Exotic hadrons are subatomic particles made of quarks (and possibly gluons), but which do not fit into the usual scheme of hadrons. While bound by the strong interaction

Strong interaction

In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...

 they are not predicted by the simple quark model

Quark model

In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons....

. That is, exotic hadrons do not have the same quark content as ordinary hadron

Hadron

In particle physics, a hadron is a composite particle made of quarks held together by the strong force...

s: exotic baryon

Exotic baryon

Exotic baryons are hypothetical composite particles which are bound states of 3 quarks and additional elementary particles. This is to be contrasted with ordinary baryons, which are bound states of just 3 quarks. The additional particles may include quarks, antiquarks or gluons.One such exotic...

s have more than just the three quarks of ordinary baryon

Baryon

A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...

s and exotic meson

Exotic meson

Non-quark model mesons include#exotic mesons, which have quantum numbers not possible for mesons in the quark model;#glueballs or gluonium, which have no valence quarks at all;...

s do not have one quark and one antiquark like ordinary meson

Meson

In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...

s. Exotic hadrons can be searched for by looking for particles with quantum numbers forbidden to ordinary hadrons. Experimental signatures for exotic hadrons have been seen recently but remain a topic of controversy in particle physics

Particle physics

Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

.

History

When the quark model was first postulated by Murray Gell-Mann

Murray Gell-Mann

Murray Gell-Mann is an American physicist and linguist who received the 1969 Nobel Prize in physics for his work on the theory of elementary particles...

 and others in the 1960s it was to organize the states then known to be in existence in a meaningful way. As Quantum Chromodynamics

Quantum chromodynamics

In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...

 (QCD) developed over the next decade; however, it became apparent that there was no fundamental reason why only 3-quark and quark-antiquark combinations should exist. In addition it seemed that gluons, the force carrying particles of the strong interaction, should also form bound states by themselves (glueballs) and with quarks (hybrid hadrons). Nevertheless, several decades have passed without conclusive evidence of an exotic hadron.

Candidates

There are several exotic hadron candidates:

• X(3872)
  X(3872)
  The X is an anomalous particle with an energy of 3871.2 MeV which does not fit into the quark model because of its quantum numbers. It was discovered by the Belle experiment in Japan and later confirmed by several other experimental collaborations. Several theories have been proposed for its...
  
   - Discovered by the Belle detector
  Belle experiment
  The Belle experiment is a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers investigating CP-violation effects at the High Energy Accelerator Research Organisation in Tsukuba, Ibaraki Prefecture, Japan.The...
  
   at KEK
  KEK
  , known as KEK, is a national organization whose purpose is to operate the largest particle physics laboratory in Japan, which is situated in Tsukuba of Ibaraki prefecture. Established in 1997. The term "KEK" is also used to refer to the laboratory itself, which employs approximately 900 employees...
  
   in Japan, this particle has been variously hypothesized to be diquark
  Diquark
  In quark–diquark models, a diquark, or diquark correlation/clustering, is the hypothetical state of two quarks grouped inside a baryon . The diquark is often treated as a single particle with which the third quark interacts via the strong interaction...
  
   or a mesonic molecule
  Mesonic molecule
  A mesonic molecule is a set of two or more mesons bound together by the strong force. Unlike baryonic molecules, which form the nuclei of all elements in nature save hydrogen-1, a mesonic molecule has yet to be definitively observed. The X discovered in 2003 and the Z discovered in 2007 by the...
  
  .
• Y(3940) - This particle fails to fit into the Charmonium spectrum predicted by theorists.
• Y(4140)
  Y(4140)
  The Y particle is a previously unpredicted particle observed at Fermilab and announced on 17 March 2009. This particle is extremely rare and was detected in only 20 of billions of collisions....
  
   - Discovered at Fermilab
  Fermilab
  Fermi National Accelerator Laboratory , located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics...
  
   in March 2009 http://www.universetoday.com/2009/03/18/new-particle-throws-monkeywrench-in-particle-physics/
• Y(4260) - Discovered by the BaBar detector
  BaBar experiment
  The BaBar experiment, or simply BaBar, is an international collaboration of more than 500 physicists and engineers studying the subatomic world at energies of approximately ten times the rest mass of a proton . Its design was motivated by the investigation of CP violation...
  
   at SLAC in Menlo Park
  Menlo Park, California
  Menlo Park, California is a city at the eastern edge of San Mateo County, in the San Francisco Bay Area of California, in the United States. It is bordered by San Francisco Bay on the north and east; East Palo Alto, Palo Alto, and Stanford to the south; Atherton, North Fair Oaks, and Redwood City...
  
  , California
  California
  California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
  
  this particle is hypothesized to be made up of a gluon bound to a quark and antiquark.