Quarks early pioneers of particle physics Murray Gell-Mann and

Quarks are defined by the strong force which hold the gluons its force carrier together. It is the only known particle that interact via Quantum Chromodynamics (QCD). The idea of “quarks” were proposed and discovered in 1964 by two of the early pioneers of particle physics Murray Gell-Mann and George Zweig, in order to explain the properties 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. Atomic nuclei collisions which are made up of protons and neutrons which are basically 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 a meson. Hadrons are usually produced and observed 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 allows “tetraquark” mesons a combination of pairs of quarks and antiquarks, four quarks and an antiquark called “pentaquark” baryons, and there also states comprised solely of gluons or “glueballs”. Furthermore, “hybrids” combinations are also possible within QCD. These hybrids, the gluonic fields entrapping the quark and antiquark are themselves excited. In recent years, several new hadrons have been discovered that do not fit well within the traditional quark model. Recently, at CERN’s Large Hadron Collider (LHC) the LHCb experiment 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. These baryon is also said to 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 are said to be according to the expectations of the charmonium models.  Also, the first clue for an exotic charmonium meson of this type came from the BaBar experiment at SLAC in the US. A clear resonant-like structure dubbed Y (4260) was discovered by the researchers. These meson has no place in the qq spectrum as interpreted by the reasearchers. 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. 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 and 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 intensively studied states is the X (3872) and the charged charmonium-like states, Z±c. X (3872) is one of the most interesting 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 also observed the P+c (4380) and P+c (4450) which are two hidden-charm pentaquark-like structures. These pentaquarks are observed in the J/?p invariant mass distribution in the decay ?0b?J/?pK. The D0 Collaboration has also recently announced the observation of a new state, the X± (5568). This X (5568) considered undoubtedly as an exotic meson 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 these hadrons are observed in the different accelerators like the Large Hadron Collider, Proton- Antiproton Collider, Tevatron, and Relativistic Heavy Ion Collider.   There are level of exoticity in exotic hadrons. The least exotic are the meson analogues of nuclei. The next one are the hybrids states where the gluonic degrees of freedom are excited in the presence of quarks and antiquarks. Lastly, the most exotic one are the combinations of compact diquarks. These exotic hadrons are like the extra-terrestrial life that we believe to exist. It is present in our universe but it is too reluctant to show themselves. Further studies and advancements may help in the discovery of these supramolecules that are believed to be existing in our universe. Then maybe sooner or later, plethora of elementary particles are yet to be discovered.