The Mystery of Ghost Particles

Written by Swarnadeep Adhikary

Ghost! The term itself is quite spooky and reading it many of you are sitting straight, right? Hoping to read something spine chilling.

But wait for a second!

What does it mean to be a Ghost particle? Are they the atoms of GHOST!! (Too remote from daily science )

Well, imagine yourself standing on an open field with your hands stretched out. And a jet of a hundred trillion minuscule bullets piercing through you every second. What would your condition be like? Surely dreadful but undoubtedly impractical. Right?

Ahhh, NOPE!!

On the contrary, it is just the actual case. Every second trillion of such particles are just piercing your body and we are constantly being bathed by these particles. And this is our Neutrino or more informally the Ghost Particles.

So let's attempt to unravel some of the greatest secrets of this mysterious particle.

But what neutrinos are?

Neutrino is a class of elementary particles (i.e. it cannot be further divided); a chargeless fermion belonging from the lepton family of the standard model and thus has an intrinsic half-spin value.

Now you may wonder what is this Standard Model?

Just like to have a systematic study of the chemical properties of the elements, we need an organized arrangement of the elements in the form of the periodic table. Similarly in particle physics, it becomes a necessity to organize the fundamental particles and their associated forces in an organized form to understand the interrelation between them. And the Standard Model is the most successful of all such approaches.

It is the theory describing three of the four known fundamental forces in nature (gravity being excluded) and classifying the elementary particles as fermions ( the building block of matter) and bosons (the force carrier particles). Fermions are again classified as quarks and leptons based on the presence or absence of color charge respectively.

And our dear neutrino with no color charge (thus indifferent to strong interaction) belongs to the lepton family.

The emergence of the idea of Neutrino

The first proposal for the existence of something like neutrino came in 1930 to explain the missing energy of Beta Decay.

In beta decay (a process involving weak interaction) what happens is a single neutron decay into a proton and an electron. However, there's always a small amount of energy missing.

Could it be a violation of the long-established principle of Conservation of Energy?

Neils Bohr, fascinated with the weird start of Quantum theory, desperately assumed the same. But, Pauli could not accept it. Rather he postulated quite radically the existence of a new and unseen elementary particle, produced together with electron and proton in the beta decay.

It must not have any electric charge and should have a tiny mass ( or no mass at all) to maintain the validity of beta decay. Subsequently, Bohr's idea was rejected by mid-1930s and Fermi published his theory of beta decay in which the neutron decays into proton, electron and an (anti)neutrino ( the name being coined by Fermi which meant'little neutral ones')

Flavors of Neutrino

Like your favorite ice-cream brand offers you various flavors of ice-cream- vanilla, chocolate or butterscotch, nature too offers neutrino in three types or flavors:

~ electron neutrino

~ muon neutrino and the

~ tau neutrino.

Which are named after their closest relatives in the weak interaction; the electron, muon, and tau.

This pairing of charged leptons with its corresponding neutrino is a part of the symmetry of standard model and is important to satisfy another fundamental principle- Conservation of Lepton Number ( which says that the type and quantity of lepton must be conserved in any event and is as fundamental as Conservation of Electric Charge or Conservation of Energy).

So...What about its Mass?

As already mentioned above; Neutrinos are quite difficult to detect and they hardly interact with matter and pass unimpeded. Also, it must have low energy as hypothesized by Pauli.

So for a long time, it was thought to be massless, sprinting at the speed of light and always coming in left-handed variety. (A right-handed neutrino is yet to be observed)

Now you may ask what it meant to be a left-handed Neutrino?

So, we know that every elementary particle has an intrinsic value associated with it. For example; Fermions value is half i.e. they obey the exclusion principle and the particles can take any one of the two available quantum states. So in a nutshell, the fermions are always exhibiting its intrinsic spin and unlike classical system; it can never be stopped from doing so (unless it is massless and it travels at the speed of light). So, as it moves in a uniform linear motion, it follows a helical path (like that of a corkscrew). By convention, if the corkscrew marches forward with clockwise spin it is said to be a Right Handed System(R) and otherwise a Left-Handed System(L). Thus, the particle is said to possess Chirality and any particle with mass oscillates between the L and R Chirality (A property arising due to Higgs Boson).

However, as it was believed that neutrinos travel at the speed of light; so time must appear to be frozen for it (due to special relativity).

So how could it spin?

Thus at the speed of light, it is either a purely L particle or purely R particle.

And this is in general applicable for all the Fermions. As they move at the speed of light they split into two different personalities.

(Fun Tip- If you ever project your Cadbury at such speed, then it too would have split character….perhaps you could enjoy its taste without devouring the bar!)

But as it turned out later, that neutrinos indeed have some minuscule mass.

To these days, the mass of a neutrino is detected and its upper limit is approximated to be about 1.1eV. However, we still lack any precise measurements.

Now, as we know that neutrinos are not massless; so it leads that they are participating in the L-R-L-R forced march; as they move in space-time (same is the case with other fermions)

But besides this, our neutrinos exhibit something additional- It oscillates between its various flavors (each with a slight difference in mass) as it propagates through space.

That is each time a neutrino takes a step in the march, it slightly changes its flavor identity. Thus if it goes through one complete cycle L-R-L an L-muon neutrino will end up mostly as a muon neutrino but will also pick up a bit of character of electron neutrino or tau neutrino. So, after it travels a considerable distance; it would have accumulated a significant probability of becoming a muon or tau neutrino!

It would be as if you were cycling to your friend's house with a gift, and every time you paddle you become a bit like an ape(i.e. you pick an ape character); and if her house is quite afar and you need to paddle a large number of times, then the bets are high that she'll open the door to find an ape with a gift! (Alas! no such properties are shown by baryons and hence no need of worry!!).

So, to sum up, neutrino oscillation arises from the mass eigenstate of the three flavors of neutrino.

Another bizarre property of neutrino is that it may be its antiparticle. Though the things are not yet too clear and there's not enough clue of whether the mass of a neutrino is of Dirac or Majorana form (for details on these consult the sources given).

A lot could be discussed upon these baffling things but here we bypass the strange realms of broken symmetry and parity, coupling with Higgs field and the feeble interaction of neutrinos with matter and let’s leap into the experimental pieces of evidence and terminates the discussion exploring the future of neutrino research and its impact on society.

Past ● Present ● Future

Since its detection for the first time in 1956 by Reines and Cowan, several experiments have been carried out to decode the reality of neutrinos and several billion dollars have already been invested in neutrino research.

Super -Kamiokande detector of Japan; is one such million dollar project, which contains the world's largest human-made vat of ultra-pure water, instrumented with thousands of large glass photo-tubes to detect light produced from neutrino interaction in ultrapure water.

This was the detector to provide the first definitive measure of neutrino oscillation and finally solve the solar neutrino problem (( getting about one-third of the theoretically estimated number of neutrino from the sun)).

However, till now most of the neutrino research has been done with the sun or cosmic rays as the source and it is desirable to have complete control over the source as well as the target. So several projects are undergoing for building more powerful colliders and sensitive detectors.

Ultimately, the NOVA project, the future LBNE Project, and the construction of a futuristic neutrino factory can provide a powerful evolutionary program in neutrino technology.

Finally, the basic research in neutrino physics can yield a literally limitless advantage to society and is the doorway to future technology.

It may account for dark matter & can have a great impact on Grand Unified Theory, hence be the window to High Energy Physics.

But most importantly; neutrinos could be the ultimate answer to the Future source of energy. It is expected that it may play a key role in the development of the futuristic 'annihilator engine'; which besides serving as the basic source of energy can also be used to build starships and make interstellar travels possible. It might also be helpful to develop technologies like neutrino tomography, the possibilities are endless.

Thus our ghost particle is not only the key to a huge treasure of knowledge hidden in the heart of nature but also has the prospects to shape and influence future technology. And, who knows perhaps the stock market value of neutrino will cross the stratosphere one day!!


1. Beyond the God particle- L.Lederman and C.Hill

2. Neutrino Oscillation and their origin- Y.Suzuki, M.Nakahata, and Y.Obayashi

3. M-S-W effect- Wikipedia

4. Standard model- Wikipedia

5. Neutrino- Wikipedia

6. The Standard Model in a nutshell- D.Goldberg.