NUCLEAR PHYSICS p0 decay precision-tests the chiral anomaly More precise neutral pion lifetime measurements probe quantum symmetry breaking
By Harvey B. Meyer point, it was assumed that a symmetry of the errors in quadrature, this amounts to a classical Lagrangian would protect p0 from tension of 1.8 standard deviations versus ost subatomic particles are decaying in the limit of massless up and the combined result of the PrimEx-I and strongly interacting composites down quarks and lead to a longer lifetime. -II experiments, 8.34 (60.13) × 10−17 s. Al- called hadrons. Most hadrons are However, it turns out to be impossible to reg- though this difference could be a statisti- unstable and decay on extremely ularize quantum chromodynamics without cal fluctuation, it provides motivation to short time scales (10−22 s) to lighter breaking that symmetry. Therefore, the latter revisit the theory prediction. The gp pp hadrons. The electrically neutral is not respected by the quantum fluctuations reaction also has a sharp low-energy pre- Mpion, p0, is the lightest hadron and decays of the quantum chromodynamics fields and diction based on the chiral anomaly (6, 7) on a time scale of 10−16 s in 98.8% of cases does not protect p0 from decaying. and is being investigated by the COMPASS into two photons, gg, through the electro- The quantum origin of the symmetry experiment (8). magnetic interaction. Historically, under- breaking leads to an exact prediction for the The gg decay width of p0 has recently standing this time scale presented a major strength of p0 coupling to gg and hence the been evaluated from first principles using Downloaded from challenge to theoreticians. On page 506 of p0 lifetime in the limit of massless up and lattice quantum chromodynamics simula- this issue, Larin et al. (1) down quarks. The product tions (9). This numerical result is in agree- report that the measure- of the p0 lifetime and the ment with the experimental measurement, ment of the lifetime of the Limits on a lifetime charged pion p+ lifetime, but with its full uncertainty of 7%, it does neutral pion has reached a The neutral pion p0 lifetime which decays through the not yet have the precision to clarify the precision of 1.5% through measured by Larin et al. can weak interaction, depends above-mentioned tension between theory http://science.sciencemag.org/ the combined results of the be compared to the calculated on no hadronic quantity and experiment. Still, lattice quantum PrimEx-I and -II experi- lifetime, which depends on the other than the pion masses chromodynamics calculations have the po- ments. Its dominant decay matrix element illustrated below. (see the figure). The matrix tential to improve in the near future. The to two photons proceeds ele ment of the divergence p0-gg coupling allows the two photons to
mainly through the chiral of the axial current, −?jA, scatter off each other through formation anomaly, and calculating g between the vacuum and a of the p0 resonance. “Scattering of light by its decay time represents p0 gg state can be calculated light” is one of the virtual processes that an important test of quan- exactly and depends only cause the magnetic dipole moment g of the tum chromodynamics in its on the fine-structure con- muon to deviate from 2. This process is one g
low-energy, nonperturba- stant and the photon mo- of the leading sources of uncertainty in the on May 13, 2020 tive regime. menta. However, the matrix predicting (g 2 2) , whose measurement Decay process µ In the 1960s, it was re- The Feynman diagram is shown element is saturated by the serves as a precision test of the standard alized that the multitude for the decay amplitude of p0 exchange of a p0, which the model of particle physics (10). Therefore, into two photons g. of hadron species could be axial current jA creates pro- the new precision measurement of the understood economically in portionally to the amplitude pion lifetime by Larin et al. contributes to terms of more elementary for the p+ decay into the consolidating the standard model predic- g + j degrees of freedom: the −?jA charged muon µ and the tion of this important quantity. quarks. This simpler and muon neutrino nµ. Thus, in REFERENCES AND NOTES deeper description is simi- g order for the two evalua- 1. I. Larin et al., Science 368, 506 (2020). lar in nature to the under- tions of the matrix element 2. S. L. Adler, Phys. Rev. 177, 2426 (1969). standing of the properties Strength of to be consistent, the cou- 3. J. Bell, R. Jackiw, Nuovo Cim. A 60, 47 (1969). 0 of the chemical elements p -gg coupling pling of p0 to gg must be in- 4. A. M. Bernstein, B. R. Holstein, Rev. Mod. Phys. 85, 49 (2013). in terms of the electronic The matrix element of the versely proportional to the divergence of the axial current, 5. K. Kampf, B. Moussallam, Phys. Rev. D 79, 076005 structure of atoms. In the p+ decay amplitude. (2009). −?jA, between the vacuum and a early 1970s, it became un- gg state can be calculated exactly A major objective of the 6. J. Wess, B. Zumino, Phys. Lett. B 37, 95 (1971). 7. E. Witten, Nucl. Phys. B 223, 422 (1983). from this triangle diagram. derstood that the force that measurement of the neutral 8. COMPASS Collaboration, J. M. Friedrich, E PJ We b C o n f . binds quarks into hadrons pion lifetime by the PrimEx 199, 01016 (2019). is mediated by a set of gauge fields. Quan- collaboration was to test its prediction at 9. A. Gérardin, H. B. Meyer, A. Nyffeler, Phys. Rev. D 100, tum chromodynamics is the fundamental the percent level. With an overall preci- 3034520 (2019). 10. F. Jegerlehner, The Anomalous Magnetic Moment of the quantum field theory that describes this sion of 1.5%, this goal has been reached. At Muon (Springer, 2017). type of interaction. this precision level, refinements must be SCIENCE The underlying cause of the short p0 life- applied to the prediction. They have been ACKNOWLEDGMENTS time was found in 1969 with the discovery of worked out by several groups and found to H.B.M.’s work is supported by the European Research Council (ERC) under the European Union’s Horizon 2020 the so-called chiral anomaly (2, 3). Up to that be on the order of +4.5 6 1.0% (4). One of research and innovation program through grant agreement the most sophisticated theory predictions 771971-SIMDAMA. −17 0 Institute for Nuclear Physics, University of Mainz, 55099 (5) obtains 8.04 (60.11) × 10 s for the p
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