Search for New Particles Leading to Z+jets Final States in $p\bar{p}$ Collisions at $\sqrt{s}=1.96$ TeV

TL;DR

This work searches for new heavy particles that decay to Z bosons plus jets in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV with the CDF II detector, emphasizing a data-driven background estimation that replaces reliance on higher-order Monte Carlo predictions. The authors develop a two-part method to predict the dominant $Z$+jets background: (1) extrapolate jet-$E_T$ distributions to predict the $N_{\rm jet}^{30} \ge 3$ yield, and (2) determine the $J_T^{30}$ shape by extrapolating the jet-$E_T$ spectra across jet multiplicities, with a parametric form $f(E_T)=p_0 e^{-E_T/p_1}/(E_T)^{p_2}$ and careful uncertainty assessment. Validation uses multi-jet data and $W$+jets samples, including signal-injection tests, demonstrating the method’s reliability in background-only and signal-like environments. Applying the technique to the data, they observe no significant excess and set a 95% C.L. limit on a fourth-generation $b'$ quark decaying to $bZ$, excluding $m_{b'}<268$ GeV/$c^2$ for $BR(b'\to bZ)=100\%$, with acceptances around 1–2% after cuts. The study provides a robust, rapid-background tool for jet-rich analyses with potential applicability to LHC searches and other multi-jet final states.

Abstract

We present the results of a search for new particles that lead to a \Z boson plus jets in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV using the Collider Detector at Fermilab (CDF II). A data sample with a luminosity of 1.06 \ifb\ collected using \Z boson decays to $ee$ and $μμ$ is used. We describe a completely data-based method to predict the dominant background from standard-model \Z+jet events. This method can be similarly applied to other analyses requiring background predictions in multi-jet environments, as shown when validating the method by predicting the background from $W$+jets in \ttbar production. No significant excess above the background prediction is observed, and a limit is set using a fourth generation quark model to quantify the acceptance. Assuming $BR(b' \to b\Z) = 100%$ and using a leading-order calculation of the $b'$ cross section, $b'$ quark masses below 268 $\gev/c^2$ are excluded at 95% confidence level.

Figures (38)

• Figure 1: Distribution of $M_{\ell\ell}$ of $Z \rightarrow ee$ and $Z \rightarrow \mu\mu$ data (black points and errors) using the baseline $Z$ selection described in the text. Overlaid are standard model $Z \rightarrow ee$ and $Z \rightarrow \mu\mu$ Monte Carlo events, normalized to the number of events expected with the given luminosities using the expected cross section of 250 pb.
• Figure 2: $E_T$ distribution of the third highest $E_T$ jet in standard model $Z \rightarrow \mu\mu$ Monte Carlo simulations. Events with $N_{\rm jet}^{30} \leq 2$ have $E_T < 30\ \rm GeV$; events with $N_{\rm jet}^{30} \geq 3$ have $E_T > 30\ \rm GeV$.
• Figure 3: $E_T$ distribution of the third highest $E_T$ jet in standard model $Z \rightarrow \mu\mu$ Monte Carlo events. The distribution is fit to Eq. (\ref{['eqn:jetetparamfinal']}) in the range $15 < E_T < 30$ GeV, and extrapolated to the $E_T > 30$ GeV region.
• Figure 4: $E_T$ of the highest $E_T$ jet in standard model $Z\rightarrow\mu\mu$ Monte Carlo events. The distribution is fit to Eq. (\ref{['eqn:jetetparamfinal']}) in the region $20<E_T<80$ GeV (dotted line), and again in the region $20<E_T<40$ GeV (solid line).
• Figure 5: $E_T$ distribution of jets in $N_{\rm jet}^{30} = 1$ events (open squares) and in $N_{\rm jet}^{30} = 2$ events (solid circles) in $Z \rightarrow \ell\ell$ data. Events with higher $N_{\rm jet}^{30}$ have harder $E_T$ spectra.