On the Critical Flavor Number in the 2+1$d$ Thirring Model

Abstract: The Thirring model in 2+1 spacetime dimensions, in which $N$ flavors of relativistic fermion interact via a contact interaction between conserved fermion currents, is studied using lattice field theory simulations employing domain wall fermions, which furnish the correct U(2N) global symmetry in the limit that the wall separation $L_s\to\infty$. Attention is focussed on the issue of spontaneous symmetry breakdown via a non-vanishing fermion bilinear condensate $\langle\bar\psi\psi\rangle\not=0$. Results from quenched simulations are presented demonstrating that a non-zero condensate does indeed form over a range of couplings, provided simulation results are first extrapolated to the $L_s\to\infty$ limit. Next, results from simulations with $N=1$ using an RHMC algorithm demonstrate that U(2) symmetry is unbroken at weak coupling but plausibly broken at strong coupling. Correlators of mesons with spin zero are consistent with the Goldstone spectrum expected from U(2)$\to$U(1)$\otimes$U(1). We infer the existence of a symmetry-breaking phase transition at some finite coupling, and combine this with previous simulation results to deduce that the critical number of flavors for the existence of a quantum critical point in the Thirring model satisfies $0<N_c\<2$, with strong evidence that in fact $N_c\>1$.