear forces which govern radioactive decay of nuclei.
Both W and Z bosons are carriers of the weak nuclear force, and they are formed through radioactive processes. The collider at CDF smashed trillions of protons and antiprotons (antimatter doubles to protons, carrying the same mass but opposite charge) into each other at high velocities as one such process, which produces a W-Boson once every 10-million collisions. However, the boson cannot be measured directly; it decays far too fast, and its presence must therefore be inferred from the particles it decays into.
It is this process that the sensitive sensors at CDF and subsequently the LHC are designed to observe. The present calculation uses 4.2 million observations of W-Boson candidate particles, almost four times as much data as the group’s last measurement of the W-Boson’s mass in 2012, and the experiment was blinded to minimize the risk of human bias, meaning that physicists analyzing its data were kept in the dark about its results until their work was completed.
"The number of improvements and extra checking that went into our result is enormous," Ashutosh V. Kotwal of Duke University, who led the work, said in a statement.
The results give the W-Boson’s mass at 80,433.5 ± 9.4 MeV (one Mega electron-Volt is about the mass-energy in one electron), whereas the Standard Model’s predictions predict the mass at 80,357 (MeV) ± 6 MeV. Although the error (approx 77 MeV) is small in magnitude, the measurement is precise to within 9 MeV, making the deviation nearly eight times the margin of error. Mathematically put, the statistical significance of the result is 7-sigma, far above the statistically accepted gold-standard of 5-sigma in particle physics. This means that if no new physics affected the W boson, discrepancies at least as large as the one observed would still arise from pure chance once every 800 billion times the experiment was run, whereas a 5-sigma result corresponds to a given effect appearing through chance once every 3.5 million runs.
This discrepancy could mean that either the math behind the analysis of the experiment or the experiment itself was wrong, or that the Standard Model is missing some parts. However, the paper produced in Science has been peer-reviewed and the experiment is the most precise one made till date. Therefore, the results have left the physics community stunned and excited, as it could potentially rewrite particle physics as it is currently known through the Standard Model.
“Nobody was waiting for this discrepancy,” says Martijn Mulders, an experimental physicist at CERN near Geneva, who was not involved with the new research but co-wrote an accompanying commentary in Science, while speaking to Scientific American. “It’s very unexpected. You almost feel betrayed because suddenly they’re sawing off one of the legs that really support the whole structure of particle physics.”
“The Standard Model is a very well-balanced structure. Therefore this measurement is not just one measurement. If this measurement changes, then the balance of the model changes which means we have to re-evaluate all the other measurements that we have done and see if, perhaps, by changing some other parameters things become natural” said Dr. Amol Dighe, professor of Physics in the Department of Theoretical Physics at the Tata Institute of Fundamental Research (TIFR) while speaking to Mint
For example, the Standard Model was used to predict the properties such as mass and charge of several particles which were subsequently discovered in what is called the great age of particle physics in the twentieth century. Most famous amongst them was the Higgs Boson, dubbed the “God-particle”, which gives mass to all other particles including the W-Boson. The Higgs, which was finally discovered at CERN’s Large Hadron Collider (LHC), had a mass predicted by the standard model as well.
However, the Standard Model had been facing challenges since before the results of this experiment as well, chief amongst them being its inability to factor dark matter, dark-energy, neutrino masses and gravity. In this context the results solidify scientists’ belief that there is “new physics” beyond the Standard Model the results are pointing to.
“As a particle physicist, I am confident in saying that there must be more physics waiting to be discovered beyond the Standard Model. It is these mysteries that give physicists new clues and new reasons to keep searching for a fuller understanding of matter, energy, space, and time” said John Conway, a part of the team that built and ran the Collider Detector at Fermilab (CDF), in an article on The Conversation.
It is not what you understand but what you don't understand that is interesting about the scientific enterprise. In the past, discrepancies like the W-Boson’s mass have pointed towards the requirement and emergence of novel theories in physics. For example, the wobble in Mercury’s orbit which Newtonian theories could not account for pointed the way from Newtonian to Einsteinian pictures of the world. Similarly, the double-slit experiments paved the way for Quantum Mechanics.
Nevertheless, scientists are doing their due diligence trying to go through the results of the Science paper and its methodology through a fine-comb. A flurry of papers attempting to account for the discrepancy in mass through novel theories and particles is expected. One of the candidates for the same is Supersymmetry -- a theory that has not produced any results so far despite experiments to find such supersymmetric particles at the LHC. More can be expected in the near future.
Binit Priyaranjan is a freelance journalist, author and poet.