Neutrinos are among the most mysterious particles in the universe. They barely interact with matter, yet trillions of them pass through our bodies every second without us noticing. Because they are electrically neutral, neutrinos do not interact with the electromagnetic force and only interact via the weak nuclear force and gravity. Their elusive nature has earned Neutrinos the nickname “ghost particles.”
Initially, scientists believed neutrinos had zero mass. However, scientists later discovered that they do have mass, though their rest mass is so small that it remains incredibly difficult to measure. Neutrinos travel at speeds very close to the speed of light, making them essential messengers of cosmic events.
Origin Of Neutrinos
Neutrinos formed just one second after the Big Bang—much earlier than the Cosmic Microwave Background (CMB), which appeared 380,000 years later. If detected, these ancient ghost particles neutrinos could provide information about regions beyond our observable universe. Additionally, some high-energy neutrinos originate from extreme cosmic events, such as active galactic nuclei (supermassive black holes) and gamma-ray bursts. These neutrinos may even carry data from beyond our observable universe, offering insights into unseen cosmic regions.
The Discovery
The existence of neutrinos was first proposed in 1930 by Wolfgang Pauli to explain an apparent violation of energy conservation in beta decay. In alpha and gamma decay, the emitted particles have fixed energies matching the expected energy differences between the initial and final states. However, in beta decay, the emitted electrons exhibit a continuous energy spectrum rather than a fixed energy value.
Physicists considered three possible explanations:
- Energy conservation is violated (a radical and unsettling idea).
- The electron carries all the energy, but measurement errors obscure this.
- A third, undetected particle carries away the missing energy.
To uphold energy conservation, Pauli proposed that an invisible, neutral particle—later named the neutrino—was emitted in beta decay. If the neutrino carried zero energy, the electron would receive the maximum possible energy. Conversely, if the neutrino carried most of the energy, the electron would receive very little. This variation explained why the electron’s energy was not fixed, leading to the observed continuous spectrum in beta decay.
Neutrino Oscillations and the Majorana Mass
Ghost particles Neutrinos come in three types, or flavours: electron neutrinos, muon neutrinos, and tau neutrinos. However, each neutrino flavour is actually a superposition of three different mass states. As a neutrino travels through space, it oscillates between different flavours due to the quantum nature of its mass states.
Neutrino oscillations occur because each mass state moves at a slightly different speed. Over time, these mass states fall out of sync, causing the neutrino to change flavour. To understand this, it is helpful to think of neutrinos as waves rather than solid particles. Each neutrino flavour is composed of three different mass waves, each with slightly different energies. As they propagate, their peaks and valleys shift, altering the overall wave pattern. Since the observed flavour depends on the alignment of these waves, any shift in their synchronization results in a change of flavour.
A useful analogy is dropping three stones into a lake at slightly different times. Each stone creates ripples that spread outward. Initially, the waves align in a distinct pattern, but as they move forward, some waves advance faster while others lag. The resulting interference pattern changes dynamically. Similarly, a neutrino might start as an electron neutrino, but after traveling some distance, it may appear as a muon neutrino due to the shifting alignment of mass waves. This phenomenon is crucial in understanding neutrino behaviour and has been confirmed through numerous experiments.
The Majorana Mass
In particle physics, the Majorana mass is a theoretical concept suggesting that certain particles, like neutrinos, could be their own antiparticles. Normally, particles and antiparticles are distinct; for example, an electron has a positively charged antiparticle called a positron. However, if neutrinos possess a Majorana mass, neutrinos and antineutrinos would be the same entity.
One way to test this idea is through a rare process: neutrinoless double beta decay. In standard double beta decay, a nucleus emits two electrons and two neutrinos to conserve energy and momentum. However, if neutrinos have a Majorana mass, the two neutrinos could cancel each other out, resulting in the emission of only two electrons without accompanying neutrinos. This process would be definitive proof that neutrinos are Majorana particles.
Detecting neutrinoless double beta decay would have profound implications for our understanding of particle physics, potentially explaining the small mass of neutrinos and shedding light on why matter dominates over antimatter in the universe.
Why Do Neutrinos Have Mass?
The Initial Prediction
Ghost Particles Neutrinos were originally predicted to have zero mass because they do not interact with the Higgs field in the usual way. Since particles acquire mass through interaction with the Higgs boson, scientists believed that neutrinos should be massless. However, experiments have confirmed that neutrinos do have a very small but nonzero mass.
The See-Saw Mechanism
The see-saw mechanism provides an explanation for why neutrinos have such tiny masses compared to other particles. It suggests that neutrinos come in two types:
- Light neutrinos – The ones detected in experiments.
- Heavy neutrinos – Hypothetical particles with an extremely large Majorana mass, yet to be discovered.
These two types of neutrinos mix with each other, leading to the see-saw effect.
Mathematical Explanation
The mass matrix of neutrinos can be expressed as:
Where m is the higgs vacuum expectation value, approximately 246 Gev. This is the average value of the higgs field in the vacuum, which helps particles gain mass.
D is the large Majorana mass term for the heavy neutrino.
Solving for the mass eigenvalues gives: Mlight = m2/Mheavy (where Mheavy is equal to D)
If Mheavy is very large, then Mlight becomes very small. This explains why neutrinos have tiny masses—they are linked to heavy neutrinos that have yet to be detected.
Implications for Matter-Antimatter Asymmetry
The existence of heavy neutrinos also helps explain why the universe is made mostly of matter rather than an equal mix of matter and antimatter.
Leptogenesis Hypothesis
The Big Bang should have created equal amounts of matter and antimatter.
However, today’s universe consists almost entirely of matter, meaning there must have been a small imbalance in their production.
A proposed explanation is leptogenesis, which suggests that In the early universe, extremely heavy neutrinos were present. These heavy neutrinos were unstable and decayed into lighter particles (such as neutrinos and other particles).
Normally, we would expect equal decay into matter (leptons) and antimatter (antileptons). However, a charge-parity (CP) symmetry violation caused a tiny excess of leptons over anti-leptons. This imbalance led to the universe having more matter than antimatter.
If the Big Bang had produced equal amounts of matter and antimatter, they would have annihilated each other, leaving only radiation. The small matter excess that remained is what formed the universe we observe today.
Sterile Neutrinos: A Warm Dark Matter Candidate
Sterile neutrinos, or heavy neutrinos, are hypothetical particles that do not interact via the weak force. They only interact with ordinary matter through gravity or neutrino mixing. If they exist, they could be an ideal dark matter candidate, as dark matter also does not interact via electromagnetic or strong nuclear forces.
If a sterile neutrino has a mass of a few keV, it could qualify as a warm dark matter (WDM) particle. Unlike cold dark matter (CDM), which moves slowly and clumps together to form small structures, WDM moves slightly faster, preventing the formation of small-scale structures. Since sterile neutrinos interact only gravitationally, they are extremely difficult to detect, much like dark matter.
The Missing Satellite Problem
One major issue with the CDM model is the missing satellite problem. In the Lambda Cold Dark Matter (ΛCDM) model, dark matter clumps together under gravity to form large structures. This model predicts that massive dark matter halos host large galaxies like the Milky Way, while smaller halos should form many satellite galaxies. Simulations suggest that the Milky Way should have hundreds to thousands of dwarf galaxies orbiting it. However, observations reveal only around 50 known satellite galaxies, far fewer than expected.
This discrepancy suggests that CDM may not fully explain the distribution of small galaxies. Since CDM assumes slow-moving dark matter, small structures can easily form. However, if dark matter moved slightly faster (as WDM suggests), it could erase small-scale structures in the early universe. This would naturally explain why we observe fewer small galaxies around the Milky Way. Sterile neutrinos, which move faster than traditional CDM candidates like Weakly Interacting Massive Particles (WIMPs), could fit into this WDM framework and help resolve the missing satellite problem.
Neutrinos and Dark Matter
Neutrinos are nearly massless particles that barely interact with matter, allowing them to pass through entire planets without colliding. The neutrinos we observe experimentally can be hot dark matter due to their speed being close to that of photons. In contrast, heavy sterile neutrinos would be warm dark matter because of their moderate speed.
Most neutrinos reaching Earth originate from the Sun, and despite their abundance, they rarely interact with atoms in our bodies or the planet itself. That’s why scientists call these neutrinos the ghost particles. Some neutrinos detected today have traveled for billions of years, originating from distant supernovae or even the Big Bang. These elusive particles remain among the most intriguing in the universe, and if sterile neutrinos are indeed warm dark matter, they could provide crucial insights into the cosmos.
Sources:
https://en.wikipedia.org/wiki/Leptogenesis
https://en.wikipedia.org/wiki/Sterile_neutrino
https://www.scirp.org/journal/paperinformation?paperid=99794