Everything about Z Boson totally explained
In
physics, the
W and Z bosons are the
elementary particles that mediate the
weak force. Their discovery 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
bosons exist with +1 and −1 elementary units of
electric charge; the is the
antiparticle of the . The Z boson (or ) is electrically
neutral and is its own antiparticle. All three particles are very short-lived with a
mean life of about .
These bosons are heavyweights among the elementary particles. With a mass of and, respectively, the W and particles are almost 100 times as massive as the
proton—heavier than entire
atoms of
iron. The
mass of these bosons are significant because they limit the range of the weak nuclear force. The
electromagnetic force, by contrast, has an infinite range because its boson (the
photon) is massless.
All three types have a
spin of 1.
The emission of a or boson can either raise or lower electric charge of the emitting particle by 1 unit, and alter the spin by 1 unit. At the same time a W boson can change the generation of the particle, for example changing a
strange quark to an
up quark. The boson can't change either electric charge nor any other charges (like strangeness, charm, etc.), only spin and momentum, so it never changes the generation or flavor of the particle emitting it (see
weak neutral current).
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. Consider, for example, the
beta decay of
cobalt-60, an important process in
supernova explosions.
» → + +
This reaction doesn't 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
antineutrino:
» → + +
Again, the neutron isn't 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
flavor of a single quark:
» → +
which is immediately followed by decay of the W
− itself:
» → +
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.
Predicting the W and Z
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
. 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 in the late
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's widely accepted as one of the pillars of the Standard Model of particle physics.
As of 2008, 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 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
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 Darriulat), 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.
Decay
The W and Z bosons decay to fermion-antifermion pairs. Neglecting phase space effects and higher order corrections, simple estimates of their banching fractions can be calculated from the coupling constants.
W bosons can decay to a lepton and neutrino or to an up-type quark and a down-type quark. The W can't decay to the higher-mass top 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 colors,
N
C = 3. The decay widths for the W boson are then proportional to
| e+νe |
1 |
| μ+νμ |
1 |
| τ+ντ |
1 |
| ud |
>Vud|2 |
| us |
>Vus|2 |
| ub |
>Vub|2 |
| cd |
>Vcd|2 |
| cs |
>Vcs|2 |
| cb |
>Vcb|2 |
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
τ) = B(τ
+ν
τ) = 1/9. The hadronic branching ratio is dominated by the CKM favored u
d and c
s final states, and the sum of the hadronic branching ratios is roughly 2/3. The branching ratios have been measured experimentally: B(l
+ν
l) = 10.80±0.09% and
B(hadrons) = 67.60±0.27%.
Z bosons can decay to a fermion and its antiparticle, with the exception of the top quark, which is too massive. The decay width of a Z boson to a fermion-antifermion pair is proportional to the square of the weak charge T
3-Qx, where T
3 is the third component of the weak isospin of the fermion, Q is the charge of the fermion, and x = sin
2θ
W, where
θ
W is the weak mixing angle. Because the weak isospin is different for left-handed and right-handed fermions, the coupling is different as well. The decay width of the Z boson for quarks is also proportional to N
C. The weak charge of the fermions is:
| (νe, νμ, ντ)L |
½ |
| (e, μ, τ)L |
-½+x |
| (e, μ, τ)R |
x |
| (u, c)L |
½-⅔x |
| (d, s, b)L |
-½+⅓x |
| (u, c)R |
-⅔x |
| (d, s, b)R |
⅓x |
The decay widths of the Z boson are then proportional to
| (νe, νμ, ντ) |
½2 |
| (e, μ, τ) |
(-½+x)2+x2 |
| (u, c) |
3(½-⅔x)2+3(-⅔x)2 |
| (d, s, b) |
3(-½+⅓x)2+3(⅓x)2 |
For x = 0.23, the branching ratios of the Z boson are predicted to be B(ν
ν) = 20.5%, B(e
+e
-) = B(μ
+μ
-) = B(τ
+τ
-) = 3.4%, B(u
u) = B(c
c) = 11.8%, B(d
d) = B(s
s) = B(b
b) = 15.2% and B(hadrons) = 69.2%. The branching ratios have been measured experimentally: B(l
+l
+) = 3.3658±0.0023%, B(ν
ν) = 20.00±0.06%, B(hadrons) = 69.91±0.06%, B(u
u + c
c) = 11.6±0.6% and B(d
d + s
s + b
b) = 15.6±0.4%.
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