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ScienceWeek
PARTICLE PHYSICS: ON THE SEARCH FOR THE HIGGS BOSON
The following points are made by Peter Renton (Nature 2004 428:141):
1) The standard model (SM) of particle physics has developed as a result of painstaking work, over several decades, by a large number of physicists. It is a quantum field theory, combining quantum mechanics and special relativity. It unifies the familiar force of electromagnetism (for example, electricity) with the weak force (such as the reactions that power the Sun) into a unified electroweak force. It also includes the strong force (such as that which binds protons and neutrons to form atomic nuclei). Despite intense experimental effort, no significant deviations from the SM(1) have been observed, so it remains the cornerstone of particle physics.
2) In the standard model, the particles are of two main types. First, we have the spin-1/2 fermions, which in turn can be split into leptons (both charged and their neutral neutrino partners) and quarks. Second, we have the spin-1 gauge bosons, which "transmit" the electroweak and strong forces between the fermions. The electromagnetic force is transmitted by the massless photon, whereas the weak force is transmitted by the very massive W and Z bosons. In the underlying theory of the SM there are three W bosons; W+ and W- and a neutral W-zero (W-0) boson, plus another neutral boson called B-zero (B-0). However, we observe physically two neutral bosons, the photon and Z boson. These are quantum mechanical "mixtures" of the W0 and B0 states. The parameter that describes this mixing is called the "weak" mixing angle, theta-w, and is an important parameter of the model. Measurements of sin^(2)(theta-w) from a variety of processes give reasonably consistent values, and this consistency is an important test of the SM.
3) Sixteen particles of the standard model have been discovered. However, with no other ingredients the SM would require them all to be massless, in clear contradiction with nature. The final particle of the SM, the Higgs boson (H), comes to the rescue. It is very important because it is responsible for the mechanism (the so-called Higgs mechanism(2,3) by which all other particles acquire mass.
4) The search for the Higgs boson is a high priority in particle physics. The Higgs boson is highly unstable and, once produced, decays very quickly to either a fermion-antifermion pair or a pair of bosons. By energy conservation, the Higgs mass, M-H, must be at least twice that of the particle in the pair to which it decays. The mass of the Higgs boson is not specified in the SM, but the strength of the "coupling" of the Higgs boson to a given particle is proportional to that particle's mass, so that the Higgs boson decays preferentially to the most massive particles that are allowed by energy and momentum conservation. The direct search involves first producing the Higgs boson and then detecting the decay products. The mass range that can be explored depends on the energy of the particle collider used. The most powerful and energetic tool available so far in this search is the electron-positron collider, LEP, at CERN in Geneva.
5) In summary: The standard model of particle physics describes the strong and electroweak interactions of fermions (spin-1/2), gauge bosons (spin-1), and a final vital ingredient -- the spin-0 Higgs boson, which gives masses to the other particles. But the Higgs boson has yet to be discovered, and its own mass is not specified by the theory. There is some evidence (although statistically not very significant) for its detection at a mass of about 115 GeV/c^(2), from electron-positron interactions at LEP (the Large Electron Positron collider). Indirect methods can also be used to constrain the mass of the Higgs boson, because it affects other observable quantities (for example, the mass of the W boson and some measurable properties of the Z boson). An indirect determination of the Higgs boson mass from the most recent measurements of such quantities yields a value compatible with 115 GeV/c^(2), but with some important caveats arising from inconsistencies in the present data.(4,5)
References (abridged):
1. Renton, P. B. Electroweak Interactions (Cambridge Univ. Press, Cambridge, 1990)
2. Higgs, P. W. Broken symmetrics, massless particles and gauge fields. Phys. Lett. 12, 132-133 (1964)
3. Higgs, P. W. Spontaneous symmetry breaking without massless bosons. Phys. Rev. 145, 1156-1163 (1966)
4. ALEPH, DELPHI, L3 and OPAL Collaborations. Search for the Standard Model Higgs Boson at LEP. Phys. Lett. B 565, 61-75 (2003)
5. The LEP Collaborations, ALEPH, DELPHI, L3 and OPAL. Combined Preliminary Data on Z Parameters from the LEP Experiments and Constraints on the Standard Model (CERN/PPE/94-187 CERN, Geneva, 1994)
Nature http://www.nature.com/nature
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PARTICLE PHYSICS: THE SEARCH FOR THE HIGGS BOSON
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 2[pi]), and bosons (all other particles) having integer spin (spin-states characterized by integer multiples of Planck's constant divided by 2[pi]). 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) The authors point out that 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 authors point out that 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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