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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
third quarks generation.
It was directly observed in 1995 during Tevatron Run I at $\sqrt{s}=1.8$
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.

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.

A direct measurement of $\left|V_{tb}\right|$ matrix element can be
measuring the single top production cross section, but a value can be
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:
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}
expected to be $0.99830^{+0.00006}_{-0.00009}$ if the same constraints are assumed.

In this analysis we measured directly the the ratio of the branching
fractions R using a data sample corresponding to 7.5 fb$^{-1}$ collected
at the CDF detector at $\sqrt{s}=$1.96 TeV. The analysis is performed in
the lepton plus jets (l+jets) channel, where one $W$ boson, coming from
$t\bar t \rightarrow W^{+}qW^{-}\bar q$, decays hadronically while the
second decays in a charged lepton and a neutrino. CDF performed several
measurements of $R$ both during Run I and Run II, combinating the l+jets
channel with the dilepton channel, where both of $W$ bosons produced by
top pairs decay leptonically. The last measurement found a central value
of $R=1.12^{+0.21}_{-0.19}$(stat)$^{+0.17}_{0.13}(syst)$ using
an integrated
luminosity of 162 pb$^{-1}$, extracting $R>0.61$ at 95\% CL. The D\O\
collaboration has measured recently $R$, using 5.4 fb$^{-1}$, with a
simultaneous fit on the top pair production cross section, in the l+jets
and dilepton channels. Their result is $R=0.90\pm0.04$(stat+syst) and
$R>0.79$ at 95\% CL.

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.

My analysis is based on the determination of the number of b-jets in $t
\bar t$ events using the l+jets sample with more than three jets in the
final state. We consider events in which the charged leptons are either
electrons or muons. Identification of jets coming from b-quark
fragmentation (b-jet \textit{tagging}) is performed by
the \textit{SecVtx}
algorithm, based on the reconstruction of displaced secondary vertices.

We divided our sample in subsets according to the type of lepton, number
of jets in the final states and events with zero, one or two tags.
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.

Our final measurement of R is obtained recursively performing a
simultaneous fit also to top
pair production cross section.
We obtain $R=0.92 \pm 0.07$ (stat+syst) and
$\sigma_{p\bar p \rightarrow t \bar t} = 7.4 \pm 0.6$ pb.
Assuming the unitarity of the CKM matrix and three generation of quarks
we obtained $\left|V_{tb}\right| = 0.95 \pm 0.04$, in agreement with the
Standard Model prediction.