After Higgs, Ramped-Up Collider Hunts for Next Puzzle

Don Lincoln is a senior scientist at the U.S. Department of Energy's Fermilab, America's largest Large Hadron Collider research institution. He also writes about science for the public, including his recent "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Things That Will Blow Your Mind" (Johns Hopkins University Press, 2014). You can follow him on Facebook. Lincoln contributed this article to Live Science's Expert Voices: Op-Ed & Insights.

Somewhere under the French-Swiss border, two protons have a date with destiny. Trapped inside the Large Hadron Collider (LHC), the world's largest and most powerful particle accelerator, they follow a circular path in opposite directions with velocities very near the speed of light.

After a hiatus of more than two years, the LHC is up and running again. After a broad program of refurbishments, retrofits and upgrades, the accelerator is essentially an entirely new facility. Operating at nearly double the energy and triple the number of collisions per second, the LHC will create collisions within the centers of four huge experiments, each ready to make the discovery of the century.

Since Einstein's 1905 papers on relativity, physicists have known of the equivalence between energy and mass. As described by Einstein's famous equation (E = mc2), energy can be converted into matter and vice versa. And that's one of the big things that happens inside a particle accelerator. The huge kinetic (i.e., moving) energy of the two incoming beam particles is converted into the mass of particles that didn't exist before the collision.

When the LHC began operations in 2010, it had a clear mission. Two large experiments, each comprised of around 3,000 scientists, were focused predominantly on finding the Higgs boson. Predicted in 1964, the Higgs boson is connected to the Higgs field, which is thought to give the mass to fundamental (i.e., pointlike) subatomic particles. Finding the Higgs boson meant that the idea of the Higgs field was validated.

While the discovery of the Higgs boson in 2012 was indeed an enormous success for the scientific community, the triumph came with a disappointment. Explaining this is simple: Essentially, the Higgs boson was like a final piece that completed the Standard Model puzzle. However, as any puzzle enthusiast will tell you, it is the tabs and blanks of pieces that allow one to build a puzzle. The hanging tab gives you a hint as to what the next piece will be. But a completed puzzle is silent on what to do next.

Another mystery stems from that famous Einstein equation, E = mc2. It actually says that when energy is converted into matter, an equal amount of antimatter will be made. During the Big Bang, the universe was full of energy, and this energy transformed into equal amounts of matter and antimatter. Yet when scientists look at the universe, they see only matter. So where did the antimatter go? While physicists have had a few hints from previous experiments, we don't really know the answer. This is another mystery.

There are other mysteries, too, like wondering if there are smaller building blocks of the universe than those with which we are now familiar. Following the history of investigations into that question, we have learned of molecules and then atoms. Research in the early 1900s revealed protons, neutrons and electrons, and the 1960s brought to light the quarks and leptons that are currently considered the smallest particles of nature. However, it is natural to ask if there might be even smaller building blocks. While scientists don't know the answer, there must be some sort of deeper and more fundamental physics that can explain the patterns seen in the quarks and leptons. The answer to that question is yet another mystery.

Because of the laws of quantum mechanics, the Higgs boson can fluctuate into other types of particles (e.g., the top quark, the W and Z bosons, and even pairs of Higgs bosons). This behavior leads to predictions of the mass of the Higgs boson that are closer to the Planck mass which is a hundred quadrillion times heavier than the mass that scientists have measured. (The Planck mass is the highest mass our current theories could possibly apply and marks a frontier beyond which we are certain that we will have to rethink everything.)

The most popular theoretical explanation is a principle called supersymmetry. This idea essentially postulates that the force-carrying bosons (particles with a subatomic spin that is integer multiple of ħ, which is the natural unit for spin in the quantum world). For example, photons of spin 1 × ħ and the matter-carrying fermions (particles with half integer subatomic spin, e.g. electrons of spin 1/2 x ħ) should appear in the theory in a symmetric way. That means if you swap all the fermion and boson symbols, the equation will remain unchanged. Essentially this puts forces and matter on equal footing, making them conceptually interchangeable.

Mind you, no direct evidence for the existence of these supersymmetric particles has ever been found. However, if they do exist, scientists can use these particles' quantum mechanical properties to cancel the contribution of the familiar particles in calculations of the mass of the Higgs boson. With supersymmetry accounting for the other particles, the calculations result in a predicted mass of the Higgs boson that is small, in accordance with measurements.

Some scientists' enthusiasm for supersymmetry has been dampened by the fact that supersymmetric particles haven't been observed. Thus, researchers are exploring other possibilities, for example, the ideas that there might exist additional dimensions of space or that the Higgs boson might contain smaller particles within it. These ideas and others are alternative approaches for taming the unruly predictions of the mass of the Higgs boson.

To quote the famous philosopher Yogi Berra, it's hard to make predictions, especially about the future. Thus it is difficult to know exactly what discoveries will be made at the LHC. However, it seems probable that the mystery of the mass of the Higgs boson is the most promising thread at which scientists can tug. Hopefully, the right tug will let us unravel the existing Standard Model and allow us to knit an even better theory. Only time will tell if we will be successful.

Don Lincoln is a senior scientist at Fermi National Accelerator Laboratory and an adjunct professor of physics at the University of Notre Dame. He conducts his research using the Compact Muon Solenoid detector located at the Large Hadron Collider. Co-author of more than 800 scientific papers, his scientific interest is broad, spanning such questions as the nature of dark matter, understanding why we see no antimatter in the universe and whether the familiar quarks and leptons are composed of even smaller particles. In addition, he has many popular science books to his credit, including "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Things That Will Blow Your Mind" (Johns Hopkins University Press, 2014). He writes for the NOVA website, has written cover articles for Scientific American and has published articles for CNN and the Huffington Post. He also produces a series of YouTube videos about particle physics and cosmology for the public. Lincoln is a Fellow of the American Physical Society and was awarded the 2013 Outreach Award from the high energy physics division of the European Physical Society. The opinions expressed in his commentaries are solely those of the author. You can follow him on Facebook (http://www.facebook.com/Dr.Don.Lincoln)