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Physics On Particle Physics May 1, 2019 Created by Riccardo Maria Bianchi Metadata Source Code

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Physics A CERN particle physicist walks through the history and science of particle physics, and why you should care about it—even outside of the laboratory.

Everything that we can see and touch is made up of particles, the basic building blocks of nature. Particles like protons, neutrons, and electrons compose the atoms of the computer or phone you are touching right now. Photons are responsible for the glow emitted from the screen. These same photons, when produced by the Sun, are responsible for the light that shines in the sky. Particle physics is the branch of science that investigates the nature and the behavior of these particles. It is one of the fundamental research fields: research that seeks to explore and explain nature at its essence. Understanding how particles interact can illuminate the innermost mechanisms of our universe. We can study how the big bang gave birth to matter, how stars behave, how cosmic rays travel from distant stars to our Earth, and how the universe itself evolves. Fundamental research fields explore the structure and behavior of the universe for the advancement of human knowledge, but that doesn’t mean they don’t have practical applications that affect peoples’ lives. Disciplines that conduct fundamental research form the base of a knowledge pyramid: their discoveries provide a foundation for practical applications developed by other disciplines. Research spinoffs and collaborations between researchers and industry bring further development with new, sometimes world-changing, applications.



Part 01

The Science of Particle Physics The Greek philosopher Democritus , who, in the 4th-century B.C., coined the term “atomos”—a privative “a” followed by “tomos”/“cut”—for the smallest, indivisible part of something. Since ancient times, people have wondered what the objects arounds them were composed of. Around the 4th century B.C., the Greek philosopher Democritus coined the term “atomos” to indicate the smallest indivisible part of something—but his concept was abandoned for hundreds of years, and it wasn’t revived until the early 19th century, when the English chemist and physicist John Dalton proposed an atomic theory of matter. After he confirmed his theory through experiments on gases, other physicists started investigating the structure of matter at a smaller scale, discovering complex substructure. Today we know that the modern atom is not the smallest component of matter, nor is it indivisible. Different materials are made up of different atoms. How those atoms organize themselves determines a material’s properties. That, in other words, is chemistry. If we looked closely at one of those atoms, we would see many electrons occupying the space around a tiny but heavy nucleus; the interactions of these atoms, and how they tie together, is the subject of a field known as condensed matter physics. The nucleus itself is composed of smaller components called protons and neutrons; the study of those and the forces acting between them is what nuclear physics addresses. Going deeper, we can see that even smaller elements compose protons and neutrons. These are called quarks. So far we cannot divide quarks or electrons into smaller elements. We currently assume that those are the basic building blocks of nature—the fundamental elements of all matter. The study of these extremely tiny objects and of the forces which bind them is subnuclear physics, also known as particle physics. The Fundamental Forces One of the great achievements of physics is the identification of four fundamental forces that form the basis of all other forces and interactions we see in our universe. The basic forces are gravity, which prevents us from drifting up into the sky and binds planets and galaxies together; the electromagnetic force, which governs the flow of electricity through a cable, the magnetism pushing a compass’ needle, and the light illuminating night; and the weak and strong nuclear forces, which act at the atomic and nuclear level. The weak nuclear force is responsible for radioactivity and roughly one-half of the warmth produced in Earth’s core. The strong nuclear force binds atoms’ nuclei together, giving matter a stable form—without it, we wouldn’t be here right now! We know today that the last three forces listed are carried out by particles: the intangible and massless photon is the carrier of the electromagnetic force; two particles, known by physicists as “W” and “Z,” carry out the weak nuclear force; and the particle called gluon is the bearer of the strong nuclear force. Studying these basic elements and forces leads to an understanding of the innermost mechanisms governing the universe. Natural sources of particles More from the Parametric Press Anything That Flies, On Anything That Moves The US covertly launched over two million bombing missions over Southeast Asian countries in the 1960s and 70s. Dig into the data behind the assault. Unraveling the JPEG JPEG images are everywhere in our digital lives, but behind the veil of familiarity lie algorithms that remove details that are imperceptible to the human eye. Let's see what our eyes can't see! The Myth of the Impartial Machine Wide-ranging applications of data science bring utopian proposals of a world free from bias, but in reality, machine learning models reproduce the inequalities that shape the data they’re fed. Can programmers free their models from prejudice? At the end of the 19th century, J.J. Thomson discovered the first fundamental particle: the electron. The discovery of the electron marked the beginning of particle physics. At the beginning of the 20th century scientists observed that particles were not only part of the structure of matter, but they were also produced by natural phenomena. Naturally produced particles were the focus of early experiments in particles physics, and many particles were discovered this way, for example the positron and the muon. Radioactive elements have unstable structures which cause them to spontaneously emit particles as byproducts of their internal readjustment. One of the most well-known is Uranium, a common fuel for nuclear power plants. Clock makers used to use Radium and Tritium to push the phosphor to glow in their glow-in-the-dark clock hands. Different radioactive atoms emit different kinds of particles including electrons, photons, and alpha particles (two protons and two neutrons bound together). Radioactive elements are naturally present in small quantities in the soil, and were the object of study by particle physics pioneers such as Henri Becquerel and Marie Curie. Particles from the cosmos continuously hit our planet and strike the atoms in the Earth’s upper atmosphere, producing additional particles. Those particles are called cosmic rays. Among them, we find photons and electrons, but also muons—a sort of heavy electron—and positrons, the equivalent anti-matter of the electron. The particles produced in the collisions travel towards the planet’s surface. Many of them get trapped in subsequent collisions, some reach the ground, and others continue traveling undisturbed. Cosmic rays are harmless to us: humanity has evolved in this environment and the human body is accustomed to those particle showers. Observing and studying particles Particles are the smallest known objects in the universe, and are intangible like light. So how is it that scientists can observe, study, and manipulate them? Particles can’t really be seen, because seeing something involves sending a beam of visible light towards an object and observing light that bounces back. But particles are so small that no visible light bounces back, even when using the most powerful microscopes. We have to find other ways to “see” particles. All particles behave differently when passing through matter. Electrons and photons rapidly lose their energy by continuously colliding with other material atoms until they get trapped. Protons travel through matter almost unnoticed until they are close to the end of their journey, when they lose all their energy in a burst and immediately stop. In Italian, Gran Sasso literally means “Big Rock”! Muons, on the other hand, can travel through layers and layers of any material, even through hundreds of meters of solid rock, without losing much of their energy. Scientists build labs, like the Gran Sasso laboratory in Italy, deep under mountains to shield sensitive experiments from cosmic muons. Neutrinos, another fundamental particle, interact extremely weakly with all other objects: they can pass through entire planets without stopping.







Source: Wikipedia. The Laboratori Nazionali del Gran Sasso is the world’s largest underground research center. It has been built under the Gran Sasso, the highest mountain of the Italian Appennini. Thousands of meters or rock shield the experiments conducted underground from most of the effects of the cosmic rays. Physicists leverage these different behaviors to develop detectors that catch and measure each type of particle. When a particle passes through a material it leaves a certain amount of its energy behind. That energy is collected and transferred to an electronic interface which visualizes a particle’s passage. By layering different detectors, physicists can reconstruct particle trajectories, and, under certain circumstances, measure the exact energy of a particle. Many particles have an electric charge. A charged particle behaves like the needle of a compass near a magnet: it points towards one of the magnet’s poles. Charged particles, like electrons or protons, are influenced by electromagnetic fields. An electric field can be used to push charged particles forward, while a magnetic field can make a particle move to one side or another, depending on the charge of the particle.

In 1933, Carl Anderson discovered the positron, an electron with positive charge. The discovery proved the existence of anti-matter.



Source: Wikipedia.

References: [1] and [2].

Arrow symbols by Freepik. Anderson’s original journal paper, The Positive Electron, in which he announces the discovery of the positron, is an interesting read! You can freely read it online. Also, if you like, you can take photos of cosmic rays too! In 1933, while photographing cosmic rays with a Wilson chamber, Carl Anderson took a photograph of a particle passing through a thin film of photographic emulsion. The particle behaved almost exactly like an electron, except for the way a magnetic field around the film bent its trajectory. If it were an electron, the magnet would have bent the path in the opposite direction. Anderson had proof that this particle wasn’t an electron. What he observed was a positron, the anti-electron, and the first proof of the existence of anti-matter. Accelerating particles Electromagnetic fields influence the speed and trajectories of charged particles. An electromagnetic field can always be divided into two distinct components: an electric field and a magnetic field. The force exerted by the field on a charged particle is called the Lorentz force: F = q [ E + ( v × B ) ] F=q[E+(v\times B)] F = q [ E + ( v × B ) ] In the above formula, q q q is the charge of the particle and v v v its velocity, E E E is the electric field, and B B B is the magnetic field. For a given electric and magnetic field, the particle experiences a force F F F from the field dependent on its charge and speed. But what if we could control the fields E E E and B B B ? Rewriting the formula to isolate the velocity of the particle we find this: v = 2 q ( V 2 − V 1 ) m v = \sqrt{ \frac{2q(V_2-V_1)}{m}} v = m 2 q ( V 2 ​ − V 1 ​ ) ​ ​ The expression describes the velocity of a particle, v v v , given the potential difference of an electric field. This means that creating a large voltage difference will increase a charged particle’s velocity. The particle in the diagram has been drastically slowed down. In reality, the particle would instantly fly out of the page, since an electron put in a small accelerator (say, of 10 meters) and pushed by an electric field (say, of 100 Volts), will be accelerated to a speed of about 6 million meters per second—that is almost 22 million km/h! That speed, while fast on its own, is still much slower than the speed of light—the fastest possible speed in nature, as far as we currently know—, which is about 1 billion km/h! Also notice how much more energy is needed to push a proton, which has the same electric charge as an electron, but contains a much higher mass. You have to choose the highest voltage setting to make it move! Accelerate a particle using an electric field electron proton Set a voltage (V) 350 1000 50000 Particle mass: 9.11e-31 Kg. Electric field magnitude: 35.00 V/m.

Choose a particleSet a voltage (V) Reset Go! If we instead rewriting the formula to isolate the radius of the bending of its trajectory, we find a description of the radius of the curvature of a particle’s trajectory, r r r , given the speed of the particle and the strength of the magnetic field: r = m v q B sin ⁡ θ r = \frac{mv}{qB} \sin\theta r = q B m v ​ sin θ Means that a magnetic field can be used to steer the path of a charged particle. In reality, this process is much more complicated. Different types of accelerating modules are used to push different types of particles, and many types of magnets (e.g., dipoles, quadrupoles, sextuples, and so on) are used to bend, squeeze, and focus beams of charged particles to maximize the their intensity (as seen in the accelerators used for medicine or industry applications) or the number of their collisions (as seen in the accelerators used for physics experiments). Steer a particle using a magnetic field positive negative Set a speed (m/s) 0.2 1 3 Set the magnetic strength (T) 0.1 1 5 Current time: 0.00 s.

Magnetic field strength: 0.10 T.

charge: -1

vx: 0.20 m/s.

Bending radius: 1 m. Set a chargeSet a speed (m/s)Set the magnetic strength (T) Reset Go! Using these two principles, physicists started building particle accelerators. By using accelerating modules one after another, we can build linear accelerators where the energy that these accelerators can produce is governed by their length. By combining accelerating and bending modules, we can build circular accelerators, in which particles are injected and forced to move in a circular path. Each time a particle goes around it is accelerated. After a given amount of cycles the particles attains their maximum energy and can be used for experiments. Circular accelerators can be further divided into two main groups: cyclotrons, which were the first to be invented around 1930, and synchrotrons. The former can be seen in hospitals today. The latter are used in machines for cutting-edge research in particle physics, such as the Large Hadron Collider (LHC) at CERN.







Source: CERN. The Large Hadron Collider (LHC) at CERN, the world’s most powerful particle collider, accelerates particles and boost their energy using radiofrequency cavities and a 27-kilometer ring of superconducting magnets. Physicists designed accelerators to study the basic characteristics of particles, but it quickly became clear that they could have many other scientific and industrial applications. For example, old television sets were driven by cathode-ray tubes, which are nothing more than compact particle accelerators! Electrons are produced and subjected to an electric field, causing them to fly towards a certain end of the tube. This end is covered by a layer of phosphorescent material that becomes luminescent when hit by electrons. The trajectory of the electrons is bent by a variable magnetic field, which forces electrons to hit the luminescent screen at different points. By modulating the intensity of the magnetic field, the electron beam paints an image on the screen. Particle factories Get updates from the Parametric Press So far, we have talked about particles produced in natural sources, but there are many more natural phenomena involving particles. All fundamental phenomena involving the electromagnetic force, for example, depend on the characteristics of electrons and photons—the elementary particle which is responsible for light. The stability of atoms, which compose all matter, depend on the interaction between two fundamental particles: quarks and gluons. These particles were discovered at the SLAC laboratory (US) at the end of the 1960s and at the DESY Laboratory (Germany) in 1979, respectively. The mechanism behind radioactive decay—which occurs in the cores of stars and planets like Earth—is determined by the behavior of the W W W and Z Z Z particles, discovered at CERN in 1983. Many fundamental mechanisms in the universe depend on particles which we haven’t discovered yet. This creates a number of interesting questions for modern physicists. For example, how did the universe form? At the beginning of time, in a fraction of a second, quarks and gluons bounced and smashed, interacting and recombining until creating the first glimpse of the matter we see today. And which particle, if any, is behind Dark Matter, which composes about 25% of the total energy of our universe? Is gravity carried by a particle like the other fundamental forces? And how are high-energetic cosmic rays that hit our planet generated, if there are no known sources for them near Earth? To answer these questions, and others not yet asked, we need particles interacting at much higher energies and with a much higher flux than those we can obtain from natural sources like radioactive elements or cosmic rays. To make progress, we would need bespoke particle factories, to create large fluxes of particles at the desired energy to study interactions and effects. That’s where Einstein comes in. In 1905, Albert Einstein proposed a theory about the equivalence of mass and energy. Contained in the well-known formula E = m c 2 E=mc^2 E=mc2 , his theory roughly states that an object has a certain amount of energy based on its mass, even when at rest. Conversely, if we create the exact amount of energy that corresponds to the mass of a specific particle, we can then create that particle and study it. Modern particle colliders are built for this: they accelerate particles up to the highest possible velocities and make them collide. The energy involved in the impact is used to create new particles which are caught by detectors and studied. The particles created in colliders are not to be considered artificial: they are naturally found under specific conditions in other places, like stars, or confined in microscopic objects, like the nuclei of atoms. Using particle colliders, we are simply creating the right conditions for them to be observed in a laboratory. Although today’s particle accelerators are impressive, they don’t compare to natural particle accelerators found in outer space. Cosmic rays from outside the Milky Way create much more energetic collisions in the Earth’s upper atmosphere than our most powerful colliders. Conducting experiments on particles In a circular collider, two beams of accelerated particles smash into one another at a collision point. From the energy of that collision, new particles arise, which physicists can catch and measure. By identifying and measuring these particles, physicists can infer how they were generated. For that, particle physics experiments use many layers of different types of detectors placed around the collision point, since different particles require different detectors (different particles act differently when traveling through matter!). Detect a particle and measure its trajectory By measuring the curve taken by the charged particle in the magnetic field we can infer its energy. Reset Start! In the simplified visualization above, three layers of detectors are placed one after the other. A magnetic field permeates the space around them. When a charged particle comes, the magnetic field bends its trajectory, as seen above, and makes it traverse the three detectors at different position. The detectors record the passage of the particle and send the information to the computers which handle the experimental data. From those data, scientists can reconstruct the trajectory of the particle in the experiment and, by knowing the strength of the magnetic field, the energy of the particle. Modern particle physics experiments can be very large due to the amount of energy involved smashing particles together. When a collision occurs, many new particles are created which move outward from the collision at extremely fast speeds in all directions. To catch and measure these particles properly, scientists use layers of detectors and magnets placed around the collision point, so that they can reconstruct the particle trajectory after the collision for analysis.

