Physicists with the BESIII Collaboration have observed an anomalous line shape around ppbar mass threshold in the J/ψ→γ3(π+π-) decay, which indicates the existence of a new bound state, labeled X(1880).
A portrait of the nucleon-antinucleon bound state. Image credit: BESIII Collaboration.
“The proximity in mass to 2mp is suggestive of nucleon-antinucleon bound states, an idea that has a long history,” the BESIII physicists said in a statement.
“Before the birth of the quark model, a nucleon-antinucleon bound state was already proposed by Professor Enrico Fermi and Professor Chen-Ning 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.”
In 2013, a new structure, X(1840), was discovered in the J/ψ→γ3(π+π-) process in the subdata sample of BESIII experiment.
“A further exploration of line shape of X(1840) was essential to have a better understanding of its nature,” the researchers said.
“Therefore, we 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.”
“An anomalous line shape of X(1840) near the ppbar mass threshold is observed for the first time.”
“After many attempts, it is found that the model with a coherent sum of two Breit-Wigner parameterizations can provide a good description of data, which reveals the new resonance X(1880) with a statistical significance greater than 10σ, and the mass and width are 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.”
The team’s paper was published in the journal Physical Review Letters.
_____
M. Ablikim et al. (BESIII Collaboration). 2024. Observation of the Anomalous Shape of X(1840) in J/ψ→γ3(π+π−) Indicating a Second Resonance Near p¯p Threshold. Phys. Rev. Lett 132 (15): 151901; doi: 10.1103/PhysRevLett.132.151901