In the last few years, stories have abounded in the press of the successes of the Large Hadron Collider, most notably the discovery of the Higgs boson. This has led some to speculate that European research is ascendant while U.S. research is falling behind. While there is no argument that U.S. particle physics budgets have shrunk over the past decade, it is also inarguable that America is still huge player in this fascinating research sector, collaborating on projects in Europe and Asia while pursuing a strong domestic program as well.
To properly appreciate the breadth of the U.S.’s contribution to particle physics research, one must distinguish between the international program and the domestic one. The international program is currently (and appropriately) focused mostly on the LHC. The ring-shaped collider is, without a doubt, an amazing piece of equipment. It is 17 miles around, took a quarter century to plan and build, cost about $10 billion, and requires about 10,000 scientists to operate and study the data it generates. Four distinct experiments (ALICE, ATLAS, CMS and LHCb) were built to use the LHC to investigate some of mankind’s oldest scientific questions.
Physicists employed by U.S. universities and national laboratories comprise about a third of the LHC experimental program, making the U.S. the single largest country involved in the project. Although the CERN laboratory itself employs more LHC scientists than any other single institution, America’s Fermilab and Brookhaven National Laboratory are uncontested seconds for CMS and ATLAS, respectively. American physicists lead many analysis efforts and the CMS collaboration even elected Professor Joe Incandela of the University of California, Santa Barbara to be the group leader.
While there is no denying the attractiveness of the LHC as a scientific opportunity, U.S. scientists also pursue an active and vibrant U.S. domestic program. Fermilab serves as the hub for the American particle physics community and the laboratory’s accelerators, both present and future, are helping scientists blaze new trails into the fascinating subatomic world.
Because the LHC is firmly ensconced as the highest energy facility in the world for the foreseeable future, Fermilab is focusing on a different technique to delve into the fundamental rules of the universe. By choosing to concentrate on making the highest intensity particle beams ever achieved, the U.S.’s domestic program is able to investigate some of the rarest phenomena ever imagined at energy scales that far exceed those accessible at the LHC. High energy means that individual beam particles are moving at unprecedented speeds, while high intensity means many particles focused on a tiny area, much like a magnifying lens can focus light. When many particles are brought into tight proximity, there is a small chance that a quantum mechanical fluctuation will allow an extremely unlikely and ultra-high energy interaction to occur.
It’s easy to explain to people why building a higher energy facility is valuable, but understanding why higher intensity beams is a leading research strategy is a little more difficult and requires two insights. The first and simpler insight is to realize that in particle physics, we look for rare collisions between beams of particles. The reason we look for rare ones is that the common ones have been studied already.
To observe the rarest collisions, one must simply make a lot of collisions and wait. It’s similar to trying to win the lottery. If you buy one ticket, you are unlikely to succeed, but if you buy many tickets there is a much higher chance that you have bought a winner.
The more subtle insight hinges on the principles of physics, specifically quantum mechanics. While it is a firm rule of classical physics that energy is conserved, this rule is not so rigidly observed in the quantum realm. According to the tenets of the Heisenberg Uncertainty Principle, energy can simply appear, as long as it disappears quickly enough. Further, the larger the temporary energy imbalance, the shorter the duration. Thus, because they persist for so short a time, the large energy imbalances are very rare. And, as I have noted above, to study very rare processes, one must employ very intense beams.
Using the current Fermilab accelerator complex, physicists are studying the interactions of neutrinos with matter. Neutrinos only experience the weak nuclear force and can pass through a lot of matter without interacting. To give a sense of scale, the sun constantly emits neutrinos. If we were determined to stop half of them, we’d need a wall composed of solid lead that is five light years thick! Given this reluctance to interact, the only way to ensure enough neutrino interactions to study is to generate incredibly intense beams and analyze them with massive particle detectors.
The Fermilab MINOS and NOVA experiments shoot unprecedentedly bright beams of neutrinos from Chicago to northern Minnesota to study an interesting phenomenon called neutrino oscillations. Neutrinos are unique in that they can change their identity, vaguely as if an electron could change into a proton and back. It is hoped that understanding this oscillatory behavior might explain why the universe is made solely of matter when we believe that matter and antimatter existed in equal quantities when the universe began.
A second bright star in the constellation of U.S. particle physics research is the use of Fermilab’s accelerator complex to study muons, the heavy cousins of electrons. Scientists of the Muon g-2 experiment will measure the magnetic moment of muons. Earlier measurements at the Brookhaven National Laboratory were very precise – with eight digits of precision. However, there is a tantalizing tension between data and theoretical predictions. While both measurement and prediction are exquisitely precise, the two numbers disagree slightly. This disagreement is small, but is about three and a half times larger than the combined experimental and theoretical uncertainty. This discrepancy could signify the onset of new physics, which could involve supersymmetry, muon substructure or something entirely unexpected. Because Fermilab can generate more intense beams of muons than Brookhaven, the g-2 apparatus was moved from Long Island, New York to Chicago to investigate this question more thoroughly.
Yet another interesting question that has been investigated relates to unconventional muon decays. Most muons decay into electrons and two neutrinos, however there are reasons to suspect that perhaps muons might decay into electrons without neutrinos. The Mu2e experiment at Fermilab is scheduled to start recording data in a couple of years and this experiment will be sensitive to energy scales far higher than the LHC can achieve. Since neutrinos transform into other types of neutrinos and quarks can change into other quarks, physicists think that the transition of muons into electrons might be possible. Because this decay is expected to be very rare (if it exists at all), this is another reason to make high intensity muon beams.
A multi-year study of the pressing physics questions by the entire U.S. particle physics community resulted in a firm recommendation to upgrade the Fermilab accelerator complex to further increase the amount of beam it can supply. Thus, the long term plan for the Fermilab laboratory is to increase the intensity of its neutrino beams by at least 50 percent and shoot these beams off to a detector to be located in South Dakota. Because neutrinos change their identity (i.e. oscillate) in flight, having detectors at different distances from Fermilab gives a complementary view of neutrino oscillations and it will shed more light on the phenomenon.
But the U.S. community hasn’t forgotten the energy frontier. Eventually, there will be an accelerator that replaces the LHC as the energy leader. It will be a long time before any decisions are made on where that facility might be located (or even what kinds of beams will be needed: protons or electrons). But, to be prepared, several institutions across the U.S. are expanding their accelerator development programs. Whether the future facility is located in the U.S., Europe or Asia, U.S. accelerator scientists will be heavily engaged in developing the required technology.
Even with tight budgets, the American particle physics community has continued to have a huge impact in humankind’s investigations of some of the oldest scientific questions, and continued support is key to maintaining this leading role.