Neutrinos: The ghost particles of the Universe

Neutrinos: Ghost particles of the Universe

We are all bombarded with flux of tiny particles which pass through our bodies uninterrupted and unnoticed.  Tens of trillions of them are passing through our bodies every second.

These ubiquitous particles called neutrinos are extremely light and extremely fast.  They travel at speeds which are very close to the speed of light.  After photons, neutrinos are the most abundant of all the known elementary entities in the universe.  Neutrinos come from different sources.  They are produced by the sun, within the earth, by the exploding stars, by cosmic rays interacting with the atmosphere, and at the nuclear reactors and particle accelerators, among other sources.

Since neutrinos have no electrical charge, they interact very weakly with matter, and it is hard to detect them.  Tens of trillions of neutrinos pass through every square cm if earth every second. These mysterious particles are nicknamed ‘ghost particles’.

According to the standard model of particle physics, all matter is made of spin ½ particles known as fermions.  There are 12 such elementary particles which are grouped into quarks and leptons.  There are six quarks and six leptons.  The lepton group consists of electron, muon, tau, electron neutrino, muon neutrino and tau neutrino.

We see that there are three types or three flavors of neutrinos.  Since they do not have electrical charge, they do not get influenced by the electromagnetic force.  Neutrinos interact only through the weak force and the gravitational force.

Our understanding of neutrinos started in 1930 when theoretical physicist Wolfgang Pauli suggested the need for a new particle to explain β- decay of radioactivity.  At that time, β- decay appeared to break the law of conservation of energy.

In β- decay, a neutron transforms into a proton and emits an electron.  The energy of the emitted electron does not have a definite value, but it follows a continuous spectrum.  Wolfgang Pauli proposed that in β- decay, another unknown particle was also emitted along with the electron, which shares the energy released in the process, with the electron.  Pauli called this particle ‘neutron’ and admitted that this was a ‘desperate remedy’ to preserve the law of conservation of energy.

In 1932 James Chadwick discovered the particle which we now call the ‘neutron’.  This particle was much heavier than the particle suggested by Pauli.

In 1933, Enrico Fermi formulated his theory of β- decay, or the theory of weak interaction, in which Pauli’s particle played a crucial role.  Fermi renamed this particle as ‘neutrino’ meaning ‘little neutral particle’.

For the next quarter of a century, nobody could actually find a neutrino, but the search was on.  In the early 1950s in the United States, Frederick Reines and Clyde Cowan, two scientists at Los Alamos Laboratory set out to detect the mysterious neutrinos.  The search for these ‘ghost particles’ was named ‘Project Poltergeist’.  They set up a neutrino detector near a nuclear reactor at Savannah River in South Carolina, USA.

In 1956 Reines and Cowans detected neutrinos for the first time.  These were antineutrinos emitted from the fission reactor.  The detector had 400 liters of water with Cadmium Chloride dissolved in it.  The antineutrinos interacted with protons in water, creating neutrons and positrons.  Annihilation of positrons with electrons in water produced gamma rays, which were detected by scintillators in the detector.  The neutrinos created were absorbed by Cadmium, which also produced gamma rays.  These gamma rays were separately detected.

1955, Frederick Reins was awarded the Nobel Prize in Physics for his work on neutrino physics.

Reins and Cowan observed electron neutrinos.  The separate identity of muon neutrinos was proved in 1962.  The third kind, the tau neutrino was observed only in 2001.

The Kolar Gold Field (KGF) experiments in India identified atmospheric neutrinos in 1965.  Experiments led by Ray Davis at Brook haven National Laboratory detected the solar neutrinos for the first time in 1968.

Physicists detected neutrinos from a supernova for the first time 1987.  The Kamiokande detector in Japan and the IMB experiment in Ohio, USA detected neutrinos from supernova 1987A, some 150000 light years away in the Large Magellanic Cloud, the galaxy nearest to the milky way. These neutrinos were detected three hours before light from the explosion reached the earth.  This event marked the birth of neutrino astronomy.

Of all the known fundamental particles, neutrinos are the least understood ones.  For half a century, Physicists thought that neutrinos, like photons, had no mass.  The standard model of particles physics treats neutrinos as massless particles.  But experimental evidence now shows that neutrinos have mass.

Only electron neutrinos are produced in the thermonuclear fusion reaction in the Sun.  The solar neutrino flux at the earth is theoretically calculated to be 70 billion/cm²/sec.  This can be calculated from solar luminosity.  But experimental observations showed that only a third of the solar neutrinos predicted by the solar luminosity models reached the earth.  The mysterious disappearance of the remaining electron neutrinos became a puzzling problem for scientists.  This is known as the solar neutrino problem.

In the late 1960s the Homestake experiment of Ray Davis and John N Babcall was the first to measure the flux of solar neutrinos and detect a deficit in the number of neutrinos reaching the earth.  The experiment took place in the Homestake gold mine in Lead, South Dakota.

Ray Davis and Masatoshi Koshiba won part of the physics Nobel Prize in 2002 for experimentally establishing that only a third of the neutrinos predicted by solar luminosity models actually reached the earth.  Masatoshi Koshiba’s experimental studies were instrumental in establishing neutrino studies and neutrino astronomy on a firm footing in Japan.

The solar neutrino problem was solved by improved theoretical understanding of the properties of neutrinos and sophisticated experiments.

According to the standard model of particle physics there are three flavors of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos.  All three are considered as massless entities.

Only electron neutrinos are produced in the Sun and the experiments like Homestake could detect only electron neutrinos.

In 1968 the Italy born Soviet nuclear scientist Bruno Potencorvo proposed that if neutrinos had mass, then they could change from one flavor to another.  Then the missing solar neutrinos could be electron neutrinos changed into other flavors, which were not detectable by Homestake and other early detectors.  Pontecorvo’s idea was not immediately accepted by particle physicists, mainly because it went against the standard model.

In 1998 the Super Kamiokande experiment led by Takaaki Kajita announced the first evidence of neutrino oscillations, the transformation of one flavor of neutrino into another.  The super Kamiokande has the world’s largest Water Cherenkov detector for detecting the neutrinos.  It consists of a stainless steel tank filled with 50,000 tones of water.  13000 photomultipliers are installed surrounding the huge tank.  The detector is located in the Kamioka Mine in Japan.  The Super Kamiokande team looked for neutrinos that were produced when cosmic rays bombarded oxygen or nitrogen nuclei in the atmosphere.  These atmospheric neutrinos are mostly muon neutrinos.  When these neutrinos pass through the detector, they interact with the atomic nuclei in water to produce electrons, muons and tau particles.  When these particles travel in water, they produce Cherenkov radiation which can be detected by photo multiplier tubes surrounding the water tank.

The detector could record neutrinos coming directly from the atmosphere above, and also the neutrinos that come passing through the earth from the other side.  The team found out that about half of the atmospheric neutrinos reaching the detector after passing through the earth were lost, while those coming from the atmosphere above were not.  This happened because a portion of the muon neutrinos changed to tau neutrinos while travelling through the earth.  This was a definite indication of neutrino oscillations.

Further observations at Sudbury Neutrino Observatory (SNO) conclusively established the phenomenon of neutrino oscillations.  SNO is a neutrino observatory located 2100m underground in a mine in Sudbury, in Canada.  It was designed to observe solar neutrinos with the help of a large tank filled with 1000 tones of heavy water.

SNO could detect all three flavors of neutrinos collectively, and electron neutrinos specifically.  After extensive statistical analysis SNO scientists established that only 34% of the electron neutrinos started from the Sun reached the earth in that same flavor.

The 2015 Physics Nobel prize was given to the scientists who led the SNO and super Kamiokande teams; Arthur B. McDonald and Takaaki Kajita, “for the discovery of neutrino oscillations, which shows that neutrinos have mass”.

The discovery of neutrino oscillations challenged the standard model’s assumption that neutrinos are massless entities.  A massless particle travels at the speed of light and hence cannot change into another form.  So it is now established that neutrinos have mass.  However the masses of the three flavors of neutrinos are not yet determined.  The standard  model of particle physics now stands as an incomplete theory.