Muon

Tiny bits fly through the air.

They are like heavy electrons. They have a lot of weight. Because they are heavy, they move slowly in fields. This helps them go deep into things. They can reach the ground and even deep mines.

These bits do not stay the same. They break apart into other bits. They turn into an electron and two tiny things. These bits are very small. They move very fast. It is amazing to find them!

A muon is a tiny particle. It is a type of lepton. Leptons are basic parts of the world. They are not made of smaller bits. A muon is like an electron. It has a negative charge. But a muon is much heavier. It has about 207 times the mass of an electron.

Muons can travel through many things. They can reach the ground. They can even reach deep mines. This happens because they are heavy. They do not slow down easily in fields.

Muons are unstable. This means they do not last long. They break apart in a set of steps. This is called decay. A muon turns into an electron. It also makes two neutrinos. Neutrinos are very small particles.

Scientists found muons in 1936. Carl Anderson and Seth Neddermeyer saw them. They were studying cosmic rays. These rays come from space. They hit our air and make muons. These muons move near the speed of light. They can survive the trip to Earth. This is due to a rule called special relativity.

A muon is a tiny, fundamental building block of our universe. Scientists call it an elementary particle, which means it is not made of smaller bits. It belongs to a group of particles known as leptons. A muon is very much like an electron because it carries a negative charge. However, the muon is much heavier than the electron. In fact, it has about 207 times the mass of an electron.

Muons are unstable, so they do not last very long. They eventually break apart in a process called decay. This happens through something called the weak interaction. When a muon decays, it always turns into an electron. It also produces two types of tiny particles called neutrinos. One is a muon neutrino and the other is an electron antineutrino.

Scientists first discovered the muon in 1936 at Caltech. Two researchers named Carl D. Anderson and Seth Neddermeyer found them. They were studying cosmic radiation when they saw something strange. They noticed particles that curved differently in a magnetic field. These particles were negatively charged like electrons. But they were heavier than electrons and lighter than protons.

In the past, people thought muons were a type of particle called a meson. A meson is a particle that helps hold an atom together. However, experiments in 1946 in Rome proved this was wrong. Researchers Marcello Conversi, Oreste Piccioni, and Ettore Pancini showed muons do not interact with the nuclear force. This discovery was a huge moment for science. It helped create the modern understanding of how all particles work.

Even though they decay quickly, we see muons on Earth every day. They are created when cosmic rays hit our atmosphere. These rays create pions, which then turn into muons. Because muons are heavy, they can travel through many things. They can pass through the air and reach deep mines. They can even be detected deep underwater.

A muon is an elementary particle that belongs to a group called leptons. It is a fundamental building block of the universe, meaning it is not made of any smaller constituent particles. While it shares many properties with the electron, such as a negative electric charge of −1 e and a spin of ħ, it is much more massive. Specifically, a muon has a mass of 105.66 MeV/c², which is approximately 207 times the mass of an electron. Because it is an unstable particle, it does not last forever; instead, it undergoes a process called decay. This instability makes the muon a vital subject for studying the fundamental forces of nature.

Muon decay occurs through the weak interaction, which is one of the four fundamental forces. This process is relatively slow compared to other unstable particles because the mass difference between the muon and its decay products is quite small. This small difference provides few kinetic degrees of freedom for the decay to occur. When a muon decays, it always produces an electron (or a positron if it is an antimuon) and two types of neutrinos. Specifically, the decay produces one muon neutrino and one electron antineutrino. This ensures that charge and leptonic family numbers are conserved during the process.

There are different types of decay modes for the muon. The most common is known as Michel decay, which is the simplest path where the muon becomes an electron, a muon neutrino, and an electron antineutrino. Other, less common paths exist, such as five-body decays, which have a branching ratio of about 0.01%. There are also radiative decays, where additional particles like photons are produced. While some other decay modes are technically possible in certain theories, they are considered forbidden in the Standard Model. For example, a decay that produces only an electron and a neutrino without following specific flavor rules is extremely unlikely, occurring in fewer than one in 10^50 decays.

Scientists first discovered the muon in 1936 at Caltech. Researchers Carl D. Anderson and Seth Neddermeyer were studying cosmic radiation when they noticed something unusual. They observed particles that curved differently than electrons when passing through a magnetic field. These particles had a negative charge, but they curved less sharply than electrons and more sharply than protons. Based on this curvature, they concluded the mass was greater than an electron's but smaller than a proton's. Anderson initially called this new particle a "mesotron," using the Greek prefix "meso-" to mean "middle."

History shows that the muon was once misunderstood. In 1935, theorist Hideki Yukawa predicted a heavy particle that would mediate the nuclear force. Because the muon had an intermediate mass, many scientists, including Niels Bohr, thought it was Yukawa's predicted particle. They even called it the "yukon." However, in 1946, Marcello Conversi, Oreste Piccioni, and Ettore Pancini conducted an experiment in Rome. They proved that muons from cosmic rays decay without being captured by atomic nuclei. This showed they did not interact with the nuclear force like the pi meson did. This discovery was so important that physicist Luis Walter Alvarez called it the start of modern particle physics.

In the modern Standard Model, we distinguish between leptons and hadrons. Hadrons, such as the pi meson, are made of quarks and interact via the nuclear force. Muons are leptons, meaning they have no quark structure and do not feel the nuclear force. This distinction changed how scientists used the word "meson." Previously, a meson was defined by its mass range. Now, a meson is defined as any particle composed of exactly one quark and one antiquark. Because the muon is a fundamental lepton, it is no longer classified as a meson. This realization was so surprising that physicist I. I. Rabi famously asked, "Who ordered that?"

Muons are also famous for their ability to penetrate deep into matter. This is because their greater mass causes them to emit less bremsstrahlung, which is radiation caused by deceleration. This allows them to travel much further than electrons of the same energy. Many muons we see on Earth are "secondary muons." These are created when cosmic ray protons hit the Earth's atmosphere, creating pions that then decay into muons. These muons can reach the land surface and even penetrate deep into mines.

Muons provide a unique way to observe the effects of special relativity. In 1941, the Rossi–Hall experiment used muons to observe time dilation and length contraction. From Earth's perspective, time dilation allows the fast-moving muons to have a longer half-life, letting them survive the trip from the upper atmosphere. From the muon's own perspective, length contraction makes the distance through the atmosphere appear much shorter. Because they are so penetrative, muons are used in detectors located 700 meters underground, such as the Soudan 2 detector, and can even be found deep underwater.