Charm quark

The charm quark is a fundamental building block of our universe. It is an elementary particle, meaning it cannot be broken down into smaller parts.

This particle belongs to the second generation of quarks. It carries a specific property known as a quantum number called charm. It also possesses an electric charge of +e. Scientists classify it as an up-type quark. It is quite heavy compared to other particles. In fact, it is the third-most-massive quark known to science.

Charm quarks exist within larger composite particles called hadrons. These hadrons are divided into two main types: mesons and baryons. Mesons are made of a quark and an antiquark. One specific type is called charmonium. This occurs when a charm quark and a charm antiquark form a bound state. There are two forms of charmonium based on particle spin. In orthocharmonium, the spins are parallel. In paracharmonium, the spins align in opposite directions. Baryons are different because they contain three quarks. Some charmed baryons include the lambda charm or the xi charm.

These particles are produced through several complex processes. They can be created during collisions in particle accelerators. Scientists often use electron-positron colliders to produce charmonium. Hadron colliders are also used because they produce charm quarks at a higher cross section. Other heavy particles can also produce them through a process called decay. For example, the W and Z bosons can decay into charm quarks. The Higgs boson can also decay into them through a mechanism called Yukawa coupling. The charm quark can also decay into other quarks via the weak interaction.

Discovery of the charm quark was a major triumph for theoretical physics. In 1961, Murray Gell-Mann introduced the Eightfold Way to group particles. By 1964, Gell-Mann and George Zweig proposed that hadrons were made of quarks. Initially, scientists only knew about up, down, and strange quarks. In 1964, James Bjorken and Sheldon Glashow theorized the existence of charm. They wanted to create a parallel between quarks and leptons. In 1970, Glashow, John Iliopoulos, and Luciano Maiani proposed the charm quark specifically. They created the GIM mechanism to explain why certain particle decays were so rare. This mechanism helped unify the weak and electromagnetic forces.

In 1974, the existence of the charm quark was finally confirmed. Two separate teams made independent discoveries at almost the same time. Samuel C. C. Ting led a team at Brookhaven National Laboratory. They used an electron-pair detector to find a particle they named the J meson. Meanwhile, Burton Richter led a team at the Stanford Linear Accelerator Center. They found the same particle and named it the psi meson. Together, these particles are known as the J/psi meson. This discovery was so important that Ting and Richter shared the Nobel Prize in Physics in 1976.

Since that discovery, many other charmed particles have been identified. In 1975, researchers found evidence for charmed baryons. In 1976, Gerson Goldhaber and François Pierre identified the D meson. This was described as finding "naked charm" because it was a neutral charmed meson. In 1977, the charmed strange meson was also discovered. In 2002, the SELEX Collaboration at Fermilab found a doubly charmed baryon. This is a rare particle containing two charm quarks. Recent studies in 2022 have even found evidence of intrinsic charm quarks inside the proton.

Modern research continues to explore how the charm quark interacts with the universe. Scientists use the Large Hadron Collider to study the Higgs boson. They have looked at how the Higgs boson decays into charm quarks. They found the Higgs-charm coupling is weaker than the Higgs-bottom coupling. There is also evidence of CP violation in the decay of the D0 meson. This involves the way particles and antiparticles behave differently. Understanding these tiny interactions helps scientists map the entire Standard Model of physics. Every new measurement brings us closer to knowing how matter is built.