All known observations in subnuclear physics are described
with an astonishing precision
by the Standard Model. In spite of this success the Standard Model
has theoretical shortcomings, to cite a few of them:
it does not include the gravity, it
does not unify the four known interactions and it has too many parameters
whose value is not predicted. All these problems indicate
that phenomena should exist which the Standard Model does not deal with.
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Figure 1 : D0 Liquid Argon Calorimeter
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Figure 2 : Particle generations
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One of the ways these phenomena may be discovered is to search
for heavy new particles/resonances, with mass in the range of
several hundreds of GeV. Being the highest energy accelerator
in the world until the startup of the LHC, the Tevatron is
the best machine today to carry out such searches. Besides the
collision energy and luminosity, mass resolution and low
background level are of crucial importance in this matter.
The liquid argon calorimeter (figure 1.) of the D0 detector has an
excellent energy resolution for
electrons
and jets providing
almost a background free Z signal (figure 3.). Combining the reconstructed
Z's in the two-electron decay channel with the jet
of the highest transverse energy in the event
a mass resolution of ~9% is achieved. This, together
with the high luminosity delivered in Run II, offers unprecedented
good conditions to observe high mass resonances decaying
into a Z and a quark (figure 4.).
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Figure 3 : e+ e- invariant Mass
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Figure 4 : Feynman graph of the q* production
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An example of such resonances could be
an excited quark if quarks would have internal structure.
As of today the ultimate building blocks of the matter is
thought to be the leptons and the quarks (figure 2.). If
these blocks, e.g. the quarks would have constituents, called
"preons", there would exist excited states of the quarks,
which would be heavy, and which would de-excite, i.e. return
to the quark ground state, emitting an intermediate boson:
a gluon, a photon, a W or a Z boson. The invariant mass of the
particle jet produced by the quark and the emitted boson
would then exhibit a resonance shape.
In the present analysis we have searched for such an enhancement
in the invariant mass distribution of the leading particle jet
and a Z boson in the collision of protons with antiprotons.
The Standard Model predicts an exponentially falling background.
The data (once again!) confirm the prediction of the Standard Model and do not show any enhancement at masses in the 100-700 GeV
energy range as shown in Figure 5. One can also see in that
Figure the expected shape of a 500 GeV excited quark.
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Figure 5 : Z + jet invariant Mass
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Figure 6 : Upper limits on resonance cross section times branching fraction at 95% CL
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We have therefore determined upper limits of the cross section
times the branching fraction into the
Z+q (Z->e+e-)
final state
of such a hypothetical resonance as functions of the resonance
mass and width (figure 6.). Excited quarks, if they exist, must
have smaller production
cross section
in the
Z+q (Z->e+e-)
final state
at a 95% confidence level.
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