The author gives a brief overview of the present status of Neutrino Physics.
(Download full article in PDF format here)
Posted by: Newton Nath, Instituto de Fisica, Universidad Nacional Autonoma de Mexico, A.P. 20-364, Ciudad de Mexico 01000, Mexico.
Email: newton@fisica.unam.mx
ABSTRACT: Neutrino, an elementary fundamental particle like electron but has no electric charge. It is the lightest among all the Standard Model (SM) particles (except photons and gluons which are massless) and interact only via the weak interactions. Neutrinos can be produced both naturally and artificially. Naturally produced neutrinos come to the Earth from the Sun, supernovae, collisions of cosmic rays with nuclei in the atmosphere, natural radioactivity, etc. On the other hand, those produced in accelerators and nuclear reactors are examples of artificial neutrinos. The naturally produced neutrinos carry information from deep inside stars and hence can help to understand the stellar evolution processes. neutrinos are massless in the SM of particle physics. Thus generation of neutrino mass signifies physics beyond the SM (BSM). This can also be related to some of the unresolved fundamental issues, like, the unification of forces, the matter-antimatter asymmetry, etc. Hence neutrinos are believed to hold the key to a deeper understanding of nature
1. THE DISCOVERY OF NEUTRINOS:
The history behind the discovery of the neutrino goes back to year 1914, when J. Chadwick first demonstrated the observed beta-decay spectrum from radioactive sources. An unstable radioactive nucleus undergoes beta-decay where a neutron decays to a proton and an electron. However, experimental observation showed a continuous pattern of the electron energy spectrum, unlike alpha or gamma-decay spectrum where a discrete behavior have been observed [1]. This continuous nature of the spectrum questioned the hypothesis of energy conservation in nuclear beta-decay. As a solution, in 1930, W. Pauli came with an idea to explain the observed beta-decay, which was hypothesized as `neutrino hypothesis', where neutrinos are electrically neutral and spin half fermions 1. Thus, saving the principle
of conservation of energy, electric charge and angular momentum in nuclear beta-decay. Pauli also postulated the neutrinos to be massless. These neutrinos take part only in the weak interactions and hence can escape detection 2. Therefore, it is not astonishing that the first neutrino was discovered 26 years after it was first proposed. In 1956, Reines and Cowan were the first to observe neutrinos using the Savannah River Nuclear Reactor [2]. Only neutrino that was known at that time was the electron neutrino. After that two more types of neutrinos were discovered corresponding to the two other charged leptons, namely muon and tau. The second type of neutrino, named as the muon neutrino, was discovered by L. Lederman, M. Schwartz and J. Steinberger in 1962 [3]. In July 2000, the DONUT collaboration [4] at Fermilab, USA announced the discovery of the third type of neutrino called the tau neutrino. By observing the neutrinos coming from the Sun, in the mid-1960s R. Davis proposed the Homestake experiment to test the hypothesis of solar energy generation [5]. This detection was based on the reaction of the neutrinos with chlorine giving rise to an isotope of argon. It was observed that the detected neutrino ux was lower than the expected theoretical predictions of the Standard Solar Model (SSM) developed by J. Bahcall [6]. There could be three reasons to explain this mismatch: (i) the experimental results were not correct, (ii) the calculation of neutrino uxes from the SSM were wrong, and (iii) the electron neutrinos coming from the Sun were getting transformed into some other avors of neutrino and they simply crossed the detector undetected since the detector was sensitive only to the electron neutrino. Neutrino oscillation proposed by B. Pontecorvo in 1957 was considered as a possible solution of number (iii) as mentioned above [7, 8]. Furthermore, neutrino oscillation requires non-zero neutrino mass which is a signal of new physics beyond the SM. During last a few decades, numerous phenomenal neutrino oscillation experiments [9 -18] have established the fact that the neutrinos carry non-zero masses and uncover their avor mixing patterns. The discovery of neutrino mass led to the prestigious `Nobel Prize 2015' in the _eld of Neutrino Physics for T. Kajita of the SK, Japan and A. B. McDonald, Canada of the SNO collaboration. (Continued...)
(Download full article in PDF format here)
Join us to create relevant content:
Writing on an academic issue either in form of study-report, news or in journal brings your work to the attention technical people of your field and also to the commoners. We invite you to be associate with the research and educational programs undertaken by CASILab and captivate the readers’ attention through your positive thought & activities.
Comments