1. CHAPTER 14 Elementary Particles
14.1 Early Discoveries
14.2 The Fundamental Interactions
14.8 Accelerators
14.3 Classification of Elementary Particles
14.4 Conservation Laws and Symmetries
14.5 Quarks
14.6 The Families of Matter
14.7 Beyond the Standard Model
Steven Weinberg ( )
“I have done a terrible thing: I have postulated a particle that cannot be detected.”
- Wolfgang Pauli (after postulating the existence of the neutrino)
“If I could remember the names of all these particles, I’d be a botanist.”
- Enrico Fermi

2. Elementary Particles
Particle physics tries to answer the most fundamental questions about nature:
What’s inside the nucleus?
What are the basic building blocks of matter?
What are the forces that hold matter together and break it apart?
What new physical laws are required to describe these forces?

3. Discovery of the Neutron
By the 1920s, most atomic nuclei were known to be heavier than Z, the atomic number. But only protons and electrons were known, so many thought that the nucleus contained extra protons and an equal number of extra electrons, too, to compensate.
But electrons can’t exist within the nucleus:
Nuclear size The uncertainty principle puts a lower limit on its kinetic energy that’s much larger that the kinetic energy observed for any electron emitted from nuclei.
Nuclear spin Deuterons were known. If a deuteron consisted of protons and electrons, then it must contain 2 protons and 1 electron. A nucleus composed of 3 fermions must have half-integral spin. But it had been measured to be 1.

4. Nuclear Properties
The nuclear charge is +e times the number (Z) of protons. Other Hydrogen atoms: Deuterium: Heavy hydrogen. One proton and one neutron in its nucleus. Tritium: Heavier hydrogen! One proton and two neutrons. The respective nuclei are called deuterons and tritons. Atoms with the same Z, but different mass number, are called isotopes.
Hydrogen
Number of nucleons
Number of protons

5. Because there’s no negative charge in the nucleus, it’s clear that a new force is involved.
The nuclear force is called the strong force for the obvious reason!
The Nuclear Force
The nuclear potential energy vs. distance
Coulomb repulsion
The angular distribution of nucleons scattered by other nucleons tells us the nuclear potential.
Nuclear potential

6. Summary of Early Discoveries
Thomson had identified the electron in 1897, and Einstein had defined the photon in 1905.
The proton is the nucleus of the hydrogen atom (let’s give Rutherford credit for its discovery).
In 1932 James Chadwick identified the neutron, actually first seen by Bothe and Becker.
That seemed sufficient…
In 1932, Irene Joliot-Curie, one of Madame Curie’s daughters, and her husband, Frederic Joliot-Curie, decided to use their strong polonium alpha source to further investigate Bothe’s penetrating radiation. They found that this radiation ejected protons from a paraffin target. This discovery was amazing because photons have no mass. However, the Joliot-Curies interpreted the results as the action of photons on the hydrogen atoms in paraffin. They used the analogy of the Compton Effect, in which photons impinging on a metal surface eject electrons. The trouble was that the electron was 1,836 times lighter than the proton and, therefore, recoiled much more easily than the heavier proton after a collision with a gamma photon. We now know that gamma photons do not have enough energy to eject protons from paraffin.
When James Chadwick reported to Lord Rutherford on the Joliot-Curies’ results, Lord Rutherford exclaimed, "I do not believe it!" Chadwick immediately repeated the experiments at the Cavendish Laboratory in Cambridge, England. He not only bombarded the hydrogen atoms in paraffin with the beryllium emissions, but also used helium, nitrogen, and other elements as targets. By comparing the energies of recoiling charged particles from different targets, he proved that the beryllium emissions contained a neutral component with a mass approximately equal to that of the proton. He called it the neutron in a paper published in the February 17, 1932, issue of Nature. In 1935, Sir James Chadwick received the Nobel Prize in physics for this work. You can read his lecture as he received his Nobel prize. It is interesting to note that the Joliot-Curies’ misinterpretation of their results cost them the Nobel Prize. (Not to worry; in 1935, they received the Nobel Prize in chemistry for their discovery of artificial radioactivity.) Above text from:
Image from
Protons, neutrons, electrons, and photons—what do you give the universe that has everything?

7. The Positron
In 1928 when Dirac combined quantum mechanics with special relativity, he introduced the relativistic theory of the electron.
He found that, in free space, his wave equation had negative, as well as positive, energy solutions.
His theory can be interpreted as the vacuum being filled with an infinite sea of electrons with negative energies.
Exciting an electron from the “sea,” leaves behind a hole with negative energy, that is, the positron, denoted by e+.
Paul Dirac ( )
Photo of Dirac:
Other is from TRex.
Electron & positron
Vacuum
Positron! E

8. Anti-Particles
Dirac’s theory yields anti-particles for all particles, which:
Have the same mass and lifetime as their associated particles.
Have the same magnitude but opposite sign for such physical quantities as electric charge and various quantum numbers.
All particles, even neutral ones, have anti-particles (with some exceptions like the neutral pion, whose anti-particle is itself).

9. Discovery of the Positron
In 1932, Carl Anderson identified the positron in cosmic rays. It was easy: it had positive charge and was light.
Photo of particle track:
Other is from TRex.
Anderson’s cloud chamber photo of the first recorded positron track
Carl Anderson ( )

10. Electron-Positron Interaction
The ultimate fate of positrons (anti-electrons) is annihilation with electrons.
After a positron slows down by passing through matter, it’s attracted by the Coulomb force to an electron, where it is annihilated through the reaction:
All anti-matter eventually meets the same fate. A lot of energy is released in this process: all of the matter is converted to energy.
Star Trek’s “dilithium crystals” supposedly contain anti-matter, which powers the Enterprise.

11. Quantum Field Theory and Feynman Diagrams
Richard Feynman presented a particularly simple graphical technique to describe many-particle interactions in what we now call quantum field theory.
Electromagnetism can be interpreted as the exchange of photons.
We say that the photons are the carriers or mediators of the electromagnetic force.
Example of a Feynman space-time diagram. Electrons interact through mediation of a photon. The axes are normally omitted.

12. Virtual photons mediate electromagnetism.
Quantum field theory predicts that, when two charged particles interact, they actually exchange a series of photons called virtual photons, which cannot be directly observed.
Two charged particles and their virtual photons:
The strong, weak, and gravitational interactions are assumed to operate in the same manner, but with their mediating particles.

13. Yukawa’s Meson
The Japanese physicist Hideki Yukawa developed a quantum field theory that described the force between nucleons (protons and neutrons)—the strong force.
To do this, he had to determine the carrier or mediator of the nuclear strong force analogous to the photon in the electromagnetic force, which he called a meson (derived from the Greek word meso meaning “middle” due to its mass being between the electron and proton masses).
Hideki Yukawa ( )

14. Yukawa’s Meson
Yukawa’s meson, called a pion (or pi-meson or p-meson), was identified in 1947 by C. F. Powell (1903–1969) and G. P. Occhialini (1907–1993) in cosmic rays at sites located at high-altitude mountains, first at Pic du Midi de Bigorre in the Pyrenees, and later at Chacaltaya in the Andes Mountains.
Charged pions have masses of 140 MeV/c2, and a neutral pion p0 with a mass of 135 MeV/c2 was later discovered.
Feynman diagram indicating the exchange of a pion (Yukawa’s meson) between a neutron and a proton.

15. The Mass of a Mediator Particle and the Range of the Force
The Uncertainty Principle allows energy conservation to be violated by a time:
If we set DE = mc2, the mass energy of the particle (allowing for its creation), we have:
But the particle can travel up to the speed of light, so, if r is the range of the force, Dt ~ r/c:
Or:
the strong and weak forces
So if r = ∞, then m = 0. But if the force is short-range, m can be large.
For the strong force, r ~ 4 × m, so mc2 ~ 150 MeV for the meson.

16. Accelerators
Particle accelerators generate particles with energies >1 TeV.

17. There are several types of accelerators used presently in particle physics experiments: cyclotrons, linear accelerators, and colliders.
Accelerators
They’re all based on the same idea: as the particles move, apply a voltage that accelerates them to higher a speed.

18. Cyclotrons and Synchrotrons
A charged particle in a mag-netic field travels in a circle. Accelerating it with voltage yields a cyclotron. A problem with cyclotrons, however, is that, when charged particles are accelerated, they radiate
electromagnetic energy called synchrotron radiation. This problem is particularly severe when electrons, moving very close to the speed of light, move in highly curved paths. If the radius of curvature is small, electrons can radiate as much energy as they gain.
Physicists have learned to take advantage of these synchrotron radiation losses and now build special electron accelerators (called light sources) that produce copious amounts of photon radiation used for both basic and applied research.

19. Linear Accelerators
Linear accelerators or linacs typically have straight electric-field-free regions between gaps of RF voltage boosts. The particles gain speed with each boost, and the voltage boost is on for a fixed period of time, so the distance between gaps becomes increasingly larger as the particles accelerate.
Linacs are sometimes used as pre-acceleration device for large circular accelerators.

20. Colliders
Head-on collisions are twice as energetic as those involving hitting an object at rest, so physicists began building colliding-beam accelerators, in which the particles meet head-on.
If the colliding particles have equal masses and kinetic energies, the total momentum is zero and all the energy is available for the reaction and the creation of new particles.

21. Large Hadron Collider
Counter-propagating protons each have an energy of ~7 TeV, giving a total collision energy of 14 TeV. The LHC can also be used to collide heavy ions such as lead (Pb) with a collision energy of 1,150 TeV.
Wikipedia:
The Large Hadron Collider (LHC) is a particle accelerator located at CERN, near Geneva, Switzerland. It lies in a tunnel under France and Switzerland.
It is currently in the final stages of construction, and commissioning, with some sections already being cooled down to its final operating temperature of ~2K (−271°C). The first beams are due for injection mid June 2008 with the first collisions planned to take place 2 months later.[1] The LHC will become the world's largest and highest-energy particle accelerator.[2] The LHC is being funded and built in collaboration with over two thousand physicists from thirty-four countries as well as hundreds of universities and laboratories.
When activated, it is theorized that the collider will produce the elusive Higgs boson, the observation of which could confirm the predictions and "missing links" in the Standard Model of physics and could explain how other elementary particles acquire properties such as mass.[3][2] The verification of the existence of the Higgs boson would be a significant step in the search for a Grand Unified Theory, which seeks to unify the three fundamental forces: electromagnetism, the strong nuclear force and the weak nuclear force. The Higgs boson may also help to explain why gravitation is so weak compared to the other three forces. In addition to the Higgs boson, other theorized novel particles that might be produced, and for which searches[4] are planned, include strangelets, micro black holes, magnetic monopoles and supersymmetric particles.[5]
The collider is contained in a circular tunnel with a circumference of 27 kilometres (17 mi) at a depth ranging from 50 to 175 metres underground.[6] The tunnel, constructed between 1983 and 1988,[7] was formerly used to house the LEP, an electron-positron collider.
The 3.8 metre diameter, concrete-lined tunnel crosses the border between Switzerland and France at four points, although the majority of its length is inside France. The collider itself is located underground, with many surface buildings holding ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants.
The collider tunnel contains two pipes enclosed within superconducting magnets cooled by liquid helium, each pipe containing a proton beam. The two beams travel in opposite directions around the ring. Additional magnets are used to direct the beams to four intersection points where interactions between them will take place. In total, over 1600 superconducting magnets are installed, with most weighing over 27 tonnes.
The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV. It will take around ninety microseconds for an individual proton to travel once around the collider. Rather than continuous beams, the protons will be "bunched" together, into approximately 2,800 bunches, so that interactions between the two beams will take place at discrete intervals never shorter than twenty-five nanoseconds apart. When the collider is first commissioned, it will be operated with fewer bunches, to give a bunch crossing interval of seventy-five nanoseconds. The number of bunches will later be increased to give a final bunch crossing interval of twenty-five nanoseconds.[citation needed]
LHC Accelerators
Prior to being injected into the main accelerator, the particles are prepared through a series of systems that successively increase the particle energy levels. The first system is the linear accelerator generating 50 MeV protons which feeds the Proton Synchrotron Booster (PSB). Protons are then injected at 1.4 GeV into the Proton Synchrotron (PS) at 26 GeV. Finally the Super Proton Synchrotron (SPS) can be used to increase the energy of protons up to 450 GeV.
The ions will be first accelerated by the linear accelerator Linac 3, and the Low-Energy Injector Ring (LEIR) will be used as an ion storage and cooler unit. The ions are then further accelerated by the Proton Synchrotron (PS) and Super Proton Synchrotron (SPS).
Six detectors are being constructed at the LHC. They are located underground, in large caverns excavated at the LHC's intersection points. Two of them, ATLAS and CMS, are large, "general purpose" particle detectors.[2] ALICE is a large detector designed to search for a quark-gluon plasma in the very messy debris of heavy ion collisions. The other three (LHCb, TOTEM, and LHCf) are smaller and more specialized. A seventh experiment, (Forward Physics at 420m), has been proposed which would add detectors to four available spaces located 420m on either side of the ATLAS and CMS detectors.[8]
The size of the LHC constitutes an exceptional engineering challenge with unique safety issues. While running, the total energy stored in the magnets is 10 GJ, and in the beam 725 MJ. Loss of only 10−7 of the beam is sufficient to quench a superconducting magnet, while the beam dump must absorb an energy equivalent to a typical air-dropped bomb. For comparison, 725 MJ is equivalent to the detonation energy of approximately 157 kilograms (350 lb) of TNT, and 10 GJ is about 2.5 tons of TNT.
Research
A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two quarks each emit a W or Z boson which combine to make a neutral Higgs.
A simulated event in the CMS detector, featuring the appearance of the Higgs boson.
When in operation, about seven thousand scientists from eighty countries will have access to the LHC, the largest national contingent of seven hundred being from the United States. Physicists hope to use the collider to test various grand unified theories and enhance their ability to answer the following questions:
Is the popular Higgs mechanism for generating elementary particle masses in the Standard Model realised in nature? If so, how many Higgs bosons are there, and what are their masses?[9]
Will the more precise measurements of the masses of the quarks continue to be mutually consistent within the Standard Model?
Do particles have supersymmetric ("SUSY") partners?[2]
Why are there apparent violations of the symmetry between matter and antimatter?[2] See also CP-violation.
Are there extra dimensions indicated by theoretical gravitons, as predicted by various models inspired by string theory, and can we "see" them?
What is the nature of dark matter and dark energy?[2]
Why is gravity so many orders of magnitude weaker than the other three fundamental forces?
As an ion collider
The LHC physics program is mainly based on proton-proton collisions. However, shorter running periods, typically one month per year, with heavy-ion collisions are included in the programme. While lighter ions are considered as well, the baseline scheme deals with lead (Pb) ions.[10] This will allow an advancement in the experimental programme currently in progress at the Relativistic Heavy Ion Collider (RHIC).
Proposed upgrade
CMS detector for LHC
After some years of running, any particle physics experiment typically begins to suffer from diminishing returns; each additional year of operation discovers less than the year before. The way around the diminishing returns is to upgrade the experiment, either in energy or in luminosity.
A luminosity upgrade of the LHC, called the Super LHC, has been proposed,[11] to be made after ten years of LHC operation. The optimal path for the LHC luminosity upgrade includes an increase in the beam current (i.e., the number of protons in the beams) and the modification of the two high luminosity interaction regions, ATLAS and CMS. To achieve these increases, the energy of the beams at the point that they are injected into the (Super) LHC should also be increased to 1 TeV. This will require an upgrade of the full pre-injector system, the needed changes in the Super Proton Synchrotron being the most expensive.
Cost
The construction of LHC was originally approved in 1995 with a budget of 2.6 billion Swiss francs, with another 210 million francs (140 M€) towards the cost of the experiments. However, cost over-runs, estimated in a major review in 2001 at around 480 million francs (300 M€) in the accelerator, and 50 million francs (30 M€) for the experiments, along with a reduction in CERN's budget pushed the completion date out from 2005 to April 2007.[12] 180 million francs (120 M€) of the cost increase has been the superconducting magnets. There were also engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid, due to, in part, the allegedly "faulty" parts lent to CERN by fellow laboratory and home to the world's largest particle accelerator, (until CERN finishes the Large Hadron Collider) Argonne National Laboratory, or FermiLab, located in Batavia, Illinois, outside of Chicago.[13] The total cost of the project is anticipated to be between $5 and $10 billion (US Dollars).[2]