Why are Neutrinos so Difficult to Detect?
Updated: 5 days ago
This is the first of four articles about neutrinos.
Neutrinos are the smallest of all the fundamental particles and are by far, the most numerous particles (with mass) that we know about in the universe. Once considered to be massless and only travel at the velocity of light, they are now known to travel at velocities below that of light and have minute amounts of rest mass.
How do they Interact with Matter?
They contain no colour charge and so cannot interact through the strong nuclear force. They have no electric charge and so cannot undertake electromagnetic interactions. They do have weak hypercharge and and this allows them to interact through the weak nuclear force. However, there are only two types of weak nuclear interactions that they can undergo in which the neutrino type can be identified and they are both very rare.
As its name implies, the weak force appears to be incredibly weak, so that's a clue to why they are difficult to detect, coupled with the fact that there are only two interactions that give away their identity. Let's look in a bit more detail at this strange clan of fundamental particles, which are thought to have no internal structure.
The following classification of fundamental particles is taken from Part 1 of A Journey into Modern Physics by Kieron Conway.
Note that, all quarks and all leptons are fermions with spin 1/2 and must obey the Fermi exclusion principle.
Three Types of Neutrino
In summary; the three type of neutrinos are the electron-neutrino, the muon-neutrino and the tau-neutrino. Each one forms a doublet with its corresponding lepton. The first thing to note is that the neutrinos are incredibly light and their mass is given as a maximum rather than a definite value. All neutrinos are affected by gravity and must follow the geodesics of spacetime in the presence of gravitational fields.
In each generation of fundamental particles, neutrinos are the lightest and as you'll find out in the third article, they can shape-shift from one type to another and back, a unique capability that has fascinating ramifications in the quantum world.
The most common type is the first generation - electron-neutrino. There are countless billions of them and their anti-particle partner, the electron-anti-neutrino, passing through the earth and us, every second. They only interact with baryonic matter in very rare collisions that invoke the weak nuclear force. As indicated, there are only two interactions where electron-neutrinos interact with protons or neutrons giving information about the neutrino's type, and these are incredibly rare. The vast majority of neutrinos, coming from the sun and other sources in space, pass straight through the earth without interacting with anything.
Only Two types of "Tell-Tale" Neutrino Interactions
Actually, a neutrino can be involved in a total of three types of interaction, but only two from which the identity of the neutrino can be determined. We will concentrate on the two that tell you what neutrino type was involved.
The electron-neutrino can only interact with a neutron and the electron-anti-neutrino can only interact with a proton, in which they give their identity away. And that's it!
These two interactions can be defined as follows;
The reason that the interactions are so rare is that the neutrinos have to get ridiculously close to the quarks inside the nucleons to trigger the weak nuclear interaction. In fact, we're talking about a force that has a range of just 10-18 metres! This is why almost 100% of neutrinos and anti-neutrinos pass right through the earth - they don't get close enough to a bound quark to invoke the weak nuclear force. To put this distance of 10-18 metres in context, the diameter of a nucleon is about 10-15 metres.
With reference to the two possible interactions of electron neutrinos;
1 - In the first case, an electron-neutrino is absorbed by a neutron and, through the weak interaction, changes the flavour of a DOWN-quark to an UP-quark, converting the neutron (ddu) to a proton (uud). In order to preserve charge, the electron-neutrino becomes an electron. The lepton number, L(electron), is also preserved in the interaction as +1 on both sides of the equation.
2 - In the second case, an electron-anti-neutrino is absorbed by a proton and, through the weak interaction, changes the flavour of an UP-quark to a DOWN-quark, converting the proton (uud) to a neutron (ddu). In order to preserve charge, the anti-electron-neutrino becomes a positron. The lepton number, L(electron), is also preserved in the interaction as -1 on both sides of the equation.
These extremely rare collision interactions can occur naturally in the colossal neutrino detectors scattered about the planet, or in particle accelerators where neutrinos are produced. The world's detectors capturing neutrinos from space, detect up to several hundred neutrinos in a day, even though countless trillions pass by in a day. Detectors, sitting in the path of artificially created neutrino beams from particle accelerators, can detect up to 10,000 neutrinos in a day.
The Two Feynman Diagrams
The Feynman diagrams, defining the two collisions, are shown below. In both cases, the interaction is mediated by a W+ boson. Electron-neutrinos interact with a DOWN-quark in a neutron (ddu) and this is represented on the right-hand side of the Feynman diagrams. Alternatively, anti-neutrinos interact with an UP-quark in a proton (uud) as shown on the left- hand side of the diagrams.
These are the only two interactions that neutrinos can initiate with the bound quarks inside neutrons and protons. Note how the interactions result in an electron-neutrino turning into an electron or an electron-anti-neutrino turning into a positron: the light neutrino turns into its heavier partner lepton. The only way that this is allowed to happen, by conservation of energy, is for the in-bound neutrino to have sufficient kinetic energy to create the rest mass of the heavier lepton. And it's these leptons that give the neutrino's identity away.
Second & Third Generation Neutrino Events
In particle accelerators, where muon-neutrinos and tau-neutrinos can be produced, there are equivalent collision interactions as follows;
Note how similar the reactions are to those of the electron-neutrino. Muon-neutrinos only ever produce muons, muon-anti-neutrinos only ever produce anti-muons and the same applies to the tau particles. In fact, the Feynman diagrams will all be identical with the exception that there is one pair of diagrams for each lepton type, either electron, muon or tau, but never a mixture. In other words, the kinetic energy of an electron-neutrino can never produce a muon or a tau.
Attempts to detect neutrino oscillations (shape shifting) rely on the fact that each type of interaction can only yield the same generation lepton as the inbound neutrino. In accelerators, these reactions, where the neutrino transforms into its partner lepton, are referred to as “charged current interactions”.
Electron-neutrinos from stars and from violent supernovae in space, always have sufficient energy to produce electrons or positrons as the case may be.
Charged Current (CC) & Neutral Current (NC) Interactions
As well as the two CC interactions already discussed, there's also a third type of interaction.
The so called "neutral current" interaction (NC) is a scattering event where a neutrino is scattered by a nucleon. Neither the type of the neutrino nor the nucleon are changed. The weak interaction is again involved, resulting in a transfer of energy from the neutrino to the nucleon and NC interactions cannot be used to identify the neutrino causing the event.
CC interactions are mediated by a charged W boson, whereas NC interactions are mediated by a neutral Z0 boson.
Do neutrinos annihilate with anti-neutrinos?
We live in a world swamped by neutrinos, mainly coming from our sun and anti-neutrinos, coming from nuclear reactors and interactions in the upper atmosphere with cosmic rays, as well as extreme cosmic events such as supernovae. We live in a veritable soup of neutrinos and anti-neutrinos.
What happens if a neutrino bumps into an anti-neutrino?
Neutrinos can't produce photons or gluons, as they don't contain electric or colour charges. So, the only option open to a neutrino annihilating with an anti-neutrino is to produce a neutral Z boson.
However, the rest-mass of the Z boson is so high in comparison to the tiny masses of neutrinos that a neutrino and anti-neutrino would have to have colossal amounts of kinetic energy to produce a Z boson. Then the Z boson has to decay into a pair of fermions, one with a positive charge and one with a negative charge so that charge is conserved before and after the decay.
As a consequence of such a high kinetic-energy requirement, neutrino/anti-neutrino annihilation is going to be exceptionally rare. The only place where such an event is likely to occur will be in high flux neutrino/anti-neutrino production events involving unfathomable amounts of kinetic energy. Prime contenders for such events are obviously super novae and high-mass mergers. As it's difficult enough studying neutrino interactions, it is quite probable that annihilation between neutrinos and anti-neutrinos has not been seen in particle detectors.
There is evidence that neutrinos and anti-neutrinos behave differently to normal matter and anti-matter particles. If true and verified as well as quantified, then this research could throw light on how come quarks and anti-quarks, in the early universe, didn't annihilate each other into total oblivion.
Conclusion: If neutrinos do annihilate with anti-neutrinos, there's a good chance it's unlikely we'll ever detect it.
Final Takeaways
There are three generations of neutrinos. Each neutrino is coupled with its corresponding lepton in a lepton doublet. So, we have electron-neutrinos, muon-neutrinos and tau-neutrinos.
Neutrinos all have anti-matter cousins. Because they are so light, a neutrino and its anti-neutrino would have to have absurd amounts of kinetic energy to annihilate each other to produce a Z0 boson via the weak nuclear interaction. Consequently, no one has seen them annihilate each other and this is a topic of study in modern physics.
A neutrino can interact with a neutron by getting the weak nuclear force to change a DOWN-quark to an UP-quark (the neutron turns into a proton) and changing itself into the corresponding lepton. For this to work, the neutrino must have sufficient kinetic energy, to change a light neutrino into its corresponding, much heavier lepton. These CC Interactions can be used to determine the type of the neutrino that caused them.
An anti-neutrino can interact with a proton by getting the weak nuclear force to change an UP-quark into a DOWN quark (the proton turns into a neutron) and changing itself into the corresponding anti-lepton. Again, the anti-neutrino must have sufficient kinetic energy, to change a light anti-neutrino into its much heavier anti-lepton. These CC Interactions can be used to determine the type of the anti-neutrino that caused them.
To invoke the weak nuclear force, the neutrino or anti-neutrino must be in the range of the weak nuclear force, about 10-18 metres, which involves penetration into the nucleon. Neutrinos can also scatter off nucleons in NC interactions, where only energy is transferred from neutrino to nucleon, mediated by a Z boson and this type of event cannot reveal the identity of the neutrino.
No interaction has ever been seen where the in-bound neutrino and out-bound lepton are from different generations in CC interactions.
In all neutrino CC interactions, energy, charge and lepton number are always conserved.
For all the above reasons, it's no wonder that it is so difficult to detect neutrinos of any type.
How Do You Detect Neutrinos?
How Do You Detect Neutrino Oscillations?
&
What is the ICECUBE Experiment?
These are the topics of the second, third and fourth articles about neutrinos, due to be published on the blog soon.
© 2026 Kieron Conway - All rights reserved.
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