Evidence of a new subatomic particle observed

A portrait of nucleon-antinucleon bound state. Credit: Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.151901

The BESIII collaboration have reported the observation of an anomalous line shape around ppbar mass threshold in the J/ψ→γ3(π⁺π^–) decay, which indicates the existence of a ppbar bound state. The paper was published online in Physical Review Letters.

The proximity in mass to 2m_p is suggestive of nucleon-antinucleon bound states, an idea that has a long history. Before the birth of Quark Model, a nucleon-antinucleon bound state was already proposed by Prof. E. Fermi and Prof. C. N. Yang.

There is an accumulation of evidence for anomalous behavior in the proton-antiproton system near the ppbar mass threshold, e.g., J/ψ→γppabr , J/ψ→γπ⁺π^–η’ and the proton’s effective form factor determined from e⁺e^–→ppbar, exhibiting a narrow peak or a very steep falloff around the ppbar mass threshold, which inspired many speculations and renewed the interests on the nucleon-antinucleon bound state.

X(1840) is a new structure discovered in the J/ψ→γ3(π⁺π^–) process in 2013 with subdata sample of BESIII experiment, which is also located near the ppbar mass threshold. A further exploration of line shape of X(1840) is essential to have a better understanding of its nature. Therefore, the BESIII experiment performed an investigation on the 3(π⁺π^–) mass spectrum with 10 billion J/ψ events, which is about 45 times larger than the subdata sample used in the previous measurement.

The anomalous line shape of the resonant structure around ppbar mass threshold in 3(π+π-) mass spectrum. Credit: Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.151901

An anomalous line shape of X(1840) near the ppbar mass threshold was observed for the first time. After many attempts, it was found that the model with a coherent sum of two Breit-Wigner parameterizations could provide a good description of data, which revealed a new resonance X(1880) with a statistical significance greater than 10σ, and the mass and width were determined to be 1882.1±1.7±0.7 MeV/c² and 30.7±5.5±2.4 MeV/c, respectively.

The proximity of its mass to the ppbar mass threshold supported the existence of a ppbar bound state. After publication, this result was selected as “Featured in Physics” by Physical Review Letters.

More information: M. Ablikim et al, Observation of the Anomalous Shape of X(1840) in J/ψ→γ3(π⁺π^–) Indicating a Second Resonance Near pp¯ Threshold, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.151901

Provided by Chinese Academy of Sciences

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