Precision measurement of the neutral pion lifetime

Journal Article · 30 April 2020 · Science

DOI: https://doi.org/10.1126/science.aay6641 · OSTI ID:1616652

Larin, I.  ^[1]; Zhang, Y.  ^[2]; Gasparian, A.  ^[3]; Gan, L. ^[4]; Miskimen, R. ^[5]; Khandaker, M.  ^[6]; Dale, D.  ^[7]; Danagoulian, S.  ^[3]; Pasyuk, E. ^[8]; Gao, H. ^[2]; Ahmidouch, A.  ^[3]; Ambrozewicz, P.  ^[3]; Baturin, V.  ^[8]; Burkert, V. ^[8]; Clinton, E. ^[5]; Deur, A. ^[8]; Dolgolenko, A.  ^[9]; Dutta, D.  ^[10]; Fedotov, G.  ^[11]; Feng, J. ^[4] ; Gevorkyan, S. ^[12]; Glamazdin, A. ^[13]; Guo, L. ^[14]; Isupov, E.  ^[15]; Ito, M. M.  ^[8]; Klein, F.  ^[16]; Kowalski, S. ^[17]; Kubarovsky, A. ^[8]; Kubarovsky, V. ^[8]; Lawrence, D. ^[8]; Lu, H.  ^[18]; Ma, L. ^[19]; Matveev, V.  ^[9]; Morrison, B.  ^[20]; Micherdzinska, A. ^[21]; Nakagawa, I. ^[22]; Park, K.  ^[8]; Pedroni, R.  ^[3]; Phelps, W. ^[23]; Protopopescu, D. ^[24]; Rimal, D.  ^[14]; Romanov, D. ^[25]; Salgado, C.  ^[6]; Shahinyan, A. ^[26]; Sober, D. ^[16]; Stepanyan, S. ^[8]; Tarasov, V. V.  ^[9]; Taylor, S. ^[8]; Vasiliev, A. ^[27]; Wood, M. ^[5]; Ye, L. ^[10]; Zihlmann, B. ^[8]

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1. Alikhanov Institute for Theoretical and Experimental Physics, National Research Center (NRC) “Kurchatov Institute,” Moscow, 117218, Russia., Department of Physics, University of Massachusetts, Amherst, MA 01003, USA.
2. Department of Physics, Duke University, Durham, NC 27708, USA., Triangle Universities Nuclear Laboratory, Durham, NC 27708, USA.
3. Department of Physics, North Carolina A&T State University, Greensboro, NC 27411, USA.
4. Department of Physics and Physical Oceanography, University of North Carolina Wilmington, Wilmington, NC 28403, USA.
5. Department of Physics, University of Massachusetts, Amherst, MA 01003, USA.
6. Department of Physics, Norfolk State University, Norfolk, VA 23504, USA.
7. Department of Physics and Nuclear Engineering, Idaho State University, Pocatello, ID 83209, USA.
8. Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA.
9. Alikhanov Institute for Theoretical and Experimental Physics, National Research Center (NRC) “Kurchatov Institute,” Moscow, 117218, Russia.
10. Department of Physics and Astronomy, Mississippi State University, Mississippi State, MS 39762, USA.
11. Department of Physics, Moscow State University, Moscow 119991, Russia., B. P. Konstantinov Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute,” Gatchina, St. Petersburg, 188300, Russia.
12. Joint Institute for Nuclear Research, Dubna, 141980, Russia.
13. Kharkov Institute of Physics and Technology, Kharkov, 310108, Ukraine.
14. Department of Physics, Florida International University, Miami, FL 33199, USA.
15. Department of Physics, Moscow State University, Moscow 119991, Russia.
16. Department of Physics, The Catholic University of America, Washington, DC 20064, USA.
17. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
18. Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA.
19. School of Nuclear Science and Technology, Lanzhou University, Lanzhou, 730000, China.
20. Department of Physics, Arizona State University, Tempe, AZ 85281, USA.
21. Department of Physics, George Washington University, Washington, DC 20064, USA.
22. RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama 351-0198, Japan.
23. Department of Physics, Computer Science and Engineering, Christopher Newport University, Newport News, VA 23606, USA.
24. School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK.
25. Department of Physics, Moscow Engineering Physics Institute, Moscow, Russia.
26. Yerevan Physics Institute, Yerevan 0036, Armenia.
27. Institute for High Energy Physics, NRC “Kurchatov Institute,” Protvino, 142281, Russia.

The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π ⁰ into two photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the ${\pi }^0\to \gamma \gamma$ process. The differential cross sections for π ⁰ photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding $\mathrm{\Gamma }\left({\pi }^0\to \gamma \gamma \right)=7.798\pm 0.056\left(\mathrm{stat}\mathrm{.}\right)\pm 0.109\left(\mathrm{syst}\mathrm{.}\right)\mathrm{ eV}$ , where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of $\mathrm{\Gamma }\left({\pi }^0\to \gamma \gamma \right)=7.802\pm 0.052\left(\mathrm{stat}\mathrm{.}\right)\pm 0.105\left(\mathrm{syst}\mathrm{.}\right)\mathrm{ eV}$ . Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.

• Sponsoring Organization: USDOE
• Grant/Contract Number: AC05-06OR23177
• OSTI ID: 1616652
• Journal Information: Science, Journal Name: Science Journal Issue: 6490 Vol. 368; ISSN 0036-8075
• Publisher: AAASCopyright Statement
• Country of Publication: United States
• Language: English