The Search for Quark Essay

This essay has a total of 3484 words and 15 pages.

The Search for Quark

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 ½ 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 and a photomultiplier. The photomultiplier changes weak flashes of
light into an electric pulse and it amplifies it. It can count single protons. It is a
very fast particle counter. It can also tell the difference between particles with
different ionization.

Chapter 3 How to Find Quarks
The instruments in chapter 2 have to be used somewhere. Scientists and theorists look in
specific places for them. There are a few methods of searching for quark.

The first search for quarks in accelerators was in 1964. An accelerator uses electric
fields to propel charged particles to big energies. The CERN intersecting storage rings
became available. This accelerator collides together to beams of protons moving in
opposite directions. Two tubes carry the proton beams. The protons are bent into a
circular path using magnets. The two beams then clash. Experiments have had the
accelerator at energies of 1500 to 2000GeV. None of the new experiments found quarks
either. Telescopes are placed to detect the quarks in these high energy interactions. The
telescopes have scintillation counters. The counters are used to determine where the
particles are as they pass the telescope. The courses of the particles are straight lines
because no magnetic fields are used. Two experiments were done using this method, one led
by Fabjan and the other led by Antonio Zichichi, and they found two particles that could
have been a quark. It had all the characteristics that a quark was supposed to have. But
since it was only two events that might have found a free quark it was not that big of a
deal.

Scientists in the 1960's used a revised version of the Rutherford scattering experiment to
search for quarks. Rutherford probed the atom using a beam of tiny entities. The beam was
changed into one of high energy electrons. This method was used with the Linear
Accelerator at Stanford. Electrons travel on an electromagnetic wave. They collide with
the protons and their scattering angles are measured.

The last method is to look for new particles and see if the fitted the description for a
quark. A lot were found that did fit. When one did not fit the description of a quark but
it could an extended model for a quark that contained different types of quarks they made
a new model.


Chapter 4 Unsuccessful Experiments
Lots of experiments were tried to find a free quark. All of the experiments were unsuccessful.

The first search for quarks in accelerators was in 1964. The two accelerators used were
the CERN proton synchrotron and the Brookhaven synchrotron. They looked for quarks with a
charge of 1/3e or 2/3e. They did not find anything.

A team of people from Columbia University led by Leon Lederman realized that if they took
advantage of Fermi motion or internal motion of protons and neutrons they could make
bigger particles. In 1965 they slammed proton beam into copper and studied the debris off
a five degree angle. They looked for heavy particles and quarks with whole number charges.
They did not find anything. All they found was evidence of an antideuteron. If quarks did
exist they had to be heavier than five GeV.

In 1965 five CERN scientists led by Antonino Zichichi looked for fractionally charged
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