Volume 149B, number 6 PHYSICS LETTERS 27 December 1984 CHIRAL PHASE TRANSITION IN LATTICE QCD WITH DYNAMICAL QUARKS R.V. GAVAI Zentrum jTtr inrerdiszipliniire Forschung and Fakultiit flir Physik, Universit~t BielefeM, Bielefeld, Germany and M. LEV and B. PETERSSON Fakultiit flit Physik, Universitiit Bielefeld, Bielefeld, Germany Received 31 July 1984 The effect of virtual light quark loops on the chiral phase transition in lattice QCD with staggered fermions is investi- gated by employing the pseudo-fermion Monte Carlo method on a 63 × 2 lattice. The variation in the order parameter ( ~ ) is found to become less sharp than the quenched case, indicating a second order chiral phase transition in the full theory. One way to learn more about a theory is to investi- gate its consequences in unusual environments such as at Finite temperatures or finite densities. The advent of Monte Carlo simulation techniques for lattice gauge theories has enabled one to obtain interesting results this way [1 ]. In the approximation of neglecting vir- tual quark loops (quenched approximation) it has thus been shown that the spontaneously broken [2] chiral symmetry at low temperatures is restored at high temperatures [3] and that for SU(3) a first order chiral phase transition separates these two regimes. At almost the same value of the temperature (or the cou- pling constant of the lattice theory), there is also a first order deconFmement phase transition in the SU(3) Yang-Mills theory, as has been shown by stu- dying the order parameter (L), the average thermal Wilson loop. In view of the importance of such results for the early universe or ultra-relativistic heavy ion collisions, it appears desirable to study how dependent they are on the quenched approximation. Indeed, arguments have been presented [4] to suggest that the decon- finement phase transition may be washed away due to the inclusion of the virtual quark loops. Such ar- guments may, however, depend on the approxima- tions such as strong coupling, heavy quarks, mean field theory etc. On the other hand, no similar argu- 492 ments have been presented for the chiral phase transi- tion so far. To study QCD in the presence of light dynamical quarks, we, therefore, undertook a numeri- cal study of the SU(3) lattice gauge theory with dyna- mical staggered fermions. In this letter, we report our results on (~), the total energy density and the aver- age thermal Wilson loop, obtained by using the pseudo- fermion method [5,6] to include the virtual quark loops *x . Let us begin by defining our action: S = S G + S F , (1) with SG=/~ ~ (1 1 It, V x_~U X ) -~RetrU~Ux+ hU ut vt X,t.t <v =/3 ~ Px ~v , (2) X,/.t<v ,1 Result s obtained at m = 0.1,but with different boundary conditions, have been published elsewhere, where more de- tails of the method may b.e found (ref. [7 ]). While the im- portance O f these boundary conditions is unclear, the ones used here are usually more favoured, and although using the other boundary conditions on our lattice seems to sharpen the variation in (~-~0), no significant difference is expected to be found on bigger lattices. 0370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)