Beyond the God Particle (39 page)

A multi-TeV Muon Collider has many potential advantages over electron colliders, most of which arise from the lack of synchrotron radiation emission by muons, due to the heaviness of the muon compared to the electron. This allows a compact circular design of a synchrotron with multi-pass acceleration and multi-pass collisions. This could make for a cost-effective approach to reaching high energies with point-like lepton beam particles. Also, the Muon Collider would have a very narrow and well-defined beam energy. These are things that proton and electron colliders do not have. Electron linear colliders at very high energies, greater than 1 TeV, simply consume too much power due to not having the advantage of multi-pass collisions in a circular machine, since electrons lose their
energy to synchrotron radiation. There is no physical problem in principle with a Muon Collider energy scale approaching in excess of 10 TeV (the equivalent proton collider energy scale for the same energy of point-like quark and gluon collisions would be about 100 TeV).

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.
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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-falsifiable 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.
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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)!