  If a global chiral symmetry is explicitly broken by anomalies in nonabelian
gauge theories, a pseudo Nambu-Goldstone boson (axion) associated with a
spontaneous breakdown of such a global symmetry acquires a mass through
nonperturbative instanton effects. We calculate the axion mass assuming a
supersymmetric SU(2) gauge theory and show that the axion obtains an extremely
small mass when the SU(2) gauge symmetry is broken down at very high energy,
say at the Planck scale. We identify the axion with a hypothetical ultralight
boson field proposed to account for a small but nonzero cosmological constant
suggested from recent cosmological observations.
