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The elusive neutrinos

The two possible orderings of neutrino masses. The colours represent the electron, muon and tau flavour content of each neutrino mass eigenstate.

The two possible orderings of neutrino masses. The colours represent the electron, muon and tau flavour content of each neutrino mass eigenstate.

A brief introduction to neutrinos

Neutrinos are some of the most elusive particles in the universe. They are the second most abundant particles in the universe, and trillions of them pass through us every second without us even realizing it. At the same time, they help the sun shine, make stars explode, and allow us to see places from where light cannot reach us.

First postulated by Pauli in 1932 to explain the energy-momentum conservation in nuclear beta decay, neutrinos took a quarter of a century to be actually observed. In the five decades since then, neutrinos have continued to spring surprises, challenging our knowledge of fundamental interactions.

Neutrinos are elementary particles, and come in three ``flavours'': electron neutrino, muon neutrino and tau neutrino. They form an integral part of the Standard Model of particle physics, and are the particles that have only weak interactions. The interactions are so weak that the neutrinos from the Sun can pass through light years of lead shielding without getting obstructed.

The recent excitement in the field of neutrinos arose from the solutions to two long-standing problems: the solar neutrino problem and the atmospheric neutrino problem. In the former, almost half of the electron neutrinos produced inside the Sun appeared to get lost on their way to the Earth. In the latter, when muon neutrinos produced in the atmosphere travelled through the Earth to emerge on the other side, almost half of them appeared to get lost.

The solutions to both these problems were obtained through ``neutrino oscillations', in which neutrinos changed their flavours while travelling. This however required that neutrinos have different masses, and neutrino flavours mixed among themselves. This indeed was the first confirmed signal of physics beyond the Standard Model of particle physics. These solutions have now been confirmed by experiments where known fluxes of neutrinos were allowed to travel long distances before being detected.

The results of the ongoing neutrino experiments have now given us a consistent picture of neutrino masses and mixing. There are three neutrino ``flavors'', $ u_e, u_mu, u_tau$. These mix among themselves, giving rise to three mass eigenstates $ u_1, u_2, u_3$, which have masses $m_1, m_2, m_3$, respectively. However only partial infomation about these masses and mixing is known. For example, the neutrino oscillation experiments cannot tell us about the absolute neutrino masses, but can only measure the ``mass squared differences'', $Delta m^2_{solar} = m_2^2 - m_1^2$ and $Delta m^2_{atmospheric} = m_3^2 - m_1^2$. On top of that, we are yet unable to determine the sign of the latter. Among the three possible mixing angles, two have been measured and found to be large, whereas for the third one we only have an upper bound.

There are many open questions about the nature of neutrinos, that need to be settled with future experiments. What are the absolute neutrino masses ? Is the sign of $Delta m^2_{atmospheric}$ positive (this situation is called ``normal ordering'') or negative (``inverted ordering'') ? Is the ``third'' angle exactly zero ? Are neutrinos their own antiparticles ? Do their interactions violate the charge-parity (CP) symmetry ? The field of ``neutrino phenomenology'' deals with obtaining the answers to these questions. It involves coming up with observable quantities that can be measured at the current and proposed experiments, and analyzing the data to determine the neutrino properties.

Determination of neutrino properties is important not only for understanding the fundamental interactions in particle physics, but also for understanding the role the neutrinos play in many astrophysical and cosmological phenomena, like supernova explosions, cosmic microwave background radiation (CMBR), and baryon asymmetry in the universe. For example, the explosion of a core collapse supernova in our galaxy would serve as a large source of neutrinos and antineutrinos of all species. The spectra of these neutrinos can shed light not only on neutrino mixing parameters but also on the supernova explosion mechanism, which is an unresolved problem in astrophysics. Neutrinos will allow us to locate the supernova in the sky hours before it is seen optically, and they can also tell us about the propagation of the shock wave while it is still deep inside the star. On the other hand, the determination of neutrino parameters depends crucially on our understanding of the primary neutrino spectra produced inside the star. Moreover, the density of the matter that neutrinos pass through affects their mixing and hence the signals observed at the detectors. All these issues are studied in the field of ``neutrino astrophysics''.

Since neutrinos, charged leptons (electron, muon and tau), and quarks are components of the same Standard Model of particle physics, one expects that the mass generation mechanism for neutrinos would be related to that for quark and lepton masses through grand unified theories. However, even the partial information we currently have about neutrinos tells us that the pattern of neutrino mixing is significantly different than that of quark mixing, where all the mixing angles are very small. Moreover, data from cosmology as well as nuclear beta decay indicate that the neutrino masses are very small as compared to the masses of the charged leptons (electron, muon and tau) and quarks. Thus neutrino mass generation should involve something more than the ``Higgs mechanism'' that gives masses to quarks and charged leptons. The theoretical understanding of the observed pattern of neutrino masses and mixing is an open question that is a part of the deeper problem of the generation of fermion masses in general.

Most of the promising contenders for neutrino mass generation, like the seesaw mechanism, give masses to neutrinos via their interactions with supermassive particles that have masses close to the Planck scale. Such mechanisms allow us to probe physics at very high energy scales that are impossible to reach with particle accelerators. Some other mechanisms, for example those that involve large extra dimensions, also provide signals that may be observed at the experiments at laboratory energies. Constructing mass generation models consistent with all the constraints from data and finding ways of testing them using observable phenomena is an important line of investigation in this regard.

The field of neutrino physics has emerged as one of the most active areas of research over the past decade. Given the central position of neutrinos in particle physics and astrophysics, and their tendency to spring surprises, it is bound to remain so for many more years.


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