On its discovery in 2007, there were suggestions that the Y(4660) particle (mass 4,660MeV) could be a tetraquark.  This remains unconfirmed.  On 17 March 2009, Fermilab announced they had found a new particle that they called the Y(4140) particle.  It has a mass of 4,140 MeV.  Because it appears to decay into two J/Y mesons, they suggest it may comprise charm quarks and charm antiquarks, possibly four quarks, making it a candidate for being a tetraquark.  Then in 2010, a group of physicists suggested that the Y(5S) meson has a well-defined tetraquark resonance. 

In June 2013, two independent groups reported on the
Zc(3900) particle.  The Zc(3900) is seen when the Y(4260) particle decays.  The Zc(3900) then itself decays into a charged pion, implying that the Zc(3900) contains at least four quarks.  In 2015, BES III, the Beijing Spectrometer III, observed the neutral Zc(3900); it decays into a neutral pion (p0) and a J/? Meson, again suggesting that it is a tetraquark.


The existence of pentaquarks was first reported in July 2003, and again in 2005.  The particle quickly decayed (in about 10-20 seconds) into a meson and a neutron.  The particle would comprise two up and two down quarks and one strange antiquark, if it were to exist.  A number of subsequent experiments failed to find the particle.  In July 2015, the LHCb collaboration identified pentaquarks in the decay of the bottom lambda baryon into a J/? meson, a charged kaon, and a proton. The results showed that occasionally, the particle decayed via intermediate pentaquark states {Pc(4450)+ and Pc(4380)+}. Both states were seen to decay to J/?p that implies there are two up quarks, a down quark, a charm quark, and an anti-charm quark.  The statistical significance is a very strong 15 s.

Weakly Interacting Massive Particles (WIMPs)

WIMPs include particles like the photino, the axion, and the squark, none of which has been observed. If they exist, they are more massive than neutrinos, and would travel at relatively low, non-relativistic velocities.  They interact using the weak nuclear force and gravity, but as they do not interact through electromagnetism, one cannot observe them directly.  In addition, they do not interact through the strong force, so do not interact with atomic nuclei.  WIMPs are one possible explanation for Dark Matter, when they are, generally, referred to as "cold dark matter" as opposed to "hot dark matter", which would include particles like neutrinos that move at relativistic (close to the speed of light) velocities. 

Supersymmetric Partner Particles

These are the particles that come out of supersymmetry theories.  This postulates that all the Standard Model particles, plus the Higgs Boson and the Graviton, have more massive partners with spin differing by 1/2.  None of these particles has been observed.  They include: the neutralino, chargino, photino, wino, zino, Higgsino, gluino, gravitino, sleptons, sneutrinos, squarks.  You will note some overlap here with the WIMPs discussed in the previous paragraph. 


Theoretically, glueballs are bound states of gluons; that is, composite particles comprising only gluons.  This is possible because, as well as carrying the color charge, gluons are subject to the the strong nuclear interaction between eachother.  Calculations of their mass indicate that they should be observable using current colliders, but this has not happened.  Typical theoretical masses are around 1.73 GeV, 2.4 GeV and 2.59 GeV.  Some evidence exists for the lightest (scalar) glueball mixed in with nearby mesons, but this is tentative and it is hard to determine whether the meson is a glueball or a normal quark-antiquark particle. 

Dark Matter Particles

A discussion about dark matter particles is included in the Dark Matter section in Cosmology.

Exotic Particles



Composite Particles