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8.10.2.3. W Bosons

Table of Contents

Role in the Weak Interaction

W bosons are the particles that carry the charged weak interaction. There are two of them, $W^+$ and $W^-$. They are responsible for processes in which electric charge is transferred from one particle to another in a way that changes particle type, especially in weak decays.

A simple example is beta decay. In beta minus decay, a neutron changes into a proton, an electron, and an antineutrino. At the quark level, one down quark changes into an up quark by emitting a $W^-$ boson, and that $W^-$ then produces the electron and antineutrino.

$$
d \to u + W^-
$$

$$
W^- \to e^- + \bar{\nu}_e
$$

In beta plus decay, the opposite charged boson appears:

$$
u \to d + W^+
$$

$$
W^+ \to e^+ + \nu_e
$$

This is why the W bosons are called charged weak bosons. Unlike the photon, which has no electric charge, the W bosons themselves carry charge.

The W bosons mediate the charged weak interaction and come in two types:
$$
W^+, \quad W^-
$$
They are exchanged in processes that change particle flavor and transfer electric charge.

Electric Charge and Basic Properties

The electric charges of the two W bosons are

$$
Q(W^+) = +e, \qquad Q(W^-) = -e
$$

where $e$ is the magnitude of the elementary charge. They are very massive compared with many other force carriers. Their mass is about

$$
m_W \approx 80.4 \, \text{GeV}/c^2
$$

Because they are so heavy, the weak interaction has a very short range. A heavier exchange particle leads to a force that acts only over extremely small distances.

They are also spin 1 particles, so they belong to the class of bosons. As force carriers, they are gauge bosons of the weak interaction.

Property$W^+$$W^-$
Electric charge$+e$$-e$
Spin11
Approximate mass$80.4 \, \text{GeV}/c^2$$80.4 \, \text{GeV}/c^2$
Interaction typeCharged weakCharged weak

Important facts about W bosons:
$$
Q(W^\pm) = \pm e
$$
$$
m_W \approx 80.4 \, \text{GeV}/c^2
$$
Their large mass is a key reason why the weak force is short-ranged.

How W Bosons Change Particle Type

One special feature of the weak interaction is that it can change one flavor of particle into another. W bosons are central to this.

For quarks, a W boson can convert an up-type quark into a down-type quark, or the reverse. For example,

$$
u \leftrightarrow d
$$

with the emission or absorption of a $W^\pm$. Similar processes can happen with other quark families as well.

For leptons, W bosons connect a charged lepton with its corresponding neutrino. For example,

$$
W^- \to e^- + \bar{\nu}_e
$$

$$
W^- \to \mu^- + \bar{\nu}_\mu
$$

$$
W^- \to \tau^- + \bar{\nu}_\tau
$$

and similarly for $W^+$ with antiparticles reversed.

This means the W boson is involved whenever the weak interaction changes charge and often when it changes flavor.

Typical Interaction Patterns

A useful way to think about the W boson is as a link between members of weak pairs. For leptons, the pairs are

$$
(\nu_e, e^-), \qquad (\nu_\mu, \mu^-), \qquad (\nu_\tau, \tau^-)
$$

The $W^-$ boson can connect the neutral member to the negatively charged member, and the $W^+$ boson does the reverse.

For quarks, a similar connection exists between up-type and down-type quarks.

W boson in beta minus decay

This drawing shows the sequence at the quark level. A down quark emits a $W^-$ and becomes an up quark. The $W^-$ then decays into an electron and an electron antineutrino.

Real and Virtual W Bosons

In particle interactions, a W boson may appear as a real particle if enough energy is available, or as a virtual particle that exists only during the interaction.

In ordinary radioactive decay, the W boson is usually virtual. It is not directly observed as a free long-lived particle. In high-energy collisions, however, real W bosons can be produced and then detected through their decay products.

Because W bosons are unstable, they decay very quickly. They do not travel far before decaying into lighter particles.

W bosons are unstable and short-lived. In many weak processes, the exchanged W is virtual rather than directly observed.

W Boson Decays

Once produced, a W boson can decay into a lepton and neutrino, or into a quark and antiquark pair, if allowed by energy and other conservation laws.

Examples include

$$
W^- \to e^- + \bar{\nu}_e
$$

$$
W^- \to \mu^- + \bar{\nu}_\mu
$$

$$
W^- \to \tau^- + \bar{\nu}_\tau
$$

and hadronic decays such as

$$
W^+ \to u + \bar{d}
$$

These decay patterns are very important in particle detectors, because the W boson is identified from what it decays into.

Why W Bosons Matter

W bosons are essential because they make possible processes that would not occur through electromagnetism or the strong interaction alone. They are responsible for many forms of radioactive decay and for reactions involving neutrinos.

Without W bosons, the Sun’s nuclear reactions, beta decay, and many particle transformations would not happen in the same way. They are therefore central to both nuclear physics and particle physics.

Comparison with Other Exchange Particles

It is helpful to compare the W bosons with a few other force carriers.

ParticleInteractionElectric chargeMass
PhotonElectromagnetic00
GluonStrong00
$W^\pm$Charged weak$\pm e$Large
$Z^0$Neutral weak0Large

The most distinctive feature of the W bosons in this table is that they are both massive and electrically charged.

Key identity of the W bosons:
They are the charged, massive carriers of the weak interaction, and they allow processes such as
$$
d \to u + W^- \to u + e^- + \bar{\nu}_e
$$
which underlies beta minus decay.

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