Welcome to this month’s science links thingy. This time we are going to do something different; in honor of the upcoming start up of the Large Hadron Collider at CERN, I will be doing a short series of articles on this project. Why? Because the LHC may represent the biggest scientific event of our lifetimes.
What is the LHC? Well, the first term needs no introduction. A collider is a machine that collides particles at high energy states in order to study the debris that they produce. It has been described as ‘crashing two garbage cans together, and looking at what flies out of them.” Subatomic particles can only be studied in this way, as they are too small to see directly or indirectly. Basically a collider consists of several basic components; the particle source, the accelerator components, and the detector. More on these later. A ‘Hadron’ is a particle that is made up of two or more quarks, for example, a proton or a neutron (there are others). The particular hadron that the LHC is going to work with are protons. Uniquely among colliders, the LHC will smash beams of protons going in opposite directions to produce collisions of unprecedented energy; 14 TerraElectronVolts. An electronvolt being a unit of energy and of mass used in physics experiments. It is so called because it is the amount of energy an electron would gain if it passed through from the negative to the positive end of a 1 volt battery (Lederman, 200) It is also used as a unit of mass for subatomic particles, because energy and mass are equivalent, and because of the small scale of the particles involved, there is often some ambiguity over what is ‘energy’ and what is ‘mass.’ A TerraElectronVolt (TeV) is a trillion electron volts. This is not really a lot of energy in everyday terms. Turn on your computer, and you are using many TeV of energy. However, to concentrate all that energy into a single collision of subatomic particles is a tremendous feat.
As I mentioned, the LHC will be unique in smashing two moving proton streams together. Other colliders use one proton beam to smash into a stationary target, or a beam of protons colliding with a beam of antiprotons going in the opposite direction (every particle has an anti particle. Why there are more particles around than antiparticles is one of the great mysteries of physics…one which the LHC will hopefully help illuminate) The LHC’s beams will each be moving with an energy of 7 TeV, for a total energy of the collision of 14 TeV, more than 10 times what has been achieved before (Close, 207)
Why do this?
To put it simply, the smaller a particle you want to see, the more energy you have to use. When I use the term ‘small’ I am of course being extremely imprecise, as these particles are so small that they don’t really have dimensions per se. Suffice it to say that some are easier to find than others, and the more energy you have, the more likely you are to see them.
Our general model of Physics, called the Standard Model, is a very successful collection of a number of well supported theories, including Quantum Mechanics, but NOT including General Relativity, which has not, at present, been explained in a way that is consistent with the Standard Model. But the Standard Model has other problems as well; The test of a scientific model is its ability to predict experimental results. The Standard Model does this quite well at low energy levels, but at high ones, it produces results that don’t make sense. Another problem is its inability to explain why certain particles have the masses that they do, and why hadrons, like protons, are more massive than the sum total of their quarks. Clearly, something is missing; something not yet observed. It is that thing that the LHC was built to find.
What are they looking for?
The name of the particle which physicists believe will resolve all (or many) of the problems with the Standard Model is called the Higgs Boson. Higgs, after physicist Peter Higgs, who, among others, noted the symmetry violations and postulated that there was a particle responsible for it, and Boson, because that is the name for a (electrically neutral) force carrying particle. In this case, the force being carried is known as the Higgs Field, and the property it produces is none other than ‘mass’ itself.
A ‘field’ is any area of space where movement within the field causes an increase or decrease in potential energy. In everyday life, the most familiar is the Earth’s gravitational field. A rock lifted high has more potential energy than one sitting on the ground, for example; release the rock and the energy will become manifest in motion. Magnetic fields have similar properties. Since Energy is equivalent to Mass, changing an object’s potential energy increases its mass. Thus objects are said to have a ‘rest mass’ which is its ‘own’ mass, and a ‘total’ mass, which includes its potential energy.
The Higgs Field is thought to be a field that permeates all space, and that the property of ‘rest mass’ above is in fact analogous to the potential energy in a smaller field; Mass is to the Higgs field as Weight is to a gravitational field. The different masses of the various particle are explained by how closely the particles are ‘coupled’ to the Higgs field.
Besides this, there are lots of other mysteries that physicists hope to solve using this machine of unprecedented power. More on that later.
For now, let’s explore the LHC itself. It is quite simply the largest machine ever built.
It is a ring 27 km in circumference. The beam is contained by 7,000 superconducting magnets. Its two main detectors, ATLAS and CMS, weigh 7,000 and 12,500 tons, respectively, and the CMS breaks the record for the size of its superconducting coil, which can store 25 billion joules of energy. (Close, 207-8) It uses two already existing CERN colliders as its proton sources; the Proton Synchrotron and the Super Proton Synchrotron.
The ATLAS and CMS detectors are similar to tried and true detectors at other colliders, but are much bigger. They consist of three parts. First is the charged particle detector, which detects the paths of any charged particle that passes through it. Then there is the calorimeter, which is a metal plate that stops any large particles passing through, and registers their momentum. Then there is the muon detector, which is specialized to detect muons, that pass through unstopped by the other detectors. The ATLAS detector does the first two jobs, and the CMS detects the Muons. The reason that the CMS is so powerful is that physicists believe that the tell tale sign of the Higgs boson will be a stream of muons. (among other things) A muon is a kind of lepton (an electrically charged particle related to the familiar electron) which is about the same mass as a proton, but is very short-lived. The collider is expected to generate a billion collisions a second. (while its turned on) In all this, scientists hope to get maybe one Higgs boson every day! (and zillions of non-Higgs events). This is expected to add up to 15 million gigabytes of data each year. (see CERN’s website below) So, as you can expect, there will be a lot of work to do just filtering out the useful data from the not so useful data…especially as no one is quite sure what the useful data will look like.
There are also other experimental devices; ALICE, which will attempt to simulate Big Bang like conditions using the high energy particle streams generated by the LHC, the LHCb will explore the causes of asymmetry between matter and anti-matter, TOTEM is meant to monitor the workings of the LHC itself, and also study the protons, and finally, LHCf, which will attempt to recreate cosmic rays in a laboratory setting.
That’s all for this month. If there is sufficient interest, I will continue on this topic for a few issues. Next month will be a history of colliders and the particles they produce. Here are links for further reading:
A good link for the LHC.
The bibliographic sources I referred to above are:
Leon Lederman, The God Particle, Houghton Mifflin Company, Boston, 1993.
Frank Close, Michael Marten, Christine Sutton, The Particle Odyssey; a Journey into the Heart of Matter, Oxford University Press, Oxford, 2002.