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The BESIII physics programme

Nature Reviews Physics volume 1, pages 480–494 (2019)Cite this article

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Abstract

The standard model of particle physics is a well-tested theoretical framework, but there are still a number of issues that deserve further experimental and theoretical investigation. For quark physics, such issues include the nature of quark confinement; the mechanism that connects the quarks and gluons of the standard model theory to the strongly interacting particles; and the weak decays of quarks, which may provide insights into new physics mechanisms responsible for the matter–antimatter asymmetry of the Universe. These issues are addressed by the Beijing Spectrometer III (BESIII) experiment at the Beijing Electron–Positron Collider II (BEPCII) storage ring, which for the past decade has been studying particles produced in electron–positron collisions in the tau-charm energy-threshold region, and has by now accumulated the world’s largest dataset that enables searches for non-standard hadrons, weak decays of the charmed particles and new physics phenomena beyond the standard model. Here, we review the contributions of BESIII to such studies and discuss future prospects for BESIII and other experiments.

Key points

  • Precision measurements of the tau-lepton mass, |Vcd| and |Vcs| Cabibbo–Kobayashi–Maskawa (CKM) matrix elements, low-energy electron–positron (e+e−) annihilation cross-sections and complex phases in D-meson decays test the standard model and provide important input for experiments at other energies.

  • Measurements of the purely leptonic and semileptonic decay rates of D and Ds mesons provide sensitive tests of lattice quantum chromodynamics calculations of charmed-meson decay constants and form factors.

  • Studies of directly produced Y(4260) mesons led to discoveries of the Zc(3900) and Zc(4020) non-standard, four-quark hadrons and a new production mode for the X(3872).

  • Huge samples of radiative J/ψ decays have provided a deeper understanding of scalar, pseudoscalar and tensor glueballs, and the intriguing but poorly understood resonant structures at the proton–antiproton mass threshold.

  • The discovery of substantial polarization of strange baryons produced in J/ψ decays provides new and unique opportunities for high-sensitivity searches for non-standard-model sources of charge conjugation–parity (CP) symmetry violation.

  • Investigations of time-like electromagnetic form factors of Λ0 and \({\varLambda }_{c}^{+}\) baryons revealed unexpected threshold behaviours and motivated measurements for other stable baryons.

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Fig. 1: Comparison of cross-section measurements.
Fig. 2: Signals of purely leptonic charmed-meson decays.
Fig. 3: Signals of XYZ states observed at BESIII.
Fig. 4: The states close to the \(p\bar{p}\) mass threshold.
Fig. 5: BESIII measurements on baryon properties.

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Acknowledgements

This work is supported in part by National Natural Science Foundation of China (NSFC) under contract nos 11835012, 11521505 and 11475187; the Ministry of Science and Technology of China under contract no. 2015CB856701; Key Research Program of Frontier Sciences, CAS, grant no. QYZDJ-SSW-SLH011; the CAS Center for Excellence in Particle Physics (CCEPP); and the CAS President’s International Fellowship Initiative (PIFI) programme. The figure in Box 1 was redrawn by Xiao-Yan Ma for this Review.

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    Chang-Zheng Yuan

  2. University of Chinese Academy of Sciences, Beijing, China

    Chang-Zheng Yuan & Stephen Lars Olsen

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Glossary

B-factory

An electron–positron collider that operates at centre-of-mass energies of 10.6 GeV, just above the threshold for producing particle–antiparticle pairs of mesons containing a b quark.

‘Super tau-charm’ factories

Proposed high-performance electron–positron colliders that would operate at energies that include the thresholds for tau-lepton and charmed-meson particle–antiparticle pairs.

Non-standard hadronic states

Hadronic states with a more complex substructure than the quark–antiquark \((q\bar{q})\) mesons and three-quark (qqq) baryons of the original quark model (for example, tetraquark \((q\bar{q}q\bar{q})\) and pentaquark \((qqqq\bar{q})\) states).

Luminosity

(\({\mathscr{L}}\)). The collision intensity that relates a reaction’s event rate, dNevt/dt, to its cross-section \(\sigma :{\rm{d}}{N}_{{\rm{evt}}}/{\rm{d}}t={\mathscr{L}}\sigma \).

Charmonium

Mesons that are comprised of a charm quark and a charm antiquark and considered to be the ‘hydrogen atom’ of hadron spectroscopy.

Hyperons

Baryons that contain one or more strange quarks but no charm, bottom or top quark.

(g − 2)μ

The magnetic moment of the muon is g(eħ)/(2mμ). The Dirac theory value g = 2 is modified by quantum electrodynamics corrections that have been calculated at the ppm level.

Unitarity

A principle of quantum mechanics that requires the sum of probabilities to be exactly equal to one.

Lepton flavour universality

Also called ‘lepton universality’. An assumption in the standard model of particle physics that the interactions between leptons and gauge bosons (such as photons) are the same for all leptons.

Hadronic vacuum polarization

The process where a virtual photon momentarily materializes as a virtual system of quarks and gluons.

Helicity

The helicity of a particle is the direction of its spin relative to its momentum.

Light cone sum rule

A theoretical method developed to calculate strong interactions in quantum chromodynamics when the coupling is strong and the conventional perturbative techniques fail to apply.

Four-momentum transfer regions

Regions of final-state phase space populated by processes that experience similar values of energy-momentum changes.

Breit–Wigner function

A functional form that is commonly used to parameterize the mass-dependence of the amplitude for unstable particle resonances. A commonly used (relativistic) form is \({f}_{{\rm{BW}}}(M)\propto 1/\,[({M}_{0}^{2}-{M}^{2})-iM\varGamma ]\), where M0 is the resonance mass and Γ is its decay width.

Gluons

Virtual spin = 1 particles that mediate the strong colour force in quantum chromodynamics, analogous to the photons of quantum electrodynamics.

Glueballs

Hadronic states comprised only of gluons, with no quarks.

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Yuan, CZ., Olsen, S.L. The BESIII physics programme. Nat Rev Phys 1, 480–494 (2019). https://doi.org/10.1038/s42254-019-0082-y

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