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The Search For Quark

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What exactly is Quark? Quark: a fermion which is believed to be one of the fundamental constituents of matter. All quarks have a fractional electric charge1. This pretty much means quarks have Ð... spin (rotate two full rotations to get to place it started), apply to Pauli Exclusion Principle, is one of the things that make up all matter, and its electric charge is a fraction. There are three different colors of quark; red, green, and blue. The colors always up to white. Also there are three different kinds of antiquark; cyan, yellow, and magenta. Quarks are at least 330MeV.

Quarks were first proposed in 1964. It was named quark by Caltech theorist Murray Gell-Mann. He named them that from a quotation in a novel "Three quarks for Muster Mark, Sure he hasn't got much of a bark ..."2 Gell-Mann said all mesons, baryons, and hadrons are made of quarks. He also said they are made of three types of quarks (up, down, and strange). That makes a total of nine types of quarks. George Zweig called them aces. Not many people believed in it at this time. From 1968 to 1973 MIT bombarded protons and neutrons with electrons. Electrons ricocheted off protons and neutrons as if it hit a hard, tiny object. The hard object was a quark. Over the years experiments and researches have led to a lot of indirect evidence that quarks exist.

Despite all this indirect evidence they could not find a single free quark. No particle detector detected one. This led to a lot of non believers. As more proof has been shown that quarks exist it became more popular and less doubted.

Chapter 1: Over coming Skepticism

Doubters did not believe in quarks. They thought of quarks just as a math equation that could explain a couple of things. They had good reason. The quark was never found free or even revealed itself.

That was until 1974 when two discoveries occurred at the Brookhaven Laboratory and Stanford. They had found a new particle. Stanford called it the psi and Brookhaven called it the J. The new particle had to be a new kind of quark. Two years later Harvard theorist Sheldon Glashow named the new particle the charmed quark. This discovery shattered any doubts about the quark being real or not.

The discovery also shattered the bootstrap model theory. This theory said that protons, neutrons, and other particles were the smallest units. From 1964 to 1976 this theory was very popular. Research associate Michael Riordan said "Supposedly we had finally reached the innermost layer of the cosmic onion, the lowest rung of the quantum ladder, where every particle was built from all others"3.

In 1973 theorists proposed a new force might explain how quarks are locked together. The theory was known as Quantum chromodynamics. The theory gave a natural reason for why quarks seemed only to exist inside protons, neutrons, and other particles.

There was a problem with the first quark model. The original model meant that you had two exact quarks in the same quantum state. This violated the Pauli Exclusion Principle.

O.W. Greenburg proposed that there were 3 triplets of the fractionally charged paraquarks. This meant you could build up baryons from three paraquarks and not violate the Pauli Exclusion Principle. This was not well accepted.

If quarks did exist people thought they would have to be really slow. It also has to be very big. The uncertainty principle proposed that slow quarks could only fit in tiny spaces if they are big. People thought that it had to be 4 to 10GeV.

There was a problem with that. There a three quarks in a proton. This would make it 12 to 30GeV for a single proton. A proton only has a mass of less than 1GeV. Quarks have to be packed tightly in protons. To take them apart there has to be lots of energy. Binding energy makes it negative towards the total mass. It represents energy that is needed to make quarks free. Binding energy has to be around -11 to negative -29GeV. This is probably why no one has seen a free quark.

Chapter 2 Instruments to find Quarks

There are many ways to detect quarks. There are chambers and other devices used in detecting quark. Some devices help in the search for quarks.

In 1899 C.T.R Wilson at Cambridge found a way of measuring ionization. It is a called a cloud chamber. Moist, dust free air is saturated with water vapor. A diaphragm expands the air in the chamber. The air cools and the water vapor condenses. Water droplets form on any ions present in the chamber. If too saturated water droplets will form anywhere. If not saturated at all droplets will not form. When droplets grow big enough they can be photographed and counted.

The bubble chamber is similar to the cloud chamber. It is a big vessel filled with a hot liquid. It detects ionized particles that pass through it. When a particle enters the pressure is decreased by a piston. This heats the liquid. The particle boils along the path and forms a string of bubbles. A camera is on the top of the chamber and takes a picture. Charged particles travel in a twisted path. The path is determined by the ratio of charge and mass of the particle. This means the mass can be measured. The pressure is returned to normal.

Photographic emulsions make particle tracks visible. They are continuously sensitive though. This is good for other things but not for finding quarks. It should be triggered only when a particle that could be a quark passes through. It would only detect quarks with charge 2/3e. Anything smaller is too hard to spot.

Neon flash tubes, invented by Marcello Conversi, are very simple. It is a long glass tube filled with neon. If it is between to electrodes it will glow when a high voltage pulse is applied to the electrodes after a charged particle has passed through the tube. The ionization produced by the particle makes the tube glow. If you have a lot of the tubes a two-dimensional layout of the path of a particle is shown and can be photographed by a camera at the end. It is good at singling out quarks.

The neon tube detectors led to the invention of spark chambers. A spark chamber has two parallel electrodes in an appropriate gas and pressure. If a high voltage pulse is applied across electrodes after a particle has passed, a spark will follow the ionized column of gas left by the particle. Can determine where spark is by photographing the chamber from two different angles. By using a few spark chambers spread out you can determine the course of a particle. The spark chamber is not good for figuring out the ionization made by a particle. It has been used though in quark searches to find out the path of the quark.

A scintillator measures the ionization of a particle. It is a combination of a block of scintillating material



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