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ScienceWeek
PARTICLE PHYSICS: ON ELECTRON ACCELERATION BY PLASMA WAVES
The following points are made by Thomas Katsouleas (Nature 2004 431:515):
1) Huge particle accelerators have been at the vanguard of research in particle physics for more than half a century. Through high-energy collisions of accelerated particles, the fundamental building-blocks and forces of nature have been revealed. The latest project, the Large Hadron Collider (LHC) currently under construction at CERN in Geneva, will attempt to find the Higgs boson, a particle associated with the mechanism through which all other known particles are thought to acquire their masses. But the size and cost of such machines -- for the LHC, a 27-km circumference and several billion euros -- are fuelling a serious effort to develop new and more compact accelerator technologies. Fresh progress is being made using a principle known as "plasma wakefield acceleration".(1-3)
2) Plasmas -- gaseous "soups" of dissociated electrons and ions -- offer a means of acceleration that could be realized on a table top(4). Waves can be generated in a plasma using short laser pulses; electrons or their antimatter counterparts, positrons, can then "surf" the electric field of a wave's wake. Particles have been accelerated in wakefields at rates that are more than a thousand times higher than those achieved in accelerators based on conventional large-scale technology. However, whether plasma wakefield accelerators could produce the high quality of beam needed for applications in high-energy physics, and in other areas of research and medicine, remained in question. Recent results (1-3) are a milestone in this regard. They provide the first demonstration that a beam of electrons can be accelerated in a wakefield to a single energy. Moreover, these beams are of high quality (having a small angular divergence) and significant charge (about 10^(9) electrons).
3) In a conventional accelerator, charged particles such as electrons, protons or their antiparticles are accelerated by an alternating radio-frequency electric field through long metallic cavities (around a meter long for medical applications, but several kilometers long for high-energy physics). The rate of acceleration is limited by the peak power of the radio-frequency source and ultimately by electrical breakdown at the metal walls of the accelerator. Laser-driven plasma waves overcome both of these limitations: the high peak power of lasers is unmatched, and the plasma, as it is already an ionized gas, is impervious to electrical breakdown. In 1995, Modena et al.(5) made clear the remarkable potential of this scheme, and it has been confirmed by subsequent experiments. Using the radiation pressure of a laser to drive a compressive oscillation in the plasma (like a sound wave, but with electrostatic repulsion rather than pressure as the restoring force), electrons have been accelerated from rest to an energy of 100 megaelectronvolts (MeV) within a distance of 1 mm -- more than 5000 times shorter than the distance required to reach that energy in a conventional accelerator.
References (abridged):
1. Geddes, C. G. R. et al. Nature 431, 538-541 (2004)
2. Mangles, S. P. D. et al. Nature 431, 535-538 (2004)
3. Faure, J. et al. Nature 431, 541-544 (2004)
4. Joshi, C. & Katsouleas, T. Physics Today 56, No. 6, 47-51 (2003)
5. Modena, A. et al. Nature 377, 606-608 (1995)
Nature http://www.nature.com/nature
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Related Material:
PARTICLE PHYSICS: THE SEARCH FOR THE HIGGS BOSON
Notes by ScienceWeek:
The current "Standard Model" in particle physics is essentially a combination of two theories into a single framework to describe all interactions of subatomic particles, except those interactions due to gravity. The two theories are a) "*electroweak theory", which describes interactions via the electromagnetic force and the "*nuclear weak force", and b) *quantum chromodynamics, which is the theory of the *strong nuclear force. Both of these theories describe the interactions between particles in terms of the exchange of intermediate "messenger" particles that have one unit of intrinsic angular momentum ("spin").
In addition to these force-carrying particles, the Standard Model encompasses two families of subatomic particles that comprise matter and that have spins of one-half unit. These particles are the *quarks and the *leptons, of which there are 6 varieties ("flavors") of each, related in pairs in three "generations" of increasing mass. Ordinary matter is built from the lightest generation; heavier types of quark and lepton have been discovered in studies of high-energy particle interactions.
According to current physics, all particles in nature are either fermions or bosons, with fermions (always elementary particles) having half-integer spin (spin-states characterized by half-integer multiples of Planck's constant divided by 2pi), and bosons (all other particles) having integer spin (spin-states characterized by integer multiples of Planck's constant divided by 2pi). In general, bosons are particles that obey *Bose-Einstein statistics, and they include photons, *pi mesons, all nuclei having an even number of particles, and all particles with integer or zero spin.
In theory, there are 6 types of quarks, the so-called "top quark" the most massive and with an electric charge of +2/3. In 1995, two independent groups reported they had found the top quark, and they established the mass of the top quark as approximately 176 billion electron volts (176 Gev).
The "Z particle" is an electrically neutral carrier of the weak nuclear force that acts upon all known subatomic particles. The mass of the Z particle (93 GeV) is approximately 100 times the mass of the proton, and it has a lifetime of only 10^(-25) seconds. The Z particle is essentially the neutral partner of the related electrically-charged "W particle". The W particle has either positive or negative charge, both with a mass of 83 GeV. Direct evidence for both Z and W particles was obtained in 1983.
In general, the "Higgs particle" (named after Peter Higgs, who proposed the particle in 1964, is the carrier of an all-pervading fundamental field ("Higgs field") hypothesized to endow elementary particles with mass via interactions of the field with the elementary particles. The idea is that the variety of masses of elementary particles arises because the different particles have different strengths of interaction with the Higgs field. The Higgs field is without direction (scalar), and the Higgs particle, which "carries" the Higgs field (the particle corresponding to perturbations in the Higgs field), is a spin-zero particle with a non-zero mass. Since it is spin-zero, the Higgs particle is thus a boson, and so it called the "Higgs boson."
The following points are made by M. Riordan et al (Science 2000 291:259):
1) A critical requirement of the Standard Model is a means to endow elementary particles with the property of mass. According to the Higgs mechanism, particle masses are the result of an invisible energy field (Higgs field) that permeates space and confers inertia upon most elementary particles. Without such an all-pervasive ethereal medium, the elementary particles would remain forever massless like the photon -- "racing about at light speed and never coalescing into galaxies, stars, planets, and people."
2) According to the wave-particle duality of quantum mechanics, the Higgs field should become manifest as a spinless particle called the "Higgs boson", which corresponds to disturbances in the field. Searches for this object have occurred at ever higher energies since the late 1970s, and possible evidence for its existence was recently obtained at the European Center for Particle Physics (CERN). New searches are about to begin on the recently upgraded Tevatron collider at the Fermi National Accelerator Laboratory (US).
3) The evidence so far indicates that the Higgs boson is a very massive particle, heavier than all known elementary particles except possibly the top quark. There are indications of the direct production of a Higgs boson near 115 Gev, a mass-energy which would be in a agreement with recent indirect evidence that such a particle should have a mass less than approximately twice that of the Z particle.
4) The authors conclude: "The advanced linear electron-positron colliders now being designed in Germany, Japan, and the United States are ideally suited for detailed studies of such a relatively light Higgs boson."
Science http://www.sciencemag.org
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Notes by ScienceWeek:
electroweak theory: In 1979, Sheldon Glashow, Abdus Salam, and Steven Weinberg shared the Nobel Prize in Physics for their formulation of electroweak theory, which is essentially the mathematical unification of the electromagnetic force and the nuclear weak force.
nuclear weak force: The nuclear weak interactions are the interactions involved in radioactive decay.
quantum chromodynamics: Quantum chromodynamics (QCD) is a theory that describes the strong interaction (strong nuclear force) in terms of quarks and antiquarks and the exchange of massless "gluons" between them. The "chromo-" in chromodynamics derives from the use of designated "color" attributes of quarks, the various "colors" labels for various quark properties.
strong nuclear force: The current view among nuclear physicists is as follows: each nucleus contains a population of protons and neutrons, collectively known as nucleons, as well as a host of other particles that bind the nucleons together. Each nucleon, in turn, is made up of three quarks bound by what is called the strong nuclear force.
quarks: A quark is a hypothetical fundamental particle, having charges whose magnitudes are one-third or two-thirds of the electron charge, and from which the elementary particles may in theory be constructed.
leptons: A class of elementary particles. Although they are affected by electromagnetic and gravitational forces, apart from that they are involved only with weak interactions, acted upon by weak forces but not by strong forces, as opposed to quarks, which are acted upon by strong forces but not by weak forces. One further difference between leptons and quarks is that leptons can be isolated as single particles, whereas quarks apparently cannot. The leptons include the electron, the muon, the tau, and their associated neutrinos. The mass of the tau is approximately 3484 times the mass of the electron; the mass of the muon is intermediate.
Bose-Einstein statistics: Bose-Einstein statistics is the statistical mechanics of a system of indistinguishable particles for which there is no restriction on the number of particles that may simultaneously exist in the same quantum energy state. Particles that obey Bose-Einstein statistics are called "bosons".
pi mesons: (pions) Pi mesons are subatomic particles with masses approximately 270 times the mass of the electron.
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