A comprehensive search for the \Theta^+ pentaquark on the lattice

TL;DR

This study uses extensive lattice QCD simulations with a large operator basis to search for the $ heta^+$ pentaquark, finding no evidence of its existence at higher quark masses but leaving open possibilities at physical quark masses.

Contribution

It introduces the largest operator basis in lattice QCD for pentaquark searches, including spatially non-trivial operators, to improve the identification of potential pentaquark states.

Findings

No pentaquark bound states found at $m_$=400--630 MeV.

Clear distinction of scattering states from potential pentaquark signals.

Results do not exclude pentaquarks at physical quark masses.

Abstract

We study spin 1/2 isoscalar and isovector, even and odd parity candidates for the $\Theta^+(1540)$ pentaquark particle using large scale lattice QCD simulations. Previous lattice works led to inconclusive results because so far it has not been possible to unambiguously identify the known scattering spectrum and tell whether additionally a genuine pentaquark state also exists. Here we carry out this analysis using several possible wave functions (operators). Linear combinations of those have a good chance of spanning both the scattering and pentaquark states. Our operator basis is the largest in the literature, and it also includes spatially non-trivial ones with unit orbital angular momentum. The cross correlator we compute is 14$\times$14 with 60 non-vanishing elements. We can clearly distinguish the lowest scattering state(s) in both parity channels up to above the expected location of the pentaquark, but we find no trace of the latter. Based on that we conclude that there are most probably no pentaquark bound states at our quark masses, corresponding to $m_\pi$=400--630 MeV. However, we cannot rule out the existence of a pentaquark state at the physical quark mass corresponding to $m_\pi$=135 MeV or pentaquarks with a more exotic wave function.