Search for Pair Production of a New $b^{\prime }$ Quark that Decays into a $Z$ Boson and a Bottom Quark with the ATLAS Detector

A search is reported for the pair production of a new quark $b^{\prime }$ with at least one $b^{\prime }$ decaying to a $Z$ boson and a bottom quark. The data, corresponding to $2.0{\mathrm{fb}}^{-1}$ of integrated luminosity, were collected from $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ with the ATLAS detector at the CERN Large Hadron Collider. Using events with a $b$-tagged jet and a $Z$ boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum $b^{\prime }$ candidates is tested for an enhancement. No evidence for a $b^{\prime }$ signal is detected in the observed mass distribution, resulting in the exclusion at a 95% confidence level of $b^{\prime }$ quarks with masses $m_{b^{\prime }}<400\mathrm{GeV}$ that decay entirely via $b^{\prime }\to Z+b$. In the case of a vectorlike singlet $b^{\prime }$ mixing solely with the third standard model generation, masses $m_{b^{\prime }}<358\mathrm{GeV}$ are excluded.

Figure 1

$e^{+}{e}^{-}$ invariant mass distribution for events passing the $Z+\ge 1$ jet selection, before imposing the $|m_{ee}-m_Z|<15\mathrm{GeV}$ requirement. The predicted contributions of the SM background sources are shown stacked. The lower panel shows the ratio of the data to the SM prediction, and the solid band denotes the systematic uncertainty on the SM prediction.

Figure 2

$e^{+}{e}^{-}$ invariant mass distribution for events passing the $Z+\ge 1$ $b$ jet selection, before imposing the $|m_{ee}-m_Z|<15\mathrm{GeV}$ requirement.

Figure 3

Transverse momentum distribution of the $b^{\prime }$ candidate in events passing the $Z+\ge 1$ $b$ jet selection. The predicted contributions of the SM background sources are stacked, while the distributions for the two signal scenarios described in the text are overlaid.

Figure 4

Mass distribution of the $b^{\prime }$ candidate in events passing the $Z+\ge 1$ $b$ jet selection and satisfying $p_T(Zb)>150\mathrm{GeV}$. The highest mass bin also includes the data and prediction for $m(Zb)>1\mathrm{TeV}$.

Figure 5

The expected and observed 95% C.L. cross section limits as a function of $b^{\prime }$ mass. The signal cross section is shown with uncertainties arising from PDFs and renormalization and factorization scale choice. The prediction is also multiplied by the $\beta$ factors described in the text.