Continuing on with our series on the Large Hadron Collider (LHC), today we'll address its energy aspects. Specifically, two primary questions are why high energies are needed, and how they are achieved. First, though, let's briefly go over the terminology for energy scales of magnitude.
The unit of energy referred to in particle physics is the electronvolt (eV). The most straightforward explanation of an electronvolt comes from this Fermilab document, which characterizes 1 eV as "very tiny." As illustrated on the page, if an electron passes through a 1.5 V battery (which many of you probably have in your home), that would represent 1.5 eV of energy.
Further, from dealing with computer memory sizes (bytes) or other measurment contexts, many of you are probably familiar with the terminology for thousandfold orders of magnitude, such as kilo (1,000 or 10^3, where ^ stands for raising to a power) and mega (1 million or 10^6). Continuing onward, we have giga (1 billion or 10^9) and tera (1 trillion or 10^12). This University College London document lists these milestones.
In fact, the Tevatron collider at Fermilab is so named because of its ability to reach 1 tera electronvolts ("TeV") of energy. As discussed below, the LHC will go even higher.
So why do we need ever-increasing energy performance from our particle colliders? It's because the new particles scientists are looking for tend to be very massive and, as a result of the direct relationship between energy and mass in Einstein's E = mc^2 (c standing for the speed of light, which gets squared), the creation of high-mass particles requires high energy. This Canadian document makes the basic point:
It is realized that the mass-energy relation (E = mc^2) provides a new way to get information about particles. If particles could be made very energetic and then used to collide with other particles, some of their energy could be converted into the creation of previously unknown particles. When particles are produced in a collision, they are not particles that were somehow inside the colliding ones. They are really produced by converting the collision energy into mass, the mass of other particles… Which particles will be produced is partly determined by their mass - the lighter they are, the easier it is to produced them [sic], other things being equal - and also by the probabilities calculated from the Feynman diagrams.
One type of particle that is eagerly being sought at the LHC is something known as the Higgs boson, about which I will have much to say later. The New Yorker magazine article that I linked to in the opening essay of this series talks about how the Higgs has not been found at the Tevatron, so the LHC, with its higher energy, is the next hope.
By now, the Higgs has been sought for so long that physicists have a pretty clear idea of how much it must weigh. The lower bound is around 120 times more than a proton... The upper bound is about 210 times as much as a proton. The most powerful collider currently in operation is Fermilab’s Tevatron, outside Chicago. The Tevatron, which smashes protons into antiprotons, can accelerate particles to an energy of just under a trillion electron volts, or one TeV... So far, the Tevatron has failed to reveal the Higgs, though physicists there are actively looking for it. The L.H.C. will accelerate particles to seven TeV, which means that it will be seven times as powerful as the Tevatron. This should be more than enough energy to produce the Higgs, if there is a Higgs to produce. It may also be enough to uncover much more than the Higgs... (page 3 of online New Yorker article).
OK, so finally, how have accelerators and colliders over the years been able to keep raising the energy levels capable of being studied? This Stanford Linear Accelerator Center (SLAC) document provides an excellent summary of the historical evolution of accelerators and colliders, including their increasing energy levels. Referring to a graph of accelerators’ particle energy plotted against a timeline from 1930-1990, the report states the following:
One of the first things to notice is that the energy of man-made accelerators has been growing exponentially in time. Starting from the 1930s, the energy has increased – roughly speaking – by about a factor of 10 every six to eight years. A second conclusion is that this spectacular achievement has resulted from a succession of technologies rather than from construction of bigger and better machines of a given type (pp. 38-39).
Further:
The [e]nergy that really matters in doing elementary particle physics is the collision energy – that is, the energy available to induce a reaction, including the creation of new particles… If two particles of equal mass traveling in opposite directions collide head on, however, the total kinetic energy of the combined system after collision is zero, and therefore the entire energy of the two particles becomes available as collision energy. This is the basic energy advantage offered by colliding-beam machines, or colliders (p. 40).
The brief summaries and excerpts above are meant simply to convey rudimentary ideas on the topic. The linked documents are, of course, available for you to read in toto, should you wish further information. Additional web documents and books are also available, and shouldn't be too hard to find via some web searching.