Starting in the spring of 2010, the LHC—the largest machine ever built by mankind—produced particle collisions with 7 TeV center-of-mass energy, on its way to its design energy of 14 TeV (see Fig. 16.1). Just like the pioneers of the American west, we enter an unknown territory: a wilderness where we don’t know what to expect, but where we hope to encounter new exciting physics. The hierarchy problem and the need for dark matter in the universe provide a strong motivation for these hopes.

But is there any connection with neutrinos? There are actually good reasons for such a connection, and they follow directly from a 700-year-old principle known as Occam’s Razor: “Entia non sunt multiplicanda praeter necessitatem,” or “Do not add new things without necessity.”

The Unification Frontier

Typically the energy scales where a GUT theory or even a version of quantum gravity such as string theory is expected to crop up are in the range of 10¹⁶ to 10¹⁹ GeV, that is thirteen to sixteen orders of magnitude above the TeV or terascale now being investigated at the LHC. In certain models with extra dimensions, or in string theories where different theory versions with different numbers of space dimensions can be related via so-called duality relations, however, the energy scale can be much lower. If the mechanism of neutrino mass generation is linked to the physics at such a “low” energy scale, it should be possible to probe it directly at the LHC.

Figure 16.1. LHC detector ATLAS, before it was closed around the collision point. A sky-high engineering marvel, 45 meters long, 22 meters high, weighing 7,000 tons, dedicated to the search for Higgs, SUSY, extra dimensions and … the unexpected. (Courtesy CERN)

But even in theories where the unification is realized at a large energy scale, the origin of neutrino masses can be at the TeV scale. This shows up in models where the masses of the right-handed neutrinos are generated via loop quantum fluctuations into two other particles. In SUSY models, such quantum fluctuations cancel above the energy scale where SUSY particles can be produced—similar to the way in which the cancellation of the contributions to the Higgs mass solves the hierarchy problem. Such theories thus need a mechanism for neutrino mass generation at or below the mass scale of SUSY particles, such as the inverse seesaw. Models of this kind have been investigated recently by Sören Wiesenfeldt, my PhD student Christophe Cauet, and me.

The Majorana Frontier

Is the neutrino a Majorana particle after all? The best possibility for finding out is in the search for neutrinoless double beta decay. As soon as one measures neutrinoless double beta decay, one can, according to the Schechter-Valle theorem, con clude that neutrinos are definitely Majorana particles. What can’t be concluded, though, is what exact kind of lepton-number-violating physics has triggered the decay, and how it is related to the mechanism of neutrino mass generation. One possibility for obtaining this information could be to measure neutrinoless double beta decay in different isotopes, as has been proposed by Frank Deppisch and me and, shortly after, by Steve Elliott and Victor Gehman of the Los Alamos National Lab. But this approach is extremely intricate experimentally, and promising in only very few cases. A better method, probably, is to search for the corresponding like sign di-lepton decay at LHC, which arises in various models, and to compare it with a possible signal for neutrinoless double beta decay. Ben Allanach, his PhD student Steve Kom of Cambridge University, and I have studied such signals in R-parity-violating SUSY and found very promising results. As of this writing, I am engaged with Martin Hirsch, Sergey Kovalenko, and his PhD student Juan Carlos Helo Herrera in a more general, model-independent analysis.

The SUSY Frontier

If supersymmetry exists at the TeV scale, some of the neutrino properties, such as lepton-flavor violation and lepton-number violation, would be transmitted onto the SUSY particles. The exact details of the transmission, such as quantum fluctuations of SUSY particles into neutrinos and sneutrinos via lepton-flavor-violating couplings in the seesaw mechanism, again depend on the concrete mechanism of neutrino mass generation. SUSY particles would then undergo lepton-flavor- or lepton-number-violating processes, which do not arise in the Standard Model and could shed light on the mechanism of neutrino mass generation. Such decays are studied, for example, by Werner Porod and Andreas Redelbach in Würzburg, Germany; Martin Hirsch in Valencia, Spain; Frank Deppisch in Manchester, UK; and their respective working groups. In addition, the same interactions could reveal themselves in the search for lepton-flavor violating decays of charged leptons, such as that of a muon into an electron and a photon being searched for in the MEG experiment at the Paul Scherrer Institute, Switzerland.

A relationship between neutrino physics and supersymmetry is also suggested by the lepton number and R-parity violating couplings that arise naturally in SUSY models and make it possible to generate neutrino masses via quantum fluctuations. Such models and their phenomenology have been extensively studied for years now in the Valencia group around Jose Valle and Martin Hirsch. Moreover, such models also have interesting consequences for the next point, the flavor frontier, which Gautam Bhattacharyya, Daniel Pidt, and I have been studying.

The Flavor Frontier

One of the most interesting and least understood aspects of neutrino physics is the question of why neutrinos exhibit such large mixing among different flavors as compared with the rather small mixing of quarks. This question is part of the flavor puzzle addressing the relations among the three particle families and their respective members; many groups around the world work on models that explain these relations with the help of symmetries. In particular the symmetry of the tetrahedron, proposed in 2001 by Ernest Ma at the University of California, Riverside, as well as the symmetry of the equilateral triangle, could play an interesting role here. My PhD student Philipp Leser, who has studied a model of this kind proposed by Ma, Michele Frigerio, and Shao-Long Chen, found that in such models the neutrino mixing can induce characteristic, lepton-flavor- and quark-flavor-violating couplings to the Higgs particle, which lead to exotic Higgs decays observable at the LHC. If such a signal could shed light on the solution of the flavor puzzle, this would definitely be one of the most spectacular discoveries at the LHC.

The Extra-Dimensions Frontier

Finally, there are searches for extra dimensions at the LHC. Such a discovery would definitely be highly relevant for neutrino physics. If researchers at the LHC find extra dimensions, for example by detecting microscopic small black holes or Kaluza-Klein excitations, then extra dimensions almost necessarily have to play a role in the process of neutrino mass generation. If it finally turns out that neutrino masses indeed are small because the right-handed neutrinos propagate in extra dimensions, then the step to shortcuts in extra dimensions and neutrino time travel isn’t that far off anymore.

The discovery potential of the LHC alone will make the next decade one of the most exciting eras in the history of particle physics, with implications for neutrinos. But in parallel there are exciting projects upcoming in neutrino physics itself: First and foremost, experimenters will put an effort into the determination of the so-far unknown observables in the neutrino sector. Among them are the masses of neutrinos and the question of whether the neutrino is a Majorana particle. Exciting news in this respect can be expected by new double beta decay experiments such as EXO-200 in New Mexico, KamLAND-Zen in Japan, and GERDA in Italy, which, as of this writing, are taking data and have reported first results. Another experiment in Italy, CUORE, is scheduled to start taking data in 2014. Complementary information will be obtained at the tritium beta experiment KATRIN, and from cosmology.

The smallest mixing angle, called θ₁₃, among the parameters that describe neutrino mixing has recently been determined in reactor experiments—a promising neutrino source, since Reines and Cowan used reactors to discover neutrinos in the first place. A typical reactor emits 10²⁰ antineutrinos per second. Until recently, the most sensitive test for θ₁₃ had been provided by the CHOOZ experiment in the French Ardennes. A problem with this measurement was, however, that the neutrino flux at the source could be determined only from the heat emission of the nuclear power plant, a method that suffers from large uncertainties. And if one has only a vague knowledge about how many neutrinos are being emitted, it is rather complicated to conclude that one is missing some.

The new generation of experiments, a successor of CHOOZ called Double-CHOOZ, DAYA BAY in China, and RENO in Korea, incorporate both near and far detectors, where the near detectors determine the original source flux at a distance of a few hundred meters, while the far detectors measure the flux after the neutrinos have propagated and oscillated a distance of about a kilometer. In March 2012, the DAYA BAY experiment actually reported a measurement of θ₁₃, which confirmed previous hints from other experiments and has since been further confirmed by RENO and Double-CHOOZ. The fact that the actual value of the small mixing lies close to its upper bound raises hopes that other unknown fundamental neutrino properties can be measured soon as well.

These properties include the question of whether the electron neutrino is composed mainly of the heavier or the lighter neutrino masses (known as inverse or normal mass hierarchy), as well as the question of whether the symmetry between neutrinos and antineutrinos of the opposite helicities, the so-called CP-symmetry, is violated. The answers to these questions are important for the flavor structure and thus for a solution of the flavor puzzle, and a new generation of experiments improves on the already crazy-appearing efforts firing neutrino beams hundreds of kilometers through the earth. Among these ambitious projects is the T2K project, with its 295-kilometer neutrino beam. There, neutrinos are shot into the earth from the J-Parc accelerator complex in Tokai north of Tokyo with a hundred times greater intensity than in the previous experiment, K2K, which confirmed the atmospheric neutrino oscillations. Super-Kamiokande saw the first T2K neutrino on February 24, 2010. While T2K probably has the best chances of observing CP violation among neutrinos, also under construction in the United States is the competing NOVA experiment, with a neutrino beam originating at Fermilab close to Chicago and aimed at a 15,000-ton detector in Ash River, Minnesota—812 kilometers away. Even farther in the future one could have superbeams (neutrinos from conventional pion-decay sources but with optimized intensity), beta beams (neutrinos from the decay of highly accelerated radioactive ions), and neutrino factories (neutrinos from the decay of highly accelerated muons) as neutrino sources. In addition, a new anomaly has appeared recently in the reanalysis of the combined data of various older reactor experiments: They seem to indicate the existence of a fourth, sterile neutrino (compare the discussion in Chapter 15). While it is not clear at all right now whether this result will hold, it could have crucial impact on the attempts to understand the results of LSND and MiniBooNE, if they are not due to faulty measurements. A direct test of LSND, which would also be valid in models with neutrino shortcuts in extra dimensions, needs to be executed at comparable energies and detector distances (“baselines”) from the source. Such experiments had been proposed for the Spallation Neutron Source in the US nuclear research center in Oak Ridge, Tennessee, as well as for Fermilab, but didn’t meet the test of convincing the relevant funding bodies. Also interesting in this context are measurements that deviate from LSND in energy and baseline but correspond to the situation at LSND if one looks at a combination of these quantities, such as their product. For neutrino shortcuts in extra dimensions, measurements at the reactor experiments Double-CHOOZ, RENO, and DAYA BAY could possibly deliver exciting results.

Vigorous work is also proceeding regarding the cosmological role of neutrinos: In May 2009, the ESA satellite PLANCK was launched; combined with astrophysical observations, it promises a significant improvement of the cosmological neutrino mass bounds. The data released in March 2013 report a stringent limit on the sum of neutrino masses of 0.23 eV, which, however, is subject to uncertainties related to the many parameters affecting cosmology. This will, at the same time, improve our knowledge about the role neutrinos played in cosmic evolution. And if neutrinos, via their SUSY partners, the sneutrinos, really are related to the inflationary epoch and acted as key players in the very creation of the universe, PLANCK may help to substantiate these models as well; this knowledge naturally will produce feedback to aid in the comprehension of neutrino properties themselves.

In contrast, the direct detection of the primordial cosmic neutrino background, for example with Tom Weiler’s Z-bursts, still seems to lie in the distant future because of insufficient neutrino fluxes of highly energetic cosmic-ray neutrinos. On the other hand, there exists an interesting idea by John F. Beacom and Mark R. Vagins on observing the diffuse background of antineutrinos emitted by supernova explosions. The proposal suggests adding gadolinium in the Super-Kamiokande water tank, which could capture neutrons produced in the inverse beta decay triggered by these neutrinos, and is called GADZOOKS.

Finally, in astrophysics, neutrinos are mutating more and more from research objects to be investigated themselves into probes delivering information about astrophysical processes. The advantage of neutrinos compared with light consists particularly in the property of their being hardly ever absorbed or deflected thanks to their weak interactions. That means that neutrinos can directly image not only processes on the surface, but also processes deep in the interior of stars and supernova explosions. In this sense, neutrinos are like better X-rays. And—a further advantage—the flight direction of neutrinos is not altered by magnetic fields in, for example, the Milky Way, so that neutrinos deliver a faithful instead of a blurred image of astronomical objects.

This research field, neutrino astrophysics, started with the measurement of solar neutrinos and the spectacular detection of nineteen neutrino events from a supernova explosion on February 23, 1987, in the detectors IMB and Super-Kamiokande—about three hours before the visible light of the explosion hit the earth. (Unlike light, the neutrinos interact only weakly with dense matter when propagating out from the supernova core. This makes the supernova in its early stages transparent for neutrinos while being opaque for light. The neutrinos thus can escape unhampered while light can escape only when the explosion reaches the collapsing star’s surface.) John Learned was one of the members of the IMB team and is, beyond any doubt, one of the brightest and most creative protagonists in neutrino history. Learned, a bandy-legged character with a big bushy beard, slouch hat, and colossal spectacles, who cruises the streets of Honolulu in an ancient Cadillac known as “the whale,” was involved in the development of numerous key experiments in neutrino physics. He had started out by sinking strings of photo multipliers—sensitive light detectors—off the Hawaiian shoreline to measure the traces of astrophysical neutrinos. Later Learned proposed, together with Francis Halzen, burying detectors also in the Arctic ice—in holes that he originally intended to fill with alcohol to keep them ice-free so that the detectors could be accessible at any time. The US National Science Foundation (NSF) asked Learned, however, to choose one of the two experiments, and he chose Hawai‘i. When Learned lost his first large detector string in the depth of the Pacific Ocean, officials at the NSF may have been relieved that they managed to keep Learned away from the North Pole.

In the meantime, the IceCube Neutrino Observatory is close to full completion in the Antarctic. Almost eighty strings equipped with photo multipliers have been sunk between 1,450 and 2,450 meters deep in the ice around the Amundsen-Scott South Pole station. For comparison, the Eiffel tower is barely 325 meters high. The physicists at the station defy extreme cold (temperatures of minus 40 to minus 80 degrees Celsius), dryness, and, due to an altitude of 3,000 meters, also thin air and disturbed sleep in military tents occupied by twenty people.1 The only distractions are events like the annual “three-times-around-the-world” run on Christmas day, occasional excursions to a crashed airplane at the end of the runway, and the breathtaking aurora australis in the endless winter night. When leaving the station, they have to stay away from a dangerous area dubbed the “death sector,” where the ice could break. The IceCube Neutrino Observatory now constitutes the largest neutrino telescope in the world and has—together with the subsea telescope ANTARES in the Mediterranean surveying the southern hemisphere, and the competing projects NESTOR (Greece) and NEMO (Italy)—a good chance to propel neutrino astrophysics into new realms. As of this writing, in April 2013, the IceCube Collaboration has reported two candidate events, nicknamed Erni and Bert, with PeV (that is, 1,000 TeV) energies that most probably are of extragalactic origin and could be the first indication of an astrophysical neutrino flux. Moreover, extended versions of the IceCube and the KM3NET telescopes (the latter being a joint endeavor of the ANTARES, NEMO, and NESTOR collaborations) with additional detector strings called PINGU and ORCA also have good prospects of solving the issue of neutrino mass hierarchy.

One of the greatest hopes for neutrino astrophysics is to observe neutrinos emitted from a supernova within the Milky Way (Fig. 16.2). Such an event could make possible revolutionary insights in both neutrino physics and astrophysics. From observations in other galaxies one concludes that such a spectacular death of a star should occur in our own galaxy roughly twice a century. The last observation of a supernova in the Milky Way dates back about 400 years to the German astronomer Johannes Kepler. Our own galactic supernova is more than overdue!

The unique ability of neutrinos to penetrate matter has even inspired ideas for useful applications, for instance in communication and imaging as well as in distinct sciences such as geology. For example, the neutrino detectors KamLAND and BOREXINO have recently detected neutrinos created in nuclear decay processes in the earth’s interior. Such processes contribute to the generation of heat in the deeper layers of earth, which are responsible for both hot lava streams and geothermal energy. A three-dimensional image of the earth’s chemical composition might be possible, which would also shed light on the origin and stability of the earth’s magnetic field. As early as 1983, the idea of using neutrinos to search for underground sources of oil and ore was advanced by Alvaro De Rujula, Robert Rathbun Wilson (Fermilab founder and nuclear weapon pioneer), and the Nobel Prize winners Georges Charpak and Sheldon Glashow.

Figure 16.2. A supernova remnant in the crab nebula: beacon of hope for neutrino physics and astrophysics. (Courtesy NASA/Defense Video & Imagery Distribution System)

The idea of using neutrinos for communication has been discussed in various forms: John Learned, Sandip Pakvasa, and Tony Zee proposed using neutrino beams to send messages to other stellar systems in the Milky Way as well as to search for alien civilizations that might have harnessed neutrinos for communication. Patrick Huber, on the other hand, has imagined utilizing neutrino communication on earth. Modern submarines can remain submerged for very long periods but then have very limited ability to communicate. They return to the sea surface now and then in order to communicate at higher band width. If neutrinos could be used for communication, submarines could remain under water longer. Prasanta Panigrahi and Utpal Sarkar have pursued this idea in reverse, so to speak. They proposed using the neutrino radiation that always accompanies nuclear reactions and can’t be shielded in order to spot secret nuclear weapons tests, nuclear submarines, aircraft carriers, or even UFOs. Even more intense neutrino beams could also be used, according to Hirotaka Sugawara, Hiroyuki Hagura, and Toshiya Sanami, for the destruction of nuclear weapons. They argue that neutrinos could be fired at buried armories in order to trigger a hadronic shower in the rock beneath the weapon, which would then induce nuclear fission inside the weapon’s material with an explosive power of a few percent of what the weapon was designed for.

Such ideas (some of them admittedly “far out”) suggest that neutrinos could develop from objects of basic research into crucial elements of future technologies. But useful application is not the point of neutrino physics. Rather, it is a quest for a better basic understanding of the universe, thereby ultimately contributing to a solid foundation for an incorruptible, rational world view. At times when intolerance, religious fanaticism, gut instincts, and irrational esotericism are flourishing, this is an effort whose importance should not be underestimated. Perhaps in a few years we will know how neutrinos acquire their masses. We may know their exact mass and the origin of the mixing pattern. We may understand what symmetries are responsible for the unique properties of neutrinos. We might even find extra dimensions that could make it possible to send particles back in time. If such a thing turns out to be possible, the neutrino will definitely be the prime candidate to make the journey.

The times remain exciting, and there is a good chance that we may soon discover new physics within the particle desert, a step forward that will inspire us to echo the words of William Clark who, together with Meriwether Lewis, led the first expedition across the North American continent to the Pacific coast: “Ocean in view! O! the joy.”

The fundamental symmetries and laws that underlie a theory for all events happening in the universe may reveal themselves a little more completely, the picture behind it all may become a little clearer, and, as sure as death and taxes, neutrinos will contribute in one way or another.