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Particle Decays and Annihilations

Particle decay refers to the transformation of a fundamental particle into other fundamental particles.

In the late 1800s the German physicist, Wilhelm Röntgen, discovered a strange new ray produced when an electron beam struck a piece of metal. Since these were rays of an unknown nature, he called them "x rays". Later on, Becquerel realized that some materials, which included uranium, emitted energetic rays without any energy input. He found out that some elements are inherently unstable, because these elements would spontaneously release different forms of energy. This release of energetic particles due to the decay of the unstable nuclei of atoms is called radioactivity. The three types of radioactivity are named alpha (+), beta (-), and gamma (neutral).

There is a tiny, tiny chance that a conglomeration of two protons and two neutrons (which form an alpha particle) may, at the same instant, actually migrate outside the nucleus. The idea that "if it can happen, it will happen!" is fundamental to quantum mechanics. For some atoms there is a certain probability that it will undergo radioactive decay due to the possibility that the nucleus may --for the shortest of instants-- exist in a state that allows it to blow apart.

When nuclei undergo radioactive decay, some of their mass is converted into kinetic energy (the energy of the moving particles). This conversion of energy is observed as a loss of mass.

It turns out that when a fundamental particle decays, it changes into a less massive particle and a force-carrier particle. In many cases, these temporary force-carrier particles seem to violate the conservation of energy. However, these particles exist so briefly that, because of Heisenberg's Uncertainty Principle, no rules are broken. These are called virtual particles. The kinetic energy plus mass of the initial decaying particle and the final decay products is equal.

There are three different types of the decay of fundamental particles: weak, electromagnetic, and strong. Only weak interactions can change a fundamental particle into another type of particle.

A neutron (udd) decays to a proton (uud), an electron, and an antineutrino. This is called neutron beta decay.

When an electron and positron (antielectron) collide at high energy, they can annihilate to produce charm quarks which then produce D+ and D- mesons. This process is called electron/positron annihilation.

A quark (from within a proton) and an antiquark (from an antiproton) colliding at high energy can annihilate to produce a top quark and a top antiquark, which then decay into other particles. This process is called top production.

Unsolved Mysteries

While the Standard Model provides a very good description of phenomena observed by experiments, it is still an incomplete theory. Isaac Newton's laws of mechanics are not wrong, per se, but his theory only works as long as velocity is much smaller than the speed of light.

There are three "sets" of quark pairs and lepton pairs. Each "set" of these particles is called a generation, or family. The up/down quarks are first generation quarks, while the electron/electron neutrino leptons are first generation leptons. In the every-day world we observe only the first-generation particles (electrons and up/down quarks). We do not know why the natural world "needs" the two other generations, and we do not know why there are exactly three generations in total.

Physicists have theorized the existence of the so-called Higgs field, which in theory interacts with other particles to give them mass. The Higgs field requires a particle, the Higgs boson. The Higgs boson has not been observed, but physicists are looking for it with great enthusiasm.

Today, one of the major goals of particle physics is to unify the various fundamental forces in a Grand Unified Theory which could offer a more elegant understanding of the organization of the universe.

Every fundamental matter particle should have a massive "shadow" force carrier particle, and every force carrier should have a massive "shadow" matter particle. This relationship between matter particles and force carriers is called supersymmetry. For example, for every type of quark there may be a type of particle called a "squark."

String Theory, one of the recent proposals of modern physics, suggests that in a world with three ordinary dimensions and some additional very "small" dimensions, particles are strings and membranes.

We infer from gravitational effects the presence of this dark matter, a type of matter that we cannot see. There is strong evidence that it might not be made up of protons, neutrons, and electrons. Perhaps it is composed of neutrinos, or even more exotic forms of matter, like neutralinos, one of the theoretical supersymmetric particles.

How Do we Detect What's Happening

Photons, like all particles, have wave characteristics. For this reason, a photon carries information about the physical world because it interacts with what it hit.

However, the wavelength of visible light is too wide to analyze anything smaller than a cell. To observe things under higher magnification, you must use waves with smaller wavelengths. That's why people turn to scanning electron microscopes when studying sub-microscopic things.

As the probe size got smaller, the images became "sharper". This quality of "sharpness" is called the resolution of an image.

Physicists can't use light to explore atomic and sub-atomic structures because light's wavelength is too long. A particle's momentum and its wavelength are inversely related. High-energy physicists apply this principle when they use particle accelerators to increase the momentum of a probing particle, thus decreasing its wavelength.

How Do we Interpret Our Data

Modern detectors consist of many different pieces of equipment which test for different aspects of an event. These many components are arranged in such a way that physicists can obtain the most data about the particles spawned by an event.

The reason that detectors are divided into many components is that each component tests for a special set of particle properties. These components are stacked so that all particles will go through the different layers sequentially.

The inner parts of the detector, especially the tracking device, are in a strong magnetic field. The signs of the charged particles can easily be read from their paths, since positive and negative particles curve in opposite directions in the same magnetic field.

The momenta of particles can be calculated since the paths of particles with greater momentum bend less than those of lesser momentum. This is because a particle with greater momentum will spend less time in the magnetic field or have greater inertia than the particle with lesser momentum, and thus bends less in a magnetic field.

When e- and e+ beams meet and are annihilated. The resulting quarks and antiquarks combined to produce mesons and baryons, whose tracks are shown. This is called a quark/gluon event

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