We study development of singularities for the spherically symmetric
Yang-Mills equations in d+1 dimensional Minkowski spacetime for d=4 (the
critical dimension) and d=5 (the lowest supercritical dimension). Using
combined numerical and analytical methods we show in both cases that generic
solutions starting with sufficiently large initial data blow up in finite time.
The mechanism of singularity formation depends on the dimension: in d=5 the
blowup is exactly self-similar while in d=4 the blowup is only approximately
self-similar and can be viewed as the adiabatic shrinking of the marginally
stable static solution. The threshold for blowup and the connection with
critical phenomena in the gravitational collapse (which motivated this
research) are also briefly discussed.Comment: 4 pages, 3 figures, submitted to Physical Review Letter