Fermilab leads the national Muon Accelerator Program (MAP) aimed at developing and demonstrating the concepts and critical technologies required to produce, capture, condition, accelerate, and store intense beams of muons.15 Critical technologies are under study, including conducting experiments to demonstrate “muon cooling” (necessary to make a refined beam of muons and anti-muons), the study of RF cavity performance in the presence of high magnetic fields required for muon cooling, and the study of very high-field solenoids. MAP is also conducting advanced studies of beam dynamics, simulations of the muon production, capture, cooling, acceleration, and collision processes. The initial application of these new technologies might be the construction of a Neutrino Factory based on a muon storage ring.
Fermilab's expertise in high-field superconducting magnets will also be critical to any future synchrotron, such as a Muon Collider or Very Large Hadron Collider (VLHC), which both benefit from magnets capable of achieving the highest possible fields. For example, one design for a Muon Collider requires enormous 50-tesla focusing solenoids, while a 40-TeV VLHC in the LHC tunnel would demand 25 to 30-tesla dipole fields. Such magnets could be based on high-temperature superconductors operating at low temperatures, where they can carry high currents in high magnetic fields. Fermilab is engaged in R&D leading to the construction of the first high-temperature superconductor-based magnets for future energy frontier accelerators.
Q: HOW TO BUILD A STARSHIP? A: START AT THE BEGINNING
With Becquerel's discovery of radioactivity, the weak interactions were seen for the first time. The methodology was quite different than collider physics today. For Becquerel and the Curies, one began with pitchblende. In pitchblende, there is uranium, and the radioactive disintegration of the radium atom reveals the physics indirectly. By analyzing lots of pitchblende, one could observe very rare processes and classify them. This is, after all, how all science begins—observation of phenomena followed by classification.
The key to the search for any rare processes is to have a large quantity of data. The data can be collider data at the LHC, where the search is now on for the various decay modes of the Higgs boson and any particles beyond the Standard Model. Higgs factories will aim at even more copious and cleaner samples of Higgs bosons. But in a world of relatively low-energy physics there are ultra-rare processes that can be studied to probe the fundamental laws of physics, and that could reveal new and previously unanticipated forms of physics. This would provide the necessary arguments to build the next collider.
This is the quarry of Project X. It is the logic of a world in which there's only a Standard Model Higgs boson, but no evidence of anything else “nonstandard” at the LHC. We believe it is a “no-brainer” that now we begin to pursue the search for new and beyond-the-Standard-Model physics with the high sensitivity and diverse program afforded by Project X, and the eventual capability of a return to the energy frontier with a Muon Collider.
We have told you the story of the Higgs boson. We have tried to give you an idea about why it exists, based upon what we've learned about the nature of mass in the previous century. We've seen how the understanding of the basic concept of “mass,” known only as the “quantity of matter” since the ancients, became more profound in the late twentieth century at the deepest level of the basic building blocks of nature, the elementary particles.
We have seen that the masses of quarks and leptons involve the interaction of two disparate and different massless particles, a left-handed particle that has a “weak charge,” together with a right-handed particle that has zero weak charge. Mass is an “oscillation” between left and right. The interaction of left and right requires a new particle that also has the weak charge of the left-handed component to maintain the conservation law of charge. This is the Higgs boson. It is mandated by profound symmetries that are fundamental and immutable in nature.
The masses of particles are generated when the Higgs field develops a “field” in the vacuum, inferred from Fermi's theory to have a value of about 175 GeV. The Higgs field, like an enormous magnetic field, extends uniformly in all directions throughout all of space and time. The Higgs field is effectively a great reservoir, filling the vacuum with its weak charge. Into this reservoir a left-handed particle can discard its charge to become an uncharged right-handed particle; likewise, the right-handed uncharged particle can acquire the weak charge from the vacuum to become left-handed. This leads to the oscillation in time—left-right-left-right—for all quarks and leptons; this is the phenomenon of mass. And like ordinary electric and magnetic fields, whose particle constituents are photons—the particles of light, the quantum of the universal Higgs field that binds left and right is the Higgs boson.
On July 4 of 2012, the discovery of the Higgs boson was announced at the home of the world's largest particle accelerator, the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. The Higgs boson has weighed in with a mass of about 126 GeV.
CONFUSED ABOUT BIG SCIENCE
Our fellow citizens often get confused about what big science is trying to do, perhaps because of what we tell them, usually in the media. For example, all too often we hear that colliders are built “to discover extra dimensions,” to “confirm string theory,” “to discover supersymmetry.” False! Colliders are built to uncover whatever is happening in nature at the shortest distances, and not to accommodate the agendas of various sects of theorists. Often we hear that colliders are built “to re-create the conditions in the early universe (the big bang).” There's some element of truth to that, but in fact colliders don't re-create the thermal plasma in the hot, dense early universe; if they did, we wouldn't see the remarkable phenomena of quark jets (see Appendix) or CP violation in our collider experiments.
That this is confusing and mixing messages is best illustrated by something that happened about the time the Superconducting Super Collider (SSC) was terminated. We recall, long ago (but we can't remember exactly where) hearing a radio interview with a nurse who had just exited one of the large hospitals in Houston after a long day at work. A microphone was suddenly thrust in her face, and she was asked by a radio reporter, “Tell us, what do you think about today's cancelation of the Super Collider?” The nurse paused for a moment then replied, “We already have one universe, so I don't see why we have to create another one.” The problem is that when people are told in a public presentation about all the latest and hottest gee-whiz theoretical and cosmic things, they often ask at the end of the talk, “What is the practical benefit of this?” “Why should I pay for this?” “What good is this?”
In fact, it's all about the world's most powerful microscopes. We have learned, by doing the experiment over the years, that people seem instinctively to “get it” when we tell them this simple fact: particle physics is the exploration of the smallest things in the world with the most powerful microscopes we humans have ever built. The audience then asks intelligent questions, such as “How big is a quark?” or “What is the magnification power of the LHC compared to the Tevatron?” They start to think like physicists. People have an inherent notion that microscopes are useful and important to humanity—that these are powerful scientific instruments studying the tiniest things and not antecedents to weapons of mass destruction or the end of the universe. Microscopes, to our friends and neighbors, are useful. They never then ask, “Why should I pay for this with my tax dollars?” (It's true—we've done this experiment many times in our talks and colloquia!)
Through this book we wanted to tell it straight. We have focused to a large extent upon the accelerators that have been built, the world's most powerful microscopes, how we have peeled away the layers of the great onion of nature, and the machines that we contemplate for the future. In any case, we've veered away from the “theories” as much as possible because, nowadays, accelerators and experiments are few and expensive, while theories are plentiful and cheap. Science is ultimately about measurement and observation, not just pure mathematics and wild, non-fal
sifiable speculations.
Particle physics is really the ultimate “materials” science, the study of the shortest-distance scale, the fabric of all matter—even the very fabric of the vacuum that fills all of space and time. The job of the world's most powerful microscopes is to reveal the smallest structures in nature, to tweak them and call them out of the depths of their sea, so we can understand them and, perhaps, see how it all works. The essential question of particle physics is: “What is matter and how does it work?” This was the question Democritus first asked in a scientific manner over two millennia ago, and beyond the immediacy of the discovery of the Higgs boson, we still have a lot of unanswered questions and a long way to go to find the answer.
THE CONNECTIONS
To be sure, the science of particle physics is indeed connected to other sciences in glorious ways. Since it deals with the quantum attributes of matter, it is intimately, conceptually connected to the study of “condensed-matter physics,” and the weird and otherworldly ways that matter can behave under certain circumstances. We have probably learned the most about the possibilities for our vacuum and its various excitations (that's what particles are—“excitations” of the vacuum) from “superconductors,” systems made of lead, or niobium, or nickel, which are cooled down to a few degrees above absolute zero, at which point they have absolutely zero electrical resistance. Such systems are “toy” universes that can be made by hand and variously studied in the lab. There is a sort of Higgs boson–like excitation found in superconductors, and the physics of a superconductor parallels and predates the theory of the Higgs boson of particle physics.
Particle physics is also connected to the study of cosmology in a fundamental way. In fact, the major breakthroughs in particle physics, culminating in those of the Standard Model revolution of the 1970s, allowed us for the first time to understand the big bang. The great discoveries, such as the “gauge principle” shared by all forces in nature, allowed us to speculate about “grand unification” and led to the idea of “cosmic inflation” and canonized the field of cosmology.1 Suddenly cosmology became respectable. The leading cosmologists are all particle physicists. This has a certain irony because cosmology uses telescopes to look at big things that are very far way, while particle physics uses the most potent microscopes and studies the smallest things that are right under our noses and, in fact, that are us!
Indeed, the early universe is a place dominated by very high-energy collisions among particles, way up to and beyond the energy reach of our most powerful accelerators. Particle accelerators therefore yield fundamental information that is essential to understand the early universe. And particle physicists also know that there is valuable information about the elementary particles to be gleaned from the fossil record of the universe, i.e., the stuff that's left over from the big bang.
Perhaps one of the most interesting open questions is the existence of a mysterious and unaccounted for form of matter, called “dark matter,” permeating the universe that is unseen by light but is nonetheless indirectly inferred from its gravity. It surrounds galaxies and great clusters of galaxies way out in the universe. The bigger the cluster of galaxies, the more dark matter we infer is there by studying the motion of the visible galaxies in the clusters. We can indirectly “see” dark matter as it bends light by its own gravitation, making enormous cosmic lenses in the sky.
But, as of this writing, while there are more theories of dark matter than there are feral cats in Chicago, the particle that constitutes dark matter has not yet been produced and detected in a particle accelerator experiment—dark matter hasn't yet been seen under a microscope (and dark matter may be plural)!
Dark matter therefore remains a mysterious quarry of the two conjoined sciences of cosmology and particle physics of our present day. So, these two sciences—particle physics, the ultimate microscopy, and cosmology, the ultimate “telescopy”—very much overlap, as they did in the era of Hans and Zacharias Jannsen and of Galileo, as the optical microscope and telescope were developed side-by-side. These sciences are intimately connected and symbiotically benefit from one another. Dark matter definitely informs us that there are things out there that we do not yet understand and that go beyond the philosophy contained in our Standard Model. There definitely is something beyond the Standard Model and beyond the Higgs boson. And there are so many unanswered questions within the Standard Model that clearly some deeper organizing principle(s) lie beyond it.
In many ways, cosmology is like studying the fossil record of dinosaurs, learning what once existed and what questions such things may pose for the overall structure of particle physics. Cosmology is an essential subdiscipline of modern physics. However, if you want to study the detailed processes that define what we call active “life,” you need to go into the biology lab and use electron microscopes. Likewise, to understand what the basic constituents of matter are, and what the forces that control them are, you need to build a powerful particle accelerator, like the LHC or Project X, eventually, perhaps, a Muon Collider.
THE UNHEALTHY WEALTHY STATE
The health and wealth of nations critically depends upon the activity of basic research, including the seemingly more abstract construction of powerful particle accelerators. It is a no-brainer that powerful and able governments should fund it, even at the seemingly enormous costs it demands. The fact is that a world-class particle collider, nowadays, will cost some multiple of $10 billion. That multiple may be 1 ×, or 1.5 ×, or even 3 ×. But on the scale of government spending, and of the scale of the wealth of nations, this is almost a trivial expenditure. Yet the US Congress is showing little interest in healthy science funding. Europe, Japan, and China are forging on.
To get a sense of scale, the US Navy's new Gerald R. Ford–class (CVN-21) aircraft carriers cost about $15 billion for R&D and construction. These will replace the 10 Nimitz-class nuclear aircraft carriers the US Navy currently operates and that cost about $50 billion just for construction (nuclear reactors, operations, etc., drive the cost up a lot more).2 Moreover, the US sits on top of an estimated total $200 trillion—that's $200,000 billion—of coal, gas, and oil.3 The total assets of households and businesses in the US is about $200,000 billion = $200 trillion,4 while the top 100 richest US citizens have a combined wealth of about $1,000 billion = $1 trillion.5 Particle physics gave us the World Wide Web, which creates an annual revenue stream globally measured in tens of trillions of US dollars. Yet endless squabbles persist in Congress over a national debt of $17 trillion (at this writing) and a deficit of less than $1 trillion. Meanwhile, the economy and the American standard of living falters, and science wilts on the vine.
HOW DOES THE HIGGS BOSON GET ITS MASS?
The Higgs boson of the Standard Model does explain (though some may prefer to say “accommodate”) the masses of quarks, the charged leptons, the neutrinos, and the W and Z bosons. But it does not explain its own mass, about 126 GeV. It is the Higgs boson mass that determines Fermi's scale in the Standard Model. But we're still in the dark about the origin of the Higgs boson mass.
Where does the Higgs boson mass itself come from? That question has now moved to the forefront of the unanswered questions we have “beyond the Higgs boson.”
This is rather frustrating for a significant reason: our very successful theory of quarks and gluons and the strong interactions, known as “quantum chromodynamics” (QCD), emerged from a series of breakthroughs in 1974. Once it was understood, and the quarks and gluons were confirmed, the theory neatly explained where the strong masses come from (see the Appendix). These are the masses of a long list of particles found in the 1950s and 1960s, and most of the masses of the proton and neutron. In fact, we should apologize for not telling you this fact earlier, but strong mass, through the proton and neutron masses, actually makes up most of the visible mass in the universe—the masses of stars, planets, and large clouds of dust and debris of supernovas seen through telescopes. Very little of this actually comes from the fundamental and r
elatively tiny masses of the up quark, down quark, and electron. Strong mass comes from the inherent mass scale found in QCD, and not from the Higgs boson!
But QCD explains the strong mass scale in a remarkable and beautiful way—it is due to quantum mechanics itself. QCD starts out at extremely short distances (high energy) as a scale-invariant theory—that means it has no inherent mass scale at the outset—and the coupling of gluons to quarks is very feeble. However, due to quantum effects, the coupling of gluons to quarks becomes stronger and stronger as we descend to lower energies, or to larger distances. Finally, at a certain energy scale, about 100 MeV, or equivalently, a distance scale of about 0.0000000000001 centimeters (that's 10-13 cm), this coupling strength becomes virtually infinite. This causes the quarks and gluons to form composite states—the protons, the neutrons, the pions, and all the other strongly interacting particles. The quarks and gluons are then “confined” and are never observable outside of a composite state in the laboratory. This mass scale of 100 MeV is determined by the quantum interactions themselves—nature creates strong mass through its own dynamics, essentially out of no mass! It has nothing to do with any other scale of the onion of physics.
This leads to a beautiful conjecture about mass: all masses in nature are generated by quantum effects.6 That is, if we could somehow “turn off” quantum theory, somehow make Planck's constant go to zero, we would live in a world with no mass—the particle utopia we described in earlier chapters. This is exactly how the strong interactions, as described by QCD, work. It is a natural idea to extend this hypothesis, and it immediately implies that the weak interactions would work the same way.
Beyond the God Particle Page 26