J/psi meson

Tiny bits make up our world. Some bits are very small. One tiny bit is called a charm quark. Two of these bits can join together. They form a new tiny thing. This thing is called the J/psi. It lives much longer than scientists thought. Two groups found it at the same time. This was a big deal for science. It changed how we see the world. Now we know more about how tiny bits work.

The J/psi is a tiny particle. It is a type of meson. A meson is a particle made of two parts. This one uses a charm quark and a charm antiquark. These two parts stay joined together. Scientists call this pair charmonium.

Two groups found this particle in 1974. One group was at SLAC. They were led by Burton Richter. The other group was at Brookhaven. They were led by Samuel Ting. Both groups found it at the same time. This led to the November Revolution. This name describes big changes in physics. Richter and Ting won a Nobel Prize for this.

Richter chose the name psi. He picked a Greek letter. Ting chose the name J. He wanted to use a Roman letter. People joined the names to make J/psi. This particle lives a long time. It lasts much longer than scientists expected. It can turn into other things. It can turn into leptons, which are tiny particles. In very hot places, the J/psi can melt. This might show a quark-gluon plasma. This is a special state of matter.

The J/psi meson is a tiny subatomic particle. It is a special type of meson. A meson is a particle made of two parts. This specific particle uses a charm quark and a charm antiquark. When these two parts stay joined together, scientists call them charmonium. This particle is very common in the charmonium family. It has a spin of 1 and a low rest mass. Its mass is about 3.1 GeV. This is just a little bit more than the mass of a D meson.

This particle works in a very interesting way. It stays together for a long time. In fact, it lasts about a thousand times longer than scientists expected. This happens because of something called the OZI rule. This rule makes it hard for the particle to decay into hadrons. Because of this, the particle can also turn into leptons. These are tiny particles like electrons. It can even melt in very hot places. If the temperature gets high enough, the particle might melt into a quark-gluon plasma. This is a special state of matter.

Two different groups found this particle at the same time. One group worked at the Stanford Linear Accelerator Center. Burton Richter led this team. The other group worked at the Brookhaven National Laboratory. Samuel Ting led this second team. They both announced their big discovery on 11 November 1974. This event was so important that people called it the "November Revolution." It caused very fast changes in the world of physics. Richter and Ting both won the Nobel Prize in Physics in 1976.

Before this discovery, scientists had many questions. In the 1960s, they proposed models using quarks. These quarks were thought to be mathematical fiction back then. They only knew about three types of quarks: up, down, and strange. Later, scientists added the charm, top, and bottom quarks. In 1970, three scientists named Glashow, Iliopoulos, and Maiani had an idea. They suggested a fourth quark called the charm quark existed. This idea helped explain why certain decays did not happen. This helped lead the way to finding the J/psi.

Even the name of the particle has a funny story. Richter wanted to use a Greek letter. He chose "psi" after talking to a friend. Ting wanted to use a Roman letter. He chose "J" for his discovery. To be fair to both men, people started using both names together. This is why we call it the J/psi. It is the only particle with a two-letter name. This name helps us remember the two groups who found it.

The J/psi meson is a unique subatomic particle. It is a flavor-neutral meson. This means it is made of a charm quark and a charm antiquark. When a charm quark and a charm anti-quark form a bound state, scientists call them charmonium. The J/psi is the most common form of charmonium. This is because it has a spin of 1 and a low rest mass. Its rest mass is 3.1 GeV. This is just slightly higher than the mass of a D meson. Understanding this particle helps scientists study the fundamental building blocks of our universe.

This particle behaves in a very surprising way. It has a mean lifetime of about 10^-18 seconds. This lifetime was about a thousand times longer than scientists expected. This happens because of the OZI rule. This rule strongly suppresses hadronic decay modes. A decay mode is a specific way a particle breaks apart. Because hadronic decays are suppressed, electromagnetic decays can compete with them. This allows the J/psi to have a significant branching fraction to leptons. Leptons are fundamental particles like electrons or muons. The particle can decay into several different things, such as three gluons or a pair of leptons.

There are different states of charmonium. The J/psi is the most common one. There are also excited states. The first excited state is called the psi(2S). The next excited state is called the psi(3770). These names indicate their specific quantum states or their mass in GeV. Other vector charm-anticharm states are named using the psi symbol and their quantum state or mass. These different states show how quarks can bind together in various ways. Scientists use these states to map out the rules of the strong interaction.

In the 1960s, the first quark models were proposed. These models suggested that protons and neutrons were made of quarks. At that time, scientists only knew about three flavors of quarks: up, down, and strange. Many people thought quarks were just mathematical fiction. They were seen as tools for calculation rather than real physical objects. However, experiments at the Stanford Linear Accelerator Center (SLAC) in 1969 showed evidence of particles inside protons. This helped prove that quarks were likely real. Later, the model expanded to include six quarks: charm, top, bottom, up, down, and strange.

The discovery of the J/psi happened in 1974. Two different research groups found it at almost the same time. One group was at the Stanford Linear Accelerator Center, led by Burton Richter. The other group was at the Brookhaven National Laboratory, led by Samuel Ting. Ting's group saw a peak at 3.1 GeV in production rate plots. They announced their findings on 11 November 1974. This moment was so impactful that it is called the "November Revolution." It caused rapid changes in high-energy physics. Richter and Ting were awarded the 1976 Nobel Prize in Physics for this work.

The name of the particle is also quite special. It is the only particle with a two-letter name. Richter wanted a Greek name. He chose "psi" after consulting with a friend. Ting wanted a Roman name. He chose "J" because stable particles often used Roman characters. To be fair to both groups, the scientific community combined them into "J/psi." This name honors both the Stanford and Brookhaven discoveries. It is a rare example of a name that represents a joint victory in science.

Scientists also study how the J/psi behaves in extreme heat. In a hot QCD medium, the particle can melt. This happens when the temperature rises well beyond the Hagedorn temperature. When it melts, it is a signal that a quark-gluon plasma has formed. This is a special state of matter. Experiments at CERN and Brookhaven have studied this phenomenon. While results have been complex, researchers look for these signals to understand how quarks move freely. This helps us understand the very early history of our universe.