Evidence of a new subatomic particle has been observed

The BESIII collaboration reported the observation of an anomalous line shape around the ppbar mass threshold in J/ψ → γ3(π⁺π^–) decay, indicating a condition associated with ppbar. the paper It was published online at Physical review letters.

Proximity in block to 2m_s It suggests that there are states associated with nucleons and antinucleons, an idea with a long history. Before the quark model was born, a state associated with nucleons and antinucleons had already been proposed by Professor E. Fermi and Professor C. N. Yang.

There is accumulating 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 effective shape factor of the proton are determined from e⁺H^–→ ppbar, showing a narrow peak or very sharp dip around the ppbar mass threshold, which has inspired many speculations and renewed interests in the nucleon and antinucleon states.

X (1840) is a new structure discovered in J/ψ → γ3(π⁺π^–) in 2013 with the data subsample of the BESIII experiment, which also lies near the ppbar block threshold. Further exploration of the shape of the X (1840) line is essential to gain a better understanding of its nature. Therefore, the BESIII experiment investigated 3(π⁺π^–) A mass spectrum containing 10 billion J/ψ events, which is about 45 times larger than the data subsample used in the previous measurement.

An anomalous line shape of X (1840) was first observed near the threshold of the ppbar mass. After many attempts, it was found that a model containing a coherent sum of the two Breit-Wigner parameters could provide a good description of the data, revealing a new statistically significant X(1880) resonance 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 supports the existence of a ppbar-bound state. After publication, this result was selected as “Featured in Physics” by Physical review letters.