Ghost lighthouses of new physics / Martin Hirsch, Heinrich Fass Warner Forud

Neutrinos, the strangest creatures in the particle zoo, may soon break through into uncharted realms of physics.

Few physicists have had the privilege of bringing into the world a new elementary particle. However, when the idea came to Wolfgang Pauli's mind in 1930, he was gnawed by worries that tempered his reaction. "I did a terrible thing," Pauli later told his colleagues. "I hypothesized a particle that cannot be discovered."

The neutrino is indeed elusive: its properties allow it to penetrate like a ghost through almost all physical barriers, including the materials that physicists use in their particle detectors. In fact, most of the neutrino particles that come from space to the Earth pass through it smoothly without even slightly rubbing against any other particle. But it turned out that Pauli's fears were somewhat exaggerated: it is possible to discover neutrino particles, although for this it is necessary to invest great efforts and genius in planning the experiments.

The neutrino particles are the strangest of the elementary particles in other respects as well. They are not sealed components and have nothing to do with chemistry. They are electrically neutral. They are incredibly light, less than a millionth the mass of the lightest component of matter besides them, the electron. And likewise, neutrino particles, more than other particles, take shape and take shape: each of the three types of neutrinos, called "flavors", can become a different type.

For more than 80 years these tiny particles have not ceased to amaze physicists. Even today, there remain fundamental questions about neutrino particles that have yet to be answered: Are there only three flavors of neutrino particles, or are there more? Why are all neutrinos so light? Do neutrino particles have antimatter counterparts? Why do neutrino particles change their shape at such a brisk and dizzying rate?

In particle accelerators, nuclear reactors and abandoned mine shafts around the world, new experiments are starting to work that can address these questions. The answers that come from them should provide vital clues to the internal mechanisms of nature.

The peculiarities of the neutrino make it the beacon that guides particle physicists in their arduous journey towards that grand unified theory, which describes all particles and all forces, except gravity, within a coherent mathematical framework. The Standard Model of particle physics, the best theory of particles and forces to date, is unable to accommodate all the complexities of the neutrino. It needs to be expanded.

light and tasty

The most common way to develop the neutrino's part in the Standard Model is to introduce new entities called right-handed neutrino particles. The characteristic known as right-handedness or left-handedness is a certain equivalent to an electric charge. It determines whether the particle will react with the weak force, which is responsible for radioactive decay: a particle must be left-handed to feel the weak force. Those putative right-hand particles will therefore be even more elusive than their left-hand counterparts, those neutrino particles of the Standard Model, whose existence has been proven experimentally. All neutrino particles are classified as "leptons", the extended family of particles that includes, among other things, electrons, and this means that they do not feel the strong force that binds the protons and neutrons together inside the atomic nucleus. Also, in the absence of an electric charge, the neutrino particles do not directly feel the electromagnetic forces either. Thus only gravity and the weak force remain for the three known flavors of neutrino, but a right-handed neutrino will be impenetrable even to the weak force.

If right-handed neutrinos exist, they would provide a very plausible explanation for another neutrino puzzle: why all three left-handed types, electron neutrinos, muonic neutrinos, and tawny neutrinos, have such tiny masses.

Most elementary particles obtain their mass through interaction with the Higgs field that prevails in all space. (The Higgs became a brand last year when physicists at the Large Hadron Collider, LHC, at the CERN laboratory near Geneva announced that they had identified a new particle that fits the description of the long-sought-after Higgs boson. This boson is the particle companion of the Higgs field, just as that the photon is a companion of the electromagnetic field.) In this process, the Higgs field removes the weak force equivalent of an electric charge from the particles. Since right-handed neutrinos have no such charge, their mass does not depend on the Higgs field. It may arise from a completely different mechanism at the incredibly high energies of the grand unification (a very high energy where, according to some theories, all forces except gravity are unified into one force). In that case, the right-hand neutrino would be terrifyingly heavy.

Quantum effects can link right-handed neutrino particles to their left-handed brothers in such a way that the enormous mass of one "contaminates" the other. However, this infection will be very weak, for example, if the right neutrino falls to bed with pneumonia, the left one will only catch a mild cold, and this means that the mass of the left one will be very small. This relationship is known as the wobble mechanism, because a large mass lifts a smaller mass, as in a children's swing.

An alternative explanation for the masses of neutrino particles emerges from supersymmetry, a leading candidate for new physics that will break the boundaries of the standard model. According to the supersymmetry hypothesis, every particle in the Standard Model has an undiscovered sibling. Those "super-brothers" (superpartners), who must have a very large mass in view of the fact that so far they have managed to escape detection, will at once increase the number of elementary particles by at least two times. If supersymmetric particles exist, the LHC may be able to produce them and measure their properties.

One of the most attractive features of supersymmetry is that one of the superparticles known as neutralinos is a very serious candidate for the role of dark matter, that mass found in galaxies and galaxy clusters that exerts a gravitational pull but does not emit light or reveal itself in other visible ways. The neutralino will meet the requirements of dark matter only if it turns out to be stable for sufficiently long periods of time, and not quickly decay into some other particle.

A short-lived neutralino will therefore send dark matter researchers back to their drawing tables but will be a great boon to neutrino physicists. The neutralino's stability depends on a putative property known as R-parity, which prevents the super-siblings from decaying into one of the regular Standard Model particles. But if the R-parity does not hold, the neutralino will become unstable, and its decay will depend in part on the neutrino mass.

Two of us (Hirsch and Porud), in collaboration with Jose Valle from the University of Valencia in Spain and Jorge C. Romano from the Technical University of Lisbon in Portugal, showed that it is possible to examine the relationship between neutrinos and neutralinos at the LHC. If neutralino stability does depend on neutrino particles, it would be possible to predict the neutralino's lifetime from known neutrino properties. And it just so happens that this superparticle should exist long enough to allow physicists to monitor its entire life, from formation to decay, inside the LHC's detectors.

Light and anti-matter

All conceivable explanations for the minuscule masses of neutrinos point towards unknown regions in the realm of physics. However, one of these explanations, the wobble mechanism, may also contribute to the solution of a central mystery in physics: how it happened that matter was able to defeat antimatter, a victory that enabled the creation of the cosmic structure, and ultimately the development of life.

Each particle in the standard model has an antimatter counterpart, like a mirror version of itself, with the opposite charge. An electron, for example, has an electrical charge of -1, and an anti-electron, known as a positron, has a charge of +1. When an electron and a positron collide, their charges cancel each other out, and the particles are ionized in a burst of radiation. The perfect chargelessness of the right neutrino may have an important implication: it may mean that, in the case of neutrino particles, matter and antimatter are the same. In physics terminology, the electron and the positron are called Dirac particles. But a particle that is its own antimatter partner is called a Majorana particle.

If the hand-and-hand theory correctly reflects the internal mechanisms of the particle world, then the left neutrino particles are affected not only by the mass, but also by the "mass" of the right neutrino particles. In other words, if some neutrino particles are their own antiparticles, then all neutrino particles are.

A reality in which neutrinos and their antiparticles are the same would have a variety of fascinating consequences. For example, neutrinos could trigger a transition between particles and antiparticles. In most particle reactions, the quantity known as the "lepton number" is preserved, which represents the number of leptons minus the number of anti-leptons, meaning it does not change. However, neutrinos may break this rule, creating an imbalance between matter and antimatter. For us humans, this imbalance is a very good thing because if matter and antimatter had been split equally after the Big Bang, they would have completely ionized each other and left nothing behind from which to build galaxies, planets, and life forms. The explanation for the dominance of matter over antimatter has eluded physicists and cosmologists for many years.

The secret of the disappearing neutrino

The connection between neutrino particles and their antiparticles does not have to remain confined to the world of a tempting but not completely resolved physical theory. Many experiments, past and present, have sought to provide an unequivocal answer to the question of whether neutrino particles are indeed their own antiparticles, by searching for a type of radioactive event known as nuclear double beta decay.

Neutrinos and antineutrinos were first observed in nuclear beta decay, in which an atom emits an electron along with an antineutrino. In some nuclear isotopes, two simultaneous beta decays may occur, which under normal conditions emit two electrons and two antineutrinos. But if the neutrino is a Majorna particle, then the same antineutrino emitted in the first decay can be absorbed in the second decay. The result is double beta decay that does not emit any neutrinos or antineutrinos [see box on previous page]. In one moment, where before there were no leptons, two leptons (the electrons) are formed, without the usual anti-leptons that balance them (the anti-neutrino particles). In other words, that neutrinoless double beta decay violates lepton number conservation.

Today, the search for neutrinoless double beta decay is the best test we have for murine neutrinos in particular and the lepton number in general. Basically, the neutrinoless double beta decay experiment is simple: collect nuclear isotopes such as Germanium 76, where simultaneous beta decays may occur, and wait for the formation of two electrons that are not accompanied by neutrino particles. But in fact, these experiments are very difficult. Double beta decays of any kind are incredibly rare, so experimenters need to collect large amounts of germanium or other source material to hope to record the neutrinoless variety. And as if that weren't enough, the constant stream of subatomic particles showering the Earth from cosmic rays tends to drown out the tiny signal of double beta decays. And so the experimenters must bury their detectors in the depths of the earth, or in abandoned mines or other underground laboratories, where the overlying layers of rock filter out almost all cosmic rays.

Unfortunately, the only report so far of neutrinoless double beta decay, from the Heidelberg-Moscow double beta decay experiment in Italy, has been strongly criticized by other physicists. The next generation of detectors that are just starting to collect data or are still in the process of being set up will search more thoroughly. An experiment conducted in New Mexico, named EXO-200, and another experiment in Japan known as KamLAND-Zen, recently published the first data from their searches for neutrinoless double beta decay, which did not agree with the previous claim but did not categorically disprove it either. meaningful

The GERDA experiment in Italy, which began operating in 2011, uses the same isotope used in the Heidelberg-Moscow array, but in a new model that aims to directly confront the controversial finding of its predecessor. The EXO-200 and KamLAND-Zen experiments both continue to operate, and another facility called CUORE is scheduled to begin collecting data in Italy in 2014. The number of advanced experiments currently underway gives us reasonable hope that neutrinoless double beta decay May qualify for confirmation by the end of this decade.

Female exchange

Finding an undiscovered neutrino, or proving that neutrinos and antineutrinos are the same, would add a whole new layer of mystery to these already enigmatic particles. But even as we physicists hunt for new properties of these particles, we also continue to wrestle with the mechanism underlying a well-documented but largely misunderstood property of neutrinos: their strong tendency to deform and take shape. In the literature it is common to say that the amount of leptonic flavor violation, or neutrino mixing, is large compared to the mixing between flavors of quarks, the elementary particles that make up protons and neutrons.

Many research groups around the world are investigating how symmetries of nature, key properties shared by seemingly distinct forces and particles that have only recently occurred to scientists, can explain such behavior. One possible example is the symmetries inherent in the ways in which the known particles transform from one to another. Gautam Bettacharya from the Saha Institute of Nuclear Physics in Calcutta, Philip Lesser from the Technical University of Dortmund in Germany and one of us (Fass), recently discovered that such symmetries will clearly affect the Higgs field. The interaction of flavor-changing quarks and neutrinos with the Higgs field will manifest in exotic decay products of Higgs bosons, which we can certainly observe at the LHC. Such a signal could teach about the mechanism underlying the hyperactive shape changes of the neutrino particles: a discovery that will undoubtedly be one of the most spectacular discoveries of the LHC.

Until that happens, another family of experiments is trying to definitively determine exactly how often the particles change their identity. Long-term experiments such as T2K in Japan, MINOS in Minnesota and OPERA in Italy detect neutrino beams originating from particle accelerators located hundreds of kilometers away, to measure flavor changes that occur as the neutrino particles travel great distances through the Earth [see box on page 43]. The scales of these experiments are so large that neutrinos may cross interstate or even international borders in their journey. (In 2011, the OPERA experiment made headlines when physicists from that collaboration announced that neutrinos from their experiment had apparently traveled from the CERN laboratory to the underground Italian laboratory at speeds faster than the speed of light, a measurement that was soon shown to be flawed.) Complementing these long-range neutrino experiments, the double Schuze project In France, the Dae Bay nuclear reactor neutrino experiment in China and the RENO experiment in South Korea all measure the short-range oscillations of neutrino particles coming from nuclear reactors.

Only in 2012 did these experiments finally manage to determine the size of the last and smallest of the parameters known as mixing angles, which control the flavor exchange of neutrino particles. The last mixing angle that needed to be determined, known as the "reactor angle", describes the probability that an electron neutrino or an electron antineutrino will undergo a short-term change. Thanks to the measurements of the reactor angle, it is possible that future neutrino experiments will be able to compare the properties of neutrino particles and the properties of antineutrinos. Asymmetry between particles and their antimatter partners will be called CP symmetry breaking (Charge-Parity violation) and together with studies of neutrinoless double beta decay, you will be able to shed light on the mystery of why there is more matter than antimatter in our universe.

Among the searches being conducted today, the first serious chance to see hints of CP symmetry breaking may belong to the T2K experiment. But the race between the new generation experiments to answer key questions about neutrino particles has not yet been decided and it seems that it will be exciting. The long-term NOvA experiment, whose establishment in the USA is underway, also has a chance to reveal CP symmetry breaking in neutrino particles. NOvA will shoot neutrino beams through the earth, from the Fermi National Accelerator Laboratory in Batavia, Illinois ("Fermilab"), in a straight path that will cut through the state of Wisconsin and the shore of Lake Superior, to the detector in the Ash River in Minnesota, a distance of 810 kilometers. The neutrino particles will travel this way in less than 3 milliseconds.

NOvA plans, among other goals of its research, also to clarify the mass hierarchy of the neutrino particles, that is, to determine which of the neutrino particles is the lightest and which is the heaviest. Currently, physicists only know that at least two types of neutrinos have non-zero masses, but, as with so many aspects of these ghost particles, the details elude us.

Stubborn mysteries

In view of the multitude of neutrino experiments currently being conducted, each of which is structured differently, has different goals and uses different particle sources, the diverse data emerging from different places around the world sometimes provoke conflicting interpretations. One of the most exciting experimental clues, and the most controversial, raises the possibility of the existence of a new particle, known as the sterile neutrino.

The barren neutrino evokes Pauli's fears from 1930 that it would be possible to detect it only indirectly, just like the right neutrino from the hand-and-hand mechanism, which is much heavier than it. (However, from a theoretical point of view, it is almost impossible for these two proposed particles to co-exist.) However, two experiments may have picked up echoes from the fluttering footsteps of the barren neutrino. The LSND experiment, which operated at Los Alamos National Laboratory in the 90s, revealed early but controversial evidence for an elusive type of flavor conversion in neutrinos: ionized antineutrinos that become electron antineutrinos. Premilab's MiniBooNE experiment, which began producing scientific results in 2007, also hinted at such conversions. But the oscillations of LSND and MiniBooNE fail to fit neatly into the usual picture of three neutrino particles.

Quantum mechanics allows neutrino particles to oscillate between flavors only if they have mass and only if each flavor has a different mass. The different neutrino masses could trigger neutrino conversions capable of explaining the anomalies discovered by LSND and MiniBooNE, but only if there is a mass difference in addition to the already known differences, or in other words, only if there are four types of neutrinos and not three. Another neutrino attached to the weak force will cause a boson Z, the carrier of the weak force, disintegrate too quickly, so such a particle must not undergo any reaction with the weak force. Hence the classification of the neutrino as "barren": this assumed neutrino will be almost completely uncoupled to the rest of the zoo of particles.

Even in detectors of a completely different type, which capture neutrino particles from nearby nuclear reactors, surprising results were recorded that may hint at barren neutrinos. In the data coming from several reactor experiments there are signs of anomalous disappearance of electron antineutrino particles over very short distances, a phenomenon that, if interpreted in terms of neutrino oscillations, would imply the existence of sterile neutrino particles. This anomaly has been known for a long time, but recent recalculations of the output of the neutrino particles from the various reactors provided reinforcement for the argument regarding a new particle.

The evidence for the existence of barren neutrino particles, as of now, is still in the form of a general outline, and is indirect and contradictory - all the expected characteristics in the case of a pursuit of a particle notorious for its elusiveness and which may not even exist. But it's possible that MiniBooNE and a companion experiment known as MicroBooNE, which is under construction at Primilab, will have something more solid to say on the subject. And a new crop of proposed experiments, which will investigate the reactor anomaly, is also discussed.

One of the amazing things is that the mighty LHC and the relatively low-energy experiments on the humble neutrino provide complementary pathways to explore the inner workings of nature. More than 80 years after Wolfgang Pauli invented his "undetectable particle", the neutrino particles continue to guard their secrets jealously. And yet, the potential reward of uncovering these secrets justifies the decades-long effort to pry further into the neutrinos' private lives.

_________________________________________________________________________________________________________________________________________________________________

About the authors

Martin Hirsch is a professor in the team of high energy and astroparticle physics at IFIC, in the Center for Particle Physics shared by the University of Valencia and the National Research Council of Spain.

Heinrich Päs is a professor at the Technical University of Dortmund in Germany. "The Perfect Wave", his book dealing with neutrino particles, is about to be published by Harvard University Press.

Werner Porod is a professor at the University of Würzburg in Germany.

in brief

The neutrino is the strangest chicken among the elementary particles. The neutrino particles seem to break all the precedents set by better understood types of particles, such as electrons and quarks.

The neutrino, the light-weight, elusive and most difficult to detect, has been on the nerves of experimenters for decades.

Even today, the neutrino's fundamental properties remain controversial. Some of the key questions concern the origin of their tiny mass, the nature of antimatter in the case of neutrinos, and the number of types of neutrinos that exist. And it goes without saying that they concern their hobby, changing their identity on the go.

Uncovering the true nature of the neutrino may pave the way for a more unified theory of physics.

 

Measuring mass / Sudeep Das and Tristan L. Smith

The secrets of the neutrinos, written in the stars

Measuring the tiny mass of the neutrino has so far proved an impossible task, and we can't say we haven't tried. Many laboratory experiments conducted over the past decades have only succeeded in placing general limits on the three neutrino masses.

We have very compelling reasons to expect that the best way to measure the mass of these tiny particles is, surprisingly, to look for their effect on the largest scales of the universe. For, although neutrinos are actually massless and almost invisible, their enormous number, about 10⁸⁹ in the universe, making them key players in the cosmos.

Our logic is this: at some time early in the history of the universe, when everything was very hot and very dense, nuclear reactions forged helium out of hydrogen, releasing huge amounts of neutrino particles as a byproduct. As the universe evolved, expanded and cooled, slight deviations in the density of this primordial soup of particles were amplified; In areas of higher than average density, gravity tried to pull more material in.

Dark matter, that invisible stuff responsible for the bulk of the universe's mass, was the first to collapse into clumps, because its interactions are purely gravitational. These primordial clumps of dark matter formed the galaxy seeds and galaxy clusters we see today. Neutrinos, which are incredibly light, began to accumulate a little later in the evolution of the universe. In fact, the ability of the neutrino particles to pass so freely through the cosmos slowed down the accumulation of dark matter, and we should be able to discover the traces of this effect nowadays.

The greater the mass of the neutrino particles, the greater the delay they created in the accumulation of matter, a phenomenon that in practice will manifest itself in the blurring of the boundaries of the structure of the universe on a large scale. If we measure the distribution of matter in the universe, we can deduce how large a mass of neutrino particles is.

Mapping the distribution of matter, most of which is dark matter, is not an easy and simple task. However, the researchers saw that the remnants of radiation from the Big Bang, known as the cosmic background radiation (CMB), are slightly distorted due to the bending of light resulting from the gravitational effects of the dark matter clumps that fill the space between the background radiation and us. Probing this "gravitational dusting" effect on the CMB is a very promising way to measure the distribution of dark matter in the universe.

New precise measurements of the CMB now underway will allow us to measure the distortions of the universe with very high precision, effectively mapping the otherwise invisible dark matter. If the distribution of dark matter is limited to structures with sharp boundaries, separated from each other by empty spaces, we can conclude that the neutrino masses are small; Alternatively, if the boundaries are blurred, we will know that the neutrino masses are larger. The new generation of CMB experiments will allow us to accurately determine the sum of the masses of the three types of neutrinos with an accuracy of up to five millionths of the electron mass.

The very fact that we may be able to measure the mass of the lightest and most elusive of all subatomic particles, by observing the universe as a whole, is nothing more than another example that the study of physics, across all scales, continues to surprise and encourage astrophysicists to dive deeper and deeper into the intricacies of the mechanisms of the natural world.

Sudip Das is a postdoctoral student, student of the David Schram research scholarship, at the Argonne National Laboratory.

Tristan L. Smith is a postdoctoral fellow at the Berkeley Center for Cosmological Physics at the University of California, Berkeley.

And more on the subject

Testing Neutrino Mixing at Future Collider Experiments. W. Porod, M. Hirsch, J. Romão and JWF Valle in Physical Review D, Vol. 63, no. 11, Article No. 115004; April 30, 2001.

Neutrino Masses and Particle Physics beyond the Standard Model. H. Päs in Annalen der Physik, Vol. 11, no. 8, pages 551-572; September 2002.