a1 (1420) resonance as a tetraquark state and its isospin partner

We systematically construct tetraquark currents of I(G)J(PC) = 1(-)1(++) and classify them into types A ( antisymmetric), S ( symmetric) and M ( mixed), based on flavor symmetries of diquarks and antidiquarks composing the tetraquark currents. We use tetraquark currents of type M to perform QCD sum rule analyses, and find a tetraquark current eta(M)(5 mu) with quark content qs (q) over bar(s) over bar (q = u or d) leading to a mass of 1.44 +/- 0.08 GeV consistent with the a(1)(1420) state recently observed by the COMPASS Collaboration. Our results support tetraquark explanations for both a(1)(1420) and f(1)(1420), assuming that they are isospin partners. We also study their possible decay patterns. As tetraquark candidates, the possible decay modes of a(1)(1420) are S- wave a(1)(1420) -> K*(892)K and P-wave a(1)(1420) -> f(0)(980)pi, while the possible decay patterns of f(1)(1420) are S-wave f(1)(1420) -> K*(892)K and P-wave f(1)(1420) -> a(0)(980)pi. We speculate that a(1)(1420) is partly responsible for the large isospin violation in the eta(1405) -> f(0)(980)pi(0) decay mode, which is reported by BESIII Collaboration in the J/psi -> gamma 3 pi process.