Thetarch

Thetarch, a pretender-king in tarnished violet plate, wears a crown of five jewels, four warm and one cold, whose linking thread is snapping.

The Physics

In 2003 the LEPS collaboration at SPring-8 reported a narrow peak near 1540 MeV in the K⁺n system, read as an exotic five-quark baryon, the Θ⁺, with minimal content uudd s̄. Its positive strangeness could not be made from three quarks, which is what made it exotic. Several other groups quickly reported sightings, but higher-statistics searches found nothing, and the Particle Data Group’s 2008 review judged the case that the Θ⁺ does not exist to be compelling, describing the whole story as a curious episode in the history of science.

Θ⁺ = u u d d s̄: Q = +1, B = +1, S = +1

• u, d: up and down quarks, charges +2/3 e and −1/3 e
• s̄: strange antiquark, charge +1/3 e, strangeness +1
• Q: electric charge in units of e
• B: baryon number (each quark +1/3, antiquark −1/3)
• S: strangeness

Ordinary baryons qqq cannot have S = +1, so a narrow K⁺n resonance would have been unambiguous. The early peaks came from small samples and background modelling. Sources: T. Nakano et al. (LEPS), Phys. Rev. Lett. 91, 012002 (2003); C. Amsler et al. (PDG), “Pentaquark update”, Phys. Lett. B 667, 1 (2008).

Honest caveat: Only this pentaquark is a ghost. Hidden-charm pentaquarks of a different kind were observed by LHCb in 2015. The 1540 MeV mass is approximate (about 1.53 to 1.54 GeV in early reports).

In character

Five jewels, they said, and a throne of my own. But the thread would not hold under more light, and the crown was only ever borrowed.

On the hoodie

Front: a five-jewel ring with one jewel hollow. Back: Thetarch in full pixel art with “BORN 2003 - DIED 2008” and “FIVE QUARKS, ZERO PARTICLE.” Gildan 18500 Heavy Blend hoodie, Black, S to 5XL.

Part of the Ghost Particles That Were Wrong set with Sir Aetheron, Enrayne, Calorus, Diphantom, Prematura. Browse all physics hoodies, more particle & nuclear physics designs, or the Deep Cuts.