Phys. Rev. Lett. 108, 102001 (2012)

A proton is made of three quarks — except that it isn't. Those three 'valence' quarks are in fact adrift in a sea of virtual quarks and gluons, according to the rules of quantum chromodynamics. To understand the proton — for example, its mass, its spin — the contribution of the sea, and particularly of strange quarks in the sea, must be taken into account. It isn't easy to study the sea in isolation, but Zafar Ahmed and colleagues, of the HAPPEX collaboration at Jefferson Laboratory, in Virginia, USA, have a valuable data point to contribute.

Using a polarized electron beam and a liquid-hydrogen target, Ahmed et al. have measured the asymmetry of the scattered electrons to access 'form factors' in the equations that describe the scattering and are sensitive to the strange-quark sea. Previous data from other experiments have suggested that the combined electric and magnetic form factors could have a value that is non-zero. However, this measurement (although only for a single value of four-momentum transfer, Q2) has sufficiently small statistical and systematic uncertainty to signal that the strange contribution to the form factors is consistent with zero after all.