Great science to come
24 June 2022
The discovery of the Higgs boson in 2012 was something that all of us involved will remember for the rest of our lives. But we won’t just spend the rest of our lives looking back: there’s a lot of science yet to be done at the Large Hadron Collider, which will begin its third data run on July 5, ten years and a day after the observation of the Higgs was announced. Run 3 will see the LHC collide protons at even higher energies than previously achieved, and will be more than doubling the size of the current data sample (reaching ten times as much data as was used for the Higgs discovery). In fact, 95% of the particle collisions planned for the LHC program have yet to happen!
Will we have another moment as dramatic as the Higgs discovery? Quite possibly not, but that doesn’t mean that there isn’t a lot of great science to come. The future LHC program should be compared to some of the sky surveys being performed by major telescope observing programs. The Rubin Observatory, for instance, will be conducting the Legacy Survey of Time and Space, which will spend ten years taking images of the entire visible sky every few nights, allowing for a time-lapse movie of the universe that will capture billions of objects. The result will be a giant catalogue of information about stars and galaxies, which will be used for a greater understanding of dark matter, dark energy, and millions of objects within our own solar system and galaxy. The greatest science won’t be obtained from any single one of those images, but from the entire dataset collected. Similarly, the LHC experiments are building up a similar catalogue of collision events that will help us explore the properties and interactions of particles with great precision, allowing us to find subtle deviations from predicted behaviors that will allow us to exclude many speculative theories — or perhaps demonstrate that one of them may be correct. We’ll need all the data we can get to have a full picture of particle physics.
All that being said, with the higher collision energy, larger datasets, upgraded detectors, and thousands of really creative scientists, maybe something distinctive will be found during Run 3! What are some of the possibilities?
One thing that we are confident we will gain from the coming run is a much better understanding of the Higgs boson. So far, we have observed Higgs interactions with the vector bosons (photons, Zs and Ws) that are responsible for particle interactions, and with the heaviest of the matter particles, the top and bottom quarks and the tau lepton that are in the third generation of fermions. We have not yet established that the Higgs interacts with the second-generation fermions, namely muons and charm quarks (although there is now evidence for Higgs decays to muons). Additional data and improved event reconstruction techniques make these clear targets for Run 3.
There’s also much to learn about other particles that have already been discovered. Earlier this year the CDF collaboration released a new, very accurate measurement of the mass of the W boson, and it was seven standard deviations different from previous measurements and from theory predictions. If it’s correct, the measurement is an alarm bell in the night for new physics. This measurement is a big challenge at the LHC, but it’s important for us to do our best to confirm or refute the CDF result. Also, we will be producing huge numbers of the other standard model particles, which will allow us to test that model ever more stringently through high-statistics measurements and searches for very rare phenomena.
At the same time, the great hope is always that we might discover truly new phenomena. Perhaps we have started to see these already? CMS, my experiment, has a number of results that are out of line with the standard model; these are not sufficiently statistically significant to be meaningful at the moment, but maybe they will become so as we double the data sample. We’ll also be pushing to use the detector in creative ways to look for types of particles that can be harder to observe, such as those that have long lifetimes and thus lead to interactions taking place far from the proton collision point.
On July 4, 2032, we will be celebrating the 20th anniversary of the announcement of the observation of the Higgs boson. By that time, we expect to have completed both Run 3, which will double our data sample, and also Run 4, the first run of the High-Luminosity LHC, which will double the sample yet again. We’ll know if any of the things I suggested above have come to pass. I hope we’ll be writing about it for Quantum Diaries once again!
I thank my US CMS colleagues Lothar Bauerdick (Fermilab) and Tulika Bose (University of Wisconsin) for discussions that stimulated the above ideas.