We have performed an experiment in a 100-GeV/c negatively charged-pion beam at the Fermi National Accelerator Laboratory to measure the form factor of the pion by elastically scattering pions from the atomic electrons in a liquidhydrogen target. The square of the pion form factor as a function of the square of the four-momentum transfer, q 2 , is defined to be the tr-e elastic (el) differential scattering cross section after radiation correction, divided by that predicted for a point (pt) pion;(da/dqX^ida/drtplFrtf)?In the second line the point-pion cross section is written in terms of the fine-structure constant ot and F w (q 2 ) has been expanded in powers of q 2 . In our experiment the maximum recoil energy of the electron was 84 GeV corresponding to q max 2 = 0.086 (GeV/c) 2 .In this Letter we present data University Report No. OITS-73, 1977 (unpublished). 7 Glennys R. Farrar and Darrel R. Jackson, Phys. Rev. Lett. 35, 1416Lett. 35, (1975.on the pion form factor in the range 0.03 ^ q 2 <0.07 (GeV/c) 2 . 3The charged pion is a particularly simple sysr-tern compared to the proton. Its isovector character implies that it couples almost exclusively to the p meson. In the timelike domain the pion o-form factor has been well measured experimentally. 1 Measurements of the pion form factor at i spacelike momentum transfers, coupled with analyticity, provide a useful test of the vectordominance model. Furthermore, the size of the pion can be extracted directly from these spacelike measurements. I 2The first direct measurement 2 of the pion form 0 • factor by n-e elastic scattering obtained (r n 2 ) = 0 o 61±0.15 F 2 . Our experiment provides an imis proved measurement through refinement of the 01 techniques employed in the earlier experiment [n and by exploiting the higher-momentum-transfer "he values available at Fermilab. The apparatus is shown in Fig. 1. The princii pal features of the experiment are as follows:We have measured the electromagnetic form factor of the charged pion by direct scattering of 100-GeV/c 7r" from stationary electrons in a liquid-hydrogen target at Fermilab. The deviations from the pointlike pion-scattering cross section may be characterized by a root-mean-square charge radius for the pion of (r^2) 1^2 = 0 e 56± 0,04 F.

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Two groups of measurements have been made on the elastic scattering of electrons by deuterium; in each case we observed the recoil deuteron instead of the scattered electron. In the first case the spectrometer was set at 45° so that magnetic scattering was unimportant (about 10%) and we deduced the electric form factors of the deuteron. In the second case deuterons were observed at 0°, allowing us to measure directly the magnetic form factor of the deuteron. Form factors of the neutron were deduced from these measurements for the transfer values q 2 = 3, 4, and 5 (F~2). Preliminary results were given in a first paper. Here we also include a description of the experimental setup and discuss relativistic and exchange-current corrections.

An experiment to measure the electromagnetic forin factor of the negative r meson has been carried out at Fermilab by elastically scattering 100-GeV/c pions from the atomic electrons in a liquid-hydrogen target. We find that the elastic differential scattering cross section is characterized by a root-mean-square pion charge radius of 0.56 +0.04 fm. This paper described our experimental design, measurement resolutions, event triggering logic, event reconstruction, experimental corrections, and form-factor results.

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