Pentaquarks $uudd\overline{s}$ with One Color Sextet Diquark

The masses of pentaquarks $uudd\overline{s}$ are calculated within the framework of a semirelativistic effective QCD Hamiltonian using a diquark picture. This approximation allows a correct treatment of the confinement, assumed here to be similar to a $Y$ junction. With only color antitriplet diquarks, the mass of the pentaquark candidate $\Theta$ with positive parity is found around 2.2 GeV. It is shown that, if a color sextet diquark is present, the lowest $uudd\overline{s}$ pentaquark is characterized by a much smaller mass with a negative parity. A mass below 1.7 GeV is computed if the masses of the color antitriplet and color sextet diquarks are taken similar.