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John Norris Bahcall, passed away on August 17, 2005, in NewYork City, USA. He was born on December 30, 1934, in Shreveport, Louisiana, USA. He was Richard Black Professor of Astrophysics in the School of Natural Sciences at the Institute forAdvanced Study (IAS) in Princeton, New Jersey, USA and a recipient of the National Medal of Science. In addition, he was President of the American Astronomical Society, President-Elect of the American Physical Society, and a prominent leader of the astrophysics community.

John had a long and prolific career in astronomy and astrophysics, spanning five decades and the publication of more than five hundred technical articles, books, and popular papers.

John's most recognized scientific contribution was the novel proposal in 1964, together with Raymond Davis Jr, that scientific mysteries of our Sun `how it shines, how old it is, how hot it is' could be examined by measuring the number of neutrinos arriving on Earth from the Sun. Measuring the properties of these neutrinos tests both our understanding of how stars shine and our understanding of fundamental particle physics.

However, in the 1960s and 1970s, the observations by Raymond Davis Jr showed a clear discrepancy between John's theoretical predictions, based on standard solar and particle physics models, and what was experimentally measured. This discrepancy, known as the `Solar Neutrino Problem', was examined by hundreds of physicists, chemists, and astronomers over the subsequent three decades. In the late 1990s through 2002, new large-scale neutrino experiments in Japan, Canada, Italy, and Russia culminated in the conclusion that the discrepancy between John's theoretical predictions and the experimental results required a modification of our understanding of particle physics: neutrinos must have a mass and `oscillate' among different particle states.

In addition to neutrino astrophysics, John contributed to many areas of astrophysics including the study of dark matter in the Universe, properties of quasars, structure of the galaxies, the evolution of stars, and the identification of the first neutron star companion.

John was an active member of the International Advisory Committee of the Nobel Symposium 129 on Neutrino Physics in Enköping, Sweden between August 19 and August 24, 2004, but he was unfortunately not able to attend the Symposium himself due to his illness. He will be hugely missed in the scientific community and especially among neutrino physicists. We, the members of the Local Organizing Committee of the Symposium, will always remember his large enthusiasm and creativity, warm friendship, and sharp intellect.

Nobel Symposium 129 on Neutrino Physics was held at Haga Slott in Enköping, Sweden during August 19–24, 2004. Invited to the symposium were around 40 globally leading researchers in the field of neutrino physics, both experimental and theoretical. In addition to these participants, some 30 local researchers and graduate students participated in the symposium.

The dominant theme of the lectures was neutrino oscillations, which after several years were recently verified by results from the Super-Kamiokande detector in Kamioka, Japan and the SNO detector in Sudbury, Canada. Discussion focused especially on effects of neutrino oscillations derived from the presence of matter and the fact that three different neutrinos exist. Since neutrino oscillations imply that neutrinos have mass, this is the first experimental observation that fundamentally deviates from the standard model of particle physics. This is a challenge to both theoretical and experimental physics. The various oscillation parameters will be determined with increased precision in new, specially designed experiments. Theoretical physics is working intensively to insert the knowledge that neutrinos have mass into the theoretical models that describe particle physics. It will probably turn out that the discovery of neutrino oscillations signifies a breakthrough in the description of the very smallest constituents of matter. The lectures provided a very good description of the intensive situation in the field right now. The topics discussed also included mass models for neutrinos, neutrinos in extra dimensions as well as the `seesaw mechanism', which provides a good description of why neutrino masses are so small.

Also discussed, besides neutrino oscillations, was the new field of neutrino astronomy. Among the questions that neutrino astronomy hopes to answer are what the dark matter in the Universe consists of and where cosmic radiation at extremely high energies comes from. For this purpose, large neutrino telescopes are built deep in the Antarctic ice, in the Baikal Lake, and in the Mediterranean Sea.

Among prominent unanswered questions, highlighted as one of the most important, was whether neutrinos are Dirac or Majorana particles. By studying neutrino double beta decay, researchers hope to answer this question, but it will put very large demands on detectors.

The programme also included ample time for lively and valuable discussions, which cannot normally be held at ordinary conferences.

The symposium concluded with a round-table discussion, where participants discussed the future of neutrino physics.Without a doubt, neutrino physics today is moving toward a very exciting and interesting period.

An important contribution to the success of the symposium was the wonderful setting that the Haga Slott manor house hotel and conference center offered to the participants.

The early history of neutrino mixing and oscillations is briefly reviewed.

Super-Kamiokande (SK) is able to measure neutrino interactions over the 5 decades of the energy from 5 MeV to a few hundreds GeV. In 1998, SK has obtained strong evidence of neutrino oscillations by observing a deficit of the atmospheric ν_μ flux coming upward to the detector. This discovery has revealed that neutrinos have finite masses. The mass difference is ~0.002 eV² and a mixing angle is nearly maximal. The experimental evidence on the atmospheric neutrino oscillation has been strengthened by the observation of the oscillatory pattern as a function of L/E.

The non-observation of the day-night flux differences and the non-observation of the spectrum distortions of solar neutrinos have placed a strong constraint on the oscillation parameters. In 2000, the so called small mixing angle solutions were rejected and it was shown that the solar neutrino oscillation should be large mixing.

Conclusive evidence of the solar neutrino oscillation was obtained by comparing the precisely measured flux by the neutrino-electron scattering in SK and the flux obtained by the charged current interaction of electron neutrinos in SNO in 2001. The result of the fit by using all the solar neutrino experiments has selected a MSW Large Mixing Angle solution.

The Sudbury Neutrino Observatory uses 1000 tonnes of heavy water in an ultra-clean Cherenkov detector situated 2 km underground in Ontario, Canada to study neutrinos from the Sun and other astrophysical sources. The Charged Current (CC) reaction on deuterium is sensitive only to electron neutrinos whereas the Neutral Current (NC) is equally sensitive to all active neutrino types. By measuring the flux of neutrinos from ⁸B decay in the Sun with the CC and NC reactions it has been possible to establish clearly, through an appearance measurement, that electron neutrinos change to other active neutrino types, properties that are beyond the Standard Model of elementary particles. The observed total flux of active neutrinos agrees well with solar model flux calculations for ⁸B. This provides a clear answer to the "Solar Neutrino Problem". When these results are combined with other measurements, the oscillation of massive neutrinos is strongly defined as the primary mechanism for flavor change and oscillation parameters are well constrained.

Data corresponding to a KamLAND detector exposure of 766.3 ton-years to reactor neutrinos was used to search for neutrino oscillations. Based on this data-sample, KamLAND demonstrates the reactor _e disappearance at the 99.998% significance and finds the reactor _e spectrum distortion at the 99.6% significance. The reactor neutrino anomaly defined as the flux disappearance and spectrum shape distortion is confirmed at the 99.999995% significance. A two-flavor neutrino oscillation analysis including the rate and energy spectrum yields a best fit at Δm² = 7.9 × 10^-5 eV² and sin² 2θ = 0.86. A global analysis of data from KamLAND and solar neutrino experiments yields Δm² = 7.9^+0.6_-0.5 × 10^-5 eV² and tan² θ = 0.40^+0.10_-0.07. The present result gives the most precise determination of Δm² to date. To test the goodness of the oscillation hypothesis, the ratio of observed anti-neutrino spectrum to the expected for no-oscillation as a function of L₀/E is compared with the predictions which give neutrino-flavor changes. The neutrino oscillation is much favored with more than 99% C.L., while the decay and decoherence are excluded at the 95% and 94% C.L.

If history is any guide, then there is a good chance that our present description of neutrinos is wrong. Over the past 30 years, neutrino physics has again and again produced results which took us by surprise, and forced us to change our theoretical direction. In light of this, it is worth examining the experimental clues we now have to see if they lead to ideas outside our "New Standard Model."

Our present paradigm says that there are only three neutrinos participating in the weak interaction. They are light, with masses less than 0.1 eV, and predicted to be Majorana neutrinos, that is, each is its own antiparticle. The light mass is motivated by the see-saw model, which postulates partners which are very massive (~hundreds of GeV).

By contrast with this "conventional wisdom," this talk discusses five examples of experimental results which, if confirmed, would cause us to rethink our theory. It focuses only on oscillations, and on those questions which can be addressed at accelerators. Other ways neutrinos may be surprising, such as the anomalous measurement of sin²θ_W from NuTeV, are left for other talks. The ideas discussed here are best explored through accelerator-produced neutrino beams. So the reasons why beam-based experiments are becoming the venue-of-choice are then examined. Lastly, the first two of the five ideas are explored in detail.

I provide a summary of the current theoretical knowledge of solar neutrino fluxes as derived from precise solar models.

Starting with a historical review, I summarize the status of calculations of the flux of atmospheric neutrinos and how they compare to the measurements.

The MSW (Mikheyev-Smirnov-Wolfenstein) effect is the adiabatic or partially adiabatic neutrino flavor conversion in media with varying density. The main notions related to the effect, its dynamics and physical picture are reviewed. The large mixing MSW effect is realized inside the Sun providing a solution of the solar neutrino problem. The small mixing MSW effect driven by the 1–3 mixing can be realized for the supernova (SN) neutrinos. Inside collapsing stars new elements of the MSW dynamics may show up: non-oscillatory transition, non-adiabatic conversion, time dependent adiabaticity violation induced by shock waves. Effects of the resonance enhancement and the parametric enhancement of oscillations can be realized for atmospheric and accelerator neutrinos in the Earth. Precise results for neutrino oscillations in low density media with arbitrary density profile are presented and the attenuation effect is described. The area of applications is the solar and SN neutrinos inside the Earth, and the results are crucial for the neutrino oscillation tomography.

Some theoretical aspects of 3-flavour (3f) neutrino oscillations are reviewed. These include: general properties of 3f oscillation probabilities; matter effects in ν_μ ↔ ν_τ oscillations; 3f effects in oscillations of solar, atmospheric, reactor and supernova neutrinos and in accelerator long-baseline experiments; CP and T violation in neutrino oscillations in vacuum and in matter, and the problem of U_e3.

In this talk I review the present status of neutrino masses and mixing and some of their implications for particle physics phenomenology.

Future neutrino oscillation experiments will lead to precision measurements of neutrino mass splittings and mixings. The flavour structure of the lepton sector will therefore at some point become better known than that of the quark sector. This article discusses the realistic potential of future oscillation experiments on the basis of detailed simulations with an emphasis on experiments which can be done in about ten years. In addition, some theoretical implications for neutrino mass models will be briefly discussed.

The results obtained so far in searches for double beta decay (DBD) are reviewed with special emphasis on the neutrinoless channel which could indicate a non-zero effective neutrino mass under the hypothesis that this particle is of Majorana type. The connection of the value of this mass with the indications coming from the recent results of experiments on neutrino oscillation is underlined. The difficulties and uncertainties are emphasized of the calculation of the nuclear matrix elements relevant for DBD. It is stressed that, as a consequence, different nuclear candidates for double beta decay should be investigated. There is an additional need to do so: a peak indicative of neutrinoless double beta decay could be mimicked by some radioactive contamination. Detection of the different peaks expected for DBD in two or more candidate nuclei becomes therefore mandatory.

The experimental methods to search for DBD, particularly in its neutrinoless channel, are compared and discussed together with the results obtained so far. The two most constraining experiments presently running (NEMO 3 and CUORICINO) are described in some detail and their preliminary limits discussed. The report is concluded with a description of the presently proposed experiments aiming to reach a sensitivity of a few tens of milli-electronvolts as required by the results coming from oscillation experiments. Special emphasis is given to possible improvements in increasing active mass and resolution, and decreasing background. This comparison refers in particular to the role played with respect to the classical methods of detection (scintillation and ionisation counters, TPC and tracking chambers, large arrays of diodes) by new detecting methods.

The compelling experimental evidences for oscillations of solar and atmospheric neutrinos imply the existence of 3-neutrino mixing in vacuum. We briefly review the phenomenology of 3-ν mixing, and the current data on the 3-neutrino mixing parameters. The open questions and the main goals of future research in the field of neutrino mixing and oscillations are outlined. The predictions for the effective Majorana mass |⟨m⟩| in (ββ)_0ν-decay in the case of 3-ν mixing and massive Majorana neutrinos are reviewed. The physics potential of the experiments, searching for (ββ)_0ν-decay and having sensitivity to |⟨m⟩| ≳ 0.01 eV, for providing information on the type of the neutrino mass spectrum, on the absolute scale of neutrino masses and on the Majorana CP-violation phases in the PMNS neutrino mixing matrix, is discussed.

Observing a high-statistics neutrino signal from a galactic supernova (SN) would allow one to test the standard delayed explosion scenario and may allow one to distinguish between the normal and inverted neutrino mass ordering due to the effects of flavor oscillations in the SN envelope. One may even observe a signature of SN shock-wave propagation in the detailed time-evolution of the neutrino spectra. A clear identification of flavor oscillation effects in a water Cherenkov detector probably requires a megatonne-class experiment.

Kilometer-scale neutrino detectors such as IceCube are discovery instruments covering nuclear and particle physics, cosmology and astronomy. Examples of their multidisciplinary missions include the search for the particle nature of dark matter and for additional small dimensions of space. In the end, their conceptual design is very much anchored to the observational fact that Nature accelerates protons and photons to energies in excess of 10²⁰ and 10¹³ eV, respectively. The cosmic ray connection sets the scale of cosmic neutrino fluxes. In this context, we discuss the first results of the completed AMANDA detector and the reach of its extension, IceCube. Similar experiments are under construction in the Mediterranean. Neutrino astronomy is also expanding in new directions with efforts to detect air showers, acoustic and radio signals initiated by neutrinos with energies similar to those of the highest energy cosmic rays.

This talk reviews status and results from the two presently operating underwater/ice neutrino telescopes, NT-200 in Lake Baikal and AMANDA-II at the South Pole. It also gives a description of the design and the expected performance of IceCube, the next-generation neutrino telescope at South Pole.

Neutrino astronomy offers the possibility to perform extra-galactic observations well beyond the photon absorption cutoff above 5 × 10¹³ eV. Based on observations of cosmic rays, we already know that astrophysical sources produce particles with at least a million times more energy than this photon cutoff. Once discovered, either the nature of the sources themselves or the cross sections of ultra-high energy neutrinos with terrestrial matter may reveal exotic physical processes that are inaccessible to modern accelerators. Some of these processes may be due to as-yet unknown physics at the grand unification scale or beyond. Neutrino telescopes based on optical techniques currently operating and under construction have apertures measured in several km³-sr. Radio and acoustic detection techniques have been demonstrated in laboratory experiments and are currently used for instrumentation of apertures 10 to 10,000 times larger than optical techniques for neutrinos above 10¹⁶ eV. I discuss the status of current and proposed neutrino telescope projects based on these techniques. These telescopes have already ruled out some of the more exotic predictions for neutrino intensity. The upcoming generation of radio-based and acoustic-based detectors should be sensitive to cosmic neutrinos above 10¹⁸ eV originating through the so-called GZK process. A comparison of different neutrino telescopes using a common aperture variable shows how they are complementary in the trade-offs of volume versus threshold. I include a proposal for how neutrino telescopes should report their sensitivities to facilitate direct comparisons among them and to allow testing of neutrino brightness models that appear even after publication of the experimental results.

High energy neutrinos can arise from a variety of processes. Interactions of ultra high energy cosmic rays with radiation and matter lead to secondary particles, some of which create neutrinos. Interactions of the 2.7 K CMB radiation with protons gives rise to pions and neutrons, and neutrons produced in the photodisintegration of heavy nuclei are also a source of neutrinos. There is speculation that super-heavy relic particles with masses of ~10²¹ eV are created in the early Universe: if such particles exist then their decay channels are expected to contain neutrinos. The different sources of neutrinos are summarised. High energy neutrinos can be detected through the extensive air showers that they create in the atmosphere and the potential of the Pierre Auger Observatory, now nearing completion, and of the planned EUSO and ASHRA instruments, to detect neutrino-induced air showers, will be described.

Neutrinos from the Big Bang are theoretically expected to be the most abundant particles in the Universe after the photons of the Cosmic Microwave Background (CMB). Unlike the relic photons, relic neutrinos have not so far been observed. The Cosmic Neutrino Background (CνB) is the oldest relic from the Big Bang, produced a few seconds after the Bang itself. Due to their impact in cosmology, relic neutrinos may be revealed indirectly in the near future through cosmological observations. In this talk we concentrate on other proposals, made in the last 30 years, to try to detect the CνB directly, either in laboratory searches (through tiny accelerations they produce on macroscopic targets) or through astrophysical observations (looking for absorption dips in the flux of Ultra-High Energy (UHE) neutrinos, due to the annihilation of these neutrinos with relic neutrinos at the Z-resonance).

We concentrate mainly on the first possibility. We show that, given present bounds on neutrino masses, lepton number in the Universe and gravitational clustering of neutrinos, all expected laboratory effects of relic neutrinos are far from observability, awaiting future technological advances to reach the necessary sensitivity. The problem for astrophysical searches is that sources of UHE neutrinos at the extreme energies required may not exist. If they do exist, we could reveal the existence, and possibly the mass spectrum, of relic neutrinos, with detectors of UHE neutrinos (such as ANITA, Auger, EUSO, OWL, RICE and SalSA).

The presence of superheavy Majorana neutrinos is an important prediction of unified theories beyond the standard model. An integration of the heavy Majorana neutrinos induces very small masses for neutrinos via the seesaw mechanism. Thus, small neutrino masses is a reflection of a unification at very high energy scale.

It is quite natural to consider that the heavy Majorana neutrinos are produced thermally in the early universe. Then, the heavy neutrinos begin to decay into Higgs and lepton, or Higgs and anti-lepton when the temperature of the universe cools down to the masses of heavy Majorana neutrinos. The decays of the heavy neutrinos may produce the lepton-number asymmetry if CP invariance is violated in the decay processes. This lepton-number asymmetry is converted into the baryon-number asymmetry in the present universe (leptogenesis) through nonperturbative effects of the electroweak gauge theory. We show, in this talk, that the neutrino masses suggested from atmospheric and solar neutrino-oscillation experiments are just in the range favorable for the thermal leptogenesis. With the aid of the observed neutrino masses we find that the thermal leptogenesis takes place at temperatures T ≳ 2 × 10⁹ GeV.

However, the above scenario suffers from the gravitino problem if one extends the standard model to the supergravity framework. This is because too many gravitinos are produced at the temperature required for the thermal leptogenesis and their decays destroy light elements created by the big-bang nucleosynthesis. We show that a leptogenesis via inflaton decay is an interesting alternative to the thermal leptogenesis. This scenario is free from the gravitino problem if the gravitino has a relatively large mass as m_3/2 ≃ 3–6 TeV.

The importance of neutrinoless double β decay experiments and measurement of CP violation in neutrino oscillation experiments is also emphasized to explain the present universe's baryon asymmetry.

The early universe provides a unique laboratory for probing the frontiers of particle physics in general and neutrino physics in particular. The primordial abundances of the relic nuclei produced during the first few minutes of the evolution of the Universe depend on the electron neutrinos through the charged-current weak interactions among neutrons and protons (and electrons and positrons and neutrinos), and on all flavors of neutrinos through their contributions to the total energy density which regulates the universal expansion rate. The latter contribution also plays a role in determining the spectrum of the temperature fluctuations imprinted on the Cosmic Background Radiation (CBR) some 400 thousand years later. Using deuterium as a baryometer and helium-4 as a chronometer, the predictions of BBN and the CBR are compared to observations. The successes of, as well as challenges to the standard models of particle physics and cosmology are identified. While systematic uncertainties may be the source of some of the current tensions, it could be that the data are pointing the way to new physics. In particular, BBN and the CBR are used to address the questions of whether or not the relic neutrinos were fully populated in the early universe and, to limit the magnitude of any lepton asymmetry which may be concealed in the neutrinos.

The main goal of the construction of large volume, high energy neutrino telescopes is the detection of extra-Galactic neutrino sources. The existence of such sources is implied by observations of ultra-high energy, ≥ 10¹⁹ eV, cosmic-rays (UHECRs), the origin of which is a mystery. The observed UHECR flux sets an upper bound to the extra-Galactic high energy neutrino intensity, which implies that the detector size required to detect the signal in the energy range of 1 TeV to 1 PeV is ≥ 1 giga-ton, and much larger at higher energy. Optical Cerenkov neutrino detectors, currently being constructed under ice and water, are expected to achieve 1 giga-ton effective volume for 1 TeV to 1 PeV neutrinos. Coherent radio Cerenkov detectors (and possibly large air-shower detectors) will provide the >> 1 giga-ton effective volume required for detection at ~10¹⁹ eV. Detection of high energy neutrinos associated with electromagnetically identified sources will allow to identify the sources of UHECRs, will provide a unique probe of the sources, which may allow to resolve open questions related to the underlying physics of models describing these powerful accelerators, and will provide information on fundamental neutrino properties.

I briefly review cosmological bounds on neutrino masses and the underlying gravitational physics at a level appropriate for readers outside the field of cosmology. For the case of three massive neutrinos with standard model freezeout, the current 95% upper limit on the sum of their masses is 0.42 eV. I summarize the basic physical mechanism making matter clustering such a sensitive probe of massive neutrinos. I discuss the prospects of doing still better in coming years using tools such as lensing tomography, approaching a sensitivity around 0.03 eV. Since the lower bound from atmospheric neutrino oscillations is around 0.05 eV, upcoming cosmological measurements should detect neutrino mass if the technical and fiscal challenges can be met.

Are neutrinos their own antiparticles? We explain why they very well might be. Then, after highlighting the fact that, to determine experimentally whether they are or not, one must overcome the smallness of neutrino masses, we discuss the one approach that nevertheless shows great promise. Finally, we turn to the consequences of neutrinos being their own antiparticles. These consequences include unusual electromagnetic properties, and manifestly CP-violating effects from "Majorana" phases that have no quark analogues.

This report explores the results and implications of the weak mixing angle measurement made by the NuTeV neutrino experiment at Fermilab. The NuTeV experiment, using a technique that exploits muon neutrino and antineutrino data to determine the neutral current to charged current ratios, R^ν and R, has made the most precise measurement of the weak mixing angle using neutrinos as probes. The result gives a value of sin²θ_W(on-shell) = 0.2277 ± 0.0016 which is about three standard deviations larger than the standard model prediction of 0.2227. Various interpretations for the source of the anomaly are considered including changes to the inputs to the standard model predictions, unexpected symmetry violations, or new physics interpretations involving unanticipated neutrino properties or new particle contributions. Speculations on new precison measurements to further explore this region are also presented, including, for example, a future reactor neutrino-electron elastic scattering measurement. At present the discrepancy is unexplained, but could point to some as yet undiscovered broken quark symmetry, or towards new physics associated with neutrino interactions or mixings.

The discovery of neutrino oscillation proved recently that neutrinos have non- vanishing masses in contrast to their description within the Standard Model of particle physics. However, the absolute neutrino mass scale, which is very important for particle physics as well as for cosmology and astrophysics, cannot be revealed by oscillation experiments. Although there are a few ways to determine the neutrino mass scale, the only model-independent method is the investigation of the electron energy spectrum of a β decay near its endpoint. The tritium β decay experiments at Mainz and Troitsk using tritium have recently been finished and have given upper limits on the neutrino mass scale of about 2 eV/c². The bolometric experiments using ¹⁸⁷Re have finished the first experiments yielding a sensitivity on the neutrino mass of 15 eV/c². The new Karlsruhe Tritium Neutrino Experiment (KATRIN) will enhance the sensitivity on the neutrino mass by another order of magnitude down to 0.2 eV/c² by an ultra-precise measurement of the tritium β decay spectrum using a very strong windowless gaseous molecular tritium source and a huge ultra-high resolution electrostatic spectrometer of MAC-E-Filter type.

The wealth of new data on neutrinos is easily incorporated in the Electroweak Theory. In this "ν-Standard Model", lepton mixings are distinguished from quark mixings by a unitary matrix coming from the Seesaw. We catalog models in terms of the number of large angles (one or two) in that matrix. Pati-Salam unification implies Cabibbo effects in lepton mixings (MNS). Without such small mixings, the Solar and Atmospheric angles may well be the same, and the CHOOZ angle could vanish: in a wide class of flavor-symmetric models, it is of (λ/√2). We discuss a new approximate chiral family symmetry that preserves Froggat-Nielsen, and relates the 5-bar of the second and third families, but not the particles in the 10's. If exact, the electron and down quark are both massless and the atmospheric angle is maximal. We conclude by discussing possible Wolfenstein parametrizations of the MNS matrix, assuming various types of "Cabibbo Flops".

In this talk we show how a natural neutrino mass hierarchy can follow from the type I see-saw mechanism, and a natural neutrino mass degeneracy from the type II see-saw mechanism, where the bi-large mixing angles can arise from either the neutrino or charged lepton sector. We summarize the phenomenological implications of such natural models, and discuss the model building applications of the approach, focussing on the SU(3) × SO(10) model. We also show that in such type II models the leptogenesis asymmetry parameter becomes proportional to the neutrino mass scale, in sharp contrast to the type I case, which leads to an upper bound on the neutrino mass scale, allowing lighter right-handed neutrinos and hence making leptogenesis more consistent with the gravitino constraints in supersymmetric models.

Seesaw mechanism appears to be the simplest and most appealing way to understand small neutrino masses observed in recent experiments. It introduces three right handed neutrinos with heavy masses to the standard model, with at least one mass required by data to be close to the scale of conventional grand unified theories. This may be a hint that the new physics scale implied by neutrino masses and grand unification of forces are one and the same. Taking this point of view seriously, I explore different ways to resolve the puzzle of large neutrino mixings in grand unified theories such as SO(10) and models based on its subgroup SU(2)_L × SU(2)_R × SU(4)_c.