Discoveries of elementary particles
“Elementary particles are terribly boring, which is one reason why we’re so interested in them.” — — Steven Weinberg.
It fascinates us when we look around the world and see the matter in different varieties and different forms. Starting from dust to stars in different colors and sizes look so beautiful. But have you ever thought of how this matter is made of? If you were to take any object, say a book, cut it in half, then in half again, and keep carrying on, where would you end up? Could you keep on going forever, or you will get a building block out of which everything is made? From the age of Greek philosophers (Aristotle, Democritus, etc.) to the modern era, this question has taken our perception of observing this world to another level, and physics has made a lot of progress. Different elementary particles come in other properties and flavors to create all the matter in interactions among themselves. We call them “Standard Model” together. It will be an exciting journey to get to know the stories of their discovery.
Standard Model: -
Based on quantum statistics, all the particles are classified into (a) Fermions and (b) Bosons.
Fermions obey Fermi-Dirac statistics, and Bosons obey Bose-Einstein. Fermions are divided into three generations with four particles each. Half of them are leptons, and the other half are Quarks. Bosons are categorized into two parts. Vector bosons are mediators of the force fields, and the Higgs boson is responsible for the mass of particles.
Now let’s dive into the history of when nature called those actors on the stage for their scene.
Electron: -
Basically, particle physics was born in 1897 when J.J Thompson discovered electrons. He noticed that the magnetic field deflected cathode rays emitting from the filament. This directly implies that it carries a charge. After observing the direction of the curvature, he found the charge to be negative. After applying a magnetic and electric field, he calculated the velocity and charge to mass ratio. He concluded cathode rays to be streams of particles and called them “Corpuscles.”
George Johnstone coined the term “electron” for the fundamental unit of charge, and later that name was taken for the particles.
Proton: -
After discovering electrons, scientists were trying to find the source of an equal amount of positive charge inside of an atom. In 1919, Ernest Rutherford performed his famous gold foil experiment. After observing the deflections of the alpha particles, he concluded that most of the space inside an atom is empty. All the positive charges are confined in a small space called the
nucleus. The nucleus of the lightest atom (Hydrogen) was named proton.
Neutron: -
After discovering protons, scientists soon realized that Rutherford’s atomic model was not complete. The total mass of the nucleus was found to be twice of the protons. Then from where this extra mass is coming? Rutherford also suggested the existence of some neutral particle having almost the same mass as the proton, but he had no experimental proof. Later in 1932, after several theories and experiments, James Chadwick discovered the neutral particle as neutrons.
Photon: -
At the beginning of the 20th century, scientists struggled to explain the blackbody spectrum. Two scientists, Rayleigh, and Jeans, used classical statistics to explain the phenomenon, but their curve did not fit with experiments and resulted in “Ultraviolet Catastrophe.” Later, Max Planck considered that electromagnetic radiation is quantized and comes in tiny energy packages with a specific frequency. In 1905, Albert Einstein came up with a radical view and said that quantization is a feature of the electromagnetic field and has nothing to do with the emission process. He used Planck’s idea to explain the photoelectric effect. Later more experimental evidence was received of the particle nature of electromagnetic waves. In 1926, at the suggestion of the chemist Gilbert Lewis the particle was called “Photon.”
Mesons: -
In the mid 19’s, a conspicuous problem came into the picture. If protons are like charges, then how come they are bound inside the nucleus and do not repel each other? On the other hand, if there is any force stronger than coulombic repulsion, what could be its source? In 1935, Hideki Yukawa suggested that a meson particle might be exchanged between the nucleons, which can be the reason for the strong force (analogous to the force exchange by the photons between charged particles). But at that time, several studies on cosmic rays were going on. After detailed investigations, researchers observed a significant difference between Yukawa’s prediction and experiments. In 1947, they discovered two middle-weight particles in cosmic rays. They named them “pion” and “muon” or “mu-meson.” The true Yukawa’s meson was pion.
Antiparticles: -
The existence of the antiparticle of electrons was first predicted by P.A.M Dirac in 1928 in his relativistic quantum theory of electrons. According to his theory, the total relativistic energy of a free electron can take two possible (positive and negative) values. He postulated the idea of negative states filled by an infinite sea of electrons. However, most physicists were uncomfortable with his framework. In 1932, while studying cosmic rays, the predicted antiparticle was found — the first known antiparticle. Later in 1940, Feynman and Stackelberg modified Dirac’s notion with positive energy states of a different particle (positron or anti-electron). In 1955, the proton’s antiparticle was found at the University of California, and antineutron was discovered in the following year.
The discovery of antiparticles gives the excitement of matter/antimatter symmetry to the physics community. But it also raises the disturbing question: how come our universe is populated with only matter? When matter and antimatter come together, they get annihilated and produce pure energy. If our corner of the universe is dominated by matter, then presumably, there are other regions of the space where antimatter will predominate. But unfortunately, till now, our observable universe is found to be made with matter only.
Neutrinos: -
In the 1930s, two problems arose while studying beta decay. In beta decay, the nuclear charge increases by a unit and decreases the energy of the nucleus by a definite amount. But the electron emerges with lesser varying amounts of energy which was the most disturbing result.
Another puzzle was corresponding to spin angular momentum. Electrons and protons each have ½ spin. In those nuclei where the total number of nuclear particles is odd (e.g., 7N14), they should have an odd half-integer angular momentum. But in the case of 7N14, it was 1.
In that situation, most physicists, including Niels Bohr (not for the first time), we’re ready to give up the law of conservation of energy. However, Pauli took a firm view, suggesting another neutral particle emitting along with electrons that carries the missing energy. Pauli called it “neutron.” Within a couple of years, Fermi proposed another idea where he incorporated Pauli’s idea so subtly that Pauli’s suggestion couldn’t be overlooked. This solved our puzzles. When a nucleus undergoes beta decay, a new light particle emerges with an electron carrying away the energy that appeared to be lost. Fermi called it “Neutrino”.
After the theoretical necessity, it was essential to detect it directly. And it took two decades to accomplish the job. In 1956, researchers detected the neutrinos by using a “target” consisting of cadmium chloride dissolved in water and surrounded by large detectors filled with a liquid scintillator. In 1962, the second type of neutrino, whose existence had been suggested, was detected in New York. A third kind, the tau neutrino, was discovered in 1975 by a group at Stanford.
Since then, neutrinos have become a vital tool in studying high-energy physics.
Quarks: -
In 1964, two physicists, Murray Gell-Mann and George Zweig, worked independently on the theory of strong interaction symmetry in particle physics. They realized that the properties of the strongly interacting particles –hadrons — could be explained if they were made up of constituent particles. In 1961 Gell-Mann introduced an asymmetry scheme called the “Eightfold Way,” based on the mathematical symmetry known as SU(3).
This scheme categorized the hadrons into two main groups. Gell-Mann built this work in a new model that could successfully describe the magnetic properties of protons and neutrons. But Gell-Mann’s model required the existence of three new elementary particles (up, down, and strange), which he called “Quarks.” In less than a year, the Gell-Mann–Zweig model was extended for a better description of Weak interaction, which further predicted the existence of a fourth quark, the charm quark. In 1977, the bottom quark was observed by a team at Fermilab led by Leon Lederman. This indicated the existence of the top quark. and it was not until 1995 that the top quark was finally observed.
Strange Particles: -
In the beginnings of particle physics (the first half of the 20th century), protons and neutrons were thought to be elementary particles. However, more particles were discovered between 1930–50. Some particles’ lifetime was longer than others. Most particles decayed through the strong interaction and had lifetimes of around 10^−23 seconds, and when they decayed through the weak interactions, their lifetime was around 10^−10 seconds. In 1953, while studying these decays, Murray Gell-Mann and Kazuhiko Nishi Jima developed the “ strangeness “ concept to explain the longer-lived particles’ “ strangeness. “ Despite their effort, the physical basis behind the strangeness property of the particles remained unclear. In 1961, Gell-Mann and Yuval Neeman independently proposed the “Eightfold Way,” taking only the up, down, and strange quarks into consideration. Up and down quarks were the carriers of isospin, but the strange quark carried strangeness. But there was no direct evidence of the existence of quarks until 1968 at the Stanford Linear Accelerator Centre. The team shooted electrons at the protons and observed how they bounced off. The scattering patterns were found to be caused by point-like particles. In the subsequent years, combining these results with others from neutrino-scattering at CERN made it clear that these constituents really do have charges of 1/3 and 2/3.
The first strange particle (Kaon) was discovered in 1947, but the existence of the strange quark itself was only postulated in 1964 by Murray Gell Mann and George Zweig.
Particles of Weak and Strong interactions: -
In 1933, Enrico Fermi proposed the first theory of weak interaction, suggesting that beta decay could be explained by four fermion interactions involving no range contact force. Now it would take a powerful particle accelerator to produce W and Z bosons. The first such machine was the Super Proton Synchrotron, where the signals of W bosons were seen in January 1983 during a series of experiments. A few months later, Z bosons were detected.
Before the 1970s, physicists were unaware of how the nucleons were bound together. It was known that the nucleus was composed of protons and neutrons and that protons possess a positive charge, while neutrons were electrically neutral. So, positive charges should repel one another, and the nucleus must be unstable. However, it was never observed. A new idea was needed to explain this weird phenomenon.
A stronger attractive short-range force was postulated to explain this fact. This force was called the strong force, which was believed to be one of the fundamental forces that acted on the protons and neutrons to bind the nucleus.
It was later discovered that protons and neutrons were not the fundamental particles but were made of constituent particles called Quarks. Quarks with unlike color charges attract one another due to the strong interaction, and the particle that mediates this was named the Gluon. In 1978, Gluon was detected, and later by further experiments, its properties were confirmed.
Higgs Boson: -
The theoretical framework of the Higgs particle and Higgs Field, acceptance of the scientific community, and the discovery inside the particle collider is a long and fascinating story. But in short, it was Peter Higgs who, in 1964, first postulated that the Higgs particle associated with the Higgs field gives mass to other fundamental particles like electrons, etc. The particles that don’t have mass don’t interact with the Higgs Field.
Although the Higgs field would exist everywhere, proving its existence was not an easy task. It took more than 40 years to build particle colliders, detectors, and computers capable of looking for Higgs bosons. On 4 July 2012, the discovery of a new particle with a mass between 125 and 127 GeV/c² was announced; physicists suspected it to be the Higgs boson. By March 2013, the existence of the Higgs boson was confirmed with all its properties, and Peter Higgs was awarded the Nobel Prize.
The journey of the search and discovery of each particle is really an exciting one. It took a lot of conflicts, debates, and patience to find them. And all the particles have given us a new direction and perception to explore our universe. But our search has not stopped yet. Gravity is still waiting to be added to the Standard Model. So, there are more particles to find, more connections to make, and more debates to play.
