Physics noble prize 2015

MEANING OF NEUTRINO HAVING MASS Dr.S.S.Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106 A neutrino is a sub atomic particle with no electric charge. Three types of neutrinos, called flavors, are currently known. One type is related to the electron. Another type is related to a particle called the muon . The third type is related to a particle called the tau. The main difference between the neutrinos and their "relatives" is the fact that neutrinos are electrically neutral, while the electron, muon, and tau are electrically charged. Neutrinos are emitted in huge numbers by stars, including the sun.

They are important to astronomers and cosmologists,

Image from: http://www.kip.uni-heidelberg.de

because their behavior provides clues to the evolution of stars and galaxies. Neutrinos may give insights into how the universe was formed, how large it is, and what will ultimately happen to it.

Neutrinos are difficult to detect, because they do not readily interact with other forms of matter.

But using special equipment located in deep underground laboratories where no other cosmic particles can penetrate, scientists have detected neutrinos and discovered some of their properties.

Neutrinos are everywhere. Trillions of them are passing through our body every second, but they are so shy and we do not see or feel them. They are the least understood elementary particle we know that exist. Existing of neutrinos was suggested as a "desperate remedy" to the apparent paradox that the energy did not appear conserved in the world of atomic nuclei.

The Standard Model of particle physics can describe everything we know about elementary particles. It says that neutrinos do not have mass because they are all "left-handed" and do not bump onthe mysterious "Higgs boson" that fills our entire Universe. In 1998, convincing evidence was reported that neutrinos have mass. The Standard Model has fallen after decades of invincibility. The evidence comes from experiments deep underground in pitch darkness with many thousands of tonnes of water housed in mines. Neutrinos are found to have mass, but the mass is extremely tiny, at least million times lighter than the lightest elementary particle: 1

electron. How do we need to change the Standard Model to explain the neutrino mass? Some argue that our space time has unseen spatial dimensions, and we are stuck on three-dimensional "sheets". Other argue that we need to abandon the sacred distinction between matter and antimatter. When Universe started with the "Big Bang", there were almost equal amount of matter and anti-matter. Most of matter was annihilated by anti-matter when Universe cooled. We are leftover of one part in ten billions. Why was there a small excess matter over anti-matter so that we can exist? Once we abandon the sacred distinction between matter and anti-matter, it provides a key to understand why we exist. The mysteries about neutrinos are now being unraveled dramatically and we will learn much more in the coming years.

The history of a particle that appeared to have no charge and no mass is an interesting one. The electron neutrino (alepton) was first postulated in 1930 by Wolfgang Pauli to explain why the electrons in beta decay were not emitted with the full reaction energy of the nuclear transition.

The apparent violation of conservation of energy and momentum was most easily avoided by postulating another particle. Enrico Fermi called the particle a neutrino and developed a theory of beta decay based on it, but it was not experimentally observed until 1956. This elusive particle, with no charge and almost no mass, could penetrate vast thicknesses of material without interaction. The mean free path of a neutrino in water would be on the order of 10x the distance from the Earth to the Sun. In the standard Big Bang model, the neutrinos left over from the creation of the universe are the most abundant particles in the universe. This remnant neutrino density is put at 100 per cubic centimeter at an effective temperature of 2K (Simpson). The background temperature for neutrinos is lower than that for the microwave background (2.7K) because the neutrino transparency point came earlier. The sun emits vast numbers of neutrinos which can pass through the earth with little or no interaction. This leads to the statement "Solar neutrinos shine down on us during the day, and shine up on us during the night�. . Bahcall's modeling of the solar neutrino flux led to the prediction of about 5 x 106 neutrinos/cm2s. A remarkable opportunity for observing neutrinos came with Supernova 1987Awhen the Japanese observing team detected neutrinos almost coincident with the discovery of the light from the supernova. Neutrinos interact only by the weak interaction. Their interactions are usually represented in terms of Feynman diagrams.

Neutrinos oscillate between different flavors in flight. That is, an electron neutrino produced in a beta decay reaction may arrive in a detector as a muon or tau neutrino. This oscillation requires that the different neutrino flavors have different masses, although these masses have been shown to be tiny. From cosmological measurements, it has been calculated that the sum of the three neutrino masses must be less than one millionth that of the electron. The Standard Model of particle physics assumed that neutrinos are massless. However the experimentally established phenomenon of neutrino oscillation, which mixes neutrino flavour states with neutrino mass states (analogously to CKM mixing), requires neutrinos to have nonzero masses. Neutrinos have mass, but tiny even compared to other subatomic particles. They are the only identified candidate for dark matter, specifically hot dark matter. Massive neutrinos were originally conceived by Bruno Pontecorvo in the 1950s. Enhancing the basic framework to accommodate their mass is straightforward by adding a right-handed Lagrangian. This can be done in two ways. If, like other fundamental Standard Model particles, mass is generated by the Dirac mechanism, then the framework would require an SU(2) singlet. This particle would have no other Standard Model interactions (apart from the Yukawa interactions with the neutral 2

component of the Higgs doublet), so is called a sterile neutrino. Or, mass can be generated by the Majorana mechanism, which would require the neutrino and antineutrino to be the same particle.

The strongest upper limit on the masses of neutrinos comes from cosmology: the Big Bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background. If the total energy of all three types of neutrinos exceeded an average of 50 eV per neutrino, there would be so much mass in the universe that it would collapse. This limit can be circumvented by assuming that the neutrino is unstable; however, there are limits within the Standard Model that make this difficult. A much more stringent constraint comes from a careful analysis of cosmological data, such as the cosmic microwave background radiation, galaxy surveys, and the Lyman-alpha forest. These indicate that the summed masses of the three neutrinos must be less than0.3 eV. In 1998, research results at the Super-Kamiokande neutrino detector determined that neutrinos can oscillate from one flavor to another, which requires that they must have a nonzero mass. While this shows that neutrinos have mass, the absolute neutrino mass scale is still not known. This is because neutrino oscillations are sensitive only to the difference in the squares of the masses. In 2006, the MINOS experiment measured oscillations from an intense muon neutrino beam, determining the difference in the squares of the masses between neutrino mass eigenstates 2 and 3. The initial results indicate |Δm2 32| = 0.0027 eV2, consistent with previous results from Super-Kamiokande.

Since |Δm2 32| is the difference of two squared masses, at least one of them has to have a value which is at least the square root of this value. Thus, there exists at least one neutrino mass eigenstate with a mass of at least 0.04 eV. In 2009, lensing data of a galaxy cluster were analyzed to predict a neutrino mass of about 1.5 eV. This surprisingly high value requires that the three neutrino masses be nearly equal, with neutrino oscillations of order meV. The masses lie below the Mainz-Troitsk upper bound of 2.2 eV for the electron antineutrino. The latter will be tested in 2015 in the KATRIN experiment, that searches for a mass between 0.2 eV and 2 eV. A number of efforts are under way to directly determine the absolute neutrino mass scale in laboratory experiments. The methods applied involve nuclear beta decay. On 31 May 2010, OPERA researchers observed the first tau neutrino candidate event in a muon neutrino beam, the first time this transformation in neutrinos had been observed, providing further evidence that they have mass. In July 2010 the 3-D MegaZ DR7 galaxy survey reported that they had measured a limit of the combined mass of the three neutrino varieties to be less than0.28 eV. A tighter upper bound yet for this sum of masses, 0.23 eV, was reported in March 2013 by the Planck collaboration, whereas a February 2014 result estimates the sum as 0.320 ± 0.081 eV based on discrepancies between the cosmological consequences implied by Planck's detailed measurements of the Cosmic Microwave Background and predictions arising from observing other phenomena, combined with the assumption that neutrinos are responsible for the observed weaker gravitational lensing than would be expected from massless neutrinos. If the neutrino is a Majorana particle, the mass may be calculated by finding the half life of neutrinoless double-beta decay of certain nuclei. As of 2015, the lowest upper limit on the Majorana mass of the neutrino has been set by KamLAND-Zen: 0.12–0.25 eV. The Nobel Prize in Physics 2015 was awarded jointly to Takaaki Kajita and Arthur B. McDonald "for the discovery of neutrino oscillations, which shows that neutrinos have mass" 3

for their key contributions to the experiments which demonstrated that neutrinos change identities. This metamorphosis requires that neutrinos have mass. The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe. Around the turn of the millennium, Takaaki Kajita presented the discovery that neutrinos from the atmosphere switch between two identities on their way to the SuperKamiokande detector in Japan. Meanwhile, the research group in Canada led by Arthur B. McDonald could demonstrate that the neutrinos from the Sun were not disappearing on their way to Earth. Instead they were captured with a different identity when arriving to the Sudbury Neutrino Observatory.

Takaaki Kajita Super-Kamiokande Collaboration University of Tokyo, Kashiwa, Japan

Arthur B. McDonald Sudbury Neutrino Observatory Collaboration Queen’s University, Kingston, Canada

A neutrino puzzle that physicists had wrestled with for decades had been resolved. Compared to theoretical calculations of the number of neutrinos, up to two thirds of the neutrinos were missing in measurements performed on Earth. Now, the two experiments discovered that the neutrinos had changed identities. The discovery led to the far-reaching conclusion that neutrinos, which for a long time were considered massless, must have some mass, however small. For particle physics this was a historic discovery. Its Standard Model of the innermost workings of matter had been incredibly successful, having resisted all experimental challenges for more than twenty years. However, as it requires neutrinos to be massless, the new observations had clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe. The discovery rewarded with this year’s Nobel Prize in Physics have yielded crucial insights into the all but hidden world of neutrinos. After photons, the particles of light, neutrinos are the most numerous in the entire cosmos. The Earth is constantly bombarded by them. Now the experiments continue and intense activity is underway worldwide in order to capture neutrinos and examine their properties. New discoveries about their deepest secrets are expected to change our current understanding of the history, structure and future fate of the universe.

Implications of neutrino mass Now that neutrinos do appear to have mass, there are two problems to be solved, the first is to overcome the contradiction between left-handedness and mass. The second is to understand why the neutrino mass is so small compared with other particle masses — indeed, direct measurements indicate that electrons are at least 500 000 times more massive than neutrinos.

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When neutrinos were thought to be massless particles, these problems were not an issue. But the tiny mass is a puzzle, and there must be some deep reason why this is the case. A comparison of the masses of all the fundamental fermions, particles with spin ½, other than the neutrino, the lightest fermion is the electron, with a mass of 0.5 MeV/c 2. Neutrino-oscillation experiments do not measure the mass of neutrinos directly, rather the mass difference between the different types of neutrino. But by assuming that neutrino masses are similar to this mass difference, we can place upper limits on the mass of a few hundred millielectron-volts. Basically, there are two ways to extend the Standard Model in order to make neutrinos massive. One approach involves new particles called Dirac neutrinos, while the other approach involves a completely different type of particle called the Majorana neutrino. The Dirac neutrino is a simple idea with a serious flaw. According to this approach, the reason that right-handed neutrinos have escaped detection so far is that their interactions are at least 26 orders of magnitude weaker than ordinary neutrinos.

The idea of the Dirac neutrino works in the sense that we can generate neutrino masses via the Higgs mechanism. However, it also suggests that neutrinos should have similar masses to the other particles in the Standard Model. To avoid this problem, we have to make the strength of neutrino interactions with the Higgs boson at least 12 orders of magnitude weaker than that of the top quark. Few physicists accept such a tiny number as a fundamental constant of nature.

According to the Higgs mechanism in the Standard Model, particles in the vacuum acquire mass as they collide with the Higgs boson. Photons (γ) are massless because they do not interact with the Higgs boson. All particles, including electrons (e), muons (μ) nd top quarks (t), change handedness when they collide with the Higgs boson; left-handed particles become right-handed and vice versa. Experiments have shown that neutrinos (ν) are always left-handed. Since righthanded neutrinos do not exist in the Standard Model, the theory predicts that neutrinos can never acquire mass. In one extension to the Standard Model, left- and right-handed neutrinos exist.

These Dirac neutrinos acquire mass via the Higgs mechanism but right-handed neutrinos interact much more weakly than any other particles. According to another extension of the Standard Model, extremely heavy right-handed neutrinos are created for a brief moment before they collide with the Higgs boson to produce light left-handed Majorana neutrinos. An alternative way to make right-handed neutrinos extremely weakly interacting was proposed in 1998 by Nima Arkani-Hamed, Savas Dimopoulous Gia Dvali and John March-Russell. They exploited an idea from superstring theory in which the three dimensions of space with which we are familiar are embedded in 10- or 11-dimensional space-time. Like us, all the particles of the Standard Model — electrons, quarks, left-handed neutrinos, the Higgs boson and so on — are stuck on a threedimensional "sheet" called a three-brane. One special property of right-handed neutrinos is that they do not feel the electromagnetic force, or the strong and weak forces. Arkani-Hamed and collaborators argued that righthanded neutrinos are not trapped on the three-brane in the same way that we are, rather they can move in the extra dimensions. This mechanism explains why we have never observed a right-handed neutrino and why their interactions with other particles in the Standard Model are extremely weak. The upshot of this approach is that neutrino masses can be very small.

The second way to extend the Standard Model involves particles that are called Majorana neutrinos. One advantage of this approach is that we no longer have to invoke righthanded neutrinos with extremely weak interactions. However, we do have to give up the fundamental distinction between matter and antimatter. Although this sounds bizarre, neutrinos and 5

antineutrinos can be identical because they have no electric charge. Massive neutrinos sit naturally within this framework. Recall the observer travelling at the speed of light who overtakes a left-handed neutrino and sees a right-handed neutrino. Earlier we argued that the absence of right-handed neutrinos means that neutrinos are massless. But if neutrinos and antineutrinos are the same particle, then we can argue that the observer really sees a right-handed antineutrino and that the massive-neutrino hypothesis is therefore sound. So how is neutrino mass generated? In this scheme, it is possible for right-handed neutrinos to have a mass of their own without relying on the Higgs boson. Unlike other quarks and leptons, the mass of the righthanded neutrino, M, is not tied to the mass scale of the Higgs boson. Rather, it can be much heavier than other particles.

When a left-handed neutrino collides with the Higgs boson, it acquires a mass, m, which is comparable to the mass of other quarks and leptons. At the same time it transforms into a righthanded neutrino, which is much heavier than energy conservation would normally allow (figure 2c). However, the Heisenberg uncertainty principle allows this state to exist for a short time interval, Δt, given by Δt ~h/Mc2, after which the particle transforms back into a left-handed neutrino with mass m by colliding with the Higgs boson again. Put simply, we can think of the neutrino as having an average mass of m2/M over time. This so-called seesaw mechanism can naturally give rise to light neutrinos with normal-strength interactions. Normally we would worry that neutrinos with a mass, m, that is similar to the masses of quarks and leptons would be too heavy. However, we can still obtain light neutrinos if M is much larger than the typical masses of quarks and leptons. Right-handed neutrinos must therefore be very heavy, as predicted by grandunified theories that aim to combine electromagnetism with the strong and weak interactions.

Current experiments suggest that these forces were unified when the universe was about 1032 m across. Due to the uncertainty principle, the particles that were produced in such small confines had a high momentum and thus a large mass. It turns out that the distance scale of unification gives righthanded neutrinos sufficient mass to produce light neutrinos via the seesaw mechanism.

In this way, the light neutrinos that we observe in experiments can therefore probe new physics at extremely short distances. Among the physics that neutrinos could put on a firm footing is the theory of supersymmetry, which theorists believe is needed to make unification happen and to make the Higgs mechanism consistent down to such short distance scales.

Acknowledgement: The use of information retrieved through various references/sources of internet in this article is highly acknowledged.

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