KATRIN – A Curiosity about the tiny Ghost Particle of the Universe

The sub-atomic world is flooded with a number of fundamental particles. They are categorized into various families or classes depending on some properties like spin, mass etc. possessed by them. Among them, Neutrino is the one with extremely small mass. The existence of Neutrino was first established by Wolfgang Pauli in 1930 using the Beta decay of nuclear reaction to explain the conservation of energy and momentum. Previously they were known as massless particles. Now, we can look at the situation where the most abundant population was believed to have no mass at all. How strange that was!!

A representation of the naked photon interaction with ghost particles which slows down its speed from infinite to limited speed C (Credit: Changshen | Wikimedia).

This fact introduces the ghost nature of this particle. Thanks to various neutrino oscillation experiments (Sudbury Neutrino Observatories and Super-Kamiokande Observatory), which grabbed the 2015 Nobel prize in Physics confirming the oscillation phenomenon and which in turn indirectly says that the said particles are massive. The fundamental principle lies in the fact that some of the electron neutrinos produced in the sun are transformed into muon and tau-neutrinos as they travel across the Earth. The phenomenon is familiar as neutrino oscillation. After we got through the fact of massive neutrinos, particle physics community has started thinking about other aspects associated with neutrino mass, such as what is the absolute neutrino-mass scale, the hierarchy of the three kinds of neutrinos (which among the three is heaviest and which is lightest) and so on. Along with all these queries, we are also worried about the smallness of neutrino mass.

a physical proton-proton chain (where a positron and neutrino is produced in the final reaction) (Credit: Wikimedia)

Neutrinos pass through our bodies with a billion in number, in every second. The production of this tiny but very abundant particle occurs via a chain reaction continuously taking place on Sun. From the very beginning, i.e., since the period of Bing Bang, the universe is left with innumerable numbers of this particle. The neutrinos are a billion times smaller than any other sub-atomic particle. But their huge population let Physicists think about their role in cosmology also. On the largest scales, neutrinos act as “Cosmic Architects” and participates in forming the visible structures in the Universe, as they encourage the formation and the distribution of galaxies. Hence there must lie some unexplored cosmological consequences drawn by the combined effect of this huge abundant particle.

Now to get into the nature of neutrino in detail one has to know about their interaction with some known particles, which is very feeble in rate. They rarely interact with other matters and this fact has created many difficulties to bring out some unexplored truths and facts regarding their properties. Majority of the particles get their mass by interacting with a particle called Higgs boson, a spin-zero bosonic particle, the existence of which carried the 2013 Nobel prize in Physics. We may think that like the other particles the neutrino also procures its mass by the same mechanism, but its smallness motivates us to search for some other non-trivial mechanism by which neutrinos get mass.

This slideshow requires JavaScript.

Therefore after a period of ten years of planning and building ideas, a giant extraordinary machine has been designed and built which is familiar as Karlsruhe Tritium Neutrino Experiment or KATRIN situated in the South-West of Germany for an extensive study of the behavior of neutrinos and electrons which are emitted by a hydrogen isotope, tritium. The machine consists of a huge and perfect vacuum chamber with the provision that the inside air pressure is lower than that on moon’s surface. The experiment is designed in a way that it becomes possible to study the variations in their trajectories when they fly through the machine’s vacuum chamber. We hope that these variations can shade some light on the precision limit of some physical properties associated with neutrino mass. Apart from creating a perfect vacuum, one also has to keep eye on the temperature of the tritium which acts as the source of the neutrino in the entire experiment. Then the whole building where the apparatus has been placed is demagnetized. The decay of a tritium nucleus is carried out by the emission of a He-3 nucleus, an electron, and a neutrino. However we cannot measure the neutrino mass directly, we can determine it by looking at the energy distribution of the electrons emitted in the decay process. Inside the vacuum chamber, electrons are allowed to flow very closely by applying a powerful magnetic field in the same direction towards a powerful electric field.

The energy spectrum of the electrons emitted in tritium beta decay. Three graphs for different neutrino masses are shown. These graphs differ only in the range near the high-energetic end-point; the intersection with the abscissa depends on the neutrino mass. In the KATRIN experiment, the spectrum around this end-point is measured with high precision to obtain the neutrino mass. (Credit: Zykure | Wikipedia)

The gigantic apparatus consists of a rear section which is responsible for monitoring and calibrating pieces of equipment. The neutrino source of the experiment, i.e., the tritium source is positioned in a device called a windowless gaseous tritium source. Superconducting magnets are used to generate such a powerful magnetic field (70,000 times more powerful than Earth’s one). Among the spread out electrons inside the huge vacuum chamber, only those with the highest energy are allowed to reach the electric force set-up. Then by counting the number of electrons that reach the detector, scientists can measure the endpoint of their spectrum and hence determine the neutrino mass.

Probably after some years, the particle physics community can draw a realistic figure of these queries.

Source:The Guardian, Katrin

Research Scholar, Dept. of Physics (High Energy Physics), Tezpur University, Assam, India.