In fact, there's some real meat to this argument, and it actually predicted the observed scale of neutrino masses way back in the 1970s.8 So, neutrino masses may have some deep secrets in store. Neutrinos really seem to be probing energy scales a trillion times beyond the LHC! Neutrino masses may already be one of our best indirect probes of the scale of the grand unification, 1015 GeV or so, and perhaps the quantum effects of gravity.
NEUTRINO CP VIOLATION
The neutrino “flavor oscillations” likely also include a new form of CP violation. This means that the marching step from L to R, from (particle) to (antiparticle for Majorana masses), is slightly different than the step from R to L, from (antiparticle) to (particle). In our hamster metaphor, it means that the probability of the hamster becoming a mouse or a rat in a complete oscillation cycle, L-R-L, may be slightly different than the probability for an anti-hamster becoming an anti-mouse or an anti-rat in an R-L-R cycle. Antineutrinos would oscillate through a cycle slightly differently than neutrinos. Neutrino CP violation is of enormous interest, and it may provide the mechanism by which the matter–antimatter asymmetry observed throughout the universe was generated.9
LONG BASE LINES
The “neutrino flavor oscillation” phenomenon is such a slight effect that it requires a great distance over which the neutrino must travel in order to produce an observable change in flavor. It's just the fact that one full cycle of L-R-L mostly preserves the identity of the original neutrino, with only a miniscule probability of changing identity—the hamster then needs many, many steps on the hamster wheel to change to a mouse. The idea of neutrino flavor oscillation was first put forward in 1957 by physicist Bruno Pontecorvo.10
The first experimental evidence of neutrino oscillation was seen by Ray Davis with his experiment in the Homestake Mine in South Dakota in the late 1960s.11 He observed a deficit in the number of solar electron neutrinos arriving at Earth in comparison to the theoretical prediction. He had built a large detector that was deep underground in a mine shaft, shielded from cosmic rays. His detector was only sensitive to electron neutrinos, which are the only flavor expected from solar fusion processes.
Davis observed a deficiency in the measured signal of neutrinos. This deficiency was interpreted as electron neutrinos launched from the sun changing their identity into undetected muon neutrinos or tau neutrinos during the long transit distance from the sun to the earth. The trouble here was that one had to accept the theoretical solar calculations as a basis for interpreting the experiment—what if the sun isn't a “standard star” after all?
By 2001, neutrino flavor oscillations were conclusively identified as the source of the solar electron neutrino deficit (see note 4). Also, much larger underground detectors around the world observed a deficit in the number of muon neutrinos coming from muon decays that were produced by cosmic ray collisions in the upper atmosphere (cosmic rays have served particle physics very well indeed!). The enormous Super-Kamiokande detector in Japan provided its first definitive measurements of neutrino oscillations in 1998, using a baseline of the diameter of the earth. This propelled Japan into the forefront of neutrino physics.
This experiment could determine to high precision the arrival direction of electron neutrinos, and it could even observe those that were coming from the sun, upward through the earth at night (when the sun is below our feet shining on the opposite side of the earth, the detected neutrinos pass through the earth, and the neutrino direction was upward, it is, of course, “downward” at noon with the sun overhead). The Super-Kamiokande experiment detected a slight variation in the number of solar neutrinos over a day-night cycle. Since the electron neutrinos (almost) freely travel through the earth unimpeded, this could only be interpreted as a neutrino oscillation where an electron neutrino oscillated into some other kinds of neutrinos that weren't detected. This is called a “disappearance” experiment since we are detecting a deficit of the expected electron neutrinos. The leader of this effort, Masatoshi Koshiba, won the 2002 Nobel Prize in Physics for this work.12
The Super-Kamiokande project was mainly the problem of building an enormous particle detector. This was the world's largest human-made vat of ultra-pure water, instrumented with thousands of large glass photo-tubes to detect the light produced by neutrino interactions in the ultra-pure water. We should mention that this kind of effort is no less fraught with danger than building and operating very large particle accelerators like the LHC—indeed, it is subject to the same kinds of “Oh, $&^%” disasters:
On November 12, 2001, about 6,600 of the photomultiplier tubes (costing about $3000 each) in the Super-Kamiokande detector imploded, apparently in a chain reaction or cascade failure, as the shock wave, from the concussion of each imploding tube cracked its neighbours. The detector was partially restored by redistributing the photomultiplier tubes which did not implode, and by adding protective acrylic shells that are hoped will prevent another chain reaction from recurring (Super-Kamiokande-II).13
PUTTING NEUTRINOS UNDER THE MICROSCOPE
All of the experiments we've described thus far used the sun or the cosmic rays as a source to generate a detectable signal of neutrinos. Clearly, it is desirable to have control over the source as well as the target in a lab experiment. Therefore, it was inevitable that neutrino experiments would move into the accelerator lab (or to use nuclear reactors as sources). However, we still require moderate to enormous distance scales for neutrinos to run in order to observe the charges in flavor. This has given rise to “long-baseline neutrino experiments,” where neutrinos are made at a lab, like Fermilab in Illinois, and are detected a long distance away, such as in a deep underground mine in northern Minnesota.
The modern long-baseline experiments are after the precise details that are involved in neutrino mass oscillations, and the search for what has become the “holy grail” of the subject: the discovery of neutrino CP violation. A typical and very sensitive experiment of this type is under way at Fermilab at present, where we launch muon neutrinos from decaying pions produced by the accelerator and allow them to travel 500 miles underground (mostly under Wisconsin) and detect their conversion into electron neutrinos in an underground laboratory in northern Minnesota. The experiment is called “NOvA” (pronounced “nova”), and it seeks critical information of the values of the masses of the neutrinos that is a prelude to the actual discovery of CP violation. The NOvA website and related sites describe this in greater detail. Many labs around the world, even CERN, are contemplating future accelerator-based long-baseline experiments in neutrino physics. So, too, is Fermilab, and we hope to do it in a big way.
Fermilab is currently developing a next-generation neutrino experiment called LBNE (Long-Baseline Neutrino Experiment). The explicit mission of LBNE is to discover (or confirm) the existence of neutrino CP violation and to make precise measurements of neutrino properties. LBNE demands much from the Fermilab accelerator complex, which will be used to provide an intense beam of neutrinos. The intense neutrino beam will be sent from Fermilab, where it is produced, through the earth, to a distant detector that will be located in the Homestake Mine of South Dakota (where Ray Davis had placed the original experiment that first saw the effect of neutrino oscillations renamed the SURF laboratory). LBNE can establish definitively whether neutrino CP violation exists.
“Intensity” is the name of the game for neutrinos—maximum “proton power on target” to make lots of pions, which decay into muons and neutrinos, providing the source for the launched neutrinos. Here the energies of the individual protons are relatively low—typically 3 to 8 GeV—but we accelerate many of these, so the beam power is measured in “megawatts.” Such powerful beams are required for many other scientific quests into the deep fabric of nature where the secondary particles are of interest. These beams may be composed of muons or neutrinos, both derived from pion decays, where the pions come from the original protons slamming into a target. Or we may wish to study copious quantities of particles called kaons, or ev
en very heavy rare isotopes like radium, francium, or radon (thus giving a new meaning to heavy metal rock ’n’ roll). The applications of future intense beams of particles are coming into focus in the field of elementary particle physics.
The existing NOvA project,14 the future LBNE project, and, ultimately, the construction of a futuristic Neutrino Factory provide a powerful evolutionary program in neutrino physics. Such a program will be sensitive to surprises. There may be hidden and unexpected new phenomena in the realm of neutrinos, such as the existence of new neutrino species or new interactions that are not found in our “Horatio dream” of the Standard Model. Does the hamster morph into new species we have never seen before?
An important aspect of LBNE will be its versatile and massive distant underground detector. Unlike Super-Kamiokande, which was an enormous vat of water, this will be a vat full of pure liquid argon, a highly optically pure material that allows greater sensitivity in recording the light emitted from neutrinos that interact within the detector, permitting a superb suppression of unwanted “noise” from background events. The detector will be the world's largest application of liquid argon, weighing several tens of kilotons. Such massive detectors are crucial for collecting sufficient events from the weakly interacting arriving neutrinos over such long distances. Liquid argon detectors have not yet been realized on such large scales, but advanced detector technologies will allow for a rich physics program beyond the study of neutrinos.15 This includes a high sensitivity search for processes predicted in many grand unified theories, such as the aforementioned neutrinoless double beta decay, and the iconic process known as proton decay, which indirectly probes an energy scale of order 1016 GeV (this is a thousand trillion times beyond the scale of the LHC). It also includes the search for neutrinos that may come from any chance supernova explosions within our galaxy or its neighbors.16
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.4 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.5 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.7
Beyond the God Particle Page 24