Virtual Photon Structure from Jet Production at HERA

Physics Letters B, 1997

The transition between photoproduction and deep-inelastic scattering is investigated in jet production at the HERA ep collider, using data collected by the H1 experiment. Measurements of the differential inclusive jet cross-sections dσ ep /dE * t and dσ ep /dη * , where E * t and η * are the transverse energy and the pseudorapidity of the jets in the virtual photon-proton centre of mass frame, are presented for 0 < Q 2 < 49 GeV 2 and 0.3 < y < 0.6. The interpretation of the results in terms of the structure of the virtual photon is discussed. The data are best described by QCD calculations which include a partonic structure of the virtual photon that evolves with Q 2 .

arXiv (Cornell University), 1997

Some aspects of extracting the information on the structure of virtual photons from jet production in ep and e + e − collisions are discussed.

The European Physical Journal C - Particles and Fields, 2002

The dijet cross section in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb −1 . The events were required to have a virtuality of the incoming photon, Q 2 , of less than 1 GeV 2 and a photon-proton centre-of-mass energy in the range 134 < W γp < 277 GeV. Each event contains at least two jets satisfying transverse-energy requirements of E jet1 T > 14 GeV and E jet2 T > 11 GeV and pseudorapidity requirements of −1 < η jet1,2 < 2.4. The measurements are compared to next-to-leading-order QCD predictions. The data show particular sensitivity to the density of partons in the photon, allowing the validity of the current parameterisations to be tested.

Physical Review D, 2000

Interactions of virtual photons are analyzed in terms of photon structure. It is argued that the concept of parton distribution functions is phenomenologically very useful even for highly virtual photons involved in hard collisions. The role of longitudinal photons for proper interpretation of jet cross-section in the region of moderate virtualities accessible at HERA is investigated.

The European Physical Journal C, 1998

The European Physical Journal C, 2004

The dependence of dijet production on the virtuality of the exchanged photon, Q 2 , has been studied by measuring dijet cross sections in the range 0 Q 2 < 2000 GeV 2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb −1 . Dijet cross sections were measured for jets with transverse energy E jet T > 7.5 and 6.5 GeV and pseudorapidities in the photon-proton centre-of-mass frame in the range −3 < η jet < 0. The variable x obs γ , a measure of the photon momentum entering the hard process, was used to enhance the sensitivity of the measurement to the photon structure. The Q 2 dependence of the ratio of low-to high-x obs γ events was measured. Next-to-leading-order QCD predictions were found to generally underestimate the low-x obs γ contribution relative to that at high x obs γ . Monte Carlo models based on leading-logarithmic parton-showers, using a partonic structure for the photon which falls smoothly with increasing Q 2 , provide a qualitative description of the data.

The European Physical Journal C-Particles and Fields, 1998

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

Physical Review D, 1996

The feasibility of an experimental determination of the parton distributions of virtual photons at high energy ep colliders is studied in the context of the DESY-HERA collider.

1994

Inclusive jet differential cross sections for the reaction ep ~ jet + X at Q2 below 4 GeV 2 have been measured with the ZEUS detector at HERA using an integrated luminosity of 0.55 pb -t. These cross sections are given in the kinematic region 0.2 < y < 0.85, for jet pseudorapidities in the ep-laboratory range -1 < ~t < 2 and refer to jets at the hadron level with a cone radius of one unit in the 7/-~b plane. These results correspond to quasi-real photoproduction at centre-of-mass energies in the range 130-270 GeV and, approximately, for jet pseudorapidities in the interval -3 < r/~t(yp CMS) < 0. These measurements cover a new kinematic regime of the partonic structure of the photon, at typical scales up to ,-~300 GeV 2 and photon fractional momenta down to x~, ,-, 10 -2. Leading logarithm parton shower Monte Carlo calculations, which include both resolved and direct processes and use the predictions of currently available parametrisations of the photon patton distributions, describe in general the shape and magnitude of the measured ~et and E~ t distributions.

European Physical Journal C, 2007

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential γ+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and xγobs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the kT-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the kT-factorisation calculations are in good agreement with the data.