Jet Shapes

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 $p\bar{p}$ experiments. Jets are measured using the cone algorithm with a cone radius of 1. The jet shape, $\psi(r)$, 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 $\psi(r)$=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 $\simeq$ 70% of their transverse energy within a sub-cone radius of 0.2, consistent with well-collimated quark jets. In contrast, the $p\bar{p}$ data jets are rather broad, with only $\simeq$ 50% of their transverse energy being contained within the same sub-cone radius, consistent with predominantly gluon jets in this E_T range.

  

Figure: Comparison of jet shape measurements from ZEUS(DIS), OPAL(e⁺e^-), CDF and 0 ($p\bar{p}$). The jet energy ranges are $37<E_T^{\rm jet}<45$ GeV, 35 GeV$<E^{\rm jet}$, $40<E_T^{\rm jet}<60$ GeV and $45<E_T^{\rm jet}<70$ GeV, respectively.

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 $R_{\rm sep}$ 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 $R_{\rm sep}$ parameter which increases with increasing rapidity in the proton direction, but which is in the range $1.3 < R_{\rm sep} < 1.8$.Differential distributions of the average transverse energy in intervals of cone radius will enable $R_{\rm sep}$ to be fitted and provide further constraints on the models.