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In Search of No Neutrinos Antonio Palazzo - Low-Energy Neutrinos to Cite This Article: Giorgio Gratta and Naoko Kurahashi 2010 Phys

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In Search of No Neutrinos Antonio Palazzo - Low-Energy Neutrinos to Cite This Article: Giorgio Gratta and Naoko Kurahashi 2010 Phys Physics World FEATURE Related content - Sterile Neutrinos In search of no neutrinos Antonio Palazzo - Low-energy neutrinos To cite this article: Giorgio Gratta and Naoko Kurahashi 2010 Phys. World 23 (04) 27 Livia Ludhova - Atmospheric neutrinos Thomas K. Gaisser View the article online for updates and enhancements. This content was downloaded from IP address 171.66.161.130 on 28/07/2020 at 21:08 physicsworld.com Feature: Neutrinos Photolibrary In search of no neutrinos Neutrinos were first observed almost 60 years ago by observing beta decay. Giorgio Gratta and Naoko Kurahashi explain why physicists are now on the lookout for an exceedingly rare variation of this process in which no neutrinos are created As you sit down to relax and read this article, take a mass. But in the last 20 years, researchers at several moment to consider that more than 10 million neut- underground neutrino detectors around the world rinos created in the Big Bang are traversing your body showed that in fact these particles, which come in at any one time. These tiny subatomic particles have three different types or “flavours”, do have masses – been travelling across the universe for the last 13 bil- albeit very small ones. However, their technique, Giorgio Gratta lion years, carrying the fingerprints of the primordial which relies on a sophisticated type of interferometry, is a professor of cosmic explosion. As they course through your body is only able to determine the mass differences between physics in the department of physics they will ignore it, because – challengingly for anyone the three types of neutrino, leaving their absolute and Naoko Kurahashi wishing to study these particles experimentally – the masses unmeasured. is a postgraduate probability of neutrinos interacting with matter is so For many years it has been known that there might student in the minute that they can cross entire planets or stars with- be a way to probe the absolute mass of neutrinos: by department of applied out being disturbed. measuring the half-life of a very rare nuclear process physics, both at First postulated in 1930 by Wolfgang Pauli and even- known as neutrinoless double beta decay. This decay Stanford University, tually observed by Frederick Reines and Clyde Cowan can only occur if the quantum nature of neutrinos is of US, e-mail gratta@ in 1953, neutrinos were originally thought to have no a novel type, in which the distinction between particles stanford.edu 27 Physics World April 2010 Feature: Neutrinos physicsworld.com 1 A handy explanation 2 Double beta decay e observer what the observer sees momentum momentum e e spin spin left-handed left-handed ν e ν e e observer what the observer sees e e e momentum momentum ν ν e ν e e spin spin e ν e left-handed right-handed N Nν N Nν time time In the upper part of the figure, the handedness of a particle of finite mass is shown as seen by an observer in a reference frame that is at rest with respect to the page. Illustrations (top) and Feynman diagrams (bottom) showing (left) two-neutrino In the lower part of the figure, the same particle is analysed from the point of view double beta decay and (right) zero-neutrino double beta decay. In the Feynman of an observer travelling at a faster speed, in such a way as to overtake the particle. diagrams, the blue arrows represent the nucleus that is decaying, while the other In this reference frame the momentum of the particle appears to be flipped arrows represent particles that are emitted. Lines without an arrowhead that backwards with respect to the previous case, while the direction of the spin does connect two vertices represent “virtual” particles that cannot be seen or detected. not change. This shows that for a non-relativistic particle, the handedness depends N, nucleus before decay; N´, nucleus after decay; e, electron; νe, electron neutrino; on the choice of reference frame. ෆνe, electron antineutrino. and antiparticles is somewhat blurred. years after the first experimental observation of neut- But this is no pipe dream – hidden deep in under- rinos – found that these particles are always emitted ground sites around the world are a number of large with left-handedness, which means that a particle’s detectors that seek to observe this extremely uncom- spin is oriented in the opposite direction to its mo - mon decay. These experiments, which are now start- mentum. Given that the only way to probe a neutrino ing up, could soon not only provide values for the is to make it interact with something, handedness, like neutrino masses, but also reveal that neutrinos are any other property, can only be defined in the context their own antiparticles. of an interaction. In fact, weak-force interactions are the only method by which we can “measure” the What we know and what we do not know handedness of the neutrino, and so neutrino handed- Neutrinos make up three of the 12 known fundamental ness was incorporated into the Standard Model of matter particles, which cannot be divided into more particle physics – the ul timate rule book for how fun- parts. The neutrinos (“little neutral ones” in Italian) damental particles in teract with one another – by have no charge and fall into the category of “leptons”, requiring that only left-handed neutrinos and right- along with the much more massive charged electron, handed antineut rinos participate in interactions muon and tau leptons. When a neutrino is produced, involving the weak force. it exists in one of its three flavours as an electron-, The implications of the fact that neutrinos are left- muon- or tau-neutrino – depending on the charged handed and antineutrinos are right-handed are far lepton with which it was simultaneously created. more profound than they may at first appear. As fig- The remaining six elementary matter particles are ure 1 shows, the handedness of a particle can change quarks. Both quarks and charged leptons experience sign by moving from one reference frame to another. the electromagnetic force, and quarks also feel the In this scenario there is no way to assign an “absolute” strong force. These forces cause the particles to inter- handedness to the particle – unless the particle is mass- act frequently with other matter particles. But having less. If the mass of the particle is zero, then, according no charge, neutrinos can only feel the weak and gra - to special relativity, the particle will move at the speed vitational forces, and hence interact very little with of light and no reference frame will be able to overtake matter. Around this simple fact revolve many of the dif- it and see it. In this case, the handedness is an absolute ficulties of experimental neutrino physics. As an ex - property of the particle. Neutrinos were therefore ample, the mean free path in lead for neutrinos with “coded” in the Standard Model as particles with zero energies similar to those produced in nuclear reactors mass and fixed handedness, and for a long time no bet- Neutrino is about a third of a light-year. ter measurement was able to challenge this assertion. Neutrino detectors therefore need to be huge and But such a challenge did come, and showed us that detectors need run for a long time for researchers to stand a chance we were wrong. Neutrinos do in fact have mass. This to be huge of ob serving their prey. Combined with the tricky was discovered by observing the strange phenomenon and run for a task of building a detector that is not swamped by of “neutrino oscillation”, which involves neutrinos long time for an overwhelming background from other types of changing between their three different flavours as they researchers to nuclear radiation, this makes neutrino detection propagate through space (see box opposite). But the extremely challenging, and the data we have collected problem with neutrino oscillation, big breakthrough stand a chance preciously scarce. though it was, is that it tells us nothing about the ab- of observing A groundbreaking experiment carried out by Maur - solute masses of the neutrinos, only the mass differ- their prey ice Goldhaber and collaborators in 1957 – only four ences between the different flavours. 28 Physics World April 2010 physicsworld.com Feature: Neutrinos Enter the massive Majorana particle Neutrino oscillations and why they imply mass In nuclear physics the simplest phenomenon involving neutrinos is “beta decay”, a process within a nucleus in which a neutron decays into a proton, an electron and 1.2 an electron antineutrino. This type of nuclear decay is very common and it is responsible for much natural and 1.0 artificial radioactivity. Figure 2 shows a more exotic process in which the 0.8 beta decay repeats itself twice, in such a way that the state in between the two decays cannot be seen or meas- 0.6 ured. This “double beta decay” process can occur in two 0.4 survival probability distinct ways. Two-neutrino double beta decay is a con- Adapted from the KamLAND Collaboration ventional process in that the Standard Model predicts its existence and it does not violate any of the known 0.2 laws of physics. This is not the case, however, for zero- 0 neutrino (or neutrinoless) double beta decay, which 10 20 30 40 50 60 70 80 90 100 violates a number of physical laws enshrined in the L /E (km per MeV) Standard Model. If this decay were discovered experi- 0 mentally, it would reveal some unknown physics that Often in physics, the most sensitive measurements are obtained by interferometry – the Standard Model is currently unable to describe.
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