To allow for the storage of all this data, a worldwide grid has been created that uses tens of thousands of regular computers. This distribution of the data allows for a much greater processing capacity than could ever be achieved by a couple of supercomputers.
The other benefit is now that the data are capable of being stored all over the world; physicists do not need to be at a central location (for instance CERN), in order for them to analyze the particle events coming from CMS.
Figure 4.16
Simulation of a Higgs Boson Event in CMS Detector
The Little Neutral One
In the early 1900s a puzzling problem developed as a result of the extensive experimentation with radioactivity. When physicists looked at beta decay, they soon realized that the energy of the ejected electron was not what they expected. When a neutron decays into a proton and a neutron, due to conservation of momentum the two ejected particles should travel in opposite directions. Researchers found that this was not the case in every event. Also, they were able to determine the resulting energy of the electron and that it did not measure to be what they expected…the electron did not emerge with the same kinetic energy every time.
Figure 4.17
The Beta Decay Dilemma
This posed a serious problem for the scientists. They could choose to ignore the basic laws of physics or assume that one or more additional particles were emitted along with the electron. In 1930, Wolfgang Pauli proposed that a third particle, the neutrino (little neutral one), was involved. Due to the conservation laws, he was even able to predict its properties. The neutrino must be neutral and the neutrino’s rest mass must be very, very small.
Although many scientists did not expect it to take long for the neutrino to be detected, it took over 25 years for their existence to be confirmed. In 1956, Clyde Cowan and Frederick Reines finally detected neutrinos using radiation coming from the Savannah nuclear reactor. The properties of the neutrino were confirmed through the study of the results of this experiment. An interesting note is that the reason it took so long for the neutrino to be detected, and continues to be quite elusive to detect to this day, is due to the fact that the neutrino’s interaction with other particles is so weak that only one of a trillion neutrinos passing through the Earth is stopped.
Hadrons
With the explosion of new particles being detected from the 1950s to present times, it might appear that once again the simplified model of the early 1900s has become more complicated. It got so bad when over new particles were identified, that physicists started referring to it as the particle zoo. It isn't quite as bad as that, though.
Just like zookeepers build order in their zoos by grouping the animals based on biological categories like genus and species, particle physicists started looking for a way to group all the particles into categories of similar properties. The observed particles were divided into two major classes: the material particles and the gauge bosons. We'll discuss the gauge bosons in another section. Another way to divide the particles was through the interactions in which they participated. The material particles that participate in the strong force are called hadrons and particles that do not participate in the strong force are called leptons. The strong force is one of the fundamental forces of nature. A discussion of the properties of the leptons may be found later in this chapter.
Most of those particles are mesons and baryons, or, collectively, hadrons. The word hadron comes from the Greek word for thick. Most of the hadrons have rest masses that are larger than almost all of the leptons. Hadrons still are extremely small but, due to their comparatively large size particle, physicists think that hadrons are not truly elementary particles. Hadrons all undergo strong interactions. The difference is that mesons have integral spin , while baryons have half-integral spin . The most familiar baryons are the proton and the neutron; all others are short-lived http://hyperphysics.phy-astr.gsu.edu/Hbase/particles/baryon.html#c1 [Table of Baryons]. The most familiar meson is the pion; its lifetime is nanoseconds, and all other mesons decay even faster http://hyperphysics.phy-astr.gsu.edu/Hbase/particles/meson.html#c1 [Table of Mesons].
Quarks
The rapid increase in the number of particles soon led to another question: Is it reasonable to consider that all of these particles are fundamental? Or, is there a smaller set of particles that could be considered fundamental? To many physicists the idea of something even smaller making up hadrons seemed to be reasonable as experimental evidence supported the notion that the hadrons had some internal structure. In 1964, the most successful attempt to build the hadrons, the quark model, was developed by Murray Gell-Mann and George Zweig.
Figure 4.18
Quark Combinations for Various Hadrons
The original quark model started with three types, or flavors, of quarks (and their corresponding antiquarks). The first three quarks are currently called up , down , and strange . Each of these quarks has spin , and—the most radical claim of the model—a fractional charge when compared to the elementary charge of an electron. The fractional charge of the quark should make the quarks easy to find, but that has not been the case. No single quark has ever been detected in any particle experiment. Regardless, the quark model has been very successful at describing the overall properties of the hadrons.
In order to make a hadron, the quarks must be combined in a very specific way. The baryons are all made up of three quarks (the antibaryons are made up of three antiquarks). As an example, the proton is made up of two up quarks and one down quark and a neutron consists of two down quarks and one up quark. The mesons are all made up of one quark and one antiquark. For example, the positive pion is made up of one up quark and one anti-down quark. To make a particle out of quarks, or to determine the quarks of a known particle, it is simply a matter of checking the particle and quark properties in a chart and using some simple addition [make a Hadron Applet].
In 1974, a new particle was discovered that could only fit the quark model if a fourth quark was added. The quark was given the name charm . In 1977, a fifth quark was added, bottom , and finally in 1995 the existence of a sixth quark was confirmed, top . The six quarks of the quark model have all been verified and supported by experiments, but the existence of more quarks is still an open question in particle physics.
Flavor Symbol Charge
Down
Up
Strange
Charm
Bottom
Top
Leptons
At almost the same time that the quark model was being developed another group of particles appeared to have a similar symmetry with the quarks. These particles, called leptons (Greek for light), appeared to be fundamental and seemed to match up in number to the quarks. Leptons are particles that are like the electron: they have spin , and they do not undergo the strong interaction.
There are three flavors of charged leptons: the electron, the muon, and the tau. They all have negative charge, and with the exception of the tau, are less massive than hadrons. The electron is the most stable and can be found throughout ordinary matter. The muon and the tau are both short-lived and are typically only found in accelerator experiments or cosmic ray showers. Each charged lepton has an associated neutral lepton partner. They are called the electron neutrino, the muon neutrino, and the tau neutrino. Neutrinos have almost zero mass, no charge, interact weakly with matter, and travel close to the speed of light. Each of these six particles has an associated antiparticle of opposite charge, bringing the total number of leptons to twelve.
Flavor Symbol Rest Mass
Electron
Electron neutrino
Muon
Muon neutrino
Tau
Tau neutrino
Conservation Laws
Conservation laws apply in the particle world just as much as they apply in the macroscopic world. The conservation of momentum, mass-energy, angular momentum, and charge are all required by the particle events that have been discovered over the past
100 years. The importance of these conservation laws allowed for the prediction of the neutrino, as we saw earlier in this chapter. Any reaction that occurs must satisfy these laws. Look at the following two possibilities for beta decay:
Which of the two decays will actually occur? What conservation law(s) does the other decay violate?
The conservation of mass-energy is a little tricky. Due to Einstein’s principle of mass-energy equivalence, mass may be converted into energy and vice versa. Because energy can be converted into mass, when two moving particles collide it is possible that the incident kinetic energy will be converted into mass during the collision. In this case, the masses of the product particles may be greater than the masses of the incident particles. So, it is very difficult to determine if mass-energy is conserved in a particle interaction, because there is no way of knowing just how much kinetic energy each particle has to start with and how much of that energy is converted into mass. Although, typically when a particle decays into other particles, it can be shown that the sum of the masses of the product particles will be smaller than or equal to the rest mass of the particle that decayed.
As more and more particles were discovered and more and more particle events analyzed it became increasingly clear that more conservation laws were necessary to help explain what was seen, and maybe more importantly, what was not seen. One of the most important of these is the conservation of baryon number. Each of the baryons is assigned a baryon number , antibaryons a baryon number , and all other particles a value of . In any reaction the sum of the baryon numbers before the interaction or decay must equal the sum of the baryon numbers after. No known decay process or interaction in nature changes the net baryon number. For example, suppose a positive pion collided with a neutron, which result could not happen?
Because the baryon number in the first interaction is before and after, this interaction could occur. But, the second interaction has a baryon number before and a baryon number of zero after, so this interaction cannot take place. The decay of a proton could not proceed by the following event, because the baryon number is not conserved. As a matter of fact, because the proton is the baryon of smallest mass it may not decay at all. Conservation of baryon number would require that any product of proton decay to have greater mass than the proton, and this would not be allowed due to conservation of mass-energy. As physicists continue to explore the particle world new discoveries may be made and new conservation laws may be created to allow for the decay of a proton, but for now a proton is considered stable. Also, there is not a conservation of meson number. Mesons can be involved in any particle event as long as they do not violate the other conservation laws.
There is a conservation law for leptons, but it is slightly more complicated than for the baryons. To first see how the lepton number is conserved; let us look at this variation of beta decay:
This event has never been observed, but according to all the other conservation laws there is no reason that it could not be. Conservation of lepton numbers require that all leptons and corresponding neutrinos be assigned a lepton number of , the antileptons and antineutrinos a lepton number of , and all other particles a lepton number of . Looking at the example above, the lepton number before the event is and the lepton number after the event is , so lepton number is not conserved. How could you conserve lepton number and make a valid reaction in the decay shown above?
A look at the following decay shows that there is a little bit more to the conservation of lepton number:
Following the rules of lepton number conservation, the preceding example could be observed, but it never has been. There must be something more to the conservation of lepton number and that is each lepton and neutrino partner are assigned its own specific number. So, there is a separate conservation of electron lepton number, muon lepton number, and tau lepton number. Because there are actually three lepton numbers that need to be conserved, the above example will not happen. If this reaction were to take place, electron lepton number and muon lepton number are both not conserved. The decay begins with an electron lepton number of and ends with an electron lepton number of ; also it begins with a muon lepton number of and ends with a muon lepton number of . Clearly, this decay cannot proceed because it violates not one, but two lepton conservation laws.
A summary of the lepton numbers is shown in the table below (Note: all of the anti leptons have a lepton number of )
Lepton Conserved Quantity Lepton Number
Fundamental Interactions
There are four fundamental forces within all atoms that dictate interactions between individual particles and the large-scale behavior of all matter throughout the universe. They are the strong and weak nuclear forces, the electromagnetic force, and gravity.
Gravitation is a force of attraction that acts between each and every particle in the universe. Gravity is the weakest of all the fundamental forces. However, the range of gravity is unlimited—every object in the universe exerts a gravitational force on everything else. The effects of gravity depend on two things: the mass of two bodies and the distance between them. In more precise terms, the attractive force between any two bodies is directly proportional to the product of the masses and inversely proportional to the square of the distance between the bodies. It is always attractive, never repulsive. It pulls matter together, causes you to have a weight, apples to fall from trees, keeps the Moon in its orbit around the Earth, the planets confined in their orbits around the Sun, and binds together galaxies in clusters.
The electromagnetic force determines the ways in which electrically charged particles interact with each other and also with magnetic fields. Like gravity, the range of the electromagnetic force is infinite. Unlike gravity, electromagnetism has both attractive and repulsive properties that can combine or cancel each other out. Whereas gravity is always attractive, electromagnetism comes in two charges: positive and negative. Two positive or two negative things will repel each other, but one positive and one negative attract each other. The same rule applies for magnets, as well, and can be easily demonstrated when two magnets are placed near each other. A north pole near a north pole will cause a repulsive force and a north pole placed near a south pole will cause an attractive force to develop. The electromagnetic force binds negatively charged electrons into their orbital shells, around the positively charged nucleus of an atom. This force holds the atoms together.
The strong nuclear force binds together the protons and neutrons that comprise an atomic nucleus and prevents the mutual repulsion between positively charged protons from causing them to fly apart. The strong force is the strongest of the fundamental forces, but it is also very short range, limited to nuclear distances. It is also responsible for binding quarks into mesons and baryons. An interesting feature of the strong force is that the strength of the force behaves like a rubber band. It actually gets stronger as the quarks move apart, but just like a rubber band, it will eventually break apart when stretched too far. Unlike a rubber band, when the strong force breaks, new quarks are actually formed from the newly released energy. This process is called quark confinement. There has never been an experiment that has found a quark in isolation.
Figure 4.19
Quark confinement
The weak nuclear force causes the radioactive decay of certain particular atomic nuclei. In particular, this force governs the process called beta decay, whereby a neutron breaks up spontaneously into a proton, an electron, and an antineutrino. It operates only on the extremely short distance scales found in an atomic nucleus.
According to modern quantum theories, forces are due to the exchange of force carriers. The various fundamental forces are conveyed between real particles by means of particles described by physicists as virtual particles. Virtual particles essentially allow the interacting particles to “talk to” one another without exchanging matter. The force–carrying particles, or bosons, for each of the forces are as follows: electromagnetic force—photons; weak nuclear interaction—very mas
sive `' and `' particles; and the strong nuclear interaction—gluons. Although it has not been possible to devise a completely satisfactory theory of gravitation, it too should have an exchange particle—the graviton (which has not yet been discovered).
The Standard Model
The theories and discoveries of thousands of physicists over the past century have created a remarkable picture of the fundamental structure of matter, the standard model of particles and forces.
Figure 4.20
The Standard Model of Particle Physics
The standard model currently has sixteen particles. Twelve of the particles are fermions, or matter particles and they are the six quarks and six leptons. Each elementary particle also has an antimatter partner. The remaining four particles are called bosons and are the exchange particles through which the four fundamental interactions are transmitted. The hypothetical exchange particle for gravity, the graviton, does not currently have a place in the standard model.
Every phenomenon observed in nature can be understood as the interplay of the fundamental forces and particles of the standard model. Interesting to note, that although the standard model does a terrific job at explaining all the matter and forces that occur in nature, nearly % of all matter that makes up the universe has still not been discovered—the elusive dark matter.
But physicists know that the standard model is not the end of the story. It does not account for gravity and the mysterious dark matter. The standard model also requires the existence of a new particle, known as the Higgs boson. The existence of this particle is essential to understand why the other building blocks (the quarks, the leptons, and the gauge particles) have mass. The Higgs has not yet been seen in any experiment. As the experiments become grander in scale and the discoveries multiply, will the standard model be supported or does a new model need to be developed? The standard model raises almost as many questions as it answers. Today physicists all over the world are searching for physics beyond the standard model that may lead to a possibly more elegant theory—a theory of everything.
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