vol IV chap 13 sect 1

Previous: 12.3. Analysis of learning trajectories.

13. Development of knowledge in high energy physics.¶

Introduction.

What are the structures of the atomic nuclei and the elementary particles?

This is the first chapter dedicated to the Universe. We describe the levels of operation of the mechanisms of knowledge concerning high energy physics: nuclear physics and particle physics. Cosmology is considered in next Chapter 14.

Learning objectives of Chapter 13.

After this Chapter you should be able to:

Describe the main steps and contributions to the development of high energy physics concerning the study of nuclear and elementary particles systems.
Analize a timeline of the main developments in high energy physics that have been awarded with Nobel Prizes in Physics or in Chemistry.
Relate the levels of operation of the mechanisms of knowledge to the descriptions of the laboratory works, the experimental purposes, and the theorical consequences concerning high energy physics.

Description of content of Chapter 13.

Section 13.1. Steps in the development of Nuclear Physics.

This Section contains a review of the three steps (N1 – N3) describing the development of nuclear physics, and a selection of contributions made by Nobel Prize laureates in Physics and Chemistry.

Section 13.2. Timeline of the main developments in high energy physics.

This Section contains a description of contributions made by Nobel Prize laureates in Physics and Chemistry connected with high energy physics. We indicate the year when the activities of the laureates were initiated not when the Nobel Prizes were awarded.

Section 13.3. Levels of operation of the mechanism of knowing.

The levels of operation of the mechanisms of knowing (inter, intra and trans) are described in terms of contributions made by Physics Nobel laureates, which are organized into three categories: the laboratory works, the experimental purposes, and the theorical consequences.

At the end of the chapter, Appendix 13.1 contains all references in MLA format.

13.1. Steps in the development of Nuclear Physics.¶

Two steps describe the main developments of nuclear physics made in the period 1895 – 1953: Atomic structure and electronic properties, and Production of new particles and detection of new interaction forces. Two more steps correspond to developments of particle physics made in the period 1953 – 2012: Classification of elementary particles, and Beyond the Standard model. These four steps show the transitions from the study of the atoms towards nuclei and from them towards elementary particles. Such transitions imply significant changes in scales of energy, time, and size.

Next, we comment on some physics knowledge based on information contained in documents published in the Nobel Prize web page such as: Award ceremony speech, Facts, Speed read, Press release. Popular information, and Advanced information, Work and Nobel lecture. The corresponding MLA references are indicated in Appendix 13.1. Descriptions of all the contributions made by Nobel laureates corresponding to previously mentioned four steps are given in next section 13.2. Timeline of the main developments in high energy physics.

Atomic structure and electronic properties (1895-1927).¶

New phenomena were observed and there were no possibilities for understanding them in the frame of classical physics, for instance, the transformation of radioactive elements, the discovery of new elements, and the existence of isotopes. These phenomena lead to new structural models of atomic matter.

Henri Becquerel observed the effect of the disintegration of uranium generated the transmutation into a new element accompanied by the emissions of radiations: Alpha rays (nucleus of He atoms), Beta rays (electrons), and/or Gamma rays (radiation with the shortest wavelength of the electromagnetic spectrum).

Research on radioactivity has contributed to generate knowledge in next aspects: there is a new property of matter with the capability of spontaneously emitting certain type of radiation; specific experimental methods have been developed to produce radioactive elements, and a new source of energy has been found with no clear understanding at that time of its origin and properties.

The smallest particles with negative and positive charges known at the time were the electron and the proton, respectively. Being part of atomic structures, they came out from the decomposition of the radioactive substances and from the disintegration of atoms and molecules. Although the atom is electrically neutral, the existence of isotopes indicated that something with mass but without charge was inside the nucleus: the neutron.

Two Rutherford´ students (Hans Geiger and Ernest Marsden) performed a series of experiments for testing Thomson atomic model called the “plum pudding”. They observed that some alpha particles were dispersed after impacting a very thin foil of gold instead of passing through it, indicating that something like a coulombic repulsion was changing the trajectory of the positive incident radiation of alpha particles. This surprising result led Rutherford to propose an atomic model composed of a positive core or nucleus and a cloud of surrounding negative charges, the electrons. Consequently, radioactivity had to be interpreted as something interior to the atomic system which will not anymore be considered as indivisible.

The fact that photons are quantized energy electromagnetic radiations, and that electrons could be diffracted implied the existence of quanta of energy and a dual behavior of electrons. Furthermore, a restriction on the electronic quantum numbers was proposed by the Pauli exclusion principle and tracks created by the passage of particles indicated the presence of subatomic particles.

Production of new particles and detection of new interaction forces (1929-1953).¶

Designing and building accelerators of particles and creating appropriate detection instruments transformed the way of doing experimental research in physics. Understanding the nucleus models and discovering new particles like neutrons and positrons required the consideration of new types of forces, aside gravitation and electromagnetism: the strong force for explaining the nuclear binding between protons and neutrons (existence of mesons) and the weak force responsible of radioactivity. The artificial production of radioactivity opened new possibilities for the transformation of elements and for the liberation of energy.

Ernest Lawrence applied a method called magnetic resonance acceleration by combining a constant homogeneous magnetic field and an oscillating electrical field with constant frequency. Charged particles that moved under the influence of such fields described orbits with ever-increasing radii through stepwise acceleration in a spiral-shaped path before they collide with specific targets to penetrate atomic nucleus and produce nuclear reactions. This machine, the cyclotron, has been also used in the production of artificially radioactive substances.

When beryllium atoms were bombarded with alpha particles (nuclei of helium) a strong and penetrating radiation was observed; this radiation appeared to be radioactive γ-rays. This radiation caused a disintegration of the atoms, similar to an explosion inside the nuclei. James Chadwick studied such radiations and calculated the transformations of mass at collisions: an available procedure to detect the presence of a particle with mass but without charge: the neutrons.

Robert Millikan recorded the tracks produced by cosmic radiations in a Wilson chamber equipped with very strong magnets. By analyzing photographs of those tracks Carl Anderson discovered the positron: he found one case where the curved path showed the same deviation as the negative electrons, but in the opposite direction, which corresponded to a positively charged particle.

According to Hans Bethe, the existence of stars follows a life cycle: to be born and then to grow and develop by burning fuel, finally they die burning out all their energy sources. The dominant reaction occurs when four hydrogen nuclei links together to form one helium nucleus. However, in a more complex cycle of nuclear reactions carbon atoms acts as a catalyst. These fusion reactions create heat and light as another demonstration of the validity of Einstein´s equation \(E = mc^2\).

Otto Hahn and Lise Meitner studied the products obtained by projecting neutrons on to heaviest elements like thorium and uranium. Later, Lise Meitner and Otto Frisch proved that the uranium nucleus had been split and that the first nuclear fission reaction was obtained due to a chain reaction where matter was converted into energy.

Classification of elementary particles (1950-1964).¶

Experiments with elementary particles are quite difficult mainly for two reasons: such particles require very high energies to be produced and a very complicated system needs to be created and operated to observe and register their existence and interpret their behavior. Elementary particles are quite small in size and fast in movements; therefore, it is not possible to detect their motions. However, their trajectory can be followed when the particles leave some tracks while they pass through certain media. When the lifespan of the particles is extremely short there is no way to observe those tracks; the possibility is to register and afterwards to interpret instantaneous photographs of the reactions produced when those particles collide with other particles.

The eightfold way was proposed as an organizational scheme for classifying subatomic particles and antiparticles, including the development of the quark model. The assumption that physical laws are characterized by symmetry operations was questioned and the theory that the left-right symmetry law is violated by the weak interaction was confirmed. The main characteristics of the Standard model are the following:

Experimental observed particles are composite of smaller building blocks called quarks. Particles are classified according to their symmetry properties.
Nuclear forces maintain together protons and neutrons inside the atomic nucleus and are charge independent.
Pi-mesons predicted form a triplet of particles that have nearly the same mass but different charges; they explain the interaction among nucleons.
New particles heavier than the nucleons are produced when high-energy pi-mesons collide with nucleons. These particles form the group of baryons.
Elementary particles can be transformed in others by the strong and the electromagnetic interactions only if the total hypercharge is conserved.
The nucleons belong to a supermultiplet of eight particles (an octet). All particles are composed of only three kinds of particles: the quarks and of the corresponding antiparticles. Quarks have charges that are fractions of the proton charge.

Elementary particles are fermions (they have half-integer spin) and constitute two families, light particles (leptons) and heavy particles (hadrons) formed by quarks. Light particles form three families consisting of pairs of particles: the electron (\(e-\)) and the electron neutrino (\(ν_e\)), the muon (\(μ\)) and the muon neutrino (\(ν_μ\)), and the tau (\(τ\)) and tau neutrino (\(ν_τ\)). Heavy particles (hadrons) are composed of mesons which are formed by quarks and antiquarks, and by baryons which are formed by combinations of three quarks. Quarks are of six types or flavors each represented by the initial letter of its name; they form three generations of pairs: up (\(u\)) and down (\(d\)), charmed (\(c\)) and strange (\(s\)), top (\(t\)) and bottom (\(b\)).

Bosons (have zero or integer spin) and correspond to four types of forces: the gravitational that is of attraction and the electromagnetic that can be of attraction or repulsion, the strong nuclear force that keeps protons and neutrons together inside the nucleus, and the weak nuclear force responsible for radioactivity and that affects particles such as neutrinos. The transformation of hydrogen into heavy hydrogen (deuterium), is caused by the weak force. The weak force makes a distinction between left and right; it has not the property of being reflection symmetric. The quanta that correspond to these four forces are all bosons: gravitons for the gravitational force, photons for the electromagnetic force, gluons for the strong nuclear force and particles \(W-\), \(W+\) and \(Z^0\) for the weak nuclear force.

Beyond the Standard model (1964-2012).¶

If natural laws are perfectly symmetrical and absolute, all the time they are valid throughout the entire universe. Three general principles of symmetry apply in physics: mirror symmetry (there is no difference between left and right), charge symmetry (antiparticles have the same properties but the opposite charge as the particles) and time symmetry (events are equally independent whether they occur forwards or backwards in time). These are respectively referred as P for parity, C for charge symmetry and T for time.

Symmetries simplify many calculations; they play a decisive role for the mathematical description of the microworld and implicate the application of conservation laws at the particle level. However, phenomena like the following have shown the need for considering broken symmetries: In the radioactive decay of kaon particles, a small fraction of them did not follow the current mirror and charge symmetries, they broke the double CP symmetry. In this case each kaon particle is a combination of a quark and an antiquark and the weak force makes them switch identities: the quark becomes an antiquark while the antiquark becomes a quark.

A quantum field theory unifies the wave functions of quantum mechanics and the fields of electromagnetism. Electroweak interaction is understood as a relativistic quantum field theory where quantized fields are considered in terms of operators for the creation and annihilation of particles. The electroweak interaction refers to the unified treatment of electromagnetism and the weak force which underlies some forms of radioactivity, governs the decay of unstable subatomic particles such as mesons, and initiates the nuclear fusion reaction that fuels the Sun.

During the last fifty years several complex experimental projects have been planned and developed, showing results of consequences: the discovery of the W an Z quantized carriers of the weak interaction, the development of techniques for studying the movements of stars in the middle of the Milky Way revealing the existence of a super massive black hole, the discovery of neutrino oscillations, and the evidence about the existence of the boson Higgs serving to explain the masses of elementary particles.

The decay of the atom nucleus in the radioactive element cobalt 60 revealed that it did not follow the principles of mirror symmetry (the electrons that left the cobalt nucleus preferred one direction to another). In this case charge and mirror symmetries are broken separately, however, CP symmetry is conserved. Another spontaneous broken symmetry, the Higgs mechanism, destroyed the original symmetry between forces and gave the particles their masses in the very earliest stages of the universe.

The neutrino was a hypothetical particle introduced by Pauli in 1930 to explain the energy distribution of electrons emitted in the radioactive beta-decay process. Neutrinos are electrically neutral, weakly interacting and very light particles. There are six types of neutrinos: the electron neutrino and the positron neutrino, the muon neutrino and the antimuon neutrino, and the tau neutrino and the antitau neutrino. Neutrinos were detected and the existence of a fifth field created by the Higgs boson was proposed.

As all matter is transparent to neutrinos, to measure their action it was necessary first to produce them as a decay product of pi-mesons generated when accelerated protons collided with beryllium atoms. A special detector was built to observe the results of neutrino collisions with matter by registering produced muons in a spark chamber.

Neutrinos interact with matter only through the weak force.

The Higgs boson is the mechanism by which mass originates in the universe: everything in it is immersed in an invisible field whose quantum is the Higgs boson. When a particle moves within this field there is some resistance to movement and the faster the movement the greater the resistance, thus implying that this quantitative measure of inertia, the mass, will be greater. Depending on the intensity of the interaction of a particle with the Higgs boson, so will be the magnitude of its mass. The Higgs boson cannot be detected directly, only its fingerprints, because the instant it is produced it disintegrates into other particles that are produced at very high energies.

According to the theory of general relativity, stars containing large concentrations of masses warp space-time and generate gravitational forces. In the vicinity of this super-dense region, space-time is encapsulated, giving rise to a black hole. This process is called gravitational collapse whereby the black hole traps everything that approaches it and then prevents it from leaving, even if it is light.

Certain solutions of the equations of general relativity technically correspond to a singularity wrapped in a closed surface, which curves the space-time of four dimensions and thus delimits the so-called event horizon of the black hole. Such a singularity means that all the mass of the star is concentrated in a mathematical point, which by definition is dimensionless. The greater the mass of the black hole, the larger the size of its event horizon and therefore, the more intense the gravitational field that exists inside it. The black hole is not seen, we only perceive the imprint left by its presence.

Next: 13.2. Timeline of the main developments in high energy physics.