Quarks is the mesons, combinations of exactly one quark

Quarks are defined by the strong force which hold the gluons its force carrier together and the only known particles that interact via Quantum Chromodynamics (QCD). These particles were originally discovered in 1964 by two of the early pioneers of particle physics Murray Gell-Mann and George Zweig, who proposed the idea of “quarks” to explain the properties of plethora of particles that were discovered in the mid-twentieth century. These quarks are known to be in different kind of varieties called “flavours”. Gluons, the force carrier of QCD are said to be self- interacting forming confinement of quarks which led to the observation that quarks are never isolated, it is only observed as part of a group of other quarks/antiquarks. These groups of quarks and anti-quarks are what we call Hadrons. Collisions of atomic nuclei which are made up of protons and neutrons (hadrons) are created in the collisions of protons in some large accelerators around the world.  Any combination of three quarks or anti-quarks is called a baryon. The two famous examples of which is the proton and neutron, which are made up of up and down quarks. The other type of hadrons is the mesons, combinations of exactly one quark and one anti-quark. Pions is an example of these type of hadrons. Hadrons are usually produced in the two or three quark varieties. However, in the past years and up until now, the attention of many particle physicist was focused on hadrons with properties that look unusual which they refer as exotic hadrons.  Aside from normal baryons and mesons, colour singlets also allow combinations of pairs of quarks and antiquarks known as “tetraquark” mesons, four quarks and an antiquark called “pentaquark” baryons,and there also states comprised solely of gluons or “glueballs”. Furthermore, “hybrids” combinations in which the gluonic fields entrapping the quark and antiquark are themselves excited are also possible within QCD. And in recent years, several new hadrons have been discovered that do not fit well within the traditional quark model. Recently, the LHCb experiment at CERN’s Large Hadron Collider (LHC) reported decays of the ?b pentaquark-like baryon that revealed similar structures with a mass of around 4.4?GeV. These baryon have been interpreted as clusters of three quarks plus a charm–anticharm pair and have normal strong-interaction lifetimes. Then the X(3872) was discovered by Belle in the ?+?-J/?  mass spectrum of the reaction B ± ? K ±?+?-J/? . Neither the mass nor the decay properties were according to the expectations of the charmonium models.  Also, the first clue for an exotic charmonium meson of this type came around from the BaBar experiment at SLAC in the US. Researchers there found a clear resonant-like structure dubbed Y (4260), which has no place in the qq spectrum. This state decays into charmonium and pions with a standard strong-interaction width. After the discovery of charmonium, the charmed mesons D, D * and the mesons with charm and strangeness Ds, D*s have followed. The LHCB collaboration reanalysed the J/?? mass spectrum from the decay B ? J/??, and found evidence for X (4140) and X (4264) both with J PC = 1++, in disagreement with some models. Recently also, the discovery of the scalar mesons f0(980) and a0(980), together with f0(500) (? ) and K*0 (800) (?), have been thought to have some exotic structures, since they exhibit an inverted mass spectrum compared to what is expected if they have simple q?q configurations. Model studies have suggested that f0 (980) and a0 (980) could be compact qq?q?q systems. But the structures of these scalar mesons are still controversial. Another exotic hadron discovered is the ? (1405) which is considered to be a meson–baryon molecule. The ? (1405) has been observed in the low energy exclusive reactions. Dibaryons are also observed in many experimental collisions like the H- dibaryons. Recent progress in facilities also helped in the advancements and discoveries of other molecules particularly the investigation of hadron spectroscopy involving heavy quarks. The D*s0 (2317) which is a charmed and strange scalar meson was first observed by BaBar through its isospin violating ?0D+s decay mode. This meson has some exotic configuration besides an ordinary q?q configuration. Heavy meson spectroscopy has progressed in the recent years. New states, called XYZ, are observed above the open charm/bottom thresholds. The XYZ states are expected to have an exotic structure because the properties of these states are not well described in the conventional constituent quark model.  Among many interesting states is the X (3872) and the charged charmonium-like states, Z±c. X (3872) is one of the most intensively studied states. It is firstly observed by the Belle collaboration in the B decay. Light quark–antiquark pair is required in addition to c?c as the valence component in the charged charmonium-like states Zc. At present, eight charged charmonium-like states have been reported, although not all these states are firmly established. These are Zc (4430), Zc (4240), Zc (4050), Zc (4250), Zc (3900), Zc (4020), Zc (4200), Zc (4055). There are also bottomonium-like state called Zb reported in Belle. They are Zb(10610)+ and Zb(10650)+ with spin-parity JP = 1+. The neutral state, Zb(10610)0, was also reported. The LHCb observed two hidden-charm pentaquark-like structures, the P+c (4380) and P+c (4450) in the J/?p invariant mass distribution in the decay ?0b?J/?pK?.The D0 Collaboration has recently announced the observation of  a new state, the X±(5568). This X (5568) would be an addition to the list of undoubtedly exotic mesons because its wave function consists of four different flavors: u, b, d and s quarks. All of the above mentioned hadrons are some of the exotic hadrons recently discovered. These exotic hadrons were discovered by different scientists’ collaboration all around the world. All of these hadrons are produce in the different colliders or accelerators like the Large Hadron Collider, Proton- Antiproton Collider, Tevatron, and Relativistic Heavy Ion Collider.  Exotic hadrons have different levels of exoticity. The least exotic are meson analogues of nuclei. Next are “hybrids”: states anticipated in QCD where the gluonic degrees of freedom are excited in the presence of quarks and/or antiquarks. Finally, the most exotic of all would be colour-singlet combinations of compact diquarks, which would lead to a rich spectroscopy. It said that these exotic hadrons are like the extraterrestrial life that we believe to exist but they are just too reluctantly to show themselves, so further studies and advancements may help in the discovery of the supermolecules that are maybe existing in our universe.