1. Harvard University, 17 Oxford Street, Cambridge, Massachusetts 02138, USA
2. National Institute of Standards and Technology, 100 Bureau Drive, MS 8461, Gaithersburg, Maryland 20899, USA
3. National Institute of Standards and Technology, 325 Broadway, MS898.02, Boulder, Colorado 80303, USA
4. Hahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin, Germany
5. University of California, Los Alamos National Laboratory, PO Box 1663, Los Alamos, New Mexico 87545, USA
6. Present address: Institut Laue-Langevin, BP 156-6 rue Jules Horowitz, 38042, Grenoble (Cedex 9), France.

Correspondence to: P. R. Huffman^1,2 Correspondence and requests for materials should be addressed to P.R.H. (e-mail: paul.huffman@nist.gov).

Accurate measurement of the lifetime of the neutron (which is unstable to beta decay) is important for understanding the weak nuclear force¹ and the creation of matter during the Big Bang². Previous measurements of the neutron lifetime have mainly been limited by certain systematic errors³; however, these could in principle be avoided by performing measurements on neutrons stored in a magnetic trap. Neutral-particle and charged-particle traps are widely used for studying both composite and elementary particles, because they allow long interaction times and isolation of particles from perturbing environments⁴. Here we report the magnetic trapping of neutrons. The trapping region is filled with superfluid ⁴He, which is used to load neutrons into the trap and as a scintillator to detect their decay. Neutrons in the trap have a lifetime of 750⁺³³⁰ _-200 seconds, mainly limited by their beta decay rather than trap losses. Our experiment verifies theoretical predictions regarding the loading process and magnetic trapping of neutrons. Further refinement of this method should lead to improved precision in the neutron lifetime measurement.