The shape of jets produced in quasi-real photon-proton collisions at
centre-of-mass energies in the range 134−277 GeV has been measured using the
hadronic energy flow. The measurement was done with the ZEUS detector at HERA.
Jets are identified using a cone algorithm in the η−ϕ plane with a
cone radius of one unit. Measured jet shapes both in inclusive jet and dijet
production with transverse energies ETjet>14 GeV are presented. The jet
shape broadens as the jet pseudorapidity (ηjet) increases and narrows
as ETjet increases. In dijet photoproduction, the jet shapes have been
measured separately for samples dominated by resolved and by direct processes.
Leading-logarithm parton-shower Monte Carlo calculations of resolved and direct
processes describe well the measured jet shapes except for the inclusive
production of jets with high ηjet and low ETjet. The observed
broadening of the jet shape as ηjet increases is consistent with the
predicted increase in the fraction of final state gluon jets.Comment: 29 pages including 9 figure