Search for standard-model Z and Higgs bosons decaying into a bottom-antibottom quark pair in proton-antiproton collisions at 1.96 TeV

Abstract

The Collider Detector at Fermilab collected a unique sample of jets originating from bottom-quark fragmentation ($b$-jets) by selecting online proton-antiproton ($p\bar{p}$) collisions with a vertex displaced from the $p\bar{p}$ interaction point, consistent with the decay of a bottom-quark hadron. This data set, collected at a center-of-mass energy of $\sqrt{s}=$1.96 TeV, and corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$, is used to measure the $Z$-boson production cross section times branching ratio into $b\bar{b}$. The number of $Z\to b\bar{b}$ events is determined by fitting the dijet-mass distribution while constraining the dominant $b$-jet background, originating from QCD multijet events, with data. The result, $\ddot{I}ƒ(p\bar{p}\to Z)\times \mathcal{B}(Z\to b\bar{b})=1.11\pm 0.08(\text{stat})\pm 0.14(\text{syst})\text{nb}$, is the most precise measurement of this process, and is consistent with the standard-model prediction. The data set is also used to search for Higgs-boson production. No significant signal is expected in our data and the first upper limit on the cross section for the inclusive $p\bar{p}\to H\to b\bar{b}$ process at $\sqrt{s}=$1.96 TeV is set, corresponding to 33 times the expected standard-model cross section, or $\ddot{I}ƒ=40.6$ pb, at the 95\% confidence level.