Measurement of the rate of bbbb events in hadronic Z decays and the extraction of the gluon splitting into bb

The rate Z→bb̄bb̄ was measured using about 2×106 hadronic decays collected by the DELPHI experiment in 1994 and 1995. Events were forced into 3-jets with ymin>0.06 and a b-tag was required for every jet. The rate was measured to be: R4b=BR(Z→bb̄bb̄)BR(Z→hadrons)=(6.0±1.9(stat.)±1.4(syst.))×10−4 where the invariant mass of every bb̄ system is above twice the b quark mass. Using the value of R4b the probability of secondary production of a bb̄ pair from a gluon per hadronic Z decay, gbb, was extracted and found to be: gbb=(3.3±1.0(stat.)±0.8(syst.))×10−3.