Photon plus jet production in large-Q2 ep collisions at next-to-leading order QCD

Ridder, A. Gehrmann–De; Krämer, G.; Spiesberger, H.

doi:10.1016/s0370-2693(99)00646-2

Cited by 6 publications

(7 citation statements)

References 19 publications

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“…NLO corrections, O(α 3 α s ), are known to the γ+(1+1)-jet cross section in deep inelastic scattering [22] for some time already; this calculation was found in good agreement with experimental data recently [14]. The derivation of NLO corrections to the γ + (0 + 1)-jet cross section and the inclusive photon cross section in deep inelastic scattering is however considerably more involved.…”

Section: Discussionsupporting

confidence: 74%

“…In addition to measuring the inclusive isolated photon cross section, the ZEUS collaboration also analysed [14] the production of prompt photons in association with hadronic jets. This measured cross section was then compared with the NLO calculation [22] of the γ + (1 + 1)-jet cross section: the cross section for the production of a photon-jet and one additional hadron-jet in the final state (n-jet observables in DIS are usually denoted by (n + 1)-jet observables where the +1 stands for the unobserved jet coming from the proton remnant). Data and theory were found in good agreement.…”

Section: Introductionmentioning

confidence: 99%

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Measuring the photon fragmentation function at HERA

2006

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The production of final state photons in deep inelastic scattering originates from photon radiation off leptons or quarks involved in the scattering process. Photon radiation off quarks involves a contribution from the quark-to-photon fragmentation function, corresponding to the non-perturbative transition of a hadronic jet into a single, highly energetic photon accompanied by some limited hadronic activity. Up to now, this fragmentation function was measured only in electron-positron annihilation at LEP. We demonstrate by a dedicated parton-level calculation that a competitive measurement of the quark-to-photon fragmentation function can be obtained in deep inelastic scattering at HERA. Such a measurement can be obtained by studying the photon energy spectra in γ + (0 + 1)-jet events, where γ denotes a hadronic jet containing a highly energetic photon (the photon jet). Isolated photons are then defined from the photon jet by imposing a minimal photon energy fraction. For this so-called democratic clustering approach, we study the cross sections for isolated γ + (0 + 1)-jet and γ + (1 + 1)-jet production as well as for the inclusive isolated photon production in deep inelastic scattering.

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Section: Discussionsupporting

confidence: 74%

Section: Introductionmentioning

confidence: 99%

Measuring the photon fragmentation function at HERA

2006

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“…In [5], the isolated-photon-plusone-jet cross section was also measured and found in good agreement with the theoretical prediction [12]. This observation renders the discrepancy in the inclusive isolated photon cross section even more intriguing, since the inclusive cross section can in principle be obtained by summing all isolated-photon-plus-n-jet cross sections, starting with n = 0.…”

supporting

confidence: 75%

“…The factorization procedure of final state collinear singularities in fragmentation functions used here is of the same type as the procedure used to absorb initial state collinear singularities into the parton distribution functions [15]. In the present calculation, we use the phase space slicing method [16] to handle the collinear quark-photon singularity, as described in detail in [12,17]. As a result, the parton-level cross sectionσ(eq → eγq) contains a collinear divergence, which is compensated by adding the parton-level cross section for the fragmentation procesŝ σ(eq → eq) ⊗ D q→γ (z) to it.…”

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confidence: 99%

Isolated Photons in Deep Inelastic Scattering

2006

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Photon radiation at large transverse momenta at colliders is a detailed probe of hard interaction dynamics. The isolated photon production cross section in deep inelastic scattering was measured recently by the ZEUS experiment, and found to be considerably larger than theoretical predictions obtained with widely used event generators. To investigate this discrepancy, we perform a dedicated parton-level calculation of this observable, including contributions from fragmentation and largeangle radiation. Our results are in good agreement with all aspects of the experimental measurement.PACS numbers: 12.20. Ds, 13.60.Hb, 13.85.Qk Production of final state photons is a classical particle physics measurement, since its cross section can be computed in principle in the well-established framework of Quantum Electrodynamics. In turn, photon production cross sections were suggested as a probe of auxiliary quantities in particle physics, such as the measurement of the gluon distribution in the proton from isolated photon production at hadron colliders. With the advent of precise data on this observable (see [1] for the most recent data), it was realized that the theoretical description of photons produced in reactions also involving final-state hadrons is less straightforward than expected, resulting in extensive theoretical discussion. For isolated photon production at hadron colliders, it turns out that effects from the fragmentation of a hadronic jet into a single, highly energetic photon [2] and effects from the resummation of threshold and recoil corrections [3,4] had to be included to obtain a satisfactory description of experimental data. A very sensitive issue is the definition of isolated photons produced in association with hadrons, since a completely isolated photon is not an infrared safe observable in quantum chromodynamics (QCD). At present, this isolation is usually accomplished experimentally by admitting only a limited amount of hadronic energy inside a cone around the photon direction.Recently, the ZEUS collaboration at DESY HERA reported a measurement [5] of the inclusive production cross section for isolated photons in deep inelastic scattering (DIS). The normalization of the experimentally determined cross section turned out to exceed the cross section expected from the multi-purpose event generator programs HERWIG [9] and PYTHIA [10] by factors 7.9 and 2.3 respectively. Even after normalizing the total event rate, none of these programs was able to describe all kinematical dependencies of the measured cross section. Since the same event generator programs are used frequently to estimate photon production backgrounds for new particle searches in other collider environments, it appears to be very important to determine the origin of these large discrepancies. In a subsequent study [11] the ZEUS measurement was analyzed in view of determining the photon distribution in the proton, relevant for electroweak radiative corrections at colliders.By further analyzing the hadronic final state in isolated phot...

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“…In this case, NLO predictions are available [27,28,29], which describe the shape of the η γ distribution reasonably well, but still underestimate the data [22,26].…”

Section: Prompt Photons In Dismentioning

confidence: 97%