The Missing Pieces of Jigsaw: The Standard Model Of Particle Physics
By Devansh Dhabhai
Autumn 2022
The Lagrangian of the Standard model perfectly encapsulates our current understanding of the observable universe into a framework of equations. It is the most rigorous theory of particle physics and is incredibly precise and accurate in its predictions. The Standard Model mathematically describes the 17 building blocks of the observable universe: six quarks, six leptons, four gauge bosons, and one scalar bosonthe Higgs Boson. Despite its great predictive power, this intricate model falls short of being a complete theory of fundamental interactions due to its inability to explain phenomena such as gravity, dark matter, and the mass of neutrinos.
Gravity
The theory of gravity, first developed by Isaac Newton, described gravity as a field that could reach great distances and dictate the path of massive objects. However, Newton's definition of gravity was incomplete; it could not unravel the true nature of this field. But then, in 1915, Albert Einstein published his theory of general relativity connecting the geometry of spacetime with matter.
Although the general Theory of Relativity successfully describes gravity at the macroscopic level, some observations point out discrepancies in General Relativity and suggest a more fundamental theory of gravity. For this reason, scientists seek to understand gravity and its interactions at the microscopic level. Unfortunately, all of our theories, from quantum field theory to quantum loop gravity, have yielded no definite result in deciphering the symmetry that unifies quantum mechanics with general relativity. As of now, physicists hypothesize the existence of a "Graviton," an undiscovered elementary particle that mediates this weak force of gravity. Like the photon, the graviton is expected to be massless; furthermore, it is also predicted to be a spin2boson because the source of gravitation is the stressenergy tensor.
In essence, the incompatibility of general relativity with quantum mechanics into a single framework is still a persisting conundrum in particle physics. The underlying theory to solve this challenge remains a mystery.
Dark Matter
Astronomers believed that the universe was made from baryonic matter until around three decades ago. However, in the previous decade, there has been proof aggregating that recommends there is something in the universe that we can not see. Although it can not be seen, researchers have a few strong confirmations that over 27% of the universe's total mass is made out of this undetected mass, called dark matter.
Unlike baryonic matter, dark matter neither absorbs nor reflects electromagnetic radiation. Nevertheless, researchers have traced its existence because of its subtle gravitational interactions with baryonic matter. However, a more fundamental perspective is required to understand dark matter's nature fully. For this reason, scientists have developed a few theories to describe dark matter's enigmatic properties. One such theory suggests the existence of a "Hidden Valley," a parallel world made of dark matter having very little in common with matter we know. Other theories, such as string theory, effective field theory, superfluid vacuum theory, and supersymmetry, are also implemented to aid our understanding of elementary interactions with dark matter. If one of these theories proved to be accurate, it could help scientists better understand the composition of our universe and, in particular, how galaxies hold together.
Neutrinos
Neutrinos are elementary particles that belong to the lepton family. However, unlike electrons, neutrinos are electrically neutral. Their interaction with the other fundamental particles is solely mediated by the electroweak force and gravity.
The standard model has led us to believe neutrinos are massless particles that propagate near the speed of light. However, over the past decade, substantial evidence has led us to conclude that neutrinos have a nonzero mass. This evidence stems from neutrinos of one flavor, such as the muon neutrino, which can metamorphose into a neutrino of a different flavor, such as the tau neutrino. This observation, known as neutrino oscillation, proves neutrinos have a certain mass. This discrepancy between the theoretical predictions of the standard model and our observations further substantiates that the standard model is an incomplete theory of elementary interactions.
Currently, scientists are working to extend the standard model to make neutrinos massive. Approaches such as introducing new particles like the Dirac neutrino and the Majorana neutrino are a few approaches that aim to generate a nonzero mass of neutrinos via the Higgs mechanism. Other theories such as Superstring theory and Mtheory are also currently at work to provide us with a better foundation on the interactions of neutrinos.
Finding these missing pieces of the jigsaw is essential in our pursuit to comprehend the dynamics of our universe. These pieces will help us develop an allencompassing equation describing our universe's fundamental interactions.
Citations

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Devansh Dhabhai is a current High School Junior pursuing Physics, Mathematics and Chemistry at a HigherLevel in the IB Diploma. His primary research interest lies in exploring the fundamental constituents of Dark Matter. He also enjoys investigations into General Relativity and Archaeoastronomy.