Professor Amanda Cooper-Sarkar

Abstract:

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (xγOBS) and pomeron (βOBS) momentum participating in the production of the dijet system. The observed xγOBS dependence shows evidence for the presence of a resolved-as well as a direct-photon component. The measured cross section dσ/dβOBS increases as βOBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.

Abstract:

Dijet cross sections are presented using photoproduction data obtained with the ZEUS detector during 1994. These measurements represent an extension of previous results, as the higher statistics allow cross sections to be measured at higher jet transverse energy (E ). Jets are identified in the hadronic final state using three different algorithms, and the cross sections compared to complete next-to-leading order QCD calculations. Agreement with these calculations is seen for the pseudorapidity dependence of the direct photon events with E > 6 GeV and of the resolved photon events with E > 11 GeV. Calculated cross sections for resolved photon processes with 6 GeV < E < 11 GeV lie below the data. T T T T jet jet jet jet

Abstract:

Elastic and proton-dissociative ρ0 photoproduction (γp → p0p, γp → ρ0N, respectively, with p0 → π+π-) has been studied in ep interactions at HERA for photon-proton centre-of-mass energies in the range 50 < W < 100 GeV and for |t| < 0.5 GeV2, where t is the square of the four-momentum transfer at the proton vertex; the results on the proton-dissociative reaction are presented for masses of the dissociated proton system in the range M2N < 0.1W2. For the elastic process, the π+π- invariant mass spectrum has been investigated as a function of t. As in fixed target experiments, the ρ0 resonance shape is asymmetric; this asymmetry decreases with increasing |t|, as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant π+π- production. The cross section has been studied as a function of W; a fit to the resonant part with the form Wa gives a = 0.16 ± 0.06 (stat.) +0.11-0.15 (syst.). The resonant part of the γp → π+π-p cross section is 11.2 ± 0.1 (stat.) +1.1-1.2 (syst.) μb at 〈W〉 = 71.7 GeV. The t dependence of the cross section can be described by a function of the type Aρ exp (-bρ|t| + cρt2) with bρ = 10.9 ± 0.3 (stat.) +1.0-0.5 (syst.) GeV-2 and cρ = 2.7 ± 0.9 (stat.) +1.9-1.7 (syst.) GeV-4. The t dependence has also been studied as a function of W and a value of the slope of the pomeron trajectory α IP′ = 0.23 ± 0.15 (stat.) +0.10-0.07 (syst.) GeV-2 has been deduced. The ρ0 spin density matrix elements r0400 r041-1 and ℜe[r0410] have been measured and found to be consistent with expectations based on s-channel helicity conservation. For proton-dissociative π+π- photoproduction in the ρ0 mass range, the distributions of the two-pion invariant mass, W and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The t distribution has been fitted to an exponential function with a slope parameter 5.8 ± 0.3 (stat.) ± 0.5 (syst.) GeV-2. The ratio of the elastic to proton-dissociative ρ0 photoproduction cross section is 2.0 ± 0.2 (stat.) ± 0.7 (syst.).

Abstract:

Inclusive jet differential cross sections for the reaction e+p → e+ + jet + X with quasi-real photons have been measured with the ZEUS detector at HERA. These cross sections are given for the photon-proton centre-of-mass energy interval 134 < W < 277 GeV and jet pseudorapidity in the range -1 < ηjet < 2 in the laboratory frame. The results are presented for three cone radii in the η - φ plane, R = 1.0, 0.7 and 0.5. Measurements of dσ/dηjet above various jet-transverse-energy thresholds up to 25 GeV and in three ranges of W are presented and compared to next-to-leading order (NLO) QCD calculations. For jets defined with R = 1.0 differences between data and NLO calculations are seen at high ηjet and low EjetT. The measured cross sections for jets defined with R = 0.7 are well described by the calculations in the entire measured range ηjet and EjetT. The inclusive jet cross section for EjetT > 21 GeV is consistent with an approximately linear variation with the cone radius R in the range between 0.5 and 1.0, and with NLO calculations.

Abstract:

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 EjetT > 14 GeV are presented. The jet shape broadens as the jet pseudorapidity (ηjet) increases and narrows as EjetT 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 EjetT. The observed broadening of the jet shape as ηjet increases is consistent with the predicted increase in the fraction of final state gluon jets.