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.