Better local hidden variable models for two-qubit Werner states and an upper bound on the Grothendieck constant $K_G(3)$

We consider the problem of reproducing the correlations obtained by arbitrary local projective measurements on the two-qubit Werner state $rho = v |psi_- > <psi_- | + (1- v ) frac{1}{4}$ via a local hidden variable (LHV) model, where $|psi_- >$ denotes the singlet state. We show analytically that these correlations are local for $ v = 999times689times{10^{-6}}$ $cos^4(pi/50) simeq 0.6829$. In turn, as this problem is closely related to a purely mathematical one formulated by Grothendieck, our result implies a new bound on the Grothendieck constant $K_G(3) leq 1/v simeq 1.4644$. We also present a LHV model for reproducing the statistics of arbitrary POVMs on the Werner state for $v simeq 0.4553$. The techniques we develop can be adapted to construct LHV models for other entangled states, as well as bounding other Grothendieck constants.