  There are few good direct laboratory tests of boost invariance for electrons,
because the experiments required often involve repeated precision measurements
performed at different times of year. However, existing measurements and
remeasurements of the 2S-1S two-photon transition frequency in H--which were
done to search for a time variation in the fine structure constant--also
constitute a measurement of the boost symmetry violation parameter 0.83c_(TX) +
0.51c_(TY) + 0.22c_(TZ) = (4 +/- 8) x 10^(-11). This is an eight order of
magnitude improvement over preexisting laboratory bounds, and with only one
additional measurements, this system could yield a second comparable
constraint.
