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The shape of the transverse energy distribution of particles within
a jet produced in various interactions allows the primary parton
source of the jet to be identified. In addition, the data provide strong
constraints on the coherence properties of the showering partons and
enable tests of the universality of the fragmentation process.
In an analysis from the ZEUS Collaboration [24], the jet shapes
measured in photoproduction and DIS were compared with those from e+e-
annihilation and experiments. Jets are measured using the cone
algorithm with a cone radius of 1. The jet shape, , is defined as
the average fraction of the jet's transverse energy that lies within an
inner cone of radius r. The distributions shown in Fig. 4 are
therefore integral plots
with =1 at r=1, whose rate of fall-off measures how broad the jet
is. The data shown are for minimum jet transverse energies around 40 GeV.
It is observed that the DIS and e+e- data contain 70%
of their transverse energy within a sub-cone radius of 0.2, consistent with
well-collimated quark jets. In contrast, the data jets are
rather broad, with only 50% of their transverse energy being
contained within the same sub-cone radius, consistent with predominantly
gluon jets in this ET range.
Figure:
Comparison of jet shape measurements from ZEUS(DIS),
OPAL(e+e-), CDF and 0 ().
The jet energy ranges are GeV,
35 GeV, GeV and
GeV, respectively.
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Photoproduction data (see Fig. 2 in [24]) were also studied
as a function of
pseudorapidity and transverse energy. The observed changes in jet shape
were reproduced in models which incorporate both direct and resolved photon
processes provided that the resolved processes include the multiple
interactions discussed above.
NLO calculations from Klasen and Kramer [25]
determine the jet shape only at the lowest non-trivial order.
In order to describe the data,
an parameter is
introduced which determines when two partons are merged into a single jet.
The jet shape distribution is well described by NLO calculations with an
parameter which increases with increasing rapidity in the proton
direction, but which is in the
range .Differential distributions of the average transverse energy in intervals
of cone radius will enable to be fitted and provide further
constraints on the models.
Next: High-ET Jet Results from
Up: QCD Effects in Hadronic
Previous: Event Shapes
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