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Tesi etd-01152012-185359

Thesis type
Tesi di laurea magistrale
Measurement of the ratio R=BR(t->Wb)/BR(t->Wq) at CDF
Corso di studi
relatore Prof. Chiarelli, Giorgio
Parole chiave
  • decay
  • top quark
  • ratio
  • Measurement
Data inizio appello
Riassunto analitico
In the Stardard Model of elementary particles, the top quark completes the <br>third quarks generation.<br>It was directly observed in 1995 during Tevatron Run I at $\sqrt{s}=1.8$ <br>TeV <br>by both CDF and D0 experiments \cite{top1,top2,top3}. It is the most massive elementary known particle up until now, with a mass of 172.7 $\pm$ 1.1(stat + syst) GeV/c$^{2}$ \cite{masstop}, about 35 times larger than the mass oft the next heavy quark and very close to the scale of the electroweak symmetry breaking.<br><br>Produced in Tevatron in proton-antiproton collisions via strong interactions, top quark decays trough weak interaction to a $W$ boson and a down-type quark $q$ ($d$,$s$,$b$) before forming hadrons, giving the possibility to study the properties of a \textit{bare} quark. In the Standard Model the decay rate is proportional to $\left|V_{tq}\right|^{2}$, the Cabibbo-Kobayashi-Maskawa (CKM) matrix element. Since the assumption of three generation of quarks and the unitarity of the CKM matrix lead to $\left |V_{tb}\right|=0.99915^{+0.00003}_{-0.00005}$ \cite{PDG}, it can be assumed that top quark decays exclusively to $Wb$. On the other hand, if more than three generation of quarks are allowed, the constraint on $\left|V_{tb}\right|$ is removed and lower values are possible, affecting top cross section measurements, B mixing and CP violation. <br><br>A direct measurement of $\left|V_{tb}\right|$ matrix element can be <br>obtained <br>measuring the single top production cross section, but a value can be <br>extracted <br>from the top quark decay rate in the $t \bar t$ channel. It is possible to define $R$ as the ratio of the branching fractions:<br>\begin{equation}<br>R =\frac{\mathscr{B}(t\rightarrow Wb)}{\mathscr{B}(t\rightarrow Wq)} =\frac{\left|V_{tb}\right|^2}{\left|V_{tb}\right|^2+\left|V_{ts}\right|^2+\left|V_{td}\right|^2}<br>\end{equation}<br>expected to be $0.99830^{+0.00006}_{-0.00009}$ if the same constraints are assumed. <br><br>In this analysis we measured directly the the ratio of the branching <br>fractions R using a data sample corresponding to 7.5 fb$^{-1}$ collected <br>at the CDF detector at $\sqrt{s}=$1.96 TeV. The analysis is performed in <br>the lepton plus jets (l+jets) channel, where one $W$ boson, coming from <br>$t\bar t \rightarrow W^{+}qW^{-}\bar q$, decays hadronically while the <br>second decays in a charged lepton and a neutrino. CDF performed several <br>measurements of $R$ both during Run I and Run II, combinating the l+jets <br>channel with the dilepton channel, where both of $W$ bosons produced by <br>top pairs decay leptonically. The last measurement found a central value <br>of $R=1.12^{+0.21}_{-0.19}$(stat)$^{+0.17}_{0.13}(syst)$ using <br>an integrated <br>luminosity of 162 pb$^{-1}$, extracting $R&gt;0.61$ at 95\% CL. The D\O\<br>collaboration has measured recently $R$, using 5.4 fb$^{-1}$, with a <br>simultaneous fit on the top pair production cross section, in the l+jets <br>and dilepton channels. Their result is $R=0.90\pm0.04$(stat+syst) and <br> $R&gt;0.79$ at 95\% CL.<br><br>Since the uncertainty on the central value measured by CDF was dominated by the statistical error, we decided to perform a new measurement adding the new datasets.<br><br>My analysis is based on the determination of the number of b-jets in $t <br>\bar t$ events using the l+jets sample with more than three jets in the <br>final state. We consider events in which the charged leptons are either <br>electrons or muons. Identification of jets coming from b-quark <br>fragmentation (b-jet \textit{tagging}) is performed by <br>the \textit{SecVtx} <br>algorithm, based on the reconstruction of displaced secondary vertices. <br><br>We divided our sample in subsets according to the type of lepton, number <br>of jets in the final states and events with zero, one or two tags. <br>The comparison between the total prediction, given by the sum of the expected $t\bar t$ events and background estimate, and the observed data in each subsample is made using a Likelihood function. Our measured value for R is that one which maximizes the Likelihood, i.e. gives the best match between the observed events and prediction. <br><br>Our final measurement of R is obtained recursively performing a <br>simultaneous fit also to top <br>pair production cross section.<br>We obtain $R=0.92 \pm 0.07$ (stat+syst) and <br>$\sigma_{p\bar p \rightarrow t \bar t} = 7.4 \pm 0.6$ pb. <br>Assuming the unitarity of the CKM matrix and three generation of quarks <br>we obtained $\left|V_{tb}\right| = 0.95 \pm 0.04$, in agreement with the <br>Standard Model prediction.