Particle Physics
Particle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them. So Particle physics is the study of the basic elements of matter and the forecasting among them. It aims to determine the fundamental laws that control the make-up of matter and the physical universe.
It is also called "high energy physics", because many elementary particles do not occur under normal circumstances in nature, but can be created and detected during energetic collisions of other particles, as is done in particle accelerators.
Modern particle physics research is focused on subatomic particles, which have less structure than atoms.
These include atomic constituents such as electrons, protons, and neutrons ( protons and neutrons are actually composite particles, made up of quarks ), particles produced by radiative and scattering processes, such as photons, neutrinos, and muons, as well as a wide range of exotic particles.
Quarks emit gluons
Color charge is always conserved. ( Quantum Chromodynamics )
When a quark emits or absorbs a gluon, that quark's color must change in order to conserve color charge. For example, suppose a red quark changes into a blue quark and emits a red/antiblue gluon (the image below illustrates antiblue as yellow). The net color is still red. This is because - after the emission of the gluon - the blue color of the quark cancels with the antiblue color of the gluon. The remaining color then is the red color of the gluon.
Quarks emit and absorb gluons very frequently within a hadron, so there is no way to observe the color of an individual quark. Within a hadron, though, the color of the two quarks exchanging a gluon will change in a way that keeps the bound system in a color-neutral state.
Gluons are elementary particles that act as the exchange particles (or gauge bosons) for the strong force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles.
In technical terms, gluons are vector gauge bosons that mediate strong interactions of quarks in quantum chromodynamics ( QCD ). Gluons themselves carry the color charge of the strong interaction. This is unlike the photon, which mediates the electromagnetic interaction but lacks an electric charge. Gluons therefore participate in the strong interaction in addition to mediating it, making QCD significantly harder to analyze than QED ( quantum electrodynamics ).
The names given to the different flavors of quarks are arbitrary. There is no particular reason for these names, and most scientists just use symbols such as 'u' for up quarks and 'd' for down quarks. Scientists first discovered two quarks with different electric charges and named them up and down. Strange quarks were discovered next and they lived a lot longer than the up and down quarks before they decayed. Their name comes from their "strangely" long lifetime. The origin of the charm quark name is because of a whim, I like to think it was because it made the mathematics in the theory work like a charm. The next two quarks to be discovered had similar electrical properties to down and up quarks, and so were called bottom and top. In the past, some called the bottom quark – beauty, and the top quark – truth. Today, most scientists refer to them as bottom and top.
Strictly speaking, the term particle is a misnomer because the dynamics of particle physics are governed by quantum mechanics.
As such, they exhibit wave-particle duality, displaying particle-like behavior under certain experimental conditions and wave-like behavior in others ( more technically they are described by state vectors in a Hilbert space).
All the particles and their interactions observed to date can be described by a quantum field theory called the Standard Model.
The Standard Model has 40 species of elementary particles ( 24 fermions, 12 vector bosons, and 4 scalars ), which can combine to form composite particles, accounting for the hundreds of other species of particles discovered since the 1960s.
Mesons are intermediate mass particles which are made up of a quark-antiquark pair. Three quark combinations are called Baryons. Mesons are bosons, while the baryons are fermions. Recent experimental evidence shows the existence of five-quark combinations which are being called pentaquarks.
In the present standard model, there are six "flavors" of quarks. They can successfully account for all known mesons and baryons (over 200). The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks.
A fermion is any particle that has an odd half-integer (like 1/2, 3/2, and so forth) spin. Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions.
For reasons we do not fully understand, a consequence of the odd half-integer spin is that fermions obey the Pauli Exclusion Principle and therefore cannot co-exist in the same state at same location at the same time.
Bosons are those particles which have an integer spin (0, 1, 2...).
All the force carrier particles are bosons, as are those composite particles with an even number of fermion particles (like mesons).
The nucleus of an atom is a fermion or boson depending on whether the total number of its protons and neutrons is odd or even, respectively. Recently, physicists have discovered that this has caused some very strange behavior in certain atoms under unusual conditions, such as very cold helium.
The neutral pion decays to an electron, positron, and gamma ray by the electromagnetic interaction on a time scale of about 10-16 seconds. The positive and negative pions have longer lifetimes of about 2.6 x 10-8 s.
The negative pion decays into a muon and a muon antineutrino.
The pion, being the lightest meson, can be used to predict the maximum range of the strong interaction. The strong interaction properties of the three pions are identical. The connection between pions and the strong force was proposed by Hideki Yukawa. Yukawa worked out a potential for the force and predicted its mass based on the uncertainty principle from measurements of the apparent range of the strong force in nuclei.
Being composed of an up and an antidown quark, the positive pion would be expected to have a mass about 2/3 that of a proton, yet it's mass is only about 1/6 of that of the proton! This is an example of how hadron masses depend upon the dynamics inside the particle, and not just upon the quarks contained.
The pion is a meson. The p+ is considered to be made up of an up and an anti-down quark. The neutral pion is considered to be a combination.
Pions interact with nuclei and transform a neutron to a proton or vice versa.
The pions p+ and p- have spin zero and negative intrinsic parity.
At Caltech Murray Gell-Mann helped to lay the foundations for our understanding of the components that make up matter. He drafted a blueprint of subatomic physics that he called the Eightfold Way. At the time, physicists understood that atoms are constructed from protons and neutrons, but they had also found many other mysterious particles. The Eightfold Way made sense of this baffling menagerie, finding within it places for particles never even imagined. The work was so important that it netted Gell-Mann a Nobel Prize in 1969.
Accelerators :
The accelerator is the basic tool of particle physics. It allows us to create the particle collisions that we want to study in our own laboratories. The highenergy collisions between particles that physicists are interested in do occurnaturally but the events are unpredictable and the number that can be observed (in cosmic rays) is low.
Accelerators work by accelerating charged particles using electric fields. Alinear accelerator accelerates particles in a straight line: the biggest linearmachine, in Stanford, California, is two miles long. Circular machines are morecommon. As well as accelerating the particles using an electric field, circularaccelerators bend their p aths using a magnetic field. In a machine like LEP atCERN, where they have opposite charges, the particles being accelerated travelin opposite directions until they are forced to collide. The drawback is thatthe faster a particle travels, the harder it is to keep it moving in a circlebut, in the largest circles (LEP is the largest in the world with acircumference of 27km) less energy is wasted when accelerating particles to highspeeds.
The measurement was made using the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland, and the Tevatron at Fermilab in Batavia, Ill. Four separate experiments found a joint value for the top quark of 173.34 (+/- 0.76) gigaelectronvolts divided by the speed of light squared
Detectors :
Detectors are used to examine tracks made by the new particles that are producedwhen accelerated particles collide. In the early days photographic film, sparkchambers and bubble chambers were used. Since the late 1960s electronicdetectors have taken over. There are two basic kinds - tracking detectors whichreveal the trajectories of individual charged particles, and calorimeters whichmeasure energies. A modern electronic detector is built like an onion, withlayers of trackers and calorimeters to give as much information as possibleabout the particles produced in each collision.
Antimatter :
Antimatter is very much like ordinary matter, but it carries the oppositecharge. An anti-electron (a positively charged electron) is just another way of describing a positron.Crashing matter and antimatter together is now a daily occurence in machines like LEP. The fact that the universe seems to be full ofmatter and not antimatter is one of the most baffling problems in modern physics. At the time of the Big Bang, matter and antimatter are believed to have been produced in equal quantities. What seems to have happened is that, at a somewhat later time, collisions between the two types have destroyed all the antimatter but left a little of the matter behind, from which our universe ismade. The reason may be due to a tiny asymmetry in the way particles of matter and antimatter decay, thereby creating an excess of matter. Every particle we talked about above, both quarks and leptons, has an anti-particle. The anti-particle is exactly the same as the particle (i.e. same mass) but all the properties are opposite. For example, an up quark has a charge of +2/3, so an anti-up quark would have a charge of -2/3.
The anti-matter quarks can interact and form new particles in the same way the quarks do. The difference is that now you need a quark and an anti-quark to make a particle, instead of the three quarks like we stated above.
When a particle meets its anti-particle, they annihilate and release a large amount of energy
Dark Matter
We know from observing the rotation of galaxies that about 90% of the matterthey contain is invisible to us. The matter we can't see is called "missing" or"dark" matter. The amount of dark matter contained in the universe is crucialto its fate. If it is greater than a certain amount, the universe willeventually collapse. Below this, and it will keep on expanding for ever.
There are many ideas about what dark matter might be, ranging from exotic new paricles to black holes. One idea says that the neutrino,an abundant fundamental particle which is thought to have zero mass, actuallyhas a tiny mass. However, neutrinos generally move about the universe quickly and are not stuck together in clumps, as they would need to be to explain the rotation of the galaxies. The most recent explanations of dark matter therefore use a combination of "hot" matter, like neutrinos, and "cold" matter like black holes. The true answer has yet to be found. Underground experiments on dark matter are taking place now.
Quarks are the fundamental building blocks of nature. They combine to form larger particles, such as protons and neutrons. There are six different types of quarks:
up, down, charm, strange, top, and bottom. If you look at the table above, the mass of the particle increases as you go to the right, meaning the top quark is much heavier than the up quark. Quarks in the top row have a charge of +2/3 of an electron's charge (where e = 1.9 x 10-19 C) and quarks in the second row have a charge of -1/3 of an electron's charge. You may think this is strange because you were taught that you can't have a fraction of the charge on an electron. Your instinct is correct: because of this fractional charge, quarks can not exist independently; they must combine to form larger particles.
Quarks combine to form most of the matter in the universe. In fact, most of the matter in the universe is made from just two quarks: the up and the down. For example, a neutron is made of two down quarks and one up quark (add the charges, it makes sense! -1/3e + -1/3e + 2/3e = 0) and a proton is made of two up quarks and a down quarks (+2/3e + 2/3e -1/3e = 1e). In general, you need three quarks to make a particle, so that the charge always adds up to a whole number.
What are leptons ?
Leptons are the six particles at the bottom of the periodic table. The bottom row shows the electron, the muon, and the tau particles. You are probably familiar with the electron; the muon and the tau are the heavier, less well known cousins to the electron. The muon and tau are rarer than the electron, have the same negative charge that the electron has. The top row shows the neutrinos. There is one neutrino for each of the electron, the muon, and the tau. Leptons are much lighter than the quarks, in fact the neutrinos are so light that there is debate whether they have mass at all!
The tau (t), also called the tau lepton, tau particle or tauon, is an elementary particle similar to the electron, with negative electric charge and a spin of 1/2. Together with the electron, the muon, and the three neutrinos, it is classified as a lepton. Like all elementary particles, the tau has a corresponding antiparticle of opposite charge but equal mass and spin, which in the tau's case is the antitau (also called the positive tau). Tau particles are denoted by t- and the antitau by t+.
Tau leptons have a lifetime of 2.9×10-13 s and a mass of 1776.82 MeV/c2 (compared to 105.7 MeV/c2 for muons and 0.511 MeV/c2 for electrons). Since their interactions are very similar to those of the electron, a tau can be thought of as a much heavier version of the electron. Because of their greater mass, tau particles do not emit as much bremsstrahlung radiation as electrons; consequently they are potentially highly penetrating, much more so than electrons. However, because of their short lifetime, the range of the tau is mainly set by their decay length, which is too small for bremsstrahlung to be noticeable: their penetrating power appears only at ultra high energy (above PeV energies)
What is a muon ?
A muon is a negatively charged particle, similar to an electron, but about 200 times heavier. Muons fall into the class of particles known as leptons.
Most muons come from what are known as cosmic rays. What are cosmic rays? Cosmic rays are caused by high energy protons from stars in outer space that interact with the Earth's atmosphere. Scientists are unsure of the origin of the highest energy cosmic rays. As they fall toward the earth, they ionize the atmosphere forming a shower of matter and anti-matter particles. These particles are known as pions (p) and are made up of up and down quarks and anti-quarks. You may have never heard of a pion before and that is because they don't last very long. They quickly decay into lighter things, such as leptons and electromagnetic radiation. This is where our muons come from: they are the results of an interaction between a proton and the atmosphere that produces a particle that decays into a muon, among other things. Other leptons, such as electrons and neutrinos are also emitted, but the muons have a higher energy so are more likely to make it down to the Earth's surface. These showers are happening all the time. About 600 particles pass through your body each minute!
We said about that the muons and electrons were negatively charged, so the positive charge indicated that it is the anti-particle. As far as decays are concerned, the anti-particle behaves the same way as the particle would.
Why study muons ?
The simple answer is that we study muons because we can. Muons from cosmic rays are relatively easy to detect. The telescopes are portable and can be used in the classroom for a variety of experiments. By studying the cosmic rays, we can find how the rate that muons are detected depends on the weather, direction, or extra-terrestrial events, such as solar flares. The question to answer in your research project is: why do these factors affect the muon rate?
How can we detect cosmic rays ?
Cosmic rays, and muons in particular, are hard to detect because they are traveling very fast and pass through most materials without interacting. The trick to detecting them is to take advantage of the fact that they are charged particles. When a charged particle passes through a particular substance it can ionize the surrounding particles and leave a trail. For example, in a cloud chamber, the air is cooled to the point that when an atmospheric particle is ionized, it will cause the air to condense and thus leaves a visible trail. With the cloud chamber, you can see both muons and electrons, but to the untrained eye, it is hard to tell the difference.
Neutrinos are some of the most abundant particles of the universe, but relatively little is known about them.
Japanese Scientist Hideki Yukawa was born in Tokyo, Japan, on 23rd January, 1907, who later became Professor of Geology at Kyoto University. While at Osaka University, in 1935, he published a paper entitled "On the Interaction of Elementary Particles. I." in which he proposed a new field theory of nuclear forces and predicted the existence of the meson. Encouraged by the discovery by American physicists of one type of meson in cosmic rays, in 1937, he devoted himself to the development of the meson theory, on the basis of his original idea. Since 1947 he has been working mainly on the general theory of elementary particles in connection with the concept of the "non-local" field.
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