Author: Arpan Dey
Happy Higgs Day!
In case you are wondering what on earth that is, read on!
Four years after the historic announcement of the discovery of the Higgs boson at CERN, a collaboration between INFN and CERN has declared 4 July 2016 Higgs Boson Pizza Day.Â
So what is the Higgs boson, exactly? You may have heard of the God particle. That's it! Before we go deep into Higgs boson, first let's see what is a boson.
In high school, you must have learned that a force is something which tends to change the state of rest or state of motion, or size, shape, direction of motion of a body, etc.. There are four fundamental forces: gravitational, electromagnetic, strong nuclear and weak nuclear forces. These forces are responsible for all possible interactions that can take place in this universe, from planets orbiting a star to protons and neutrons confined in the nucleus of an atom.
In classical physics, the assumption was that an imaginary field exists, through which a force can be transmitted. For example, an electric field or a magnetic field. This idea serves a good analogy, but modern physics suggest otherwise. With the advent of quantum mechanics, this idea was changed radically. A field exists, but that is a quantum field. The field vibrates gently, and these vibrations give rise to particles and their corresponding antiparticle partners, i.e., particles with opposite charge. But these particles can exist for a limited amount of time. Antiparticles being oppositely charged as compared to particles, the overall charge remains constant. But who gives the energy? Isn't the law of conservation of energy violated if particle-antiparticle pairs can literally pop out of nothing in vacuum? Well, this can happen by temporarily 'borrowing' energy from some source. And after a small amount of time, they come in contact and annihilate each other.
According to modern physics, light can be treated as a stream of particles called photons. The exchange of photons gives rise to electromagnetic forces. Virtual photons can pop out of nowhere around an electron, by ‘borrowing’ some of the electron’s energy. If there is another electron near the virtual photon, it will absorb the photon. Thus, essentially, some energy and momentum is exchanged between the electrons, causing them to repel, since the second electron, on gaining energy, will move away from the first one. Thus, the photon is a boson, for in the above case, exchange of the photon gave rise to the force of repulsion between the two electrons. Thus, electrons, both being negatively-charged (like-charged), repel.
But wait, what if the particles drift far away before annihilation, once a particle-antiparticle pair materializes? For example, if a particle-antiparticle pair pops up near the event horizon of a black hole? The black hole's intense gravitational attraction can pull in a particle, and its antipartner, on the other side of the event horizon, drifts away. What accounts for the existence of this particle then? Well, that is a story for another day.
What gives rise to forces then? Particles called bosons. Bosons, named after Indian physicist Satyendra Nath Bose, are particles, the exchange of which give rise to forces. Bosons, along with the fermions (which make up matter), are referred to as elementary particles.
First, let us deal with fermions. Fermions can be categorized into two groups: quarks and leptons. Quarks, in turn, can be either up, down, charm, strange, top or bottom (ignore the jargon for now). Neutrons and protons, which make up the nucleus of the atom, consists of up and down quarks. On the other hand, the electron is a lepton. Then, muons and the tau particle are also leptons. Each of these three leptons has its corresponding neutrino partner. The neutrino is a particle which carries away some energy during nuclear reactions. It is extremely difficult to detect a neutrino, since it can penetrate through almost anything.
Now, instead of comprehending whether a botanist has to remember more names or a particle physicist, let us come to bosons. The W- and Z- bosons are involved in nuclear interactions. The gluon is involved in strong nuclear interaction. But other than all these, there is a very special boson, which is our main concern.
What is mass? A classical physicist would say that mass is an intrinsic and fundamental property, related to the amount of matter contained in a body. But in modern physics, mass is no more a fundamental property. It is a consequence of a particle’s interaction with a Higgs field. The omnipresent Higgs field contains Higgs bosons (named after Peter Higgs), which are responsible for mass transfer. The Higgs boson is responsible for generating the masses of the fermions by a process known as spontaneous symmetry breaking. As particles react with the Higgs bosons, mass is ‘transferred’ to them. Faster particles like photons (which is a boson) hardly interact with a Higgs field (the photon is itself a boson). But the concept of Higgs field alone can’t explain why ‘less-fundamental’ particles like protons have mass, for the sum of the masses of the constituent quarks (which are present in the proton) is unable to account for much of the proton’s actual mass. This extra mass can be accounted for by the fact that, when quarks are confined in a tiny region, they contain much more energy. This energy is expressed as the extra mass.
Now, let's come to a more pressing question. The Higgs boson was detected some years back, on this date. So, how exactly was it detected?
I mailed this question to a high-energy physicist of the Tata Institute of Fundamental Research. Fortunately, I got a reply! The Higgs boson is expected to be quite heavy, and its presence might be inferred if an elementary particle is found in numbers greater than expected, when two particles are collided at high velocities. This would indicate that the massive (i.e., mass-containing) Higgs boson has decayed into these particles. However, it cannot be conclusively assumed that such an excess of fundamental particles can occur only due to a Higgs boson decay.
The Higgs boson can decay to any quark (along with its antiquark), except the top quark, since the top quark is heavier than the Higgs boson. How strongly Higgs bosons couple with a quark depends on the mass of the quark; the heavier the quark, the more the connection with the Higgs boson. Of all the quarks other than the top quark, the bottom quark is the heaviest. Thus, the Higgs boson couples to it most strongly, and decays to bottom-antibottom pairs more than any other quark-antiquark pair. The next heaviest quark is the charm quark, and the Higgs decaying to charm-anticharm pairs has also been observed; and the ratio between these two decays go exactly as expected from their masses. The other quarks are much lighter, so it is much more difficult to see Higgs bosons decaying into them.
But how is the Higgs boson important? What are we to gain from further research on the subject? The answer is quite simple. The Higgs boson is one of the most important particles in the Standard Model. Further research on particle physics is bound to reveal further information on the fundamental nature of reality.
So, let us all celebrate this very special day, and eat pizza!