Voyage into the unknown
In the deep, dark quantum sea known as the Intensity Frontier, particle physicists expect to find everything from exotic new particles to new insights into the evolution of our universe. They are likely to discover the completely unexpected, changing our understanding of matter, energy, space and time.
At their darkest depths, the Earth’s oceans have given rise to life forms foreign enough to have come from other worlds. Scientists have discovered life that exists without light, bacteria that feast on foul-smelling chemicals fatal to humans, and colonies of giant clams and tubeworms that thrive in vents hotter than 400 degrees. The forms of life that scientists have observed in these alien ocean realms are stranger than anything in our imaginations.
The depths of the quantum seas harbor their own surprising creatures. Probing the secrets of the subatomic world, physicists have found stealth neutrinos that traverse the Earth without leaving a trace, colorful quarks that form a zoo of short-lived denizens and massive bosons that break fundamental concepts of nature such as left-right symmetry. And they know there is more to discover. The known particles account for less than one twentieth of the entire mass and energy of the universe.
In a research effort known as the Intensity Frontier, scientists are embarking on a voyage into the deepest, darkest recesses of the subatomic realm. What will they see when they shine their most intense particle beams ever at the dark, unknown and unexplored quantum universe? Will their large, massive particle detectors make discoveries that prove even more strange and startling than those made in the past?
“No one has been there before,” says SLAC National Accelerator Laboratory’s JoAnne Hewett, who co-leads US efforts to map out research opportunities at the Intensity Frontier. “There are certain to be some things we expect to see. But some things will be completely unexpected and could change our view of how the universe works.”
By probing the subatomic depths with intense particle beams and super-massive detectors, scientists hope to find undercurrents of strange new particles and behaviors in the uncharted waters called “new physics.”
“We know that there is new physics out there, and we know that there are particles and interactions that are not part of the Standard Model,” says Argonne National Laboratory’s Harry Weerts, who together with Hewett co-chaired an Intensity Frontier workshop last fall. “Only a few percent of the universe is made of matter like us. We think we should be able to explain why, but so far we haven’t been able to. With very intense particle beams and large detectors, we will be able to look for the very rarest of processes.”
In mapping out their voyage and preparing for a new generation of experiments, Intensity Frontier collaborators have identified the six straits most likely to lead to discoveries:
- Neutrinos - What can these elusive, ghost-like denizens of the quantum seas tell us about the evolution of the universe? Did they cause the overwhelming predominance of matter over antimatter throughout the most distant corners of space? Are neutrinos the reason we exist?
- Charged leptons - Why does the familiar electron, the most common species in the family of leptons, have two heavier cousins, the muon and the tau? How and why did the different species originate? Can they morph into each other?
- Hidden-sector particles - Can axions, chameleons and other strangely named creatures point the way to the most mysterious and shrouded latitudes in the vast quantum seas? Can they explain the presence of mysterious, invisible substances known as dark matter and dark energy, which make up 95 percent of the universe?
- Proton decay - Do protons exist forever? The observation of a single proton decaying would unleash a tsunami of scientific confusion and chaos and yet advance Einstein’s dream of a grand unified theory-exactly the kind of discovery these quantum voyagers hope to confront.
- Heavy quarks - These meaty creatures have always ridden the high tides of discovery, unveiling those one-chance-in-a-billion particle breakdowns that keep scientists awash in new questions and new puzzles. What else can we learn from sea devils called charm, strange and bottom quarks?
- Nucleons, nuclei and atoms - Time always seems to pass slowly on a voyage. But what if time flowed backward as well as forward? Among the crews exploring these composite particles through complex, messy collisions, one group is throwing caution to the winds as they search for processes that re-cast the concept of time.
Let the voyage begin.
Leaving no wake in the sea of neutrinos
We live in a sea of neutrinos. Every second, trillions of them pass through our bodies without leaving a trace. With their infinitesimal mass and zero electric charge, they zip straight through just about everything-even through hundreds of miles of rock. Even stranger, there are three species of neutrinos, and the ghostly little creatures are constantly transforming from one to the other. Weirder yet, a neutrino might be its own antiparticle.
These exceedingly odd particles may be just the ticket to understanding one of the biggest mysteries out there. The Big Bang that spawned the universe should have produced equal amounts of matter and antimatter, yet there is almost no antimatter anywhere, except what cosmic rays produce in the Earth’s atmosphere and physicists produce with particle accelerators. What caused this imbalance of matter and antimatter?
Scientists have discovered different behaviors between quarks and antiquarks, but those are not sufficient to explain the imbalance. If scientists were to see the equivalent symmetry breaking in neutrinos, they would have new clues about the evolution of the early universe and the dominance of matter over antimatter.
Experiments that analyze neutrinos and antineutrinos produced by the world’s best Intensity Frontier accelerators aim to do just that. Pointing the particle beams at large, massive detectors, scientists can observe the rare interactions of neutrinos and antineutrinos with matter.
“Neutrinos were major players in the early universe,” says Kate Scholberg of Duke University. “So if we see neutrinos and antineutrinos behaving differently, that will give us a hint about what happened to all that missing antimatter.”
Other experiments try to reveal the neutrinos’ secrets by looking for nuclear processes that are so rare that detecting just a few signals over the course of a year would be a triumph. If scientists observe neutrinoless double-beta decay, they will know that neutrinos are their own antiparticles, which would make them different from all other building blocks of matter. At the same time, it would provide theorists with a clue to why a neutrino weighs less than a millionth of the mass of an electron.
Stars can provide important information on neutrinos, too. The cosmos puts on a neutrino show every time a star dies and exits in what is known as the core collapse of a supernova. Neutrinos make up 99 percent of the energy released in these supernova collapses. Large, massive neutrino detectors for Intensity Frontier experiments would light up following the collapse of a star in our galaxy.
Observing these neutrinos and measuring their properties would present the ultimate test for astrophysical models that explain the nuclear processes in stars from birth to death.
“Core collapses happen only every 30 years or so in the Milky Way,” Scholberg says, “so it’s a gamble when a detector would actually observe one. But if you win, you win big.”
Muons in the charged-lepton sea
The muon is among the strangest points on the chart of the particles and forces called the Standard Model. When this plump cousin of the electron first surfaced in cosmic-ray research in 1936, noted theorist and future Nobel Laureate I. I. Rabi famously quipped, “Who ordered that?”
Today scientists use the muon as a tool in particle physics, cosmology, material science, archeology and public safety. With muons scientists can image materials such as high-temperature superconductors, examine chemical reactions and search for nuclear weapons hidden in cargo containers.
Particle physicists hope that the muon-some 200 times the mass of the electron-will bring clarity to the murky waters in the charged-lepton strata. They know that neutrinos, which are leptons without electric charge, morph into each other. And so do quarks. Can muons do it as well?
By sharing this behavioral trait, the muon could point to a fundamental concept that would be common to all particle classes.
“Our world is diverse, from the very large-the cosmos-to the very small-the subatomic realm,” says Stan Wojcicki of Stanford University. “Our goal is simplification. Could we put all of physics on the back of a T-shirt?”
Simplicity has been the goal of scientists as long as there have been scientists. Sir Isaac Newton declared: “Truth is ever to be found in simplicity.” More than 200 years later, Albert Einstein agreed: “A theory is more impressive the greater the simplicity of its premises.”
To have a chance to discover the rare occasion of a muon transforming into an electron, scientists need an accelerator that produces more muons per second than ever before. Advances in accelerator technology have paved the way.
Muons are light enough to be produced copiously, yet heavy enough to serve as a unique probe of new phenomena beyond the Standard Model. One particular muon property that scientists are interested in is its precession, or wobble, in a magnetic field. Standard Model calculations predict with great precision the muon’s behavior in a magnetic field. Experimental measurements, however, indicate a slightly different result. With Intensity Frontier accelerators that produce more muons, scientists can make more precise measurements and either confirm or refute the discrepancy.
Exploration of the darkest of seas
Axions, chameleons, heavy photons-these hypothetical particles don’t interact with the ordinary forces that we know. Instead, they could take us into the darkest realm of the universe, the realm of the dark matter that makes up 25 percent of the cosmos and the dark energy that accounts for 70 percent. If these particles do indeed exist, they are the prime movers in the evolution of our universe. Ordinary matter, less than 5 percent of our universe, provides the points of light in this dark, vast sea.
These theoretical particles could reside in what is called a “hidden sector.” As-yet-undiscovered forces might offer a link between the particles that make up this sector and the visible universe. The key for access to the hidden sector could be a yet-to-be-discovered particle called the heavy photon.
“Think of the Chronicles of Narnia,” says Rouven Essig of Stony Brook University. “The wardrobe in the attic has the doorway to the hidden magical realm of Narnia. This heavy photon could be our doorway to the hidden sector.”
The heavy photon would interact with particles having an electric charge, but would interact very weakly. Due to its heavy weight, it would act very differently from the standard, massless photon.
The lighter particles being sought in this hidden sector are grouped as WISPs-Weakly Interacting Sub-electron volt (or “slim”) Particles.
Axions, for example, are considered excellent dark-matter candidates. And chameleons could explain the origin of dark energy.
“How chameleons interact with ordinary matter could depend on their environment,” Essig explains. “For example, near the Earth where there is lots of matter around, they would be very heavy, and they could hide from terrestrial experiments. But farther out in universe, where there is not much matter around, they could be very light, and they could act as dark energy.”
Becalmed in the sea of protons
Remarkably steady, protons seem to exist forever-representing the calmest waters in the stormy seas that make up our universe. But what if even they are transient?
Theorists have long suggested that protons, while extraordinarily long-lived, may actually break down. But their mean lifetime would still exceed 1034 years-that’s the number 1 followed by 34 zeros. In comparison, our universe is about 14 billion, or 14,000,000,000, years old. But the same way that not all creatures grow old, some of these protons would decay in their early years. Only extremely large particle detectors that monitor an incredibly large number of protons for a long time have a realistic chance to record such an event.
Searching for proton decay is still a long shot-but finding it would lead to a huge payoff. It would bring us closer to Einstein’s dream of a theory that unifies all forces, combining the electromagnetic force, the weak nuclear force and the strong nuclear force; it would point the way toward the unification of quarks and leptons by linking them to a single structure.
If there were signs that protons could indeed break down over some finite amount of time, it would raise stormy seas for everything in our view of matter and the universe.
“A finite proton lifetime would imply the instability of ordinary matter and change our understanding of the future of the universe,” says Carlos Wagner of Argonne Lab and the University of Chicago. “If found, proton decay will no doubt constitute a landmark discovery for mankind.”
In the grips of the quark undertow
The six species of quarks found in the quantum seas bear unusual monikers, also called their flavors: up and down, charm and strange, top and bottom. Even the term quark, introduced by Nobel laureate Murray Gell-Mann, has a distinctive, literary origin: a line from James Joyce’s “Finnegan’s Wake.”
The oceans of the world are filled with quarks and leptons-the building blocks of matter. But only the two lightest quarks-up and down-are the main ingredients of water and other everyday things. So why are there more flavors of quarks? What can they tell us about the physics at high-energy scales, even higher than those achievable at the world’s highest-energy particle accelerator, the Large Hadron Collider in Europe?
A flood of flavor physics discoveries has come from US-based experiments that no longer run, including the CLEO program at Cornell University, the BaBar experiment at SLAC and Fermilab’s Tevatron collider experiments. Yet flavor physics research is flourishing in many countries around the world.
Together with their colleagues in Asia and Europe, US scientists are working on a new generation of experiments that focus on three quark species: strange, charm and bottom. These experiments will use intense particle beams to create far greater numbers of particles containing heavy quarks than previous ones. They will enable scientists to search for the rare and unexpected when sifting through the decays of these particles and obtain more precise information on the often surprising behavior of heavy quarks.
These findings will provide essential constraints and complementary information on measurements and discoveries made at the LHC, and they have the potential to reveal new physics that is inaccessible to the LHC.
“If the LHC does discover beyond-Standard-Model physics, there are endless studies showing how flavor physics measurements can give us important information about new physics, beyond what the direct measurements at the LHC can provide,” says Zoltan Ligeti of Berkeley Lab. “There is both complementarity and synergy between the heavy-quark physics experiments and the LHC.”
Beached in backwards bay
Trust your navigation tools: Time flies like an arrow, right? Always straight ahead, right?
Well, maybe and maybe not.
“We know that time goes from the past to the future,” says Vladimir Gudkov of the University of South Carolina, Columbia.
“However, most physics laws allow for a change in the arrow of time. There should be a fundamental reason why time cannot change its direction. This phenomenon-the impossibility of going in the opposite direction-is called time reverse invariance violation, or TRIV.”
A crew led by Gudkov and colleague Young-Ho Song plans to test the TRIV phenomenon in nuclear reactions using neutrons, one of many Intensity Frontier experiments with nucleons, nuclei and atoms under consideration. According to their calculations, the complex structures of neutrons and nuclei result in an array of collision parameters that should magnify any possible TRIV interactions by several orders of magnitude.
The stakes are high: The TRIV search is another attempt at answering the question of why we are here and why the universe works the way it does.
“The existence of our universe, and the prevalence of matter over antimatter, requires TRIV,” Gudkov says. “However, we do not know a source of TRIV that is strong enough to explain why we exist, or why there is the prevalence of matter over antimatter. So this search is a way to find a manifestation of new physics.”
A coordinated effort to explore the deepest, darkest recesses of the Intensity Frontier is now underway. Initiated by the US Department of Energy’s Office of Science and led by SLAC’s Hewett and Argonne Lab’s Weerts, scientist-explorers are charting the course. Last fall, about 500 Intensity Frontier experts from around the world gathered at a kick-off meeting in Washington, DC, including researchers from across the United States and throughout the national laboratory system. Since then they have written a report-authored by researchers from 145 institutions-that summarizes the research opportunities at the Intensity Frontier.
Jim Siegrist, director of the Office of High Energy Physics in the Office of Science, emphasized in his remarks at the workshop that a strong US particle research program is vitally important for the nation and for the world.
“This program must be diverse in order to best position our community to make the next major discoveries and follow where they lead,” he said. “It must keep the US a leader in the field, to keep the balance between the major world regions and ensure a steady flow of new talent and ideas to US physics institutions.”
In taking up that charge, scientists expect a voyage that will call up, to quote Shakespeare, the “spirits from the vasty deep:” finding something new; finding a question that can’t be readily answered; finding something that just won’t fit in the current framework of particle physics, the Standard Model. They hope for results that will illuminate realms that have never seen light before.