W and Z bosons

The W and Z⁰ bosons are the elementary particles that mediate the weak force. Their discovery was a major success for what is now called the Standard Model of particle physics.

Name

The W particle is named after the weak nuclear force. The physicist Steven Weinberg named the additional particle the Z particle, giving no explanation.^[3] It has been speculated that the Z particle was semi-humorously given its name because it was said to be the last particle to need discovery and has zero electric charge.^[4]

Basic properties

There are two types of W bosons; the W⁺ with an electric charge of +1 e and its antiparticle, the W⁻ with an electric charge of −1 e. The Z boson (or Z particle) is electrically neutral, and it is its own antiparticle. All three of these particles are very short-lived with a half-life of about 3×10⁻²⁵ s. In practice, they can be considered to be virtual particles.

These bosons are among the heavyweights of the elementary particles. With masses of 80.4 GeV/c² and 91.2 GeV/c², respectively, the W and Z particles are almost 100 times as massive as the proton—heavier than entire atoms of iron. The masses of these bosons are significant because they act as the force carriers of a quite short-range fundamental force: their high masses thus limit the range of the weak nuclear force. By way of contrast, the electromagnetic force, has an infinite range because its force carrier, the photon, has zero rest mass.

All three types of these W and Z particles have particle spin values of plus or minus one. The emission of a W⁺ or W⁻ boson either raises or lowers the electric charge of the emitting particle by one unit, and also alters the spin by one unit. At the same time, the emission or absorption of a W boson can change the type of the particle - for example changing a strange quark into an up quark. The neutral Z boson obviously cannot change the electric charge of any particle, nor can it change any other of the so-called "charges" (such as strangeness, baryon number, charm, etc.). The emission or absorption of a Z particle can only change the spin, momentum, and energy of the other particle. (See also, weak neutral current).

Weak nuclear force

The Feynman diagram for beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate heavy W boson

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 bosons are best known for their role in nuclear decay. Consider, for example, the beta decay of cobalt-60, an important process in supernova explosions.

6027Co → 6028Ni + e⁻ + ν_e

This reaction does not involve the whole cobalt-60 nucleus, but affects only one of its 33 neutrons. The neutron is converted into a proton while also emitting an electron (called a beta particle in this context) and an electron antineutrino:

n⁰ → p⁺ + e⁻ + ν_e

Again, the neutron is not an elementary particle but a composite of an up quark and two down quarks (udd). It is in fact one of the down quarks that interacts in beta decay, turning into an up quark to form a proton (uud). At the most fundamental level, then, the weak force changes the flavour of a single quark:

d → u + W⁻

which is immediately followed by decay of the W⁻ itself:

W⁻ → e⁻ + ν_e

The Z boson is its own antiparticle. Thus, all of its flavour quantum numbers and charges are zero. 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.

Predicting the W and Z

A Feynman diagram showing the exchange of a pair of W bosons. This is one of the leading terms contributing to neutral Kaon oscillation.

Following the spectacular success of quantum electrodynamics in the 1950s, attempts were undertaken to formulate a similar theory of the weak nuclear force. This culminated around 1968 in a unified theory of electromagnetism and weak interactions by Sheldon Glashow, Steven Weinberg, and Abdus Salam, for which they shared the 1979 Nobel Prize in physics.^[5] Their electroweak theory postulated not only the W bosons necessary to explain beta decay, but also a new Z boson that had never been observed.

The fact that the W and Z bosons have mass while photons are massless was a major obstacle in developing electroweak theory. These particles are accurately described by an SU(2) gauge theory, but the bosons in a gauge theory must be massless. As a case in point, the photon is massless because electromagnetism is described by a U(1) gauge theory. Some mechanism is required to break the SU(2) symmetry, giving mass to the W and Z in the process. One explanation, the Higgs mechanism, was forwarded by Peter Higgs and others in the mid 1960s. It predicts the existence of yet another new particle, the Higgs boson.

The combination of the SU(2) gauge theory of the weak interaction, the electromagnetic interaction, and the Higgs mechanism is known as the Glashow-Weinberg-Salam model. These days it is widely accepted as one of the pillars of the Standard Model of particle physics. As of mid-2010, despite intensive search for the Higgs boson carried out at CERN and Fermilab, its existence remains the main prediction of the Standard Model not to be confirmed experimentally.

Discovery

The Gargamelle bubble chamber, now exhibited at CERN

The discovery of the W and Z particles was considered a major success for CERN. 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 Super Proton Synchrotron, 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 (led by Peter Jenni)^[6], 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.^[7]

The W⁺, W⁻, and Z⁰ bosons, together with the photon (γ), build up the four gauge bosons of the electroweak interaction.

Decay

The W and Z bosons decay to fermion-antifermion pairs but neither the W nor the Z boson can decay into the higher-mass top quark. Neglecting phase space effects and higher order corrections, simple estimates of their branching fractions can be calculated from the coupling constants.

W bosons

W bosons can decay to a lepton and neutrino or to an up-type quark and a down-type quark. The decay width of the W boson to a quark-antiquark pair is proportional to the corresponding squared CKM matrix element and the number of quark colours, N_C = 3. The decay widths for the W boson are then proportional to:

Here, e⁺, μ⁺, τ⁺ denoted the three flavours of leptons (more exactly, the positive charged anti leptons). ν_e, ν_μ, ν_τ denote the three flavours of neutrinos. The other particles starting with u and d all denote quarks and anti-quarks (factor N_C is applied). V is the CKM matrix with its coefficients.

Unitarity of the CKM matrix implies that |V_ud|² + |V_us|² + |V_ub|² =  |V_cd|² + |V_cs|² + |V_cb|² = 1. Therefore the leptonic branching ratios of the W boson are approximately B(e⁺ν_e) = B(μ⁺ν_μ) = B(τ⁺ν_τ) = ¹⁄₉ (~11.11%). The hadronic branching ratio is dominated by the CKM favored ud and cs final states, and the sum of the hadronic branching ratios is roughly ²⁄₃ (~66.67%). The branching ratios have been measured experimentally: B(l⁺ν_l) = 10.80±0.09% and B(hadrons) =67.60±0.27%.^[8]

Z bosons

Z bosons decay into a fermion and its antiparticle. The decay width of a Z boson to a fermion-antifermion pair is proportional to the square of the weak charge T₃ − Q·x, where T₃ is the third component of the weak isospin of the fermion, Q is the charge of the fermion (in units of the elementary charge), and x = sin²θ_W, where θ_W is the weak mixing angle. Because the weak isospin is different for fermions of different chirality, either left-handed or right-handed), the coupling is different as well. The decay width of the Z boson for quarks is also proportional to N_C.

Here, L and R denote the chirality of the fermions, i. e. left-handed and right-handed, respectively. The right-handed neutrinos do not exist in the standard model. However, in some extensions beyond the standard model they do.^[10][11]

See also

• Standard model (mathematical formulation)
• List of particles
• X and Y bosons: analogous pair of bosons predicted by GUT
• W' and Z' bosons

References

External links

• The Review of Particle Physics, the ultimate source of information on particle properties.
• W and Z page from CERN
• W and Z particles at Hyperphysics
• Z particle at Everything2