The BESIII collaboration has marked a significant milestone: the completion of its 15-year campaign to collect 20 fb–1 of e+e– collision data at the ψ(3770) resonance. The sample, collected in two main running periods, 2010–2011 and 2022–2024, is more than 20 times larger than the world’s previous charm-threshold data set collected by the CLEO-c experiment in the US.
BESIII is an experiment situated on the BEPCII storage ring at IHEP in Beijing. It involves more than 600 physicists drawn not only from China but also other nations, including Germany, Italy, Poland, the Netherlands, Sweden and the UK from the CERN member states. The detector has collected data at a range of running points with centre-of-mass energies from 1.8 to 4.95 GeV, most of which are inaccessible to other operating colliders. This energy regime allows researchers to make largely unique studies of physics above and below the charm threshold, and has led to important discoveries and measurements in light-meson spectroscopy, non-perturbative QCD, and charm and tau physics.
The ψ(3770), discovered at SLAC in 1977, is the lightest charmonium state above the open-charm threshold. Charmonium consists of a bound charm quark and anti-charm quark, whereas open-charm states such as D0 and D+ mesons are systems in which the charm quark co-exists with a different anti-quark. The ψ(3770) can decay into D and anti-D mesons, whereas charmonium states below threshold, such as the J/ψ, are too light to do so, and must instead decay through annihilation of the charm and anticharm quarks.
The sample is more than 20 times larger than the world’s previous charm-threshold data set
Open-charm mesons are also produced in copious quantities at the LHC and at Belle II. However, in ψ(3770) decays at BESIII they are produced in pairs, with no accompanying particles. This makes the BESIII sample a uniquely clean laboratory in which to study the properties of D mesons. If one meson is reconstructed, or tagged, in a known charm decay, the other meson in the event can be analysed in an unbiased manner. When reconstructed in a decay of interest, the unbiased sample of mesons can be used to measure absolute branching fractions and the relative phases between any intermediate resonances in the D decay.
“Both sets of information are not only interesting in themselves, but also vital for studies with charm and beauty mesons at LHCb and Belle II,” explains Guy Wilkinson of the University of Oxford. “For example, measurements of phase information performed by BESIII with the first tranche of ψ(3770) data have been essential input in the world-leading determination of the CP-violating angle γ of the unitarity triangle by LHCb in events where a beauty meson decays into a D meson and an accompanying kaon.” Exploitation of the full 20 fb–1 sample will be essential in helping LHCb and Belle II realise their full potential in CP-violation measurements with larger data sets in the future, he adds. “Hence BESIII is very complementary to the higher energy experiments, demonstrating the strong synergies that exist between particle-physics facilities worldwide.”
This summer, BEPCII will undergo an upgrade that will increase its luminosity. Over the rest of the decade more data will be taken above and below the charm threshold. In the longer term, there are plans, elsewhere in China, for a Super Tau Charm Facility – an accelerator that would build on the BEPCII and BESIII programme with datasets that are two orders of magnitude larger.
