Exotic Sub-Atomic Particles

Tetraquark

On its discovery in 2007, there were suggestions that the Y(4660) particle (mass 4,660MeV) could be a tetraquark. This remains unconfirmed. Also In 2007, the Z(4430) state was identified as a tetraquark candidate comprising a charm/anti-charm pair plus a down/anti-up pair. In 2014, the Large Hadron Collider experiment LHCb confirmed the existence of the Z(4430) state.

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

In September 2018, the LHC reported evidence of a particle, called Zc-(4100). It included two charm quarks, and was detected in the decay of heavier B mesons.
However, they reported a significance of about 3 standard deviations (s) while it normally requires 5 s to report a discovery. Hopefully more evidence will be found. In 2020, LHCb announced the discovery of a tetraquark denoted X(6900) containing two charm quarks and two charm antiquarks.

In August of 2021, a new tetraquark was found that has been named Tcc+. It is the first tetraquark to contain two heavy quarks and two light antiquarks, comprising two charm quarks together with an up and a down antiquark. It is the longest-lived exotic matter particle yet discovered lasting for one-quintillionth of a second, about ten times longer than previous tetraquarks, before decaying into a pair of mesons, each comprising one heavy quark and one light antiquark. Called a "double charm" tetraquark, it is only slightly heavier than the mesons into which it decays, which is why it decays just a bit more slowly.

Pentaquark

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 {P_c(4450)⁺ and P_c(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.

In March 2019, the The LHC team reported observations of a new pentaquark, Pc(4312)+, to a significance of 7.3 sigma. They also confirmed the pentaquark observations reported in 2015; that is, the particle comprises two up quarks, one down quark, and a charm/anticharm quark pair. It decays into a proton (2u/1d quarks) and a J/? particle (charm/antcharm pair).

For a discussion on the latest thinking around pentaquarks, read this article.

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.

Glueballs

The theory of quantum chromodynamics (QCD) predicted the existence of the "odderon" in 1973. 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 indicated that they should be observable using current colliders. 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.

During a meeting at CERN on Friday 5 March, 2021, the TOTEM collaboration at the LHC, together with the DØ collaboration at the Tevatron collider at Fermilab, announced the discovery of the "odderon", comprising three gluons. Observing protons as they glance off one another in high-energy collisions has provided the first experimental evidence.

Dark Matter Particles

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