CK-12 21st Century Physics: A Compilation of Contemporary and Emerging Technologies
Page 11
References / Further Reading
http://particleadventure.org/
http://cms-project-cmsinfo.web.cern.ch/cms-project-cmsinfo/index.html
http://library.thinkquest.org/28582/history/index.htm
http://www2.sjs.org/friedman/PhysAPC/particle.htm%20
Virginia Physics Standards of Learning
This chapter fulfills sections PH.3, PH.6, and PH.18 of the Virginia Physics Curriculum.
Chapter 5: The Standard Model and Beyond
Tony Wayne. "Beyond The Standard Model", 21st Century Physics FlexBook.
Unit Overview
Colliders accelerate and steer charges to a point and collide them with a stationary target or head–on with another charge. In describing how the Large Hadron Collider at CERN works and how it will be used in experiments to explore new areas of physics, it helps to impart some basic physics principles. Many of these principles are also used to describe what the world's most energetic collider is seeking.
Terminology and Some Background Physics
Lesson Objectives
Describe kinetic energy.
Describe the unit of energy used to describe small charges.
List the mathematical representation of the prefixes used to describe small amounts of charge.
Describe the implications of Einstein’s most famous equation.
Explain how Einstein’s most famous equation is used.
The amount of charge on a particle is described using a unit called a coulomb. When the electron was believed to carry the smallest size charge, , physicists created a unit of energy to match the electron’s charge. It is called the electron volt—abbreviated An is equal to joules. Instead of saying a particle carries an energy of or , physicists now can say a particle carries an energy of or respectively.
One of the nice aspects of the electron volt is that it also relates the energy gained by an accelerating particle to the potential difference it crosses. This is the mechanism that a linear accelerator uses to accelerate a charged particle. One particle with a charge equal to an electron, changes its kinetic energy by when it accelerates between two plates connected to a volt potential difference, shown in Figure 1.
Figure 5.1
A particle with a net electric charge equal to one electron gains one of energy after crossing the metal plates. It gains instead of loses the energy because the plate (on the right) opposite the electron is oppositely charged and attracts the negatively charged electron.
Colliders are large machines designed to smash small charged particles such as protons, electrons, and the nucleus of atoms at extreme speeds. The colliders send particles into each other or into a stationary target. These moving particles have kinetic energy, where is the kinetic energy of the particle, is the particle’s mass in kilograms, and is the particle’s velocity. An object has kinetic energy as long as it has velocity. One of the ways colliders are classified is by the kinetic energy of the collisions. Because the particles are very small with masses typically in the range of the kinetic energies are measured in . But the collisions are millions or billions of not just or . The collision’s energies are listed using the prefixes listed below.
Prefix Pronunciation Number Math Expressions
Mega Millions of
Giga Billions of
Tera Trillions of
Einstein showed that a particle at rest has a rest energy given by where is the mass of the particle measured in kilograms, is the speed of light, , and is the rest energy. The rest energy is measured in the standard S.I. unit of joules. If an object of mass, , was annihilated (destroyed), then this formula would describe how much energy would be released. This equation shows that the mass and energy are equivalent: It allows physicists to quantify the mass of an object in terms of energy.
Example:
The mass of a proton is . What is the energy associated with proton’s mass in units of joules and ?
Solution:
We use with the speed of light . Then
Using the prefixes shown above, this is typically written as .
What is a “Collider?”
Lesson Objectives
Describe the purpose of a collider.
Describe what a collider is in a complete sentence.
Describe the steering mechanism for directing the charged particles.
Compare linear and ring designs.
Overview
It is theorized that when the universe began the temperatures were so hot that protons and neutrons did not exist. Instead the building blocks of these particles, quarks, roamed in space by themselves. As the universe cooled down, the quarks began to regroup into protons and neutrons. Today, the universe in our location is too cool for quarks to float around by themselves. The collider will do two things to solve this. First, it will accelerate protons or electrons to such high speeds that the energy of the charges at impact will be converted into thermal energy. Second, the energy of the particles at impact will be converted into new particles.
To generate these high levels of energy, charged particles are accelerated into each other at speeds near the speed of light. Nothing can start out slower than the speed of light and then accelerate to a speed faster than the speed of light. However, large electric fields are used to accelerate charged particles to speeds near the speed of light. There are two basic design geometries.
The linear accelerator has charged particles that travel down a straight line. The particles can start at opposite ends of a long tunnel and collide into each other.
The linear accelerator works best with electrons because they are a thousand times lighter than protons. A high percentage of the energy put into the accelerator goes into speeding up the charge (Schwartz, 1997). But electrons generate large amounts of synchrotron radiation. Protons generate less radiation but cannot achieve the same velocities.
Synchrotron radiation is caused any time a charged particle accelerates. When a particle accelerates in a straight line it is called brehmsstrahlung radiation. The (simplified) formula for calculating the radiation's power is: where is Coulomb’s constant, is the elementary charge’s value, is the speed of light, is a factor to account for relativistic speeds, and a is the acceleration. (When the speed is less than % the speed of light, ). This equation applies, for example, for the power radiated by a (radio-) antenna. When a particle accelerates in a circle or curve it is called synchrotron radiation. The same formula applies except the acceleration is found from: This means for circular motion: Because the varies with speed, the -factor for an electron moving near the speed of light can be times greater than for a proton. This means that accelerating electrons is more difficult than the accelerating protons. In order to keep synchrotron radiation as small as possible protons are used and as the speed increases the radius must also increase.
If the charges were placed in an energized ring, then they could continually be pumped up with energy to reach relativistic speeds. Because the proton generates less synchrotron radiation, it would make for a more viable candidate for acceleration in a circular collider.
Large Hadron Collider, LHC
Lesson Objectives
Describe the brief history of the CERN facilities.
Give an overview of the purpose of the experiments at the LHC.
Overview
When particles move at relativistic speeds, their energies are large enough to generate new particles when colliding with other particles. Huge amounts of energy can also overcome the strong nuclear force holding particles together. This may allow scientists to see what’s inside the protons and neutrons. To achieve these high energies, a bigger collider needs to be built.
CERN is the French acronym for European Nuclear Research Centre. This collider is located at the foot of the Jura mountains straddling the border between France and Switzerland (CERN, 2009). CERN built its first synchrotron accelerator in the late 1950s. The first synchrotron gained notoriety in 1959. Since then several new colliders have been built on top
of existing colliders at CERN. The new colliders either use the previously built colliders for pre-staging or the existing tunnels. The current LHC is no different. It uses the tunnels that were finished in 1989 for the LEP, Large Electron-Positron Collider. The LEP ceased running in November 2000 to make room for construction of the LHC (CERN Courier, 2001). The LHC is retrofitting the LEP’s tunnels with the most advanced superconducting magnets and updating its detectors to collect new data. There are currently six experiments requiring six different detectors at the LHC (CERN, 2009).
When Einstein came up with his theory of general relativity he could not foresee the practical applications of this theory today. But a hundred years later, the theory of general relativity is used to calculate your position on the planet using a GPS-enabled device, (TED, Patricia Burchat: The Search for Dark Energy and Dark Matter, 2008). The LHC is doing science for the sake of education to answer some of the big questions such as:
What causes mass?
What is dark matter?
Are there more than three spatial dimensions?
The implications in science and technology of these answers is not yet known. But in a hundred years, it may have a profound effect on society (TED, Brian Cox: An Inside Tour of the World's Biggest Supercollider, 2008).
ALICE: A Large Ion Collider Experiment
Collisions in this section will be hotter than the sun.
Looking for the particle responsible for mass.
Investigating of quarks can be freed from protons and neutrons (CERN–ALICE Collaboration).
Size: long, high, wide (CERN, 2008).
Mass: tons (CERN, 2008).
Look up “ALICE” on Google Earth to see its location.
ATLAS: A Toroidal LHC ApparatuS
It is a general purpose detector.
Looks at mass while searching for evidence of: the Higgs particle responsible for mass.
dark matter.
The ATLAS is the largest particle detector in the world (CERN–ATLAS Experiment 2008).
Size: long, high, and wide (CERN, 2008).
Mass: metric tons (CERN, 2008).
Look up “ATLAS” on Google Earth to see its location.
CMS: Compact Muon Solenoid
It is a general purpose detector.
Looks at mass while searching for evidence of: the Higgs particle responsible for mass.
dark matter.
Unlike the ATLAS it will look for this evidence using different techniques (CERN–CMS Outreach).
It generates a magnetic field 100,000 times stronger than the Earth’s.
Size: long, wide, and high (CERN, 2008).
Mass: metric tons (CERN, 2008).
Look up “CMS” on Google Earth to see its location.
LHCb: Large Hadron Collider Beauty
Looking to answer the question of why is there so little antimatter in our region of the universe (CERN–LHCb Experiment, 2008).
Size: long, high, and wide (CERN, 2008).
Mass: metric tons (CERN, 2008).
TOTEM: TOTal Elastic and Diffractive Cross Section Measurement
Looks at the size of the particles and the beam’s luminosity.
This will complement the CMS’s data and give some quality assurance.
Size: long, high, and wide (CERN, 2008).
Mass: metric tons (CERN, 2008).
LHCf: Large Hadron Collider Forward
Produces cosmic rays under laboratory conditions to look at how cosmic rays interfere with our atmosphere.
Two detectors.
Size: long, high, and wide.
Mass: each.
LHC Facility
Lesson Objectives
Describe how the proton bunch gets up to speed.
Describe some of the physics involved in the proton bunch’s motion.
Overview
Several scientists have called the LHC “the largest scientific experiment in the world” (Cox, 2008). To successfully accelerate the particles to relativistic speeds, the particles must be energized in stages. The circular geometry of the LHC and the fact that it is built using previous machines makes this possible.
When launching a rocket to the moon, the rocket has multiple stages. Each stage pushes the rocket a little faster. The LHC does something similar to get the protons up to speed (www.YouTube.com What is CERN Large Hadron Collider LHC? End of the World? Search for God Particle and Micro Black Holes, 2008).
Figure 5.2
In the early stages, hydrogen gas is ejected into a chamber. Using electricity to generate a large electric field, the electrons are stripped from the atom. Protons are then sent into the linear accelerator. This is stage one of five for the process. This collection of charges contains protons. This collection is called a The device accelerating the bunch is called the lineac 2. By the time the proton bunch reaches the end of the tube it will be traveling at the speed of light. That is fast enough to go around the Earths equator two and a half times in one second. The charge injection process is repeated to create a collection bunches. This many bunches creates a beam of protons.
Figure 5.3
Upon leaving the lineac 2, the proton bunch enters stage 2. This booster stage consists of rings with a radius of meters. The packets are accelerated by electric fields. The electric fields are pulsed in such a way to speed up the packets and more tightly pack the protons together. Powerful magnets with a B-field perpendicular to the direction of motion steer the packets in the circular rings. The packet leaves this stage at % the speed of light.
Figure 5.4
Now in stage 3 of the acceleration, the packet is in the proton synchrotron. In this ring the bunch gets closer to the speed of light. Upon leaving this ring the protons will move as if they are 25 times heavier than when they were at rest. The proton will stay in this ring for seconds and reach a speed of % the speed of light before leaving the ring. Each proton will leave the ring with .
Figure 5.5
In stage 4 of the acceleration process the bunch enters a larger ring. This ring is called the super proton synchrotron. It has a radius of about . Energy added in this ring will increase the mass of the proton to 450 times its resting mass. At this point, each proton will leave the ring with an energy of . When the bunches leave this stage, half will enter the large ring traveling clockwise. The other half will leave the ring traveling counterclockwise (See Figure 6).
Figure 5.6
The packet enters the large ring. The large ring has a radius of . The protons will travel around 11,000 times per second. This large ring contains two tunnels. The beams will travel in opposite directions until they are directed to a location for a head-on collision. Each proton will reach an energy level of while traveling at % the speed of light. This head-on energy generates a temperature of (www.YouTube.com, What is CERN Large Hadron Collider LHC? End of the World? Search for God Particle and Micro Black Holes, 2008; CERN, LHC Beams, 2008).
What is Mass?
Lesson Objectives
Describe the current theory of mass being tested at the LHC.
Describe why knowing the mechanism for mass is important.
Describe the LHC’s contribution to this search.
Overview
Inertia is one aspect of mass. The larger the mass of a resting object, the harder it is to move that object. But what causes mass? Is gravity related to particles the same way an atom’s charge depends on the protons and electrons it holds?
When you incorporate the standard model into the familiar formula for universal gravitational attraction you get a variable that keeps appearing in the mathematics.
This is a small part of a formula that is handwritten on about lines of notebook paper. And in this formula the “” variable keeps appearing. The “” variable represents a particle called the Higgs. Because the Higgs particle is responsible for a force, it is a boson. Somehow stuff attracts Higgs particles. The more Higgs particles you attract, the more your motion is retarded. This is termed inertia and it can indi
cate the mass of an object. If the Higgs particle exists then it will lend more support for the standard model of subatomic particles. If the something different from the particle is found, then the fun really begins as new theories are developed and old ones are modified (Brian Cox: An Inside Tour of the World’s Supercollider, 2008).
The ATLAS experiment at the LHC is designed to search for this particle (Cox, 2008). Previous experiments have hinted toward this particle’s existence but were inconclusive. It has been determined that a more energetic collision is needed in a chamber with more sensitive detectors in an effort to find more conclusive evidence (CERN, “History,” 1999).
Super Symmetry
Lesson Objectives
Describe some of the concepts the standard model does not describe.