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Two Beams of Protons will Accelerate in Opposite Directions to Test the History of New-Born Universe at CERN
:: 23 January, 2008
The most extensive and expensive scientific instrument in history is due to start working this summer at Cern, the European particle physics laboratory near Geneva. Two beams of protons will accelerate in opposite directions around a 27km tunnel under the Alpine foothills until they are travelling almost at the speed of light – and then smash together, reproducing on a tiny scale the intense energy of the new-born universe after the inaugural Big Bang 15bn years ago.
As exotic sub-atomic particles spring fleetingly into existence for the first time on Earth, the $8bn Large Hadron Collider or LHC will give thousands of physicists around the world a cornucopia of data with which to create and refine theories about the nature of the universe.
The immediate scientific target for the LHC is to find the Higgs boson. This is the last important particle for which physicists still have no direct evidence, but whose existence is predicted by the so-called Standard Model of the universe built up by theorists over the past 40 years.
Peter Higgs of Edinburgh University proposed the Higgs boson in the 1960s to explain how matter acquires its most basic property: mass. The idea is that a “Higgs field” pervades the universe; the more strongly an object interacts with it, the more mass it has. According to physics theory, any force field must have an accompanying fundamental particle – and the Higgs boson goes with the Higgs field.
None of this is easily explained in non-scientific language. One analogy is to imagine the Higgs field as a cocktail party in which people are evenly distributed through the room. Then a celebrity walks across the space, attracting clusters of admirers who slow her progress – a resistance to movement analogous to the mass acquired by a particle.
Now imagine that there is no famous guest, just a rumour that one will soon make an unexpected appearance at the party. This rumour is like the Higgs boson, creating a clustering effect like a real celebrity as it crosses the room. The boson arises from the Higgs field but it is slightly misleading to suggest, as many popular explanations of the effect do, that it gives mass directly to other particles.
The LHC is the first atom smasher powerful enough to make enough Higgs bosons to demonstrate convincingly that they exist and to measure their properties. A negative result would be devastating for the Standard Model – and in some ways more interesting than it would be if the LHC produced a profusion of Higgs particles.
Although Cern has made much of the Higgs boson in justifying the LHC, it is just the start of the collider’s scientific agenda. The most exciting research lies beyond the Standard Model – for instance testing “supersymmetry”, the theory that every sub-atomic particle has a far heavier super-partner or superparticle. Mathematicians insist that supersymmetry simplifies the universe by bringing out hidden links between forces and particles.
The lightest predicted superparticle, and the one most likely to be generated by the LHC, is the neutralino. Its discovery would delight those cosmologists who believe that superparticles make up “dark matter”, the mysterious substance that accounts for a quarter of the universe but does not emit or interact with electromagnetic radiation.
Beyond supersymmetry lies the hope that the LHC may confirm more speculative ideas. For example “string theory”, the most comprehensive attempt to explain the underlying nature of the universe, predicts that six extra dimensions lie hidden within the three familiar dimensions of space. Could the collisions at Cern create enough energy to give physicists a glimpse into these hidden dimensions? Best of all would be total surprises – revelations that have not occurred even to the most brilliant theorists.
Physicists have been waiting a long time for the LHC. Cern started planning it in the 1980s, as a follow-up machine to LEP, the Large Electron-Positron collider for which the 27km sub-Alpine tunnel was originally excavated. A boost for the project was the cancellation in 1993 of the planned US Superconducting Super Collider. This made the LHC a de facto global instrument and helped Cern to pull in $500m worth of US public funding, mainly for the six giant detectors that will make sense of the particles it generates.
Although the primary purpose of Cern is, of course, high energy physics, its prodigious data output has also made it a computing pioneer – indeed the worldwide web was invented there. When the LHC begins operations, it will produce 15 petabytes (15m gigabytes) of data annually, enough to fill 100,000 DVDs a year.
The long timescales involved in planning and building high-energy physics facilities mean that the leadership at Cern is already thinking about what comes next. The LHC is scheduled to receive a major upgrade in 2016, which should keep it going into the late 2020s. By then a global successor, the International Linear Collider or ILC, may be in operation.
But particle physicists cannot assume that they will continue to win the political support required to build and run their multi-billion dollar instruments, devoted after all to intellectual rather than economic enrichment, unless they promise discoveries to catch the public imagination. So they are hoping that the LHC produces inspiring answers that raise in turn even more tantalising questions about the nature of our universe.
Note for Subatomic Particle
A subatomic particle is an elementary or composite particle smaller than an atom. Particle physics and nuclear physics are concerned with the study of these particles, their interactions, and non-atomic matter.
Subatomic particles include the atomic constituents electrons, protons, and neutrons. Protons and neutrons are composite particles, consisting of quarks. A proton contains two up quarks and one down quark, while a neutron consists of one up quark and two down quarks; the quarks are held together in the nucleus by gluons. There are six different types of quark in all ('up', 'down', 'bottom', 'top', 'strange', and 'charm'), as well as other particles including photons and neutrinos which are produced copiously in the sun. Most of the particles that have been discovered are encountered in cosmic rays interacting with matter and are produced by scattering processes in particle accelerators.
In particle physics, the conceptual idea of a particle is one of several concepts inherited from classical physics, the world we experience, that are used to describe how matter and energy behave at the molecular scales of quantum mechanics. As physicists use the term, the meaning of the word "particle" is one which understands how particles are radically different at the quantum-level, and rather different from the common understanding of the term.
Note for Large Hadron Collider
The Large Hadron Collider (LHC) is a particle accelerator and collider located at CERN, near Geneva, Switzerland (46°14′N, 6°03′E). Currently under construction, the LHC is scheduled to begin operation in May 2008. The LHC is expected to become the world's largest and highest-energy particle accelerator. 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 hoped 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. 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 three of the four fundamental forces: electromagnetism, the strong force, and the weak force. The Higgs boson may also help to explain why the remaining force, gravitation, is so weak compared to the other three forces.
The collider is contained in a circular tunnel with a circumference of 26.659 kilometres (16.5 miles), at a depth ranging from 50 to 175 metres underground. The tunnel was formerly used to house the LEP, an electron-positron collider.
The 3.8 metre diameter, concrete-lined tunnel actually 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.
Note for Higgs boson
The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. It is the only Standard Model particle not yet observed, but would help explain how otherwise massless elementary particles, still manage to construct mass in matter. In particular, the difference between the massless photon and the relatively massive W and Z bosons. Elementary particle masses, and the differences between electromagnetism (caused by the photon) and the weak force (caused by the W and Z bosons), are critical to many aspects of the structure of microscopic (and hence macroscopic) matter; thus, if it exists, the Higgs boson has an enormous effect on the world around us.
As of 2008, no experiment has directly detected the existence of the Higgs boson, though the Large Hadron Collider (LHC) at CERN is hoped to be able to detect the Higgs boson. The Higgs mechanism, which gives mass to vector bosons, was first theorized in 1964 by Peter Higgs, François Englert and Robert Brout, working from the ideas of Philip Anderson, and independently by G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble. Higgs proposed that the existence of a massive scalar particle could be a test of the theory, a remark added to his Physical Review letter at the suggestion of the referee. Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The electroweak theory predicts a neutral particle whose mass is not far from the W and Z bosons.
Note for Supersymmetry
In particle physics, supersymmetry (often abbreviated SUSY) is a symmetry that relates elementary particles of one spin to another particle that differs by half a unit of spin and are known as superpartners. In other words, in a supersymmetric theory, for every type of boson there exists a corresponding type of fermion, and vice-versa.
As of 2008 there is no direct evidence that supersymmetry is a symmetry of nature. Since superpartners of the particles of the Standard Model have not been observed, supersymmetry, if it exists, must be a broken symmetry allowing the 'sparticles' to be heavy.
If supersymmetry exists close to the TeV energy scale, it allows the solution of two major puzzles in particle physics. One is the hierarchy problem - on theoretical grounds there are huge expected corrections to the particles' masses, which without fine-tuning will make them much larger than they are in nature. Another problem is the unification of the weak interactions, the strong interactions and electromagnetism. Another advantage of supersymmetry is that supersymmetric quantum field theory can sometimes be solved. Supersymmetry is also a consequence of most versions of string theory, though it can exist in nature even if string theory is wrong.
Note for Standard Model
The standard model of particle physics is a theory that describes three of the four known fundamental interactions between the elementary particles that make up all matter. It is a quantum field theory developed between 1970 and 1973 which is consistent with both quantum mechanics and special relativity. To date, almost all experimental tests of the three forces described by the standard model have agreed with its predictions. However, the standard model falls short of being a complete theory of fundamental interactions, primarily because of its lack of inclusion of gravity, the fourth known fundamental interaction, but also because of the large number of numerical parameters (such as masses and coupling constants) that must be put "by hand" into the theory (rather than being derived from first principles).
The standard model is a grouping of two major theories – quantum electroweak and quantum chromodynamics – which provides an internally consistent theory describing interactions between all experimentally observed particles. Technically, quantum field theory provides the mathematical framework for the standard model. The standard model describes each type of particle in terms of a mathematical field. For a technical description of the fields and their interactions, see standard model (mathematical formulation).
Tags: Cern , European particle physics laboratory , protons , Alpine foothills , Big Bang , sub-atomic particles , spring fleetingly , Large Hadron Collider , Higgs boson , Standard Model , Peter Higgs , Edinburgh University , supersymmetry , dark matter , electromagnetic radiation ,
