Carlo Rubbia

W and Z Bosons

In physics, the W and Z bosons are the elementary particles that mediate the weak nuclear force. Their discovery at CERN in 1983 has been heralded as a major success for the Standard Model of particle physics.

The W particle is named after the weak nuclear force. The Z particle was semi-humorously given its name because it was said to be the last particle to need discovery. Another explanation is that the Z particle derives its name from the fact that it has zero electric charge.

Basic properties

Two kinds of W boson exist with +1 and ?1 elementary units of electric charge; the W⁺ is the antiparticle of the W^?. The Z boson is electrically neutral and is its own antiparticle. All three particles are very short-lived with a mean life of about 3 × 10^?25 seconds.

These bosons are heavyweights among the elementary particles. With a mass of 80.4 and 91.2 GeV/c², respectively, the W and Z particles are almost 100 times as massive as the proton—heavier than atoms of iron. The mass of these bosons is significant because it limits the range of the weak nuclear force. The electromagnetic force, by contrast, has an infinite range because its boson (the photon) is massless.

The weak nuclear force

The W and Z bosons are carrier particles that mediate the weak nuclear force, much like the photon is the carrier particle for the electromagnetic force. The W boson is best known for its role in nuclear decay. Being its own antiparticle, the Z boson has all zero quantum numbers. The exchange of a Z boson between particles, called a neutral current interaction, therefore leaves the interacting particles unaffected, except for a transfer of momentum. Unlike beta decay, the observation of neutral current interactions requires huge investments in particle accelerators and detectors, such as are available in only a few high-energy physics laboratories in the world.

Discovery of the W and Z

The discovery of the W and Z particles is a major CERN success story. First, in 1973, came the observation of neutral current interactions as predicted by electroweak theory. The huge Gargamelle bubble chamber photographed the tracks of a few electrons suddenly starting to move, seemingly of their own accord. This is interpreted as a neutrino interacting with the electron by the exchange of an unseen Z boson. The neutrino is otherwise undetectable, so the only observable effect is the momentum imparted to the electron by the interaction.

The discovery of the W and Z particles themselves had to wait for the construction of a particle accelerator powerful enough to produce them. The first such machine that became available was the SPS, where unambiguous signals of W particles were seen in January 1983 during a series of experiments conducted by Carlo Rubbia and Simon van der Meer. (The actual experiments were called UA1 (led by Rubbia) and UA2, and were the collaborative effort of many people. Van der Meer was the driving force on the accelerator end (stochastic cooling).) UA1 and UA2 found the Z a few months later, in May 1983. Rubbia and van der Meer were promptly awarded the 1984 Nobel Prize in physics, a most unusual step for the conservative Nobel Foundation.

Carlo Rubbia was born in the small town of Gorizia, Italy, in 1934. After high school, he studied in the Faculty of Physics at the Scuola Normale in Pisa. In 1958, he went to the United States to widen his experience and to familiarize himself with particle accelerators.
Around 1960, he moved back to Europe, attracted by the newly founded CERN where he worked on experiments on the structure of weak interactions. In 1976, he suggested adapting CERN's Super Proton Synchrotron (SPS) to collide protons and antiprotons in the same ring and the world's first antiproton factory was built. The collider started running in 1981 and, in January 1983, came the announcement, first from the UA1 detector, that W particles had been created. A couple of months later the even more elusive Z particles were also observed.
The Nobel Prize in Physics 1984	The following year, 1984, Carlo Rubbia and Simon van der Meer shared the Nobel prize for physics, one of the shortest intervals ever between discovery and award.
W and Z Bosons In physics, the W and Z bosons are the elementary particles that mediate the weak nuclear force. Their discovery at CERN in 1983 has been heralded as a major success for the Standard Model of particle physics. The W particle is named after the weak nuclear force. The Z particle was semi-humorously given its name because it was said to be the last particle to need discovery. Another explanation is that the Z particle derives its name from the fact that it has zero electric charge.
Basic properties Two kinds of W boson exist with +1 and ?1 elementary units of electric charge; the W⁺ is the antiparticle of the W^?. The Z boson is electrically neutral and is its own antiparticle. All three particles are very short-lived with a mean life of about 3 × 10^?25 seconds. These bosons are heavyweights among the elementary particles. With a mass of 80.4 and 91.2 GeV/c², respectively, the W and Z particles are almost 100 times as massive as the proton—heavier than atoms of iron. The mass of these bosons is significant because it limits the range of the weak nuclear force. The electromagnetic force, by contrast, has an infinite range because its boson (the photon) is massless.	The weak nuclear force The W and Z bosons are carrier particles that mediate the weak nuclear force, much like the photon is the carrier particle for the electromagnetic force. The W boson is best known for its role in nuclear decay. Being its own antiparticle, the Z boson has all zero quantum numbers. The exchange of a Z boson between particles, called a neutral current interaction, therefore leaves the interacting particles unaffected, except for a transfer of momentum. Unlike beta decay, the observation of neutral current interactions requires huge investments in particle accelerators and detectors, such as are available in only a few high-energy physics laboratories in the world.
Discovery of the W and Z The discovery of the W and Z particles is a major CERN success story. First, in 1973, came the observation of neutral current interactions as predicted by electroweak theory. The huge Gargamelle bubble chamber photographed the tracks of a few electrons suddenly starting to move, seemingly of their own accord. This is interpreted as a neutrino interacting with the electron by the exchange of an unseen Z boson. The neutrino is otherwise undetectable, so the only observable effect is the momentum imparted to the electron by the interaction. The discovery of the W and Z particles themselves had to wait for the construction of a particle accelerator powerful enough to produce them. The first such machine that became available was the SPS, where unambiguous signals of W particles were seen in January 1983 during a series of experiments conducted by Carlo Rubbia and Simon van der Meer. (The actual experiments were called UA1 (led by Rubbia) and UA2, and were the collaborative effort of many people. Van der Meer was the driving force on the accelerator end (stochastic cooling).) UA1 and UA2 found the Z a few months later, in May 1983. Rubbia and van der Meer were promptly awarded the 1984 Nobel Prize in physics, a most unusual step for the conservative Nobel Foundation.
LINKS http://www.nationmaster.com/encyclopedia/Z-boson http://www.nationmaster.com/encyclopedia/Carlo-Rubbia http://www.nobel.se/physics/laureates/1984/

W and Z Bosons

Basic properties

The weak nuclear force

Discovery of the W and Z

LINKS

http://www.nobel.se/physics/laureates/1984/