CDF Physics analyses by the Liverpool group
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- W+photon and Z+photon Production:
The electroweak sector of the Standard Model is described by the non-Abelian SU(2)xU(1) group.
The non-Abelian structure of the group implies that the electroweak gauge bosons can interact
with each other, and their coupling to each other is precisely predicted in the Standard Model.
It is thus interesting to probe the Standard Model prediction, and test its validity. At CDF this can
be done by looking for W+photon and Z+photon events and compare the cross section and kinematic
distributions to the Standard Model prediction. The Liverpool group made a
first measurement of these
processes at CDF Run II. The publication can be found
here The cross section for W+photon
production was measured to be 18.1±3.1 pb compared to a theoretical prediction of 19.2±1.4 pb. For Z+photon
production the cross section was measured to be 4.6±0.6 pb in good agreement with the theoretical prediction
of 4.5±0.3 pb.
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- B-jet production:
The measurement of the production cross section of two bottom quarks is a fundamental
test of perturbative QCD. For this measurement two b-jets with ET> 15 GeV are identified via the presence
of a secondary decay vertex from the b-hadron. The b-fraction is fitted using the mass at the secondary vertex.
The
resulting cross section measurement is in good agreement with QCD predictions.
- photon+b and Z+ b production:
Heavy flavour production in association with a photon or Z-boson is sensitive to the heavy flavour
content of the proton which is relatively poorly known, particularly at high Bjorken-x values.
The Liverpool group has measured both photon+b and photon+c
and Z+b production.
- exclusive diphoton and dielectron production:
There has recently been a large interest in the exclusive production of Higgs
bosons as a possible mode at the LHC that could aid the Higgs boson discovery or the measurement
of the Higgs boson quantum numbers. Here, exclusive production means that the two protons stay intact, and
all there is seen in the detector are the decay products of the Higgs boson. At the Tevatron the cross
section for Higgs production is too low but the very similar process of diphoton production is accessible.
A nice QED prorcess that can possibly be used to constrain the luminosity is also the exclusive
production of electron pairs. Neither of these processes had so far been seen in proton-proton collisions.
At CDF the Liverpool group has contributed to a first measurement of the cross sections for these processes.
Here are some very preliminary results
on these studies.
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- Bs oscillations:
One of the major puzzles in our Universe is the large asymmetry between matter and anti-matter. It seems
that all the anti-matter that had been present in the early Universe has disappeared at some point, and
there is no known mechanism in the Standard Model that explains that. The only mechanism known
to produce any asymmetry is the mixing in the quark sector that has been observed with neutral kaons
and neutral Bd mesons. At CDF there is a unique opportunity to observe this mixing in Bs mesons. These
mesons contain a b-quark and a s-quark while the Bd meson contains a b- and a d-quark. This different
quark content changes the nature of the oscillation dramatically. The oscillations are
expected to be much faster in the Bs than the Bd system.
The CDF Liverpool group has contributed to the search for these oscillations in the
semileptonic and
hadronic decay modes of the Bs meson.
The
combined result is one of the world's best results.
- rare B decays:
The CDF Liverpool group studies rare decays of the Bd, Bs and Lambdab particles into a dimuon pair and
a kaon, K*, phi-meson or Lambda particle. These decays are very rare in the Standard Model but can be enhanced
in new physics models such as supersymmetry.
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- Standard Model Higgs:
The CDF Liverpool group is interested in searching for the Standard Model Higgs boson. The current focus
is the associated production of the Higgs boson with a Z boson. It is expected that with 2 fb-1 of data
a 115 GeV SM Higgs boson can be probed beyond the current world's best sensitivity of LEP.
- Supersymmetry:
Supersymmetry provides an elegant solution to many of the deficiencies of the Standard Model.
E.g. it provides a natural candidate for the cold dark matter in the Universe, it allows
for a unification of the electroweak and strong forces, and it solves the hierarchy problem.
At CDF we have searched for trilepton production which can occur when SUSY particles are produced.
It is one of the best discovery modes for SUSY at the Tevatron.
Preliminary results
are available in the mode where two leptons are electrons.
- Extra Dimensions:
An alternative solution to the hierarchy problem is the presence of extra dimensions at the TeV scale.
In the model developed by Lisa Randall and Raman Sundrum
this is achieved by having one warped (or highly curved) extra dimension.
At CDF this extra dimension can lead to the exchange of a virtual graviton that will signal
as a peak in the invariant mass distributions of two leptons or two photons. We have searched
for this signature in the diphoton decay mode.
So far no evidence for extra dimensions has been found.