The Standard Model
|2. Neutrinos meet 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. Photons (γ)
are massless because they do not interact with the Higgs boson. 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. Experiments have shown that neutrinos (ν)
are always left-handed. Since right-handed
neutrinos do not exist in the Standard Model, the theory
predicts that neutrinos can never acquire mass. (b) In one
extension to the Standard Model, left- and right-handed
neutrinos exist. These Dirac neutrinos acquire mass via the
Higgs mechanism but right-handed neutrinos interact much more
weakly than any other particles. (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
We now know that all the elementary particles six quarks
and six leptons are grouped into three families or generations.
Indeed, precision experiments at the Large Electron
Positron (LEP) collider at CERN in Switzerland have demonstrated
that there are exactly three generations. 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.
The second and third generations comprise heavier versions
of these particles with the same quantum numbers. 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. Each particle also has a corresponding antiparticle
with opposite electric charge. In
the case of neutrinos, the antineutrino is
neutral but right-handed.
The Standard Model also includes
a set of particles that carry the forces
between these elementary particles.
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
All 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. Although
the Standard Model cannot predict their
masses, it does provide a mechanism
whereby elementary particles acquire
mass. This mechanism requires us to accept
that the universe is filled with particles
that we have not seen yet.
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).
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).
For example, a left-handed electron will become right-handed
after the first collision, then left-handed following a second
collision, and so on. Put simply, the electron cannot travel
through the vacuum at the speed of light; it has to become
massive. 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.
This picture also explains why neutrinos are massless. If a
left-handed neutrino tried to collide with the Higgs boson,
it would have to become right-handed. Since no such state
exists, the left-handed neutrino is unable to interact with the
Higgs boson and therefore does not acquire any mass. In this
way, massless neutrinos go hand in hand with the absence of
right-handed neutrinos in the Standard Model.
Neutrinos are everywhere. Trillions of them are passing through
your body every second,but they are so shy and we do not see or feel
them. They are the least understood elementary particle we know
- Birth of Neutrinos
Existing 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.
- The Standard Model
The Standard Model of particle physics can describe everything we
know about elementary particles. It says that neutrinos do not 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
- Evidence for neutrino mass
In 1998, a convincing evidence was reported that neutrinos have
mass. The Standard Model has fallen after decades of invicibility.
The evidence comes from experiments deep underground in pitch
darkness with many thousands of tonnes of water housed in mines.
- Implications of neutrino mass
Neutrinos are found to have mass, but the mass is extremely tiny, at
least million times lighter than the lighest elementary particle:
electron. How do we need to change the Standard Model to explain
the neutrino mass? Some argue that our spacetime has unseen spatial
dimensions, and we are stuck on three-dimensional "sheets". Other
argue that we need to abandon the sacred distinction between matter
- Why do we exist?
When Universe started with the "Big Bang", there were almost equal
amount of matter and anti-matter. Most of matter was
annihilated by anti-matter when Universe cooled. We are
leftover of one part in ten billions. Why was there a small excess
matter over anti-matter so that we can exist? Once we abandon the
sacred distinction between matter and anti-matter, it provides a
key to understand why we exist.
The mysteries about neutrinos are now being unraveled dramatically.
We will learn much more in the coming years.
This homepage is based on Feature Article "Origin of Neutrino mass"
in Physics World, May 2002, by Hitoshi
Murayama. The whole article can be download as a PDF file.
Last modified: Fri Jul 5 11:33:01 PDT 2002