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A Brief History of Particle Physics

During the various stages of schooling a person receives, one learns that all matter is made of tiny particles called atoms. Later, we learn atoms are made of Protons, Neutrons and Electrons. In High School, we learn about the uncertainty in measuring position and velocity of subatomic particles. But are those the only subatomic particles? Or do there exist more fundamental particles which may or may not be a part of the atom?

Particle Physics looks answers to these questions. I personally find particle physics to be the most ubiquitous of the sciences. It is the path to the discovery of the explanation of all phenomena to the deepest level.


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HISTORY

The stream of particle physics picked up pace during the 1950s. At that time, governments were interested on funding in Atomic and Nuclear research, given that it was the era of Cold War and every country was running a race to build their own nuclear arsenals.

By that time, it had already been discovered that some fundamental particles, like the photon, existed independently and not inside atoms. There was a development of two statistical models- Fermi-Dirac and Bose-Einstein- to explain behavior of various particles. It was established through the spin-statistics that
any particle in our space-time may be either a boson (that means its statistics is Bose–Einstein) or a fermion (that means its statistics is Fermi–Dirac).

What gave an impetus to particle physics was the research on heavy radioactive metal synthesis, or simply, making atoms with high mass artificially. This required colliding atoms at high speeds so as to fuse them together. Thus, particle accelerators were built, which soon became the backbone tool for particle physics.

A Particle Accelerator

Soon, it was discovered that colliding nuclei at very high speeds yielded various particles. Scientists believed all of these were “elementary particles”, like electrons. By the end of the 50s, there was a huge collection of particles, most of them unstable, and scientists could not properly make head or tail of it.
Infamously, this was called “the particle zoo”. At this, Wolfgang Pauli remarked: “Had I foreseen this, I would have gone into botany”. All of these particles showed different interactions with the fundamental forces of the Universe (Strong and Weak Nuclear forces, Electromagnetic Force, Gravity).


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QUARK MODEL

Now, scientists were tasked with the overwhelming procedure of finding the composition of these particles and explaining their different behaviors. They began by looking at properties of the particles:

  • Charge
  • Strangeness: The measure of the decay rates of the particles. The quality initially referenced to abnormally stable particles in the “zoo’.
  • Mass

All of this led scientists to predict that there were fundamental particles, that they named quarks, which formed all the hadron particles. The interaction of these particles by scattering and decay provided a key to new fundamental quantum field theories.

Classification on these properties finally yielded the Quark model, hypothesizing that all the hadrons were made of 6 quarks and their respective anti-quarks (particles with opposite properties, other than mass which is equal for a particular particle and its antiparticle). These were the Up and Down quarks, the Strange and Charm quarks (I know, the names are weird, but wait for it), and the
Truth and Beauty quarks. Apparently (and luckily) physicists realized that they had to bring it a notch down and they could not just name Quarks randomly, so they renamed Truth and Beauty to Top and Bottom.

The Quark model hypothesized that the hadrons were made of quarks joined together by the strong nuclear force.

Composition of a Proton and Neutron

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TOWARDS THE STANDARD MODEL

But the Quark Model was not enough to explain everything. True, the structure of hadrons such as protons and neutrons could easily be explained by quarks, but, there was another class of fermions which did not interact with the Strong Nuclear force. These were called the leptons. This included electrons, taus, and muons and respective counterparts with no charge called neutrinos.

At the same time, bosons were being researched upon. It was hypothesized that these were formed as a result of the excitation of the fields from which Fundamental forces arose. These were:

  • Photon (Electromagnetic Field)
  • Gluon (Strong Nuclear Force)
  • W and Z bosons(Weak Nuclear Force)
  • Graviton (Hypothetical, still not discovered) (Gravitational Force)
  • Higgs Boson (Higgs Field)

By this time, physicists simplified a lot of the particles and interactions into a model which described all of these discoveries in a unified manner. This is called the Standard Model of Particle Physics, the model that we accept as of today.

The discovery of the Higgs Boson in 2012 has strengthened the belief in the Standard Model even more.

The Standard Model of Elementary Particles

This follows some basic rules:

  • All particles participate in gravitation.
  • All charged elementary particles participate in electromagnetic interaction.
  • As a consequence, neutron participates in it with its magnetic dipole in spite of zero electric charge. This is because it is composed of charged quarks whose charges sum to zero.
  • All fermions participate in the weak interaction.
  • Quarks participate in the strong interaction, along gluons (its own quanta), but not leptons nor any fundamental bosons other than gluons.

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CHALLENGES

Though widely accepted, the Standard Model is not complete. The Yang Mills Problem is one such problem that remains to be solved. The force of gravity has also not been orderly inculcated into particle physics. The solving of these problems is our key to the Theory of Everything, a complete description of our Universe. Or, more interestingly, if the Standard Model is proven false, then we can open up a chest of whole new bunch of theories and discoveries!


About the Author – MEHUL ARORA

Mehul Arora is a high school student studying Physics, Chemistry, Mathematics and Computer Science. He has worked as a Technical Head for The Red Megaphone Blog and been an Associate of Public Relations at Innerve 2019. He has been writing prominently in the fields of Fundamental Physics, Technology, Statistics, Geology, Politics and Disease Response since 2019.

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