Possibility of an axial vector isoscalar tetraquark udss is discussed. If a f 1 meson in the mass region 1.4 − 1.5 GeV consists of four quarks nsns, the mass of the isoscalar udss(ϑ +meson) state with J P = 1 + is expected to be lower than that of the f 1 meson. Within a flux-tube quark model, a possible resonant state of udss(J P = 1 + ) is suggested to appear at ∼ 1.4 GeV with the width O(20 ∼ 50) MeV. We propose that the ϑ + -meson is the good candidate for the tetraquark search, which would be observed in the K + K + π − decay channel.The possibility of multiquark states has been discussed for a long time. 1)-10) In particularly, the possible qqqq states have been suggested in many theoretical efforts to understand light scalar mesons (for example, Refs. 1), 11) and 3)). The 4q states were proposed in the description of f 0 (600) and f 0 (980), where the strong attraction between (qq)3 and (qq) 3 play an important role. 1), 11) Here, (qq)3 and (qq) 3 denote the color-anti-triplet quark pair and the color-triplet anti-quark pair, respectively. On the other hand, the KK molecule states were suggested to understand the properties of f 0 (980) and a 0 (980). 3) Even if the 4q components are dominant in a certain meson with the conventional flavor, it is difficult to find an direct evidence of the 4q components due to the mixing with the conventional qq state via the annihilation of qq pairs. Our main interest here is the possibility of "tetraquark" states which has the minimal 4-quark content.The recent observation of D sJ (2317) 12) and the reports of the pentaquark baryon Θ + (uudds), 13) revived the motivation of the experimental and theoretical researches on multiquarks in hadron physics, though the existence of Θ + yet to be well established. One of the striking characteristics of the Θ + is its narrow width. For the theoretical interpretation of the narrow Θ + state, the possibility of the spin-parity J P = 1/2 + and J P = 3/2 − have been discussed by many groups. 10), 14)-19) The unnatural spin and parity is a key of the suppressed width for the lowest decay channel.Now we turn to the discussion on the possibility of the tetraquarks. If we accept the interpretation of the pentaquark as the (ud)3(ud)3q state based on the diquark picture by Jaffe and Wilczek, it is natural to expect that a tetraquark with the udss content may exist at the nearly same energy region by replacing a ud-diquark withs quark. Firstly, one should consider the states with unnatural spin and parity, which cannot decay into two light hadrons (pseudo scalar mesons) in the S-wave channel. Second, the exotic color configurations (qq)3(qq) 3 would be essential to stabilize

We discuss in detail the shear viscosity coefficient eta and the viscosity to entropy density ratio eta/s of a hadronic gas comprised of pions and nucleons. In particular, we study the effects of baryon chemical potential on eta and eta/s. We solve the relativistic quantum Boltzmann equations with binary collisions (pi pi, pi N, and NN) for a state slightly deviated from thermal equilibrium at temperature T and baryon chemical potential mu. The use of phenomenological amplitudes in the collision terms, which are constructed to reproduce experimental data, greatly helps to extend the validity region in the T-mu plane. The total viscosity coefficient eta(T,mu)=eta^pi + eta^N increases as a function of T and mu, indirectly reflecting energy dependences of binary cross sections. The increase in mu direction is due to enhancement of the nucleon contribution eta^N while the pion contribution eta^pi diminishes with increasing mu. On the other hand, due to rapid growth of entropy density, the ratio eta/s becomes a decreasing function of T and mu in a wide region of the T-mu plane. In the kinematical region we investigated T < 180MeV, mu < 1GeV, the smallest value of eta/s is about 0.3. Thus, it never violates the conjectured lower bound eta/s= 1/4pi ~ 0.1. The smallness of eta/s in the hadronic phase and its continuity at T ~ T_c (at least for crossover at small mu) implies that the ratio will be small enough in the deconfined phase T > T_c. There is a nontrivial structure at low temperature and at around normal nuclear density. We examine its possible interpretation as the liquid-gas phase transition.Comment: 19 pages, 13 figures, reference added, figure 8 updated, minor change in the tex

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