Tetraquark

Everything is made of tiny bits.

Most bits live in small groups. Some groups have only two or three bits. But some bits form a group of four. Scientists call this a tetraquark.

These four-bit groups are very rare. Many types of these groups have been found. They can have light bits or heavy bits. Some were found in Japan. Others were found in China. It is exciting to find new bits.

Everything is made of tiny bits called quarks. Most particles are made of only two or three quarks. But some particles are made of four quarks. We call these exotic particles tetraquarks.

Scientists use a theory called quantum chromodynamics to study them. This theory explains how quarks stay together. In these groups, quarks can be light or heavy. Some tetraquarks have one light quark and one heavy quark.

Many groups have been found by big experiments. In 2003, the Belle experiment in Japan found X(3872). This was a possible tetraquark. In 2013, experiments in China and Japan found Zc(3900). This was the first confirmed four-quark state. In 2020, the LHCb experiment found X(6900). Other groups like X(6600) and X(7300) were seen later.

Finding these particles helps us learn about our world. Scientists still look for more types. They want to see if they can find groups with two heavy quarks.

Everything in our world is made of tiny bits. Most particles are made of just two or three quarks. But some particles are made of four quarks. Scientists call these exotic particles tetraquarks.

They are special because they do not fit the normal rules. These particles are a type of meson. A meson is a group of quarks. Tetraquarks are part of a bigger group called hadrons.

How do these four quarks stay together? A theory called quantum chromodynamics explains this. This theory describes the strong interactions between quarks. It shows how color charges stay connected. These connections act like tubes of force. We can call them flux tubes. These tubes act like strings that pull the quarks together.

This pull keeps the quarks in a group. This is called confinement. It is why these tiny bits do not fly apart.

Finding these particles has taken many years. Scientists have suspected they could exist for a long time. They used math to predict them first. In 2003, the Belle experiment in Japan found X(3872). This was a possible tetraquark candidate. In 2004, the SELEX experiment at Fermilab found DsJ(2632). Later, Belle found the Z(4430) state in 2007. These early finds helped scientists learn more.

Many big experiments have found new names and numbers. In 2013, BES III in China and Belle in Japan found Zc(3900). This was the first confirmed four-quark state. In 2014, the LHCb experiment confirmed the Z(4430) state. In 2020, LHCb found a particle called X(6900). The ATLAS experiment also saw X(6900) in 2022. By 2023, CMS found three more states. These were X(6600), X(6900), and X(7300).

These tiny particles help us see the rules of nature. Most tetraquarks use one light quark and one heavy quark. Light quarks include up, down, or strange quarks. Heavy quarks include charm or bottom quarks. Scientists are still looking for even more types. They want to find groups with two heavy quarks. This would be a new discovery. Every new particle tells us more about how the universe works.

A tetraquark is an exotic meson made of four valence quarks. In particle physics, quarks are the fundamental building blocks of matter. Most particles follow a conventional quark model. For example, many common particles contain only two or three quarks. A tetraquark is different because it lies outside these standard classifications. It belongs to a larger group of particles called hadrons. Scientists have long suspected that these four-quark states are possible. Their existence is supported by quantum chromodynamics. This is the modern theory of strong interactions.

To understand how a tetraquark works, we must look at quantum chromodynamics. This theory explains the strong interaction between quarks. This force is governed by color charges. In these particles, confinement occurs due to these charges. Confinement means quarks cannot exist alone. Instead, they are connected by color flux tubes. These flux tubes are produced by the four static quark and antiquark charges. They act like attractive, string-like potentials. This pull keeps the four quarks bound together in a single state.

There are many different types of tetraquark states. Most observed tetraquarks have a specific quark content. They often consist of a light quark and a heavy quark. The light quarks include the up, down, or strange varieties. The heavy quarks include charm or bottom quarks. In scientific notation, these are often written as qQ. Here, q represents the light quark and Q represents the heavy quark. Antiquarks are also part of these structures. They are denoted with an overline symbol. Theoretical physicists also discuss states made of two light quarks or two heavy quarks. However, these specific types have not been reported by experiments yet.

Discovery of these particles has happened in stages during the 21st century. The search began with several candidates. In 2003, the Belle experiment in Japan proposed X(3872). The number 3872 represents its mass in MeV. The name X is temporary because scientists are still testing its properties. In 2004, the SELEX experiment at Fermilab suggested the DsJ(2632) state was a candidate. In 2007, Belle announced the Z(4430) state. Belle also found indications for the Y(4660) state that same year. In 2009, Fermilab announced the Y(4140) particle. These discoveries slowly built a map of exotic matter.

Major milestones have confirmed these particles are real. In 2010, researchers from DESY and Quaid-i-Azam University re-analyzed data. They found a well-defined tetraquark resonance linked to the (5S) meson. A significant breakthrough occurred in June 2013. The BES III experiment in China and the Belle experiment in Japan independently reported the Zc(3900). This was the first confirmed four-quark state. Later, in 2014, the LHCb experiment confirmed the Z(4430) state. This confirmation had a significance of over 13.9 sigma. This high number shows the discovery was very certain.

Recent years have brought even more specific findings. In 2016, the LHCb experiment announced three more candidates. These were named X(4274), X(4500), and X(4700). In 2020, LHCb discovered the X(6900) tetraquark. The ATLAS experiment also observed X(6900) in 2022. By 2023, the CMS experiment reported three states: X(6600), X(6900), and X(7300). There was also some debate regarding the X(5568). The DØ experiment reported evidence for it in 2016 and 2017. However, other experiments like LHCb, CMS, CDF, and ATLAS did not observe it.

These discoveries connect to the broader study of nuclear physics. By studying tetraquarks, scientists learn about the strong force. They also learn about color confinement. Every new state, like the cu or cd states found by LHCb, provides data. These particles help us understand how matter is held together at the smallest scales. The study of exotic hadrons continues to expand our knowledge of the universe.