Jet measures and hadronic event shapes at the CERN <img src="/fulltext-image.asp?format=htmlnonpaginated&src=K052H23476586206_html\10052_2005_Article_BF01579132_TeX2GIFIE1.gif" border="0" alt=" $$\bar p$$ " />p collider

We analyze the energy density distribution in harronic final states as a function of their total transverse energy measured in the segmented central calorimeter of the UA2 detector. The energy dependence of collective shape variables is investigated. The data, collected at the CERN <img src="/fulltext-image.asp?format=htmlnonpaginated&src=K052H23476586206_html\10052_2005_Article_BF01579132_TeX2GIFIE2.gif" border="0" alt=" $$\bar p$$ " />p Collider at <img src="/fulltext-image.asp?format=htmlnonpaginated&src=K052H23476586206_html\10052_2005_Article_BF01579132_TeX2GIFIE3.gif" border="0" alt=" $$\sqrt s = 630$$ " /> GeV, exhibit strong variations in all these variables over the transverse energy range between 15 and 210 GeV, corresponding to substantial modifications in the structure of multihadronic final states. The evolution of the energy density distribution and of the collective shape variables shows a clear transition between two extreme dynamical regimes, respectively dominated byp_T-limited phase space and by collimated two-jet configurations. A study of the relative populations of two-and three-jet systems reveals two different sources of configurations having three distinct lobes in the pattern of the energy-flow. A first component, steeply falling with energy, receives substantial contributions from soft parton collisions at lower transverse energies. Above 60 GeV a new hard component emerges, characterized by an approximately constant rate with respect to the dominant two-jet structures.