The Higgs Boson Explained to School Children
One
Thursday night my 8th grader put a virtual gun to my head and demanded,
"My class teacher wants a write-up on the God Particle
by tomorrow!" I bought time till Monday and scoured the internet,
and stitched together a shameless cut-and-paste summary, for I know
next to
nothing about this stuff. If pretending to explain stuff that I
don't know wasn't temerity enough, I sent the draft to the legendary
Prof. Balakrishnan of the Physics Department at IIT Madras. He
was gracious enough to go through it and made crucial
changes. Since he didn't tear it apart to pieces, I felt the
effort was worth sharing with the online community at large.
Early scientists thought that matter was made up of indivisible
atoms. The "indivisible atom", it turns out, is actually made up
of electrons, protons, and neutrons---the so-called "sub-atomic
particles". This is as far one gets to in school physics.
Besides matter, we also encounter "force", the most familiar being
gravitational, electrical and magnetic forces. Electricity and
magnetism are part of the same phenomenon, which goes under the name
'electromagnetism'. (Light is also a form of electromagnetism.)
The natural questions are, "Are the electron, proton, and neutron the
most elementary particles? That is, are they indivisible?
Are there forces besides gravitation and electromagnetism? Is
there one model or theory that can explain all the known particles and
their interaction via various forces?" It is now known that the
proton and neutron are not elementary particles but can be divided
further into particles called quarks. The current theory, which
has been verified to an amazing accuracy, is the so-called Standard
Model. It attempts to explain everything about the world as we
know it and how it is held together. It explains how elementary
particles are bound together to create atoms and then matter, by three
of the four fundamental forces of nature.
The four fundamental forces are strong nuclear force, weak nuclear
force, gravity, and electromagnetism. Gravity is not part of the
Standard Model. The unification of gravity with the other three
forces is an open problem.
In the Standard Model, elementary particles come in two classes:
fermions and bosons. Fermions consist of 12 'matter particles' that are
further sub-divided into 6 'leptons' and 6 'quarks'. (Each of these
also has an anti-particle.) Bosons are the 'force particles' that
mediate the forces between matter particles. There is 1 boson
(the photon) that mediates electromagnetism, 3 bosons (called W-plus,
W-minus and Z-nought) that mediate the weak nuclear force, and 8 bosons
(called gluons) that mediate the strong nuclear force. There is
another boson, called the graviton, that is supposed to mediate the
force of gravity. But this particle is as yet undiscovered.
Over and above all these particles, the Standard Model requires another
special boson to exist: the Higgs boson, which was only a theoretical
prediction until the announcement of its discovery on July 4,
2012. What is special about the Higgs boson that has caused so
much excitement?
The Standard Model hypothesizes the existence of a Higgs field, which
causes the matter particles, such as electrons and quarks, to acquire
non-zero, positive masses. By a subtle mechanism, it also causes
some of the bosons (W-plus, W-minus and Z-nought) to acquire positive
masses. All these particles acquire masses by interacting with
the Higgs field. If this theory is true, a matching
particle---the smallest possible excitation of the Higgs field---must
also exist and be detectable, which is the acid test for the
hypothesis. This matching particle is the Higgs boson.
To see if such a particle exists, and to detect it if it does, was one
of the important goals of the Large Hadron Collider (LHC). The
word 'hadron' stands for 'a particle that can interact via the strong
nuclear force'. Protons are made up of quarks, and are
hadrons. The LHC collides one beam of protons against another
beam of protons. If the Higgs boson does indeed exist, to detect
is very hard because when two protons collide, the probability that a
Higgs boson is produced is extremely small---this means that one has to
set off a very large number of collisions before one can reliably
detect the Higgs boson. Even worse, the Higgs boson cannot be
detected directly, as it not only has no electric charge, but
also extremely unstable and decays into various other particles in a
very short time. It is those decay products that are detected in
the experiments. The existence of a Higgs particle is indirectly deduced from these measurements.
Some exotic particles can be detected relatively easily because they
decay modes are very distinctive. The decay products of the W
boson, for instance, are so characteristic in certain respects that its first
discovery was made with only five candidate events. But when the
decay products of a new, undiscovered particle are the same as those
arising from the decay of other, known unstable particles, it becomes
very hard to deduce the actual source of these products. It is
only after accumulating huge data sets that the details unique to the
new particles can be isolated and used to confirm their discovery.
At higher collision energies, however, the production of the Higgs
particles dramatically increases, and the ratio of Higgs signal to the
'background' (due to already known particles) improves. Other
than improved detectors, this is the huge advantage the LHC has over
older particle accelerators such as the Tevatron.
Hence, with LHC (the largest and highest energy particle collider built
so far) and with computer resources drawn from all over the world to
analyze the voluminous decay signals, it is believed that the Higgs
boson has been discovered, with the probability of error being less
than one in a million. No wonder the whole world is excited, for
now we think we know how it has come about that elementary particles,
and therefore we ourselves, have a property called mass!
Epilogue: You may feel that I haven't
really explained the Higgs boson. Perhaps so. What I found
missing when I first encountered this stuff first was the big picture:
though the Higgs particle was the one that caused the buzz, the context
in which this was set was unclear to me. That is what I've tried
to capture and explain. More details are only a mouse click away.