Particle decay

Tiny bits of stuff can change. One bit can turn into many new bits. This happens all by itself.

These tiny bits are very small. Some bits are not steady. They want to change into something else. This is called decay.

When a bit changes, it makes new bits. The new bits are smaller. But the total weight stays the same.

Some bits change very fast. Other bits stay for a long time. One bit can last for years. Another bit lasts for only a tiny moment.

This process helps us learn about our world. It is a very big wonder.

Tiny bits of matter can change on their own. This is called particle decay. It happens when one unstable particle turns into other particles. An unstable particle is one that can change.

When this happens, the new particles are smaller. Each one must have less mass than the first. But the total mass stays the same. This is called conservation. Forces in nature help make these changes happen.

Some bits change very fast. Others last a long time. A muon lasts for 2.2 seconds. A tau lepton lasts for 2.9 seconds. A neutron lasts for about 885.7 seconds. Some bits stay for a very long time. An electron can last for more than 6.6 years.

Sometimes, the new bits are also unstable. They might decay again into even smaller bits. This can happen in many different ways. Scientists use math to find the chance of each way. This chance is called a branching ratio. It shows how often one way happens compared to others.

Tiny bits of matter can change on their own. This is called particle decay. It happens when one unstable particle transforms into other particles. An unstable particle is one that can change into something else. This change is a spontaneous process. It is driven by the fundamental forces of nature. Some people call this radioactive decay when it happens to an atomic nucleus. However, particle decay is a slightly different term in physics.

There are rules for how this change works. The new particles must each be less massive than the original one. Even so, the total mass of the whole system stays the same. This rule is called conservation. One particle might have many ways to decay. Each way has its own chance of happening. This chance is called a branching ratio. A particle might decay into two new bits. These new bits might also be unstable. They could then decay into even more particles.

Scientists use math to study these tiny changes. They use something called Fermi's golden rule to find the decay rate. This rate tells us the chance a particle decays in a certain amount of time. The math also looks at the phase space. This is the space of all possible ways the particles can move. Scientists use Feynman diagrams to help calculate these paths. These diagrams help them see how particles connect to each other.

Different particles last for very different amounts of time. The Particle Data Group keeps track of these numbers. For example, an electron is very stable. It lasts for more than 6.6 years. A muon only lasts for 2.2 seconds. A tau lepton lasts for 2.9 seconds. A neutron lasts for about 885.7 seconds. Some particles like the W boson or Z boson are very short-lived. They only last for about 400 or 1000 seconds.

Understanding decay helps us see how the universe works. It is like watching a building turn into smaller bricks. We can use these ideas to understand how energy moves. The mass of an unstable particle can even be seen as a complex number. This includes a real part for mass and an imaginary part for the decay rate. When the decay rate is very large, we call it a resonance. This shows how everything in our world is always changing.

Particle decay is a fundamental process in particle physics. It occurs when an unstable subatomic particle spontaneously transforms into multiple other particles. This process is known as the final state. For a decay to happen, the original particle must be unstable. A particle is considered unstable if there is at least one allowed state it can decay into. These transformations are mediated by one or several fundamental forces of nature. While similar to radioactive decay in an atomic nucleus, particle decay is a distinct concept in physics.

There are strict physical rules that govern how these transformations occur. The total mass of the system must always be conserved during the process. However, each individual particle in the final state must be less massive than the original parent particle. An unstable particle may have several different ways to decay. Each specific path is called a decay mode or a decay branch. Every mode has its own associated probability. The chance of a particle choosing one specific mode is called the branching ratio. This ratio is found by dividing the decay rate of that mode by the total decay rate of all possible branches.

Physicists use complex mathematics to calculate the decay rate. The decay rate is the probability per unit time that a particle will decay. To find this, scientists often use Fermi's golden rule. This rule involves several mathematical components. One part is the invariant matrix element, which connects the initial state to the final state. Scientists often calculate this using Feynman diagrams. Another part is the phase space. The phase space represents the possible momenta and energies of the particles created. The calculation must also include a combinatorial factor. This factor accounts for final state particles that are indistinguishable from one another.

When a parent particle decays into exactly two particles, it is called a two-body decay. In the rest frame of the parent particle, certain conservation laws apply. Specifically, the four-momentum of the parent must equal the sum of the four-momenta of the two new particles. This allows scientists to determine the angle of the emitted particles. The angle of a particle in a laboratory frame is related to its angle in the center of momentum frame. This relationship is vital for understanding how particles move in high-speed experiments.

Different particles have vastly different lifetimes. The Particle Data Group tracks these measurements for many types of matter. For example, the electron and positron are very stable. Their mean lifetime is greater than 6.6 years. In contrast, the muon and antimuon have a mean lifetime of only 2.2 seconds. The tau lepton and antitau last for 2.9 seconds. Even heavier particles like the W boson and Z boson decay very quickly. The W boson has a lifetime of about 400 seconds, while the Z boson lasts about 1000 seconds.

Other particles fall into different categories like mesons or baryons. The neutral pion is a meson with a mass of 135 MeV and a lifetime of 8.4 seconds. The charged pion is a meson with a mass of 139.6 MeV and a lifetime of 2.6 seconds. Baryons include the proton and antiproton, which have a mass of 938.2 MeV. Their mean lifetime is 1.67 years. The neutron and antineutron are also baryons with a mass of 939.6 MeV. Their lifetime is approximately 885.7 seconds. These varying scales show the incredible diversity of the subatomic world.

In advanced quantum field theory, the mass of an unstable particle is treated as a complex number. This number has two parts. The real part represents the particle's mass in the usual sense. The imaginary part represents the particle's decay rate. If the imaginary part is very large compared to the real part, the object is called a resonance. A resonance is often thought of as a particle that is exchanged between other particles. This happens when there is not enough energy to create a permanent particle. If the time to travel between particles is short, the particle may decay before it even completes its travel.