quarta-feira, 11 de setembro de 2013



Bosons are particles which have integer spin and which therefore are not constrained by the Pauli exclusion principle like the half-integer spin fermions. The energy distribution of bosons is described by Bose-Einstein statistics. The wavefunction which describes a collection of bosons must be symmetric with respect to the exchange of identical particles, while the wavefunction for a collection of fermions is antisymmetric.

At low temperatures, bosons can behave very differently than fermions because an unlimited number of them can collect into the same energy state. The collection into a single state is called condensation, or Bose-Einstein condensation. It is responsible for the phenomenon of superfluidity in liquid helium. Coupled particles can also act effectively as bosons. In the BCS Theoryof superconductivity, coupled pairs of electrons act like bosons and condense into a state which demonstrates zero electrical resistance.

Bosons include photons and the characterization of photons as particles with frequency-dependent energy given by the Planck relationship allowed Planck to apply Bose-Einstein statistics to explain the thermal radiation from a hot cavity.

Bose-Einstein Condensation

In 1924 Einstein pointed out that bosons could "condense" in unlimited numbers into a single ground state since they are governed by Bose-Einstein statistics and not constrained by the Pauli exclusion principle. Little notice was taken of this curious possibility until the anomalous behavior of liquid helium at low temperatures was studied carefully.
When helium is cooled to a critical temperature of 2.17 K, a remarkable discontinuity in heat capacity occurs, the liquid density drops, and a fraction of the liquid becomes a zero viscosity "superfluid". Superfluidity arises from the fraction of helium atoms which has condensed to the lowest possible energy.
A condensation effect is also credited with producing superconductivity. In theBCS Theory, pairs of electrons are coupled by lattice interactions, and the pairs (called Cooper pairs) act like bosons and can condense into a state of zero electrical resistance.
The conditions for achieving a Bose-Einstein condensate are quite extreme. The participating particles must be considered to be identical, and this is a condition that is difficult to achieve for whole atoms. The condition of indistinguishabilityrequires that the deBroglie wavelengths of the particles overlap significantly. This requires extremely low temperatures so that the deBroglie wavelengths will be long, but also requires a fairly high particle density to narrow the gap between the particles.
Since the 1990s there has been a surge of research into Bose-Einstein condensation since it was discovered that Bose-Einstein condensates could be formed with ultra-cold atoms. The use of laser cooling and the trapping of ultra-cold atoms with magnetic traps has produced temperatures in the nanokelvin range. Cornell and Wieman along with Ketterle of MIT received the 2001 Nobel Prize in Physics "for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates". Cornell and Wieman led an active group at the University of Colorado, Boulder which has produced Bose-Einstein condensates with rubidium atoms. Other groups at MIT, Harvard and Rice have been very active in this rapidly advancing field.

Force carriers are particles that act like messages exchanged between other particles.

Scientists have discovered force carriers for three of the four known forces: electromagnetism, the strong force and the weak force. They are still searching for experimental evidence of the force carrier for the fourth force, gravity.

Particles communicate with one another in different languages, as defined by the kind of force carriers they exchange. Two particles can communicate with one another only if they are exchanging force carriers that convey a language they both understand. For example, a charged particle like an electron responds to force carriers for the electromagnetic force, but a neutral particle like a neutrino does not.

Sometimes two particles must be very close together to communicate via force carriers. They can “whisper” a message that would be too soft to extend over a long distance. Electrons and neutrinos can exchange W bosons, which are force carriers for the weak force, only when they are close to one another.

A force carrier can convey different messages. Protons and electrons, which have opposite charges, are attracted to one another through the electromagnetic force. The particles that carry that force, called photons, act like love notes. They draw the protons and electrons together.

When two electrons, which both have a negative charge, communicate through electromagnetism, the photons act more like hate mail. They push the electrons apart.

The Z boson is one of five particles that transmit the fundamental forces of nature. It is responsible for two of the most surprising discoveries of the 20th century-that nature has a “handedness” and that the physics of antimatter is subtly different from the physics of the matter-based world we see around us.

The W boson comes in positively and negatively charged varieties. They collaborate with another particle, the electrically neutral Z boson, to cause the force known as the weak interaction, which is responsible for some forms of nuclear decay, among other phenomena.

The W is very massive, which means its effects are very short range and very weak at everyday energies. Hence, the effects of these particles are subtle-but important! For example, the W can change the very nature of an interacting particle, turning an electron into a neutrino or a down quark into an up quark. This is important in the fusion reactions that power the sun, which involve protons turning into neutrons. Finally, the W provides the only established mechanism for allowing matter and antimatter to evolve in different ways.

When W bosons are created in particle accelerators, they live for only about 10-25 seconds, but they provide important tests of the Standard Model of particle physics.
Patricia Burchat, Stanford University

The Z boson

The Z boson is a heavy particle that is one of the carriers of the ‘weak force'. It is a partner of the W+ and W-bosons that mediate radioactive decay processes.

The Z boson was first discovered as an intermediary of a new type of neutrino reaction. This so-called ‘neutral current interaction' was the missing piece of a puzzle in which the forces created by the W bosons fit together neatly with the force of electromagnetism, due to the photon. Together, these four particles create the forces that form a beautifully unified theory of ‘electroweak' interactions.

In the 1990s, accelerators at the Stanford Linear Accelerator Center and CERN produced 12 million of these Z bosons in a controlled setting and studied the decays of the Z in great detail. The Z decays to pairs of all types of quarks and leptons, except for the heavy top quark. These experiments made high precision tests of the electroweak theory and the properties of quarks and leptons. Quarks produced from the Z radiate gluons, and so these experiments also give some of the highest-precision information about the carrier of the ‘strong' interactions.

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