Beyond the God Particle (37 page)

Our neighbors often ask, “So, what is the future of Fermilab?” Fermilab is the sole remaining single-purpose scientific laboratory dedicated to elementary particle physics in the Western Hemisphere. Fermilab no longer operates the Tevatron, which up to the time of the LHC was the world's most powerful particle accelerator. The Tevatron discovered the top quark, and in its last days it spotted the Higgs boson in a unique decay mode and production mode that only the Tevatron could explore. Alas, for funding reasons, and the impact on other planned projects, it was terminated on September 30, 2011.

Scientists first stopped the CDF and DZero detectors. They then stopped the data acquisition system and switched off the electricity to various sub-detector systems. Then they shut down the Tevatron. Helen Edwards, who was the lead scientist for the construction of the Tevatron in the 1980s, terminated the final store in the Tevatron by pressing a button that activated a set of magnets that steered the beam into the metal target. Edwards then pushed a second button to power off the magnets that guided beams through the Tevatron ring for 28 years. For about a week following the shutdown, accelerator operations worked to warm up the superconducting magnets, normally kept at 4.8 Kelvin. Once the magnets reached room temperature, crews began removing the Tevatron's cooling fluids and gases. It took about a month to fully shut down the CDF detector. Shutting down the DZero detector took longer, since the collaboration took data using cosmic rays as a way to double-check the calibration of its detector. The DZero detector was completely shut down after about three months.
¹

The termination of the Tevatron program marked the end of Fermilab's reign as “king of the energy frontier,” since the Main Ring accelerator was first turned on in the 1970s. Unfortunately, this has given rise in the press to a false perception that the laboratory no longer has a mission in particle
physics, and that its future has now become uncertain. But, in terms of future plans, the laboratory has many. Fermilab's director for Project X exclaimed to a reporter:

“We have 10 accelerators here on site,” says Fermilab physicist Steve Holmes, with the merest hint of irritation. “We turned one of them off, okay?” Like several scientists I spoke to, Holmes was keen to point out that colliding high-energy beams of particles is not the only way of discovering new physics with accelerators.”
²

FERMILAB'S PROJECT X

Fermilab has a unique and critical mission to find new ways to penetrate deeper into the fabric of nature. And, yes, we do have plans for another approach. It's a departure from the conventional “energy frontier” effort using particle colliders such as the LHC. It is complementary to the LHC. It marks a revival of an older approach—the very manner in which the science of new forces and the structure of matter at short distances began. It follows the lessons of the heroes of the grand generation, the pioneers of modern physics and discoverers of radioactivity: Henri Becquerel, Marie and Pierre Curie, Ernest Rutherford, and many others. They deeply probed inside of matter, to discover and study
ultra-rare processes
that ultimately revealed new physics.

Such an approach may reveal the first real chinks in the armor of the Standard Model. The energy scales (the short distances) that can be probed by this indirect route are hundreds to thousands of times greater (smaller) than those of the direct approach of a collider. When the style of the old physics of this pioneering generation of scientists is combined with the recent advances in the technology of accelerators and detectors, astonishing new opportunities abound. And the usual benefits to society of developing these new technologies—the “exogenous inputs” to the economy—will accrue.

As Fermilab evolves the Long-Baseline Neutrino Experiment (LBNE), which will ultimately aim a neutrino beam at the Homestake Mine in South Dakota, it is preparing in parallel for the eventual construction of the world's most intense particle accelerator: “Project X.” Project X will be the centerpiece of the future of Fermilab and the US High Energy Physics
program. Project X is a
high-intensity
proton accelerator, sometimes called a “proton driver.” Incidentally, this has the mysterious name “Project X” not because it is shrouded in some kind of secrecy but simply because no one has come up with a better one. If you have any suggestions for a better name for Project X, please don't hesitate to contact us.

Let's start with something simple: there is a profound difference between “intensity” and “energy.” At the LHC we have fewer protons in the beam, but each has the highest energy to which we have ever accelerated protons. At Project X we will have lower-energy protons in the beam (from about 3 to 8 GeV) but many, many more of them so that the overall beam power is the highest ever achieved.
³
In our microscope analogy it's like turning up the brightness of the particle beam and at the same time studying many different and exotic samples under the microscope to search for something new.

Project X is an ambitious and aggressive technological goal: The construction of about a
5-megawatt
proton accelerator with an energy of 3–8 GeV per proton. Project X would become a new enabling technology of much of the mid- to long-term research goals at Fermilab, much like the gas discharge tube was for Röntgen, or photo emulsions and phosphorescence were for Becquerel. Project X would give the US a powerful new scientific instrument to advance basic research.

The physics program with Project X is extraordinarily rich. Detailed studies of neutrinos at LBNE, which would require 30 years without Project X, can be done within a decade with Project X. The rarest decays of K-mesons, which first taught us about CP violation, become possible and may reveal new physics at energy scales approaching 1,000 TeV. Project X will open an entirely new probe of CP-violation (or, the “time mirror” in our Alice metaphor) physics by permitting the study of super-heavy atomic isotopes that may provide unprecedented sensitivity to the detailed properties of electrons, neutrons, and nuclei themselves. This enables the greatest reach for possible discovery of the
electric dipole moment
of the electron (see below), directly giving us a new window on CP violation and possibly a new window on dark matter. Project X will also enable us to build a “Muon Storage Ring Neutrino Factory” that would provide an unprecedented source of
both
electron and muon neutrinos and that would give us the capability to study the neutrino's physical properties at the highest level
of precision and to search for new physics. And, Project X sets the stage for perhaps the most exciting high-energy collider of all: the Muon Collider.

PROJECT X NEUTRINO EXPERIMENTS

Up to now, all neutrinos we study are the product of pion decays, since pions are easy to make in large numbers if you have a very high-power accelerator, such as Project X. Pions, when they decay, only yield muon neutrinos, and this limits the possible neutrino oscillation studies we can do. We would ultimately like to launch an electron neutrino underground on its way to the Homestake Mine in South Dakota to see what it morphs into (recall our description in the
previous chapter
of hamsters morphing into mice).

Muons decay into electrons, antielectron neutrinos and muon neutrinos. Therefore, if we were to capture the muons from pion decay, place them in a
racetrack-shaped “storage ring,”
where most of the muons decay in the straight sections, they would give us a powerful beam of antielectron neutrinos, as well as muon neutrinos (we could alternatively place anti-muons in the storage ring to produce anti–muon neutrinos and electron neutrinos) A neutrino factory would allow, for the first time, the study of the neutrino oscillations of launched electron neutrinos in long-baseline experiments. It would be as though we could launch a mouse instead of a hamster and see what it morphs into over a long baseline trip.

Wait a minute—did we say “capture the muons” and put them in a storage ring? They only live for two millionths of a second, so are we sure that's what we meant? Yes, we're sure. This has been done, but at nowhere near the intensity scale required for a Neutrino Factory.
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Smaller muon storage rings have operated since the early 1970s at CERN and at the Brookhaven National Lab. The latter's ring is moving to Fermilab and will be used to precisely measure the magnetic properties of the muon, known as the “g-2” experiment.
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The goal of the Neutrino Factory is to significantly scale the size of the storage ring and to increase the intensity of the muon beam circulating in the ring. The muon storage ring could provide the ultimate Neutrino Factory.
⁶
It also gives us a great deal of “batting practice” for the eventual construction of the Muon Collider.
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Fermilab has acquired enormous experience in the burgeoning science of neutrinos and currently operates several major neutrino experiments. It is upgrading its accelerator complex to improve them. As we have seen in
chapter 10
, neutrino CP violation may be of profound importance, as it may play a key role in the generation of the matter–antimatter asymmetry observed throughout the universe.

RARE KAON PROCESSES AND CP VIOLATION

CP violation was first observed in experiments with “kaons.” Kaons are strongly interacting particles that are composed of a light quark, an “up” or “down” (or anti-“up” or “down”) quark with an anti-strange (or strange) quark. Of particular interest are the anti-down, strange or anti-strange, down states. These are called the neutral K-mesons,
K
⁰
, and
(see chapter 9,
note 20
).

The neutral kaons have long been known to “oscillate” between one another as they travel through space, and they are forerunners of the neutrino oscillations. The detailed study of these kaon oscillations led to the original discovery of CP violation in physics (the fact that the time mirror takes Alice to a different world, not her own). The detailed properties of neutral kaons may reveal the surprise of a small discrepancy with the Standard Model and indicate the presence of some new physics. There are also charged kaons consisting of (up, anti-strange) or (strange, anti-up) quarks whose decays are also potentially sensitive probes that may also reveal new physics.

Kaon experiments that study the decays of these particles with trillions of produced kaons would yield a very high level of precision in monitoring the rarest processes in the standard model. These rare processes typically involve two W bosons “flickering” into existence for miniscule instants of time as quantum fluctuations. At the same time, top quarks, and possible new particles, can flicker in the same fluctuation, yielding potentially surprising signals. By measuring these processes in detail we can reach a new level of sensitivity to possible new and unknown physics beyond the Standard Model.

Of particular interest are two ultra-rare processes that involve decays
of kaons into pions and neutrinos. These are a K
⁺
→
π
⁺
v v
-bar and K
⁰
→
π
⁰
v v
-bar. The latter process has a very precisely calculated Standard Model rate, and any deviations from this would be evidence of new physics. To fully probe these requires experiments capable of detecting about 1,000 of these decays of both the charged and neutral kaons.

Future Project X–based kaon experiments will be able to probe for new physics with unprecedented precision, up to energy scales of hundreds of thousands of TeV, well beyond the reach of any foreseeable high-energy colliders. Should a kaon experiment at Project X reveal a new rare process, it would be the direct analogue of the Becquerel-Curie discovery of the weak interactions of over a hundred years ago. It would provide a clear-cut goal for the next century of particle physics.
⁸

The high-intensity proton beam of Project X would readily enable such experiments. The particular technology of the Project X accelerator design, called a “continuous-wave linac” (this means a continuous beam, rather than a more typical pulsed beam), would provide ideal conditions for these experiments, permitting major simplifications of the experimental apparatus. The measurements would reach the precision of a few percent for these extremely rare decay rates of the kaons, comparable to the uncertainty on the Standard Model prediction. This thus offers the ultimate sensitivity to any new physics in these processes that might alter the decay rates from their Standard Model predictions. The two experiments would additionally offer sensitivity to a variety of other rare kaon decays involving speculative exotic new particles.