Neutrinos are found to have mass, but the mass is extremely tiny, at least million times lighter than the lighest elementary particle: electron. The evidence comes from experiments deep underground in pitch darkness with many thousands of tonnes of water housed in mines. The Standard Model has fallen after decades of invicibility. In 1998, a convincing evidence was reported that neutrinos have mass. Neutrinos do not have mass because they are all 'left-handed' and do not bump on the mysterious 'Higgs boson' that fills our entire Universe. The Standard Model of particle physics can describe everything we know about elementary particles. They are the least understood elementary particle we know that exist.Įxisting of neutrinos was suggested as a 'desperate remedy' to the apparent paradox that the energy did not appear conserved in the world of atomic nuclei. Trillions of them are passing through your body every second,but they are so shy and we do not see or feel them. (c) According to another extension of the Standard Model, extremely heavy right-handed neutrinos are created for a brief moment before they collide with the Higgs boson to produce light left-handed Majorana neutrinos. These Dirac neutrinos acquire mass via the Higgs mechanism but right-handed neutrinos interact much more weakly than any other particles. (b) In one extension to the Standard Model, left- and right-handed neutrinos exist. Since right-handed neutrinos do not exist in the Standard Model, the theory predicts that neutrinos can never acquire mass. Experiments have shown that neutrinos (ν) are always left-handed. All particles, including electrons (e), muons (μ) and top quarks (t), change handedness when they collide with the Higgs boson left-handed particles become right-handed and vice versa. Photons (γ) are massless because they do not interact with the Higgs boson. (a) According to the Higgs mechanism in the Standard Model, particles in the vacuum acquire mass as they collide with the Higgs boson. In this way, massless neutrinos go hand in hand with the absence of right-handed neutrinos in the Standard Model. Since no such state exists, the left-handed neutrino is unable to interact with the Higgs boson and therefore does not acquire any mass. If a left-handed neutrino tried to collide with the Higgs boson, it would have to become right-handed. This picture also explains why neutrinos are massless. Similarly, muons collide with Higgs bosons more frequently than electrons, making them 200 times heavier than the electron, while the top quark interacts with the Higgs boson almost all the time. Put simply, the electron cannot travel through the vacuum at the speed of light it has to become massive. For example, a left-handed electron will become right-handed after the first collision, then left-handed following a second collision, and so on. Quantum field theory and Lorentz invariance show that when a particle is injected into the 'vacuum', its handedness changes when it interacts with a Higgs boson (figure 2a).

No matter how empty the vacuum looks, it is packed with particles called Higgs bosons that have zero spin (and are therefore neither left- or right-handed). This mechanism requires us to accept that the universe is filled with particles that we have not seen yet. Although the Standard Model cannot predict their masses, it does provide a mechanism whereby elementary particles acquire mass.

Photons mediate the electromagnetic force the massive W+ and W- particles carry the weak force, which only acts on left-handed particles and right-handed antiparticles and eight gluons carry the strong force.Īll the particles that make up matter have mass - from the lightest, the electron, to the heaviest, the top quark - and can be left- or right-handed. The Standard Model also includes a set of particles that carry the forces between these elementary particles. In the case of neutrinos, the antineutrino is neutral but right-handed.

Each particle also has a corresponding antiparticle with opposite electric charge. The analogues of the electron are called the muon and the tau, while the muon neutrino and tau neutrino are equivalent to the electron neutrino. The second and third generations comprise heavier versions of these particles with the same quantum numbers. Everyday matter is built from members of the lightest generation: the up and down quarks that make up protons and neutrons the electron and the electron neutrino involved in beta decay. Indeed, precision experiments at the Large Electron Positron (LEP) collider at CERN in Switzerland have demonstrated that there are exactly three generations. We now know that all the elementary particles - six quarks and six leptons - are grouped into three families or generations.

Beta decay neutrino