Beyond the God Particle (36 page)

Most theorists believe the
observed neutrino masses
will prove to be of the Majorana kind. The reason for this is a remarkable observation about neutrino masses in grand unified theories. In short, the idea is that at very high-energy scales, those of grand unification (about a trillion times beyond the LHC energy scale, or 10
¹⁵
GeV), neutrinos are hypothesized to start out with the 4 ingredients of Dirac masses, that is, a distinct R neutrino exists for each flavor (with its corresponding anti–R particle). Such R neutrinos, however, would be “sterile,” coupling only through the Higgs interaction (such as in
figure 6.22
) and through the gravitational interaction.

But then the sterile R neutrinos could experience extremely high-energy and extremely weird processes that the other L neutrinos cannot. The reason is that the W boson coupling of conventional L particles constrains them in many ways. For example, suppose a “mini–black hole” underwent a quantum fluctuation and came briefly into existence at a tiny distance a trillion times smaller than the scales we have ever probed at LHC (or smaller still). The black hole is like a fish in the ocean, like a big fat grouper, that appears on the scene and eats a little fish, then disappears into the undersea gloom. The mini–black hole would see a lowly R neutrino and swallow it—poof! The R neutrino is gone, and the grouper then swims away and disappears. But the mini–black hole cannot swallow the L neutrino and then simply disappear, because the L neutrino has weak charge—it couples to the W boson. The W boson is like a fishing line attached to the L neutrino—when the grouper swallows it, it is now caught
and can't swim away…there's still weak charge in the grouper's belly forbidding him from just disappearing. So, he spits the L neutrino back out immediately: “Ouch. I don't want to mess with those fishing lines,” says the grouper, and then he's gone.

The effect of this would be that the mini–black holes interfere with the mass-generation mechanism of the Higgs boson for neutrinos. All the sterile R and anti–R neutrinos can be eaten by mini–black holes in quantum fluctuations, which in physics parlance gives them effectively a very large Majorana mass. But the Higgs boson can still cause L to convert to R as a big quantum fluctuation. This then causes the L neutrinos to acquire a very tiny Majorana mass. These are the neutrinos we would see, with the fish lines of W bosons attached to them—the L and anti–L neutrinos (the ones with weak charges). So, what we believe we are seeing, here in the land of broken symmetries, are effectively Majorana masses among the ordinary L neutrinos that are produced in beta decays.

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, 10
¹⁵
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 “NO
v
A” (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 NO
v
A 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 even 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 NO
v
A 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 10
¹⁶
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