Theta meson

Not to be confused with the θ⁺ of the τ–θ puzzle, which is now identified as kaon, or with the Θ⁺ and Θ⁺
_c, formerly thought to be pentaquarks.

Theta meson

Quark structure of the theta meson
Composition	tt
Statistics	bosonic
Interactions	strong, weak, gravitational, electromagnetic
Status	hypothetical
Antiparticle	self
Electric charge	0 e
Isospin	0
Hypercharge	0
Parity	−1
C parity	−1

The theta meson (θ)^[1] is a hypothetical form of quarkonium (i.e. a flavourless meson) formed by a top quark (t) and top antiquark (t). As a P-odd and C-odd tt state, it is analogous to the ϕ (ss), J/ψ (cc) and ϒ (bb) mesons. Due to the top quark's short lifetime, the theta meson is expected to not be observed in nature.^[2]

In 2025, new research from the CMS^[3] and ATLAS^[4] collaborations demonstrated the existence of theta mesons at greater than 5-sigma significance.^[5][6]

See also

• List of mesons

References

1. Roos, M.; Wohl, C.G. "Naming Scheme for Hadrons" (PDF). lbl.gov. Retrieved 23 March 2022.
2. Fabiano, N. (1998). "Top mesons". European Physical Journal C. 2 (2): 345–350. arXiv:hep-ph/9704261. Bibcode:1998EPJC....2..345F. doi:10.1007/s100520050144. S2CID 17321939.
3. CMS Collaboration (2025). "Observation of a pseudoscalar excess at the top quark pair production threshold". Reports on Progress in Physics. 88 (8). arXiv:2503.22382. Bibcode:2025RPPh...88h7801C. doi:10.1088/1361-6633/adf7d3. PMID 40763770.
4. ATLAS Experiment. "Observation of a cross-section enhancement near the ${\displaystyle t{\overline {t}}}$ threshold in ${\displaystyle {\sqrt {s}}=13}$ TeV pp collisions with the ATLAS detector".
5. Siegel, Ethan (9 July 2025). "Starts With A Bang: The top quark isn't a loner after all: "toponium" is real!". bigthink.com. Retrieved 10 July 2025.
6. "Elusive romance of top-quark pairs observed at the LHC". home.cern. Retrieved 10 July 2025.