Beyond the God Particle (38 page)

RARE MUON PROCESSES:
μ
TO e CONVERSION

Rare decays of muons, such as μ → e
γ
, if observed, would also be a harbinger of new non–Standard Model physics, since this process does not otherwise occur at an observable level in the Standard Model. Some exotic theories predict decay rates for this process that could be within reach of experiment. A related process involves the conversion of a muon to an electron upon scattering off of an atom, known as “μ-to-e conversion.” This could also be sensitive to exotic new physics mass scales that may lie at thousands of TeV.

Fermilab is planning an experiment designated “Mu2e,” which will use the existing Fermilab 8 GeV proton beam from the old Tevatron Booster to search for the μ-to-e conversion process with sensitivity at a level 10,000 times better than previous experiments. This experiment will probe for new physics mass scales up to 10,000 TeV, significantly beyond the reach of the LHC. Project X offers the possibility of increasing the beam power to the experiment by more than a factor of 10, allowing an ultimate sensitivity ten times greater. If the Mu2e experiment discovered these exotic processes, the Project X–era experiment would offer the unique capability of distinguishing the underlying new physics by measuring the μ-to-e conversion rate using different nuclear targets.

If we're fortunate and a new process such as μ-to-e conversion is detected, we would again be dealing with the direct analogue of the Becquerel discovery of radioactivity in the 1890s. There would be much to do as a follow-on study, as this would likely be the first hint of a new force in nature, probably involving some new “X” boson that would become the bread and butter of a futuristic collider of the late twenty-first and early twenty-second centuries.

PROJECT X PROBES OF ELECTRIC DIPOLE MOMENTS USING RARE ISOTOPES

Electrons define the entire world of chemistry and biology, the world that our eyes can see and about which our brains can think. Electrons are essentially “us.”

J. J. Thompson, in 1897, had discovered that the “cathode rays,” comprising the electrical current in a gas discharge tube (sort of the precursor to the fluorescent lightbulb), were actually particles streaming through the tube. These particles had a very small mass compared to the atom (the rest of the atom's mass being the heavy nucleus), and Thompson showed that they were part of every atom in nature. These particles had a definite fingerprint: their ratio of electric charge to mass, or “e/m,” is a definite value, which can easily be measured by bending their trajectories in a magnetic field. When Rutherford later studied the beta-decay radioactivity in detail, he found the emitted particles were electrons, because they had the same charge-to-mass ratio as Thompson's electron.

Today we know more about the electron and its interaction with the photon than of any other physical system in nature. This is all codified in the magnificent theory called “quantum electrodynamics,” which was ultimately developed into a consistent, calculable theory of every aspect of electron-photon physics in 1949, due to Julian Schwinger, Richard Feynman, and Sin-Itiro Tomonaga, who shared the 1965 Nobel Prize for their efforts.
⁹

We know that electrons, because of their charge and their spin, are each little magnets. A spinning electrically charged particle is, in effect, a loop of electrical current, and currents produce magnetic fields. This is called a “magnetic dipole” field because it has the same form as a bar magnet, which has two poles, N and S. An electric charge, on the other hand, produces an outwardly directed electric field, called a “monopole field.” The equations of electricity and magnetism are such, as far as we know, that there are no magnetic monopoles in nature. But this leaves one final loophole: are there elementary particles that have electric dipoles? Does the spinning electron that produces a magnetic dipole field also produce a similar electric dipole field?

Let's return for a moment to Alice's parlor. Consider an electric field emanating from an electric charge (an electric monopole field) and a magnetic field emanating from a current loop. If we look at a mirror image of this, we see there is a difference in how electric and magnetic charges reflect in mirrors. If the electric field is emanating “outward” from the charge (a positive electric charge), then its mirror image will also show an electric field emanating “outward” from the charge—there's no change in the mirror image. On the other hand, if the magnetic field is emanating upward vertically out of the current loop, then in the mirror the field will reverse, emanating downward out of the loop. This can be understood by considering the mirror image of the current—if the loop plane is perpendicular to the plane of the mirror, then the current direction is seen to be reversed in the mirror, and this causes the reversal of the magnetic field.
¹⁰

Now, if instead of the parlor mirror, we use the antimatter mirror, C, and we change the sign of the electric charge (that is, replace electrons with positrons), then both the magnetic field and electric field change direction.

So, if we do both a reflection in the parlor mirror, P, and follow by a reflection in the C mirror, C (swap all particles for antiparticles), for a net CP reflection, then the electric field always behaves oppositely to the
magnetic field. So, if an electron produces a particular electric dipole field, then there must be a violation of CP symmetry, since the alignment of the electric and magnetic dipole fields of the electron select a preferred side of the combined mirrors. CP violation is always interesting because, since the combination of passing through the CPT mirrors always gets us back home, it is therefore associated with the arrow of time in physics, i.e., passing through the time mirror, T, undoes the effect of passing through the combined mirrors, CP.

No one has ever discovered an electric dipole of a point-like elementary particle. Certain molecules, like water, are famous because they spontaneously form a bent configuration that does have an electric dipole field. However, this is associated with the complexity of the water molecule. The bent molecule is a snapshot that is far from its pure quantum ground state. In its pure quantum ground state of rotational spin, even a water molecule has no electric dipole field. But truly elementary particles have “intrinsic spin” and by their very identity are always in their pure ground state of spin. For them, the existence of an electric dipole field is always violation of CP symmetry.

The discovery of a nonzero electric dipole field, or “electric dipole moment” (EDM), as it is more professionally called, for any elementary particle, e.g., the electron, would be of historic significance and could indicate the existence of new CP-violating physics. It would surely win a Nobel Prize for its discoverers. The CP violation in the Standard Model can produce an infinitesimal electric dipole field that is too small to be seen in the current experiments (it's about 10
^-38
in units that are the electric charge of the electron multiplied by 1 cm, or e-cm). EDMs provide potentially remarkable sensitivity to new physics.
¹¹
Indeed, the known Standard Model CP violation among quarks is also too far away to explain the creation of the matter–antimatter excess and yet exist, so there must be some other source of CP violation in the universe. The search for nonzero EDMs is an excellent way to probe nature to try to get hints, in the spirit of Becquerel and the Curies, about what the other unknown sources of CP violation may be.

There are many exciting experiments that attack the problem of electric dipole moments. We can describe only one interesting line of attack presently based upon the remarkable fact that a big atom, an atom with a very heavy nucleus, provides an “amplifier” for EDMs of electrons.
¹²
Indeed,
there are some really heavy nuclei. The heaviest ones, above uranium, such as radon, radium, americium, and francium, are all radioactive, that is, they disintegrate, and some don't live very long at all. The number of protons in the nucleus is always denoted by Z. The effect of large Z atoms is to amplify the effect of the EDM by large factors that grow as Z
³
.

Project X can yield large quantities of heavy short-lived isotopes, such as radon, radium, americium, and francium, to support precision searches for the electron EDM. These experiments could significantly improve the existing limits by a whopping factor of 100 to 1,000. We think the technologies acquired here will lead to multi-billion-dollar economies in the future as well.

RIDDING THE WORLD OF PLUTONIUM AND PROVIDING ETERNAL CLEAN ENERGY: ACCELERATOR-DRIVEN SUBCRITICAL REACTORS

Particle physics demands and drives the creation of leading-edge technologies, the capability of studying systems as small relative to the atom as a basketball is small relative to the earth. Society and global economies have greatly benefited from the development of the most powerful particle accelerators and detectors, i.e., the most powerful “microscopes” ever created by humans. The advent of such applications of these technologies, such as advanced medical imaging, the very effective proton therapy for cancer treatment,
¹³
massive data handling and computing, and the World Wide Web, has paid back many thousands of times what the investments originally cost and has played a major role in defining our modern world.

New ideas, which require Project X for testing and development, offer the prospect of virtually infinite and clean sources of energy and the means to “incinerate” nuclear waste. These are profound goals: the generation of safe, clean, and abundant electrical power through accelerator-driven thorium reactions, and the potential “incineration” of radioactive wastes from conventional nuclear power production.
¹⁴
They can be explored at the “proof of principle” level at Project X, with eventual implementation of these technologies elsewhere.

The issue of high radio-toxicity and the long lifetime of conventional
spent nuclear fuel is a global challenge. Accelerator-driven systems, like Project X, can be used to transmute spent nuclear fuel, which would significantly reduce the lifetime and toxicity of nuclear waste. Accelerators can also produce net energy by inducing fission in lower-atomic-weight elements such as thorium (Th). These are called “accelerator-driven subcritical” reactors because they don't require a “critical mass” of radioactive fuel and can never lead to an event such as the Fukushima or Chernobyl core meltdowns.

ADS reactors would have many key advantages: (1) it is estimated that the abundance of conventional reactor fuel,
²³⁵
U, is limited to about 100 years of energy production at current global demand rates, while the
²³²
Th isotope is abundant and is estimated to be able to provide 10,000 to 100,000 years of available fuel; (2) using Th fuel eliminates the production of toxic long-lived heavy actinides (such as plutonium and americium) and significantly lessens production of long-lived nuclear waste; (3) use of Th limits the possibility of nuclear weapons proliferation; (4) accelerator-driven reactors run in the “subcritical mode” and would be relatively safer to operate, i.e., one can turn off the accelerator driver and the reactor will shut down. There are no “core meltdowns” to fear.

The government of India and the US are contemplating formal cooperation in these areas of nuclear energy research in conjunction with Project X. Project X could support R&D of (1) the development of techniques for the destruction of spent fuel from conventional nuclear reactors, and (2) the development of ADS systems for safe and abundant nuclear energy production. We do not envision a full-scale accelerator-driven nuclear reactor development program at the Fermilab site, but key elements for the future of safe ADS nuclear energy can be studied and developed with Project X. Developing ADS reactors is, to us, a “no-brainer.” It should have happened “yesterday.” It is now becoming urgent.

BEYOND PROJECT X: THE NEXT COLLIDER

Over the next decade, experiments at the Large Hadron Collider will continue to explore a new energy regime and uncover the details of the Higgs mechanism that distinguishes the weak interactions from
electromagnetism. The answer appears to be the Standard Model Higgs boson, but there may be a more elaborate accomplice—perhaps a new form of physics—new forces of nature, new symmetries, new particles, or new intricacies of space and time. Highly sensitive experiments at Project X will study rare processes, such as neutrino oscillations, EDMs, and very weak transitions among different quark and lepton flavors, and will indirectly probe energies well beyond those explored directly at the LHC.

To prepare to capitalize on any discoveries of new physics from the LHC and/or Project X–based experiments, Fermilab scientists are exploring the feasibility of a multi-TeV Muon Collider. This could be the highest-energy collider for the next generation beyond the LHC. The Fermilab community is leading physics and detector studies to map out the physics potential of a Muon Collider in terms of the machine's energy and luminosity. These studies will provide details as to how experiments could be carried out at a Muon Collider.

A Muon Collider uses muons and anti-muons as the “beam and target.” The muons are produced from the intense Project X proton beam. The Muon Collider could begin life as a “Higgs factory,” providing the best determination of the Higgs boson mass and directly scanning the Higgs boson in a unique way that no other collider can. The main advantage of the Muon Collider, unlike an electron linear collider, is that it could be scaled upward to become a very high-energy collider, effectively probing energy scales ten times greater than those at LHC. Any deep questions left unanswered from the LHC may ultimately be addressable by the high-energy Muon Collider.