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What are Particle and Nuclear Physics?

Introduction

Our universe, from the stars in the sky to the pixels making up this text, is made of tiny particles. In high school, one learns of the atom, the bottommost level of matter, and its components, the proton, neutron, and electron. However, this is not all there is to the universe. There are many other types of particles always around us. Protons and neutrons themselves are made of particles known as quarks, while electrons are one of six different particles referred to as leptons. Along with force carriers, the mysterious particles that help bring about forces like electromagnetism, these particles compose what is known as the Standard Model. There are two main groups of physicists who study the Standard Model, its particles, and their interactions.

Particle physicists are interested in smaller-scale actions. By analogy, if the Standard Model is the brain, particle physicists would be analyzing it on the level of the neuron. They examine the basic components, look at the various ways they can interact, and attempt to understand what forces allow these interactions to take place. However, simply understanding how neurons work does not mean that you can understand the human mind. Thus, nuclear physicists exist, looking at larger scale phenomena occurring during interactions of large groups of particles at once. There is obviously a fair bit of overlap between the two disciplines, and neither could reach a full understanding without the other, but the main difference lies in the level at which they analyze particles.

The Standard Model

What is the Standard Model, then? It is, put simply, a chart much like the Periodic Table.

These sixteen particles, believe it or not, are the only true fundamental particles of physics. The six in the top left are known as quarks. They each have a discrete mass and charge. They also have something known as spin, which refers to angular momentum much like that of a rotating top. The bottom-left six are known as leptons. Unlike the quarks, which come together in groups three to create new like protons and neutrons, the leptons do not merge. (Quarks can also pair up to make pions and many other particles.) Additionally, not all of them have an electric charge: the three neutrino particles are electrically neutral, which, along with their tiny mass, makes them incredibly difficult to detect. In fact, neutrinos can pass freely through the entire Earth! Finally, the four particles along the right side are known as force carriers. Among them is the photon, which many know as the particle form of light. In truth, this is only partially correct. Firstly, all of these particles can actually behave as waves too, depending on the situation: this is known as particle-wave duality. Additionally, the photon also acts as the force carrier for the electromagnetic force. Similarly, the gluon carries the strong nuclear force, and the Z and W bosons carry the weak nuclear force. It has been theorized that there exist particles known as gravitons which are the corresponding carrier of gravity, but this has not yet been proven.

It should be noted that the particles in the Standard Model have counterparts known as antimatter. Antimatter particles have equal mass and spin to their analogous Standard Model particles, but opposite charge. For example, the antiproton is negative, and the antielectron, or positron, is positive. Neutral particles like photons are their own antiparticles.

Beyond the Standard Model

Other than the professed uncertainty towards the existence of gravitons, it may seem that the Standard Model does not have many holes, or that there is little research left to be done. This is actually quite far from the truth, as any number of interesting problems have emerged over the recent years. Even more excitingly, our technology is finally beginning to catch up to our theories, allowing us to work with new techniques to prove more difficult aspects of nuclear and particle physics. For instance, a major question in nuclear physics relates to the spin, or angular momentum, of a proton. The proton is made of three quarks (two up quarks and a down quark), which each have .5 h-bar spin. It had been thought that these quarks combined their spins to give the proton its .5 h-bar spin, but recently it was discovered that the quarks themselves only contribute a small part of this spin. This leaves a major open question: where does the rest of the spin come from? There have been theories proposed to make up for this, but none have been proven successfully yet. The two leading theories are that the spin is contributed either by the gluons that hold the quarks together, or by what are known as virtual quarks that pop in and out of existence instantly. These particles do not violate conservation of energy due to the Uncertainty Principle, which states that over a small enough period of time, energy can increase or decrease within a small limit.

Particle physics has many similarly interesting questions. Probably the most well known question is that of the Higgs Boson. This theoretical particle has been proposed to generate a special field which is what gives all particles their original mass. Thus, as the origin of mass, the Higgs is something of a holy grail to many particle physicists, and the LHC is currently the main force working on this project. Another big question relates to the existence and nature of dark matter, which is mass that we can detect due to its gravitational force, but cannot see, feel, or otherwise sense. Amazingly, cosmologists have theorized, with strong empirical evidence from examining gravitational interaction in space, that dark matter composes more than 80% of all the mass in the universe! This means that even if we manage to perfect the Standard Model, we will still only understand one fifth of the matter in the universe. So have no fear. There will be plenty more questions to research and more data to discover for many years to come!

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