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 $5.4~\rm{fb}^{-1}$, is used to measure the $Z$-boson production cross section times branching ratio into $b\bar{b}$. The number of $Z\rightarrow b\bar{b}$ 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, $\sigma(p\bar{p} \rightarrow Z) \times \mathcal{B}(Z \rightarrow b\bar{b})= 1.11\pm 0.08(\text{stat}) \pm 0.14(\text{syst})~\text{nb}$, 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 $p\bar p \rightarrow H\rightarrow b\bar b$ process at $\sqrt{s}=$1.96 TeV is set, corresponding to 33 times the expected standard-model cross section, or $\sigma = 40.6$ pb, at the 95\% confidence level.
Highlights
This paper reports the most precise measurement of the σðpp → ZÞ × BðZ → bb Þ production cross section times branching ratio and the first inclusive search for the pp → H → bbprocess in proton-antiproton collisions at a center-of-mass energy of 1.96 TeV
The procedure to extract the b-tagging efficiencies in data uses events with muons reconstructed inside the jet cone, which are likely to originate from semileptonic decays of b hadrons, B → μX
The search for Z → bband H → bbcandidates is conducted by looking for an enhancement over the continuum background in the invariant-mass distribution of the two leading-ET b-tagged jets, m12, in the double-tagged sample
Summary
This paper reports the most precise measurement of the σðpp → ZÞ × BðZ → bb Þ production cross section times branching ratio and the first inclusive search for the pp → H → bbprocess in proton-antiproton (pp ) collisions at a center-of-mass energy of 1.96 TeV. The identification of the Z → bbdecay is challenging at hadron colliders because of the overwhelming irreducible background from multijet production [as is predicted by quantum chromodynamics (QCD)], but it represents a benchmark capability as it validates the experimental techniques used in searches for events with b quarks in the final states It can be used for a direct test of the calibration of the jet-energy scale (JES) for b-jets, defined as a factor that measures the discrepancy between the effect of the detector response and energy corrections in real and simulated b-jets. New scalar particles decaying into bbquark pairs are foreseen in models where they act as mediators for dark matter interactions [9,10]
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