Search for the Neutral Current Top Quark Decay t-->Zc Using the Ratio of Z-Boson + 4 Jets to W-Boson + 4 Jets Production

We have used the Collider Detector at Fermilab (CDF II) to search for the flavor-changing neutral-current (FCNC) top quark decay {t\goes Zc} using a technique employing ratios of $W$ and $Z$ production, measured in \ppbar data corresponding to an integrated luminosity of 1.52 $fb^{-1}$. The analysis uses a comparison of two decay chains, {p\bar{p}\goes t\bar{t}\goes WbWb\goes\ell\nu bjjb} and {p\bar{p}\goes t\bar{t}\goes ZcWb\goes\ell\ell cjjb}, to cancel systematic uncertainties in acceptance, efficiency, and luminosity. We validate the modeling of acceptance and efficiency for lepton identification over the multi-year dataset using another ratio of $W$ and $Z$ production, in this case the observed ratio of inclusive production of $W$ to $Z$ bosons. To improve the discrimination against standard model backgrounds to top quark decays, we calculate the top quark mass for each event with two leptons and four jets assuming it is a $t\bar{t}$ event with one of the top quarks decaying to $Zc$. For additional background discrimination we require at least one jet to be identified as originating from a $b$-quark. No significant signal is found and we set an upper limit on the FCNC branching ratio Br({t\goes Zc}) using a likelihood constructed from the ${\ell\ell cjjb}$ top quark mass distribution and the number of ${\ell\nu bjjb}$ events. Limits are set as a function of the helicity of the $Z$ boson produced in the FCNC decay. For 100% longitudinally polarized $Z$ bosons we find limits of 8.3% and 9.3% (95% C.L.) depending on the assumptions regarding the theoretical top quark pair production cross-section

First Measurement of the \lowercase{\ttbar} Differential Cross Section \lowercase{d\sigma/d}M_{\lowercase{{t\overline{t}}}} in \lowercase{\ppbar} Collisions at \sqrt{\lowercase{s}}=1.96 TeV

8 pages, 1 figureWe present a measurement of the \ttbar differential cross section with respect to the \ttbar invariant mass, dSigma/dMttbar, in \ppbar collisions at $\sqrt{s}=1.96$ TeV using an integrated luminosity of 2.7\invfb collected by the CDF II experiment. The \ttbar invariant mass spectrum is sensitive to a variety of exotic particles decaying into \ttbar pairs. The result is consistent with the standard model expectation, as modeled by \texttt{PYTHIA} with \texttt{CTEQ5L} parton distribution functions.We present a measurement of the tt̅ differential cross section with respect to the tt̅ invariant mass, dσ/dMtt̅ , in pp̅ collisions at √s=1.96  TeV using an integrated luminosity of 2.7  fb-1 collected by the CDF II experiment. The tt̅ invariant mass spectrum is sensitive to a variety of exotic particles decaying into tt̅ pairs. The result is consistent with the standard model expectation, as modeled by PYTHIA with CTEQ5L parton distribution functions.Peer reviewe

ATM Based Event Building and PC Based Level 3 Trigger at CDF

. Baseline event building and Level 3 trigger processing at the CDF experiment at Fermilab is specified to process about 300 events/s with an average event size of about 150 KB. First, the event must be assembled from fragments originating from approximately 16 readout sources, with individual average fragment sizes ranging from 12 to 16 KB. Then the Level 3 processor-based trigger performs reconstruction and filtering tasks requiring in excess of 45000 MIPS of CPU power on the assembled events. We present a distributed, scalable architecture using commodity components: VME-based CPU modules for the readout, an ATM switch for event buildling, and Pentium-based PC's running Linux for event processing. Pentium-based PC's are also used for event distribution throughout and collection from the Level 3 processors via multiple 100 Mbps Ethernets. We report on preliminary studies conducted at CDF with a small-scale prototype. This architecture is also a possible candidate for the CMS experime..