Volume 193, number 1 PHYSICS LETTERS B 9 July 1987 DYNAMICAL QUARK EFFECTS ON THE HADRONIC SPECTRUM AND q~I-POTENTIAL IN LATTICE QCD M. CAMPOSTRINI INFN Sezione di Pisa, 1-56010 Pisa. Italy and Brookhaven National Laboratory, Upton, NY 11973, USA K.J.M. MORIARTY Institute for Advanced Studies, Princeton, NJ 08540, USA and Consortium for Scientific Computing, The John yon Neumann Center, 665 College Road East, Princeton NJ 08543, USA J. POTVIN Physics Department, Brookhaven National Laboratory, Upton, NJ 11973, USA and C. REBBI Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA and Boston University, Boston, MA 02215, USA Received 11 May 1987 The results of a calculation which incorporates virtual quark-antiquark pairs at a coupling offl= 5.7 and quark mass of 0.05 are compared with those of two quenched simulations at fl= 5.895 and 6. The inclusion of the dynamical quarks produces no effects in the hadron mass spectrum besides an overall shift in the coupling. In contrast, the Wilson loop factors show a non-trivial effect, in the form of a flattening of the heavy quark potential. All three studies were carried out on a 103.32 lattice. The numerical simulation of QCD, in the frame- work of the lattice formulation, has been an invalu- able tool for studying the low energy properties of the strong interactions [1 ]. One remarkable exam- ple is the direct calculation of the hadron mass spec- trum (cf. ref. [ 2 ] for a review, ref. [ 3 ] for very recent investigations). An approximation which has played a very impor- tant role in many lattice investigations of QCD, the so-called quenched or valence approximation, con- sists of neglecting vacuum polarization effects induced by the virtual creation and annihilation of qcl pairs. The quenched approximation has been widely used essentially for computational reasons, since even approximate algorithms to incorporate fermion polarization effects end up being quite costly in computer time. Moreover, its use for the calcu- lation of hadron masses can be justified on a variety of phenomenological and theoretical grounds (OZI rule, large N limit, etc.). The quenched simulations of the past six years have indeed confirmed this expectation [2,3], reproducing the lowest states of the hadronic spectrum up to 10% accuracy. How- ever, a real check of the validity of the quenched approximation can only come from the comparison of its predictions with the results of calculations which incorporate the effects of the dynamical quarks (hereafter called "unquenched" calculations). 78 0370-2693/87/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)