New accelerator technologies bring within reach the discoveries that may transform our understanding of the physical nature of the universe.
Upgrades to Fermilab’s accelerator complex, known as the Proton Improvement Plan II, or PIP-II, will enable scientists’ search for answers to pressing questions of the universe. Wielding an intense proton beam – more than a megawatt – scientists will hope to uncover rare particle physics processes and will pursue precision particle measurements.
The upgraded complex, which will make use of the latest advances in particle accelerator R&D, will open a path to discovery in neutrino and muon physics, as well as other physics that may reveal itself along the way.
Questions in need of answers
What are the most basic building blocks of the universe? What are the forces that enable these elementary constituents to form all that we see around us? What unknown properties of these particles and forces drive the evolution of the universe from the big bang to its present state, with its complex structures that support life—including us? These are the questions that particle physics seeks to answer.
Particle physics has been very successful in creating a major synthesis, the Standard Model. At successive generations of particle accelerators in the United States, Europe and Asia, physicists have used high-energy collisions to discover many new particles. By studying these particles they have uncovered both new principles of nature and many unsuspected features of the universe, resulting in a detailed and comprehensive picture of the workings of the universe.
Recently, however, revolutionary discoveries have shown that this Standard Model, while it represents a good approximation at the energies of existing accelerators, is incomplete. They strongly suggest that new physics discoveries beyond the Standard Model await us.
A set of interrelated questions defines the path ahead:
- What principles determine the recently discovered Higgs boson’s effect on other particles? Is there one Higgs particle or many? Is it fundamental or is it composed of others?
- What are the masses and properties of neutrinos and what role did they play in the evolution of the universe? How are they connected to matter-antimatter asymmetry?
- Why is the universe as we know it made of matter, with no antimatter present?
- What is the origin of this matter-antimatter asymmetry?
- What is the nature of new particles and new principles beyond the Standard Model?
- What is the dark matter that makes up about one quarter of the contents of the universe?
- What is the nature of the dark energy that makes up almost three quarters of the universe?
- Do all the forces of nature become one at high energies? How does gravity fit in? Is there a quantum theory of gravity?
- Is the building block of the stuff we are made of, the proton, unstable?
- Are there extra dimensions of space?
- How did the universe form?
Finding answers to these questions requires powerful particle beams, providing scientists with an exquisite tool to probe our universe at its most fundamental. Such beams are possible only with the most advanced accelerator technology.
The PIP-II accelerator complex, positioned at the cutting edge of accelerator R&D, will help solve these mysteries, enabling the exploration of new physics by accelerating intense particle beams for several experiments, each of which provides new windows to the subatomic world.
The upgraded accelerator complex would provide unprecedented numbers of particles to help uncover theoretically predicted – but as yet unobserved – phenomena that are calculated to be either incredibly rare or incredibly difficult to detect. The sheer numbers of particles will help draw out the subtle behaviors that would otherwise stay under wraps.
It will also pave the way for future advances in accelerator technology.
Neutrinos are famous for being able to able to fly through everything – rock, lead, us – without leaving a trace. The ghostly behavior of these neutral particles makes them exceedingly difficult to capture and thus difficult to observe. The PIP-II accelerator would generate a significantly higher neutrino intensity than previously available at Fermilab, giving us more opportunities to study these maverick particles.
The greater intensity of neutrinos would bring us closer to understanding the three different types, or flavors: electron, muon and tau. Among other mysteries that scientists are trying to solve is the question of their masses, which are so tiny they were once believed to be massless. Neutrinos are about a million times less massive than quarks and charged leptons. Why does this enormous gap in mass exist? What is the mass of each type and by how much do they differ?
Scientists also grapple with the way neutrinos appear to change from one flavor to another as they zip through space. Evidence suggests that, for example, what is seen to be an electron neutrino in one moment may be observed as a tau neutrino in the next. What are the rules of this apparent flavor-changing behavior?
Mass and flavor-changing measurements, fascinating explorations in and of themselves, are stepping stones to addressing much bigger questions about the universe: the fate of antimatter, of which we see very little, the nature of dark energy, the unification of forces. As they cruise through space at nearly light-speed, neutrinos carry with them information that could shed light on these large-looming questions.
The more neutrinos that are generated, the closer we get to answering them.
Muon-based experiments include the search for an unimaginably rare process: the direct conversion of a muon, the electron’s heavier cousin, into an electron. The PIP-II accelerator complex would provide scientists with a staggering number of muons to search for the rare muon-to-electron conversion phenomenon.
The motivation for these investigations is an unaccountable divide in the family tree of matter.
In physics, the family tree can be broadly divided into three groups of particles: charged leptons, neutrinos and quarks. Members of the neutrino family are able to morph into each other: electron neutrinos can change into tau neutrinos. This is also true of members of the quark family: an up quark can transform into a down quark. But charged-lepton family members do not appear to directly change into each other the way those of the other two families do.
Muons, along with electrons and taus, are members of this apparently constrained charged-lepton group. The transformation of a muon into an electron and two neutrinos has been observed, but there’s nothing in the laws of physics that says that neutrinos must always accompany the electron.
Because only a tiny fraction of muons are predicted to transform into an electron in the absence of neutrinos, scientists would use intense beams to make an abundance of muons available to increase the chances of observing this process. New physics could drive this direct-conversion phenomenon to occur once in every 1018 muon decays. To set some scale, 1018 is the number of sand grains in all the beaches in the entire world! Clearly, to see this exotic conversion, scientists will need lots and lots of muons.
If muon-to-electron conversion were observed in the Mu2e experiment, it would be evidence of charged lepton flavor violation above the expected Standard Model. As such, the implication for physics would be huge. It would be a clear signal of new physics beyond the Standard Model, such as supersymmetry, a parallel set of new particles in the universe, or perhaps the existence of dimensions beyond the three spatial and one temporal dimensions we live in. The intense beams from the PIP-II accelerator will provide a much higher flux of muons to extend the reach of muon-to electron-conversion research.
Accelerator technology advances
Greater demands are being placed on accelerator performance, and PIP-II will help advance the next generation of accelerator technology. In turn, scientists will be able to extend their search for new particles and interactions.
Among these forefront technologies for future accelerators is superconducting radio-frequency accelerating cavities, which efficiently generate high-energy particle beams. Another is high-field superconducting magnets, which are necessary for bending and focusing beams. Other technologies are being developed for the required higher performance and lower cost of future accelerator concepts.
In building the future PIP-II, scientists, engineers and technicians will provide a platform for cutting-edge accelerator developments.
Progress in precision physics and rare processes will be shaped partly by what particle physicists learn in the coming decade. PIP-II will offer opportunities to further our knowledge and prepare us for the surprises that surely lie ahead.