Search for Standard Model resonances decaying into a pair of $b$-jets at CDF II

The CDF Collaboration

June 27, 2017 (update of the note 11219)

Abstract:

CDF has collected $5.4~\text{fb}^{-1}$ of $p\bar p$ events using the dedicated SVT trigger path which required a displaced vertex compatible with a $b$-hadron decay. This unique dataset is used to search for Standard Model $Z$ and Higgs resonances decaying into a pair of $b$-jets. The $Z$ production cross section times the $b\bar{b}$ branching ratio is measured by extracting $Z\rightarrow b\bar b$ events from a fit to the dijet invariant mass distribution, where the dominant QCD $b$-jet background is estimated by a data-driven technique to minimize the dependence of the analysis on the Monte Carlo simulation. The measured cross section is $$\sigma_Z \times B(Z\rightarrow b\bar b)= 1.11\pm 0.08(stat) \pm 0.13(sys)~\text{nb}.$$ The analysis technique is used on the same dataset to search for $H\rightarrow b\bar b$. No signal is found and an upper limit on the $p\bar p \rightarrow H\rightarrow b\bar b$ cross section is set at 95\% C.L. resulting on 33 times the expected Standard Model value. This result constitutes the first inclusive $p\bar p \rightarrow H\rightarrow b\bar b$ limit.

Blessed plots

  trigtag
Figure 1: Data/MC scale factor distribution as a function of the jet transverse energy for tight SecVtx tags with result of the fit superimposed: in blue the constant line and in red the straight line.

  • FIGURE1.PDF

      SECVTX
    Figure 2: Trigger $b$-tag data/MC scale factor distribution as a function of the jet transverse energy with result of the fit superimposed: in blue the constant line and in red a straight line.

  • FIGURE2.PDF

      tagmass
    Figure 3: Invariant mass distribution of the charged tracks of the secondary vertex for a sample of events with at least one $b$-tag trigger jet.

  • FIGURE3.PDF

      fit
    Figure 4: Double tagged events invariant mass distribution with the result of the fit. In red the fitted $Z\rightarrow \bar{b}b$, in blue the Bb+Cb and in green the bB+bC background. Capital letter indicates the $b$-tag trigger jet.

  • FIGURE4.PDF

      figt
    Table 1: The signal and background yields as returned by the fit to the double tagged sample.

  • TABLE1.PDF

      fit
    Table 2: Summary of the systematic uncertainties.

  • TABLE2.PDF

      data-bkg
    Figure 5: Double tagged events invariant mass distribution after the background subtraction. The Z peak is clearly visible and it is in good agreement with the MC signal template, in red.

  • FIGURE5.PDF

      data-bkg
    Figure 6: Invariant mass distribution of the double tagged data sample with the result of the fit that includes the $H\rightarrow b\bar{b}$ decay. The normalization of the Higgs signal is set to $\times 10^3$ the expected SM cross section for illustrative purposes.

  • FIGURE6.PDF

      data-bkg
    Figure 7: Observed (black solid line) and expected (black dashed line) $CL_s$ as function of the cross section times the branching ratio normalized to SM $H \rightarrow b\bar{b}$. Green and yellow regions are respectively the $1\sigma$ and $2\sigma$ bands on the expected $CL_S$. The $95\%$ C.L. limits are indicated by the red vertical arrows under the red horizontal line.

  • FIGURE7.PDF

      data-bkg
    Table 3: Summary of the systematic uncertainties on the $H\rightarrow b\bar{b}$ cross section limit.

  • Table3.PDF

    Emanuele Michielin