On 22 December 2011, the DØ collaboration also reported limitations on the Higgs boson within the Minimal Supersymmetric Standard Model, an extension to the Standard Model. Proton- antiproton (p p) collisions with a centre-of-mass energy of 1.96 TeV had allowed them to set an upper limit for Higgs boson production within MSSM ranging from 90 to 300 GeV, and excluding tan β > 20–30 for masses of the Higgs boson below 180 GeV (tan β is the ratio of the two Higgs doublet vacuum expectation values).