Indian connection

The Standard Model family portrait, with three classes and three generations.

RAHUL SINHA, a professor at The Institute of Mathematical Sciences, Chennai, and his group have been working on physics beyond the Standard Model for many years. In particular, his work on the angular distribution of the particles coming from B0 meson decay results in angular observables that are highly sensitive to the effects of new particles on the decay and are crucial to the experiments being carried out in LHCb as well as Belle and Belle II experiments. He, along with his student Rusa Mandal, has developed a different way of analysing the decay that indicates a deviation from the Standard Model at a higher significance (six sigma), and the work is under review. If indeed their work is found to be correct, it will clearly signal a call for new physics beyond the Standard Model.

The family portrait

School science tells us that atoms are made up of electrons, protons and neutrons. It is now clear that while the electron is a fundamental particle, protons and neutrons are a composite of three quarks. Just as biology divides the living world into animal, plant and microbial kingdoms, the Standard Model classifies all the fundamental particles into three types, leptons, quarks and force carrier “gauge bosons”.

In addition to the familiar electron, two other particles, the muon and the tau, and their associated neutrinos, the electron neutrino, the muon neutrino and the tau neutrino, constitute six leptons .

There are six “flavours” of quarks, up , down , strange , charm , top , and bottom, and each comes in three colours, red , green and blue . Up and down quarks have the lowest masses of all quarks and are the most stable of all the six. The second generation strange and charm are the next heaviest, and the third generation top and bottom are the most heaviest and the least stable. Higher-generation quarks rapidly decay into up and down quarks through a process of particle decay. B0 mesons, one such particle composed of a bottom antiquark and a down quark, is of particular interest.

In addition to leptons and quarks, the Standard Model also has a third category of particles called gauge bosons (named after the Indian physicist S.N. Bose). We are acquainted with one of the 13 “gauge bosons”, that is, the photon. In modern physics, electromagnetic interaction is mediated by photons. In other words, if you have a magnet stuck on your refrigerator, modern physics says that every moment, billions of “virtual” photons are “exchanged” between the magnet and the refrigerator, and this exchange is what is manifested as “attraction”. In like manner, the two forces operating at the nuclear level, weak force and strong force, are also thought of as interaction of ‘virtual’ gauge bosons. The experiments have found that there are eight types of gluons enabling strong force and three W+, W- and Z0 bosons relating to weak force. Finally, the Higgs boson is what gives “rest” mass to all the fundamental particles.

What about the familiar proton and neutron or weird particles like mesons found in cosmic rays? The Standard Model says “hadrons” like proton and neutron, called “baryons”, are composed of three quarks, and another type of hadron, mesons, are composed of one quark and one antiquark. The allowable permutations and combinations of quarks and anti-quarks make up the zoo of subatomic particles observed in cosmic rays and colliders.

Also, all these fundamental particles have their own antiparticles such as anti-electron (positron), anti-quarks, and so on. Why our universe is full of “matter” and not “anti-matter” is still a deep mystery.

A proton is made up of three quarks, a blue up quark, a red down quark and a green down quark. All these three are positively charged, and as we know, “like repels like charges”, and within a jiffy, the proton should have disintegrated. But the proton is one of the more stable subatomic particles. This implies that there should be some “strong” force that keeps the positively charged quarks bound together. It is this strong force that energises stars and provides the earth with solar energy.

During beta minus decay, it was observed that a down quark within a neutron is changed into an up quark, thus converting the neutron into a proton and resulting in the emission of an electron and an electron antineutrino. As a result, an atom of one element turns into an atom of another element, natural alchemy. It is the weak force at work here. While the other forces hold things together, the weak force plays a greater role in things falling apart, or decaying, and allows for quarks to swap their flavour for another. The weak force is harnessed in nuclear power plants, in the treatment of cancer and also for making atom bombs.

The missing link

Suppose you look at me, a person with a 65-kilogram mass. Most of my mass comes from protons and neutrons. While 99.95 per cent of my mass is accounted for by protons and neutrons, only 0.05 per cent is contributed by electrons.

Let us now ask the question, how do protons and neutrons acquire the mass? Obviously from its composites, quarks. If one looks at the “rest” mass of quarks, it is only 1 per cent of the mass of the proton. The rest of the 99 per cent is the strong force that binds these quarks. In that sense, most of our mass is actually strong force.

Now, how do the electron and quarks have mass, rest mass? The invariant mass of an electron is approximately 9.109×10−31 kg or 5.489×10−4 atomic mass units. Using Einstein’s mass-energy equivalence principle, E=mc2, one can say the rest mass of the electron is 0.511 MeV. One of the questions in the Standard Model is why would the electron and its next generation particles such as muon and tau have different rest masses although they are all point particles with the same charge? In like manner, the rest mass of a photon is zero, while force carriers like W+, W- and Z0 have non-zero rest mass.

The Higgs field was postulated to explain the origin of rest mass of fundamental particles such as quarks and electrons. Imagine electrons and quarks to be some type of sponge and the Higgs field as rain. If there is no place to hide, the rain will fall on it and based on its porous capacity, the sponge will soak in the rain water and become heavier. Except for the photon and the gluon, all other fundamental particles couple with the Higgs field in various strengths, resulting in the rest mass that they have. The associated particle with the Higgs field is the Higgs particle. The Higgs particle remained elusive for more than four decades and was finally found in 2012.

T.V. Venkateswaran