Evidence of new subatomic particles observed

The BESIII collaboration reported the observation of an anomalous line shape near the ppbar mass threshold in J/ψ→γ3(π).⁺π^–) attenuation. This indicates the existence of a ppbar bound state. paper Published online on physical review letter.

Mass proximity up to 2m_p This suggests a bound state of nucleons and antinucleons, an idea with a long history. Before the birth of the quark model, the nucleon-antinucleon coupling state had already been proposed by Professor E. Fermi and Professor CN Yang.

Evidence of anomalous behavior in proton-antiproton systems near the ppbar mass threshold is accumulating. Examples: J/ψ→γppabr, J/ψ→γπ⁺π^–Effective shape factor of proton determined from η’ and e⁺e^–→ppbar, ppbar exhibiting a narrow peak or very steep decay around the mass threshold, which has led to much speculation and renewed interest in the nucleon-antinucleon bound state.

X(1840) is J/ψ→γ3(π⁺π^–) 2013 process using subdata samples from the BESIII experiment. This also lies near the ppbar mass threshold. To better understand the properties of X(1840), it is essential to further investigate the shape of the line of X(1840). Therefore, in the BESIII experiment, 3(π⁺π^–) Mass spectrum containing 10 billion J/ψ events. This is approximately 45 times the subdata sample used in the previous measurement.

An unusual line shape of X(1840) near the ppbar mass threshold is observed for the first time. After many attempts, it was determined that a model containing a consistent sum of the two Breit-Wigner parameterizations could provide a good description of the data. This revealed a new resonance, X(1880), with statistical significance greater than 10σ, as well as mass and width. It was measured to be 1882.1±1.7±0.7 MeV/c.² 30.7±5.5±2.4MeV/c and 30.7±5.5±2.4MeV/c, respectively.

Its mass close to the ppbar mass threshold supported the existence of a ppbar bound state. After publication, this result was selected as a “Special Feature on Physics.” physical review letter.